mathematical model of the central battery for a …
TRANSCRIPT
c MATHEMATICAL MODEL OF THE CENTRAL BATTERY
FOR A MAJOR OIL PRODUCING FIELD
by
DAVID RANDELL SKINNER, B.S. in E.E.
A THESIS
IN
ELECTRICAL ENGINEERING
Submitted to the Graduate Faculty of Texas Tech University in
Partial Fulfillment of the Requirements for
the Degree of
MASTER OF SCIENCE
IN
ELECTRICAL ENGINEERING
Approved
Accepted
May, 1976
hCT-l'^' '^
' 7
^
ACKNOWLEDGEMENTS
I gratefully acknowledge the direction of Dr. Darrcll
L. Vines in the preparation of this thesis. I also appreciate
the helpful criticism of the other members of my com'nittee.
Dr. Donald L. Gustafson and Dr. Herald W. Winkler.
11
CONTENTS
ACKNOWLEDGEMENTS ii
LIST OF FIGURES v
I. INTRODUCTION 1
Water, By-Product of Crude Oil Production . . 1
Central Battery Facilities in Oil Producing
Operations 7
Computer Modeling of a Central Battery. . . . 14
Method of Constructing the Model of a Central Battery 16
II. DEVELOPMENT OF MATHEMATICAL RELATIONSHIPS DESCRIBING
THE OPERATION OF THE CENTRAL BATTERY 19
Inlet Surge Tank ?2
Electrostatic Dehydrator 41
Cooling, Sales, and Recycle Tanks end LACT
Units 56
Recycle Dehydrators 64
Produced and Fresh Water Injection System . . 70
Interconnection of Vessels in the Central Battery 75 Model Flowchart Development 76
III. ANALYSIS OF MODEL 77 Comparison of Model Operation to Actual
Operation 7 7
Stability Analysis of the Continuous System . 88
Discrete System Behavioral Analysis 96
IV. CONCLUSION 101 • • •
111
LIST OF REFERENCES 1C3
APPENDIX 104
A. PARAMETER DESIGNATIONS 105
B. CENTRAL BATTERY MODEL FLOWCHART 112
IV
LIST OF FIGURES
1. Emulsion Droplet Behavior 6
2. General Central Battery Schematic Diagram 10
3. West Texas Central Battery Piping Diagram 12
4. West Texas Central Battery Schematic Diagram 20
5. Inlet Surge Tank Schematic Diagram 23
6. Analogous System for Inlet Surge Tank Temperature. . . 31
7. Electrostatic Dehydrator Schematic Diagram 42
8. Cooling, Sales, and Recycle Tanks and LACT Units Diagram 57
9. Recycle Dehydrator Schematic Diagram 65
10. Simplified Dehydration System 89
n . Continuous Analogy of Simplified Dehydration System. . 97
12. Discrete Analogy of Simplified Dehydration System. . . 99
CHAPTER I
INTRODUCTION'
In the last ten years the increase in the application
of technology to the practices of producing crude oil has
been phenomenal. This growth has been caused in part by the
evolution of knowledge in all technical fields and to a
great extent by the prediction of a shortage of petroleum
products in the next few years. This problem has made more
efficient production and handling of crude oil absolutely
necessary. The following work is a study of application of
computer modeling to field-processing of crude oil.
Water, By-Product of Crude Oi1 Production
The exact method by which the mixture of hydrocarbons
called crude oil was developed is only partially known.
Several theories have been proposed and can be supported
by geological and chemical data [1]. For this study, it
is sufficient to say that in a period of many thousands of
years and by the action of high pressure and temperature,
organic material deposited in the dim past was transformed
into liquid and gaseous hydrocarbons far below the surface
of the earth.
By whatever processes oil was created, saltwater was
almost invariably present. In present times, it can be said
that water is present to some degree in every reservoir where
oil and gas are found [2], 1
In some oil reservoirs water may be present as droplets
that are small in comparison to the size of oil droplets or
as thin films of water on the inside surface of the small,
interconnected pores of the reservoir rock or sand. This
portion of water in the reservoir is referred to as the connate
water saturation. Because of "wetting" tendencies of the rock
or surface tension of the water, this water is not expected
to flow through the reservoir into a well throughout the
producing life of the reservoir and therefore is not expected
to cause any water-related problems in the recovery of the oil.
In the vast majority of reservoirs, an appreciable volume
of water exists in the oil-bearing part of the reservoir or
is present without oil in a layer of the reservoir adjacent
to the oi1-productive portion. This layer of water-productive
reservoir is commonly called an aquifer. In either case,
water is expected to be recovered in appreciable quantities
with oil throughout the life of a reservoir.
Another cause for recovering water with oil is man-made.
In some secondary recovery techniques for increasing the amount
of oil that can be recovered from the reservoir rock water is
intentionally pumped into the oil-bearing reservoir in order
to push the oil toward a pumping well. This water eventually
reaches the well and is pumped to the surface with the oil.
The effects of water in a reservoir on recovery mechanisms
are far reaching; however, the effects of water mixed with oil
on surface equipment are more important in the actual sales of
oil to refineries.
The chemical and physical reactions involved in the
separation of crude oil into its components such as gasoline
and heating oil are quite complex. These reactions cannot
occur as required when water is present in any but the most
minute quantities. Therefore, refineries require that water
comprises a small proportion of the inlet stream.
Transportation facilities such as pipelines also present
problems when water is present in oil. Most pipeline operations
do not operate treating facilities and must impose the same
limitations on water quantity as do the refining operations.
Water generally occurs in pipelines as emulsions of oil and
water. These emulsions do not behave as pure oil or water
and cause significant problems by their viscosity effects.
Water is a corrosive agent alone and forms other par
ticularly corrosive compounds when mixed with other materials
commonly found with crude oil [31. These corrosive elements
cause eventual disintegration of pumps and pipelines. The
corrosion of metal equipment is a relatively minor problem
in comparison to the previous problems.
The three problems defined above indicate that water must
be removed from a mixture of oil and water before the oil
can be sent to a refining plant. In every crude oil production
operation, a portion of the equipment used to obtain the oil
must be used for water removal.
One reaction between oil and water deserves particular
attention because this reaction increases the difficulty of
separating oil and water. When oil and water are mixed by
agitating, an emulsion is formed. An emulsion is a collection
of droplets of oil in water or water in oil. Surface tension
of the external phase generally retains the integrity of the
droplet and prevents it from coalescing with other droplets.
Often, the external surface of an emulsion droplet is a
semisolid material that resists mechanical damage. Some of
the most difficult emulsions to separate are formed when for
eign substances such as corrosion inhibitors, spent stimulation
acid, and iron sulfide form the external skin of the emulsion
droplets. Exposure to air also causes the external shell
of the droplets to take the characteristics of solid material.
A certain amount of produced water in oil is contained
in a volume of free water not bound to the oil. This water
can be removed from the oil by gravity segregation. However,
a significant volume of water is contained in oil-water
emulsions. This portion is much more difficult to remove
from the oil-water mixture.
There are three basic mechanisms used to separate the
water and oil in emulsions. The application of heat energy
decreases the strength of the external phase of emulsion
droplets. The addition of heat energy also increases the
likelihood of collisions between particles by increasing
the velocity at which they vibrate. The combination of these
reactions increases the possibility of two droplets colliding
and coalescing into one. The particles grow with each
collision until the surface tension of the external phase
no longer retains the integrity of the droplet and it sepa
rates into small volumes of oil and water. Gravity segregation
accomplishes the final separation of oil and water phases.
Another mechanism for separation of emulsions into oil
and water phases is the reaction of chemical compounds called
demulsifiers which are added to the emulsion stream and
thoroughly mixed with it. A number of demulsifiers are avail
able for various types of emulsions. The compounds react
with the external phase to reduce its surface tension.
Coalescence, collapse of emulsion droplets, and separation
of oil and water by the action of gravity accomplish the
removal of the water phase from the oil phase.
The application of electrostatic fields to emulsions is
also effective in separating water from oil. A field of
several thousand volts per inch reversing itself at a line
frequency of sixty hertz is applied across two parallel grids
emersed in an emulsion. On each half cycle, the emulsion
droplets are polarized and distorted from their normal spher
ical shape to an eliptical shape as shown pictorially in
Figure 1. The oscillation of the field continually distorts
the droplets which in turn mechanically weakens the external
phase. On each half cycle of the field oscillation there
exists a finite probability that two droplets will be aligned
with each other in the direction of the electrostatic field
rvORl/AL El /ULSlO: j D R O f ' l C T
f . l L K \ . ' , L I il.'.M
1 x r i ^:'^A^. ( M/.r.r
EMULSION ORCiPLE'.T ISf LUCI.'CED HY F L f C T K l C f l K l - D
DROPLET DISTORTION DUE TO FIELD OSCILLATION
«s
I FIELD
0-INTENSn^
TIME
DROPLF.T INTERACTION CAUSED BY ELECTRIC FIELD
/ I r^ms
'/J
I v ^
ALIGNI.'.LNT ATTRACTlO\'/COLLlS!0N CO-'vl FSCENCE OR n'.EAKDOV.'N
Figure 1 . Emulsion Droplet Behavior
lines. The opposing charges of the adjacent ends of the
droplets attract the droplets to each other and increase the
likelihood of collision of the droplets. When collisions
occur, the mechanically weakened external phase allows drop
lets to coalesce. As in the previous mechanisms, coalescence,
separation of phases, and gravity segregation cause separation
of oil and water.
The mechanisms of heat addition, demulsifier reaction,
and electrostatic action may be used in combination to increase
the efficiency of separation of oil and water. In most modern
water removal facilities, a combination of heat and chemical
addition is used to separate oil and water. In recent years,
devices that employ all three mechanisms have been used to
increase the efficiency of water removal [ 4 ] .
Central Battery Faci1ities in Oil Producing Operations
In any oil producing operation some facility must be
provided for the separation of oil and water. Where small
volumes of liquid are handled, this facility may simply be a
heated dehydrator. In oilfields where larger volumes of liquid
are to be processed, a more elaborate system is provided
because economic considerations require that several functions
be performed. A major facility performing the functions of
gathering the produced fluid streams from several satellite
batteries and removing water from the fluid is usually called
a central battery.
8
An important function that must be performed by a central
battery is the removal of water from the liquids pumped into
the battery. It is at a central battery that nearly water-
free oil is metered as the total crude oil capacity of the
oilfield served by the central battery. A number of water
separation techniques including the use of settling tanks,
settling ponds which are aided by the heat of the sun, heated
dehydrators using demulsifying chemicals, or heated electro
static dehydrators using chemical additives can be employed
in a central battery. Obviously, when water has been removed
from the crude oil stream, some method of water disposal must
be provided.
In many major oil recovery projects, water injection is
used as a secondary recovery technique. Water is pumped into
certain wells in order that reservoir fluids are displaced
through the reservoir to nearby producing wells. Since large
volumes of water must be pumped at high pressure, a large
injection pump station is often located such that it is close
to a central battery where a water source is readily available.
Another method of disposing of water from a central battery
is to pump it into a water disposal well which is drilled so
that liquids pumped into the well enter an aquifer. This
method of disposal is used when water containing foreign sub
stances that would interfere with normal battery or injection
station operation enter the system. Also, at some time in the
life of a secondary recovery project, the rate at which water
enters the central battery is expected to exceed the demand
of the water injection system and excess water must be pumped
to disposal wells.
In some cases, a central battery is located in reason
ably close proximity to a facility handling the natural gas
which is an inherent part of the produced fluids of an oil
reservoir. Since most central batteries use gas-fired equip
ment, it is necessary to provide fuel gas as well as to send
produced gas to a processing facility.
As mentioned previously, the central battery of an oil
field producing large volumes of fluid is a complex arrangement
of equipment. Figure 2 shows the schematic flow diagram of
a general central battery. As indicated in this Figure, oil
is processed to remove water and pumped to a device called a
Lease Automatic Custody Transfer Unit (LACT Unit) which mon
itors the water proportion and diverts the oil stream for
further processing if the water content exceeds acceptable
limits. Water is routed through a coalescer which assures
that no oil is present and into the storage tanks of the water
injection or disposal facility. Fuel gas and electrical power
are provided for use in the central battery. Not shown on the
diagram but always present in physical equipment are energy
losses such as heat which is lost as a result of inefficiency.
As in the case of any process facility, design, construc
tion, and operation of the facility pose significant problems
to all concerned. Engineering personnel must consider all
10
FIELD INLET
DEHYDRATION EQUIPMENT
WATER OUTLET
, EMULSION OUTLET
UNACCEPT/^BLE EMUl 0I0.\'
WATER CONTENT DETEGTCC
^ACCEPTAf: f. ^Er.vjLSio::
WATER INJECfK','. '
Oa DISPOSAL FACIL ITY
WATER OUTI LT
Figure 2. General Central Battery Schematic Diagram
11
aspects of operation of the facility under ewery foreseeable
condition in order to design a facility capable of performing
its functions while considering economic, operational, and
safety constraints. Operating personnel must consider the
problems of training and familiarization with a facility such
that the facility performs its functions smoothly under all
foreseeable conditions. Based on previous experience in oil
field operation, all personnel must consider the factors that
can influence the performance of a central battery such as
field operations that cause fluctuations in the rate and com
position of fluid streams, long range changes in the character
istics of the reservoir that also affect rates and compositions,
atmospheric conditions that cause wide variation in ambient
temperature, and in the effect of the central battery on the
local environment.
An example of an operating central battery is shown in
the diagram in Figure 3, This battery is the process facility
for a major West Texas field containing about 700 producing
and injection wells. At the time this study was begun in 1973,
the field pumped about 18,000 barrels of oil and 12,000 barrels
of water daily. This oilfield has been in active programs of
secondary recovery, well stimulation, and drilling for the past
several years. These programs have been successful in increasing
oil as well as total fluid production which necessitated the
expansion of the central battery facilities. This study was
12
_q611*J?LftVlft
WC5T CHEM UAL
KIMP
v»\Trri r^iturAce
VAIVCS
t t t C l f ^ O S I A T i -
OCMVORATOH
-0 IK.KCTION
f'OVP
TO WAIER INJCCTlOM Wt l t o
^
RESIPOE CAS INLCT
OAS s.cf'SUi-i.in
Figure 3. West Texas Central Battery Piping Diagram
13
begun concurrent with the start of the design of new facilities,
however only the original facilities are dealt with here.
The original central battery utilized as its input vessel
a 10,000-barrel input surge tank which provided liquid settling
time as well as acting as an emergency storage vessel. Water
which separated from the oil by the action of gravity is
pumped from the tank directly to the produced water tanks.
The oil-water emulsion which floats on the water in the tank
is pumped into two parallel electrostatic dehydrators which
use the action of heat, chemical reaction, and electrostatic
fields to separate water from the oil. The oil is routed
to the cooling and sales tanks while the hot water is returned
to the inlet surge tank to transfer heat energy to the vessel
as the water settles to the bottom of the tank. The oil
travels from the cooling to the sales tank and loses heat
energy in the process. Two LACT units monitor the water
content of the treated oil and route the acceptable oil
through sales meters to pipeline pumps. Unacceptable oil is
sent to a recycle tank from which it is pumped into recycle
dehydrators whose function is to heat the oil and return it
to the inlet surge tank. Produced water is pumped through a
coalescer and mixed with fresh water to supply a source for
a water injection station. Another tank is used to store
water which contains foreign substances such as the residue
obtained from backwashing the coalescer and water filters.
This water is pumped to a water disposal well.
14
Computer Model ing of a Central Battery
There is ample justification for the development of a
model of a central battery. A need exists for a method by
which the performance of a central battery can be predicted
prior to the design and construction of such a facility.
Such a model would save many man-hours in the design phase,
facilitate the construction of the most efficient facility
within the constraints of practicality and cost, and afford
operating personnel the possibility of testing several operating
methods in order to cope with anticipated operating conditions.
The development of the model would be another step in advancing
the area of oil production technology.
The proposition that a central battery can be accurately
modeled by a digital computer is consistent with the increasing
application of technology to oil producing operations. Dig
ital computers have been used for several years to not only
model process facilities such as refineries and product
manufacturing plants but to also monitor and control them.
The models of process facilities are used to predict perform
ance, aid in design of new plants or additions to existing
facilities, and to aid in modifying operation in order to
optimize product recovery and process efficiency.
Computer models are also used to predict the performance
of hydrocarbon reservoirs. In the case of reservoir models,
the models are often based on empirical data rather than clearly
defined mathematical relationships. As the knowledge of
15
reservoir behavior increases, these models must be changed
to accommodate this additional knowledge.
Reservoir models are used to project oil recovery under
varying conditions. These models are also used to indicate
performance under various stimulation, drilling, and water-
flood projects. The newest ideas of improved secondary and
tertiary recovery techniques are being tested using computer
models prior to installation.
The engineering applications of a model of a central
battery are obvious. While a battery is being designed, many
plans could be made and tested by modeling before the best
plan is selected. The same modeling techniques could be used
to evaluate the economical feasability of every plan. It
is also possible to plan for future expansions prior to con
structing such a facility.
As mentioned previously, operating personnel can use
such a model to test several operating schemes to determine the
best method to cope with an anticipated set of conditions.
As new personnel are trained, the performance of the battery
can be shown for a set of ambient conditions and operating
techniques.
It is proposed as the basis of this work that a computer
model be constructed of the central battery described pre
viously. The primary objective is to develop a program that
approximates as closely as possible the actual operation of
the central battery. The model must operate as does the
16
battery under both normal and abnormal conditions. The
program must also be capable of being expanded exactly as is
the actual battery. The model must be able to account for
equipment failures and human errors that occur rarely in the
operation of the battery.
Method of Constructing the Model of a Central Battery
In order to develop a model of any process, a method
must be developed whereby the parameters which describe the
process can be expressed as mathematical relationships.
Regardless of what type computer or programming technique is
used, the computer is capable only of processing mathematical
variations of parameters.
Only a few of the most basic operations of the central
battery were available when this work was begun. The relation
ships of various parameters had not been derived or even
considered previously. Then, the first phase was to transform
emperical descriptions of various processes into mathematical
relationships.
It was necessary to interview engineers and operators
familiar with the actual operation of the central battery to
determine precisely what processes were taking place. Manu
facturing representatives familiar with the operation of in
dividual pieces of equipment were also interviewed. Operating
data was analyzed to determine if any conditions had been
overlooked.
17
The interview material and operating data were condensed
into a set of equations that describe processes analogous to
the processes thought to be occurring. These equations form
the basis of the model of the central battery.
Chapter II discusses the processes occurring in the ves
sels comprising the central battery. The descriptions of these
processes and the derivation of the mathematical relationships
necessary for the development of the model are dealt with in
this chapter.
A flowchart is developed as a method of describing the
operation of the central battery. This flowchart shows the
relationships of fluid flow streams in the battery. The para
meters of the battery are evaluated in an order analogous to
the order of occurrence of the actual processes described by
these parameters. The flowchart is developed so that a com
puter program may be written directly from the logic shown
on the flowchart. The flowchart is presented in Chapter II.
The performance of the model of the central battery oper
ation was analyzed by comparing the calculated values of key
parameters to the observed values of these parameters in the
actual facility. Since the necessary mathematical relation
ships had already been developed, it was possible to analyze
the processes occurring in the central battery in terms of
continuous time functions. After deriving these time functions,
it was possible to estimate the stability of some of these
processes. The behavior of the derived continuous system was
18
then treated as a sampled data system. The maximum error of
the model could then be calculated using the theories of dis
crete time systems. The comparison of the model's description
of the central battery and the continuous and discrete time
system analyses are described in Chapter III.
CHAPTER II
DEVELOPMENT OF MATHEMATICAL RELATIONSHIPS DESCRIBING
THE OPERATION OF THE CENTRAL BATTERY
The central battery to be studied served an oilfield
containing about 700 wells. This facility was designed to
process about 35,000 barrels of oil and water per day.
Under normal conditions, the battery handles about 18,000
barrels oil and 12,000 barrels water per day for a total
fluid inlet of 30,000 barrels per day. The piping diagram of
this facility is shown in Figure 3. A simplified flow diagram
of the facility is shown in Figure 4. Reference in the follow
ing discussion is to Figure 4.
A brief description gives the sequence by which fluid
moves through the central battery. This sequence is used in
the derivation of the mathematical relationships dealt with
in this study. The produced oil and water from the field are
mixed with liquid being recycled from the dehydrators and the
mixture enters the inlet surge tank. Free water is pumped
from the bottom of the tank while the oil-water emulsion
is pumped from a higher level in the tank. The emulsion is
routed through two electrostatic dehydrators where water
is removed from the emulsion. Water is returned to the inlet
surge tank while the nearly water-free oil is sent to the
cooling and sales tanks. Two LACT units take suction from
the sales tank and pump acceptable oil to pipeline pumps
19
20
r n f S l I WATCR IMCtl
OXYCtH D E a O R P T l O l l
MIXII<G VEGSEL
'•
WATER
OUTLET
riELO CMilLUlON IIA.rT
BU'.\r.".i IUJ\DL.
CLECTKOSTATIC
OEHYOnftTOn
LACT UNITS
r iPCLiwE POMIM
[ O I L
OUTLET
O
K EC VOLE
OEHYDftATCnS
Figure 4. West Texas Central Battery Schematic Diagram
K
21
but pump unacceptable oil to a recycle tank. The oil in the
recycle tank is pumped to recycle dehydrators where it is
heated and returned to the inlet surge tank.
Free water from the inlet surge tank is pumped to pro
duced water tanks. From these tanks, the water is pumped
through a coalescer and to a coalesced water tank. The
produced water is mixed in the mixing vessel with fresh
water from which free oxygen has been reduced. This water
mixture travels to the injection suction tank. The pumps of
the injection station take fluid from the suction tank and
pump at a relatively constant rate. Based on the demand of
the injection system, some water is diverted back to the
suction tank to maintain constant system pressure.
It should be apparent from the above description that
a large number of dependent and independent variables are
involved in the description of the processes in the battery.
To simplify the problem of keeping track of all parameters,
a common parameter designation is used. The following letter
designations are used throughout this work:
R - Rate of change of liquid, fluid, or power V - Volume of liquid W - Water proportion of an oil-water mixture E - Efficiency of a water-removal process T - Temperature in degrees Fahrenheit IND - Indicator, flag, or status
These variable designations with numbered subscripts in
brackets, i.e., R[15], are used to describe various parameters
Appendix A shows a complete listing of all variables used in
22
this work. Some of these parameters will not be defined until
a later chapter.
It is fortuitous that this central battery is associated
with a field that is served by a computer-monitored oilfield
automation system. This system monitors several critical oper
ating points and furnishes a warning when exceptional operation
is noted. This system exercises no control over the battery,
but it has an effect on abnormal operation beacuse it noti
fies operating personnel of problems in a matter of minutes
whereas these problems might go undetected for almost an hour
if the system were not in operation. This reduction in reaction
time is included in the model operation and results in reducing
the time that is allowed between the indication of abnormal
operation and the manual data inputs to return the model to
normal operation.
Inlet Surge Tank
The inlet surge tank is the primary input vessel of the
battery. It is a 10,000-barrel flat-topped, cone-bottomed
tank. Figure 5 shows a schematic diagram of the tank.
Two streams of fluid enter the tank. The first is the
total oil and water production of the field served by the
battery. Just before this field inlet stream enters the
tank, demulsifying chemical is pumped into the stream by what
is called the West Chemical Pump. The second stream is the
heated fluids being returned to the inlet surge tank from
23
h - 23.6*
HEIGHT FLUID VOLl'N«
(tlAr.'iELS) 16' 0" 10,046 -
6 ' l l " 3.715 6* 8" 8.267
ip<r 2,ir8 l,G64
ocHroRArns RETURN FU»O S'{'?'-y..', .'- 'I'"!'
wrsT CHEMICAL
PUMP \ . f lELO IHLET FLUID
\
Ef-",!L!.ipN
VTATCP PUMPft
Figure 5. Inlet Surge Tank Schematic Diagram
24
the dehydrators. The two inlet streams and the demulsifying
chemical are thoroughly mixed in the inlet line to the tank
and enter the top of the tank through a baffle plate which
breaks the stream into a spray covering a large area of the
upper surface of the tank fluid. This spraying effect breaks
the inlet stream into droplets and aids in distributing the
effect of the temperature of the inlet stream throughout the
tank as opposed to creating a portion of the tank volume that
remains at a temperature different from the temperature of the
surrounding fluid.
As discussed in the previous chapter, oil and water are
pumped to the central battery from satellite batteries. At
each satellite battery, demulsifying chemical is added to the
fluid streams and is thoroughly mixed with the emulsion created
by the satellite pumps.
Since the pipelines from the satellite batteries are buried
at a depth of about 36 inches, the temperature of fluids in
the lines approaches that of the ground long before the fluids
reach the central battery. The seasonally variable temperature
of the fluids in these lines has an effect on the amount of
water that separates from the emulsion. During warm or hot
weather, the effect of warm temperatures and the chemical
additives makes a fairly large proportion of water separate
in the pipes, while a smaller amount separates by the action
of the chemicals in cool or cold weather.
25
The field inlet line carries a total of about 18,000
barrels oil and 12,000 barrels water into the battery daily.
The temperature of this inlet fluid is seasonally variable
from about 40° F. in the coldest season to about 80° F. in the
hottest weather. When the temperature is 80° F., about 20%
of the total water is contained in an emulsion with all of
the oil. When the temperature is 40° F., this amount of
water increases to 50% because less water separates in the
cooler temperature mixture.
The field inlet stream will be considered an independent
input variable to the model. The input parameters and their
ranges are given below:
Input Variable Emulsion Rate Free Water Rate Water Proportion Temperature
Units Barrels/Minute
Designation Range RLIJ " i ^ R[2] 5-10,5 Barrels/Minute W[l] ,05-.3 T[13 40-80 °F.
The units of rate are expressed in barrels per minute rather
than barrels per day because this is the manner in which the
rates will be used in the model. Also, the range exceeds the
values given previously so that abnormal variations can be
used.
Occasionally several wells in a particular area of the
field may be treated with stimulation acid, corrosion inhib
itor chemicals, or scale disintegration chemicals. When this
occurs, for several hours an abnormally large amount of these
chemicals may enter the inlet surge tank. The volume of the
chemical residues is negligible in comparison to the volume
26
of the tank. However, these residues have a tendency to create
emulsions with an external skin that is virtually indestruct
ible with normal applications of demulsifying chemicals and
heat. Often, operators are alerted when residue is expected to
arrive at the battery, thus they can prepare the battery by
raising temperatures and increasing treating chemical volumes.
In some cases, however, the battery is not prepared for the
situation and a layer of this emulsion develops in the tank.
This emulsion rapidly decreases the efficiency of the tank
and dehydrators and results in an upset of the equilibrium of
the battery.
The other inlet stream to the inlet surge tank is the
return fluid from the dehydrators. This return fluid stream
can be described as one feedback path in the facility. As
will be discussed later in this chapter, each dehydrator
heats fluid and returns the fluid to the tank. From each
dehydrator, emulsion and free water come to the inlet spreader
of the tank and are thoroughly mixed with the field inlet
fluids. The parameters used to describe these streams are:
R[20] - Return Emulsion from Electrostatic W[13] - Water Proportion of Emulsion from Electrostatic T[4] - Return Fluid Temperature from Electrostatic R[33] - Return Emulsion from North Recycle R[32] - Return Free Water from North Recycle W[ll] - Water Proportion of Emulsion from North Recycle T[5] - Return Fluid Temperature from North Recycle R[35] - Return Emulsion from South Recycle R[34] - Return Free Water from South Recycle W[12] - Water Proportion of Emulsion from South Recycle T[6] - Return Fluid Temperature from South Recycle
The action of gravity settles fluids in the tank into
27
layers separated by indistinct boundaries. Water, with a
specific gravity of about 1,1 settles to the bottom of the
tank. Oil-water emulsion floats above the free water layer.
Since the water proportion of the emulsion layer ranges from
0% at the top of the layer to 100% at the bottom, the specific
gravity ranges from about 0.8, the approximate value for the
oil-water emulsion at the top to about 1.1 at the bottom.
Between the emulsion and free water layers is a layer
of fluid called interface fluid. This material is the residue
of the process of oil dehydration. It gradually develops
over a relatively long period of time as fluids are constantly
circulated in the central battery. Thi-s fluid consists of
emulsion particles whose skins are virtually indestructible
and of a suspension of microscopic particles of solid material
such as iron sulfide and sand. Under normal conditions the
volume of the interface layer remains constant because some
of the fluid is pumped out with saleable oil about as rapidly
as more fluid is created. The normal creation and loss of the
interface fluid are not included in this model because these
reactions require much more time than would ever be used to
operate the model. However, the volume of the fluid layer
is a very significant part of the model.
No reactions of interest take place in the free water
layer of the inlet surge tank. However, the processes of
heat action and chemical reaction are active in the emulsion
layer. As these actions take place, water separates from
28
the oil and settles to the bottom of the tank by the force
of gravity.
From the above discussion, it is obvious that a deri
vation of the mathematical analogies of the processes should
start with the actions occurring in the emulsion layer. The
actions to be considered are listed below:
1. The time variation of heat energy in the tank considering the input, output, and heat storage in the tank.
2. The reaction involving chemical additive and heat energy action which causes the dissolution of the external phase of the emulsion droplets, the coalescence of emulsion droplets, and the segregation of the water phase from the emulsion by the action of gravity,
3. Fluid flows in the tank as functions of time and volumes.
4. The abnormal change of volume of the interface 1ayer.
The temperature of the fluid in the tank varies because
of changes in the energy in the vessel. According to the
law of conservation of energy, the energy in a system may
be defined as
Energy In - Energy Out = Energy Stored. (1)
The energy in the inlet surge tank must vary by this relation
ship.
It is a valid assumption that the kinetic and potential
energy in the system is negligible and that energy in the form
of chemical bonding may be ignored since it does not change.
Then, the energy of the tank system may be redefined in terms
29
of heat energy as
Heat Energy In - Heat Energy Out = Heat Energy Stored.
Using the notation of Reynolds [5], the relationship
in (2) may be expressed as
(2)
M..C.T in^p'^^in ~ ^out^p^out " ^(^'^^v^out^ (3)
where
M is the mass of a system inside a boundary M is the rate of change of mass with respect to time Cp is the specific heat of a substance for constant
pressure Cy is the specific heat of a substance for constant
volume d is the differential operator
When dealing with liquid in a closed tank, it can be
assumed that the pressure is constant, and that the rate
of change in volume is negligible. Therefore, both Cy and
Cp are constant for all time. Based on values of Cy and C Q
of several liquids in the tank, it can also be assumed that
Cy and Cp are equal and can be eliminated from (3).
Since the density and composition of fluid in the tank
are almost invariant in tim?, the mass, M, can be considered
to be the volume of fljid in the tank. The M terms in the
relation are then analogous to the rates of fluid flowing
in the tank. Therefore, (3) reduces to
^ " i n ^ i n - RoutTout = "(VTout) C )
with the various constants of proportionality deleted.
The Laplace technique is used to express the function
of time in (4) as a function in the s-domain.
I R i n ( s ) T i n ( s ) - Rout(s)Tout(s) = Tout(s)sV(s) + V(s)sTout(s) (5)
30
The variable T^^^^, fluid temperature in the tank, is given
by
"•"outfs) = I [ R i n ( s ) T i n ( s ) ] / 2 V ( s )
(6) s + Rout(s)/2V(s)
This relationship of T^^j^ is shown as a continuous system
in Figure 6.
Tout is a function of the variables R i n ( s ) T ( s ) , ^out^^^'
and V ( s ) . If these variables were constant in time, the
solution for TQ^J^ would be exponential in form. However,
all of the variables are also variable with time and variable
in the s-domain.
It is not feasible to obtain a continuous solution for
the variables and the T^^^ parameter. Such a solution requires
a complete solution of virtually every variable in the central
battery. This derivation is beyond the scope of this study
because sufficient data is not available to obtain complete
solutions for all variables in the battery. Continuous relation
ships such as the one above are used in a later chapter to
study the theoretical error of the model.
All of the data available for the central battery was
of a discrete nature, that is, meters, pressure gauges, and
thermometers were checked only once per day. Since an explicit
solution for the temperature of fluid in the inlet surge tank
is not derived, an empirical solution must be derived. The
empirical solution for the temperature must be a discrete
function of time because continuous data is not available
31
-R(s>*-
RiN'(r>)TK<(3)
V(s)
^ _ i — > ToJs)
Figure 6. Analogous System for Inlet Surge Tank Temperature
V
32
for generation of the solution.
As this work was being conducted, it was decided that
if any parameter in the battery was required to be a discrete
function of time, little purpose would be served by deriving
any but the most simple relationships as continuous functions.
Therefore, all relationships are derived for discrete time
increments. A time increment of one minute was chosen as
the base time period because it is short when compared to
the time between data points but sufficiently long to use
for representing several hours of actual operation without
requiring massive amounts of computer time.
In all derivations throughout the remainder of this chap
ter, all functions are assumed to be discrete functions of
time. Where feedback paths would occur if continuous func
tions were given, the present value of a parameter is calculated
as a function of its previous value and relationships of
other parameters. The conventional method of expressing
such a variation of x, y, and z, for example, would be
yi(x,z) = yi_-,(x,z) + F(x,z)At. (7)
Since At is always one minute in duration, the tern, A t ,
can be removed. However, when any parameter with the desig
nation, R, is used, it is understood that the variable is R^t.
This notation is adopted because any R variable is the channe
of volume, voltage, or power per one minute time increment.
Using the battery data available, a relationship for the
temperature of tank fluid, T [ 3 ] , was developed.
33
T[3](V[1] + V[2] + V[3]) + T[1]R[1] + T[4]Rr20] + T[5](R[32] + R[33]) + T[6](R[34] + R[35])
- T[3](R[6] + R[7] + R[8])
T[3] = (8)
V[l] + V[2] + V[3]
It should be noted that the relationship in (8) displays the
principle of conservation of energy even though it is a dis
crete relationship.
As previously discussed, the presence of foreign sub
stances in the field inlet stream creates an emulsion in the
tank that cannot be processed easily. It was found that when
this material was present, it occupied about 10% of the volume
of the incoming fluid stream. However, this foreign material
is diluted in the tank by the mixing action of the tank.
Then, the effect of the foreign substances is not felt until
their concentration reaches approximately 10% of the volume
of the emulsion layer. This action of concentrating the
foreign emulsion is modeled by calculating a volume of fluid
separate from the volume of emulsion. This volume is used
to indicate an emulsion containing a high content of foreign
substances and is therefore not shown as being treated.
The presence of foreign substances in the inlet emulsion
stream is shown by manual input signal. When the substances
are indicated, a volume of the material begins to accumulate
by the relation
V[4] = V[4] + 0.1R[1]. (9)
When the volume reaches a limit of 371.5 barrels, the model
34
will signal the presence of sufficient foreign substances
to reduce the efficiency of water removal and the model
stops accumulating V[4]. When the manual indication of
foreign substances is removed, the model begins reducing
V[4] by the relation
V[4] = V[4] - 0.1R[1]. (10)
If the indication of a large volume V[4] is present, the
indication is not removed until V[4] becomes zero. The
action of indicating the abnormal value of V[4] is consistent
with the hysteresis effect of the emulsion containing for
eign substances. V[4] does not reduce to less than zero. The
parameter, IND[4], is used to show the high or low value
of V[4].
In this study, the term efficiency is used to indicate
the rate at which water is removed from emulsion. The
efficiency parameter is defined as
. . _ Volume of Water Removed from Emulsion Efficiency - volume of Water Originally Contained in Emulsion
The efficiency of the water separation process in the
inlet surge tank depends on three variables: fluid temperature,
the presence of a large volume V[4], and the amount of demul
sifying chemical added to the inlet stream. The chemical
input parameter, denoted as R[9], takes only two values, normal
and abnormal, and is manually entered into the model. This
operation is consistent with actual operation since the rate
at which the chemical additive enters the inlet stream is
35
manually adjusted from a normally low value to a high value
when abnormal operating conditions are encountered.
Since the parameters, IND[4] and R [ 9 ] , can each take
only two values, there are four combinations of these two
variables that will influence the efficiency of the tank.
The efficiency of the tank is continuously related to T[3]
and is related to the four combinations of IND[4] and R[9].
The defining equations for the efficiency are shown sepa
rately for the four cases:
Case 1: IND[4] and R[9] Normal E[l] = 0.05 + 0.011T[3] (11 )
Case 2: IND[4] Normal, R[9] Abnormal E[l] = 0,51 + 0.006T[3] (12)
Case 3: IND[4] Abnormal, R[9] Normal E[l] =-0.25 + 0.0125T[3] (13)
Case 4: IND[4] and R[9] Abnormal
E[l] = 0.01T[3] (14)
Water that is removed from the emulsion settles to the
bottom of the inlet surge tank. The pure water in the bottom
of the tank is a combination of the water removed from the
emulsion in the tank and the free water that is pumped into
the tank. This volume is denoted V[3].
A float that monitors the volume of the free water in
the tank is used to control two free water pumps. Since the
outlet streams of these two pumps are connected, the outlet
free water rates are combined to form a single rate, denoted
by R [ 8 ] . When V[3] is less than or equal to a limiting value
of 1884 barrels, R[8] is equal to zero since the pumps are
36
turned off by the float. When V[3] is in the range of normal
operation from 1884 barrels to 2198 barrels, R[8] takes the
val ue
R[8] = (V[3] - 1884)(0.022). (15)
When V[3] is larger than 2198 barrels, R[8] is constrained to
its upper limit of seven barrels per minute. This analogy
is consistent with the operation of the two centrifugal pumps
which are controlled by a pneumatic throttling valve. The
proportionality constant was chosen so that R [ 8 ] , a function
of V [ 3 ] , varies in accordance with emperical data.
The emulsion, which is lighter than both water and inter
face fluid, floats on these two liquids-. A float is used
to determine the height of the top of the emulsion layer.
Since the tank walls are vertical, this float also monitors
the total liquid volume of the tank. Emulsion is removed from
a point in the tank above the normal height of the interface
fluid and water layer tops by centrifugal pumps which are
throttled by a valve operated by the float. The parameter,
R [ 7 ] , is used to indicate the emulsion removal rate.
The throttling valve on the emulsion outlet stream
also has three modes of operation:
R[7] = 0 If V[l] + V[2] + V[3] < 3297 (16)
R[7] = (V[l] + V[2] + V[3] - 3297)(0.045) If 3297 < V[l] + V[2] + V[3] < 3715 (17)
R[7] = 18.75 If V[l] + V[2] + V[3] > 3715 (18)
The electrical control system can shut down the free water
37
pumps if the free water surge tank has an excessive volume.
This condition is duplicated by the use of an indicator,
I N D [ 1 8 ] , whose value is calculated later. The Dumps are
stopped if an operator inadvertently leaves a switch off.
This is an uncommon but not impossible situation and is
entered into the model by the use of IND[40].
The electrical controls of the free water pumps are
characterized in the model by manipulation of the value of
R [ 8 ] . If either IND[18] or IND[40] indicates abnormal oper
ation, R[8] is constrained to zero regardless of its
former value.
The value of R[7] depends on the value of the sum of the
values of V [ l ] , V [ 2 ] , and V [ 3 ] . V [ l ] , V [ 2 ] , and V[3] are
used to denote the volumes of the emulsion, interface fluid,
and water layers, respectively. This relationship is shown in
equations (16) through (18). However, the value of the emul
sion outlet rate can approach zero even when the charge pumps
continue operation because the layers of water and interface
fluid can become so thick that interface fluid or water is
pumped by the charge pumps. Also, excessive volumes in the
sales or cooling tanks will constrain R[7] to a value of zero.
The point at which the suction lines of the charge pumps
enter the tank is about 5'2" above ground level. This height
in the tank represents a volume of 3297 barrels. Thus, if the
volume V[3] or the sum of volumes V[2] and V[3] exceeds 3297
barrels, emulsion is not being handled by the charge pumps.
38
If the sales or cooling tanks have excessive volumes,
indicators, IND[17] and IND[15], respectively, show abnormal
operation. As mentioned in the discussion of R[8], it is
possible to inadvertently turn off the pumps and this con
dition is modeled by use of IND[40].
There are several conditions by which R[7] can take
some other value than that calculated in equations (16)
through (18). R[6], the outlet rate of interface fluid,
can also take other values than its normal value of 0.25
barrels per minute. These conditions are shown below.
Case 1-a: V[2 IND
Case 1-b: V[2 IND
Case 2-a: V[2 IND
Case 2-b: V[2 IND
Case 3-a: V[3 IND
Case 3-b: V[3 IND
+ V[3] < 3297 15], IND[17], and IND[40] Normal
R[6] = 0.25 R[7] = R[7] from (16)-(18)
+ V[3] < 3297 15], IND[17], or IND[40] Abnormal
R[6] = 0 R[7] = 0
+ V[3] > 3297, V[3] < 3297 15], IND[17], and IND[40] Normal
R[6] = R[7] from (16)-(18) R[7] = 0
+ V[3] > 3297, V[3] < 3297 15], IND[17l, or IND[40] Abnormal
R[6] = 0 R[7] = 0
> 3297 15], IND[17], and IND[40] Normal
R[6] = 0 R[7] = R[7] from (16)-(18)
> 3297 15], IND[17], or IND[40] Abnormal
R[6] = 0 R[7] = 0
(19) (20)
(21) (22)
(23) (24)
(25) (26)
(27) (28)
29 30
The variation of the volumes V[l], V[2], and V[3] can be
39
calculated based on the values of R[6], R[7], and R[8] and the
various inlet parameters discussed previously. These varia
tions depend to a large extent on the values of the volumes
in the above cases. Thus, the derivation of volume is also
based on the above cases.
Case 1-a V[2 V[5
V[6
V[3
V[l
W[3
W[2
Case 2-a V[2 V[5
V[6
V[3
V[l
W[3
W[2
Case 3-a V[2 V[5
V[6
V[3
V[l
and b = v[ = v[
= v[ +
= v[
2] 5] + 6] +
3]
5] 5]
+ (R[1]W[1] + R[20]Wri3] + R[33]W[11] R[35]W[12])(1 - E[l]) - R[7]W[31 + R[l](l - W[l]) + R[20](l - Wtl3]) 33](1 - W[ll]) + R[35](l - W[12] R[5] - R[6] - R[7](l - W[3])
+ R[2] + R[32] + R[34] + (R[1]W[1] + R[20]W[13] + R[33]W[11] + R[35]W[12])E[1] - R[8]
+ V[6]
= W[
and b = V[ = V[
= V[ +
= v[
T7 3]
21 5]
6] R 3] +
5] 5] n
= W[3]
= V[ _ V[
VT
+ R[5] - R[6] + (R[1]W[1] + R[20]W[13] + R[33]W[11]
+ R[35]W[12])(1 - Efl]) + R[l](l - W[ll) + R[20](l - W[13]) [33](1 - W[ll]) + R[35](l - W[12]) + R[2] + R[32] + (Rri1W[l] R[20]W[13] + R[33]W[111
+ R[35]W[12l)E[l] - R[8i + V[6]
and b: = V[2 = V[5
] ] + (R[1]W[1] + R[20]W[13] + R[33]W[11]
+ R[35]W[12])(1 - E[l]) = V[6] + R[l](l - W[n) + R[20](l - W[13]
+ R[33](l - W[ll]) + R[35](l - W[12]) + R[5]
= V[3] + R[2] + R[32] + R[34] + (R[1]W[1] + R[20]W[13] + R[33]W[11]
+ R[35]W[12])E[1] - R[7l - R[8]
= V[5] + V[6]
(31 )
( 3 2 )
( 3 3 )
( 3 4 ) ( 3 5 )
( 3 6 )
( 3 7 )
( 3 8 )
( 3 9 )
( 4 0 )
(41 ) ( 4 2 )
( 4 3 )
( 4 4 )
( 4 5 )
( 4 6 )
( 4 7 )
( 4 8 ) ( 4 9 )
40
W[3] = ^
W[2] = 1.0
(50)
(51)
In the above cases, the relation
V[2] = V[2]
is a shorthand notation of the lack of change in the volume
of the interface fluid layer. The volumes V[5] and V[6]
are theoretical volumes of water and oil, respectively, which
exist in the emulsion layer. The method of calculating the
value of the water volume V[5] is to take a summation of the
water content of the emulsion streams entering the tank,
i.e.,
and consider only the water that is not removed from the emul
sion by the heat and chemcial action, (1 - E ) . The form of
the equation for oil volume is a summation of the oil content
of each emulsion inlet stream,
T'^ R.(l - W,-)
The emulsion layer volume, V [ l ] , is the sum of V[5] and V [ 6 ] .
The form of the equation for the volume of free water in the
tank is calculated by adding the oil-free water streams and
the water that is removed from the emulsion layer,
The volumes are calculated by adding incremental volumes
of liquid, RAt, to the existing volume. This method of model
ing gives the best analogy to actual operation even though
it requires the use of an iterative calculation technique.
41
Electrostatic Dehydrator
In Figure 3, two electrostatic dehydrators are shown.
In order to guarantee continuous operation of the battery,
one dehydrator was considered to be a permanent standby unit.
The standby dehydrator was used so infrequently that it was
not necessary to include it in the model of the central
battery. The operating procedure called for maintaining a
standby unit until the operating dehydrator became inoperative.
At this time, the role of the two dehydrators was reversed.
The same operation is simulated in the model by making multi
ple model calculations.
The electrostatic dehydrator used in the central battery
is shown diagramatically in Figure 7, This vessel can be
visualized as two dehydration vessels, one using heat and
chemical action only and the other using electrostatic fields
and chemical action, connected in a cascade configuration.
With a few minor exceptions, the heated section and the grid
section of the dehydrator can be analyzed independently.
The first section is heated by use of a fire tube in
which natural gas is burned with air. The burner is controlled
by one of two thermostats. One thermostat is used to main
tain a temperature of 100° F. when the grid section is operating
If the grid section automatically turns off due to overload,
the second thermostat is used to maintain a temperature of
140° F,
Emulsion from the inlet surge tank, R [ 7 ] , enters the
42
EXHAUST MEAT LOSS
POWER OJT n:6o; VOLTAGE OOT K.4 9J
I BUHNER I REGULATOR
f FUEL INLET
ROOI
RiWj
RtTuntjrtmo OUTLET
Figure 7. Electrostatic Dehydrator Schematic Diagram
43
heated section at the top of the dehydrator. The emulsion
is forced to flow past the fire tube, through a baffle plate,
past the fire tube again, and over two more baffle plates
before entering the grid section. By the time the rapidly
moving fluid has followed this torturous path several actions
have been performed. First, the fluid is evenly heated to
the temperature required by the thermostat. The negligible
volume of gas that had not bubbled out of solution in the
fluid path from the wells is allowed to separate from the
oil and to be drained from the dehydrator into the gas
gathering system. Finally, the velocity of the fluid has
been lowered so that time is allowed for water which separ
ated from the emulsion to accumulate in the bottom of the
heated section.
At the time of this study, no actual data was available
to determine the parameter functions of the dehydrator.
Based on the experience of operating personnel and the
engineering staff of the manufacturer of the vessel, the
following emperical relations were developed.
1. Approximately half of the water that is removed from emulsion is removed in the heated section.
2. Depending on the concentration of residue-laden emulsion and the rate at which demulsifying chemical is added, the efficiency of the water removal process in the heated section is linearly related to the temperature of the fluid in the section.
3. If the temperature of the fluid in the dehydrator is 100° F., the efficiency of the heated section can never exceed 50%, but if
44
the temperature reaches 140° F., the efficiency of the heated section can increase to a maximum of 60%.
4. If the water proportion of the emulsion stream in the dehydrator drops to a value less than or equal to 0.5%, the efficiency of the dehydrator drops to zero since this water proportion is the limiting value at which no more water can be removed from the emulsion by the dehydrator.
Since the efficiency of the heated section depends
primarily on fluid temperature, demulsifier inlet rate, and
presence or absence of excessive foreign material in the
emulsion stream, relationships for these variables must be
derived before any other parameters can be considered. The
demulsifier has a tendency to associate itself only with
emulsion fluid. For this reason, the rate at which the demul
sifier enters the dehydrator is identical to the rate at
which it enters the inlet surge tank and the parameter, R [ 9 ] ,
is used again in the electrostatic dehydrator.
The mere presence of residue-laden emulsion in the
heated section changes the relationship of efficiency. If
the indicator, IND[1], shows an excessive concentration of
this emulsion in the inlet surge tank, the relationship of
heated section efficiency is altered. Since IND[1] can take
two values, there exist two relationships for efficiency.
Case 1 : IND[1] Normal
E[2] = 0.5 (52)
Case 2: IND[1] Abnormal E[2] = -0,53 + 0.004T[1] + 600R[9] (53)
The variation of treater fluid temperature depends on
45
two factors. First, if the grid section is disabled,
indicator, IND[5], indicates that abnormal operation is occur
ring and additional heat is required. Second, the temperature
variation depends on whether or not a change in temperature
is taking place. If the low temperature thermostat has been
activated the temperature is allowed to instantaneously drop
from any value to 100° F. On the other hand, if the heated
section is in the process of increasing the temperature,
the temperature is forced to vary linearly with the rate,
R [ 7 ] , and time. This operation corresponds to rapidly de
creasing and slowly increasing temperatures caused by the
high inlet velocity of fluid in the electrostatic dehydrator.
The model parameter of dehydrator temperature is shown for
two cases.
Case 1: Normal Grid Section Operation T[4] = 100 (54)
Case 2: Abnormal Grid Section Operation
T[4] = T[4] + 1.222 - 0.074R[7] (55)
When T[4] reaches a value greater than or equal to
140° F., it is allowed to maintain that value. It should
be noted that if R[7] is allowed to reach the upper limit
of its range, T[4] can actually decrease rather than increase.
This situation is consistent with actual operation for ex
tremely high inlet rates.
It is obvious that the gas consumption rate of the
burners depends on whether the temperature is being main
tained at 100° F. or 140° F. Only emperical data is available
46
to determine the relationship of the rate of gas consumptior
The instantaneous gas rate varies significantly due to the
operation of the thermostats. However, only average daily
gas consumption data is available. The gas consumption rate
expressed in BTU/minute is given below.
Case 1: 100° F, Thermostat in Use R[10] = 1.6 X 105
Case 2: 140° F. Thermostat in Use R[10] = 2 X 10^
(56)
(57)
No data of any kind is available to determine how much
of the energy is utilized in heating the inlet emulsion stream
It is reasonable to expect that a significant portion of this
energy is lost as heat through the stack exhaust and through
convective cooling of vessels, pipes, and pumps in the central
battery. According to the equipment manufacturers, the best
heating that could be expected is about 40%. Therefore, the
heat loss, which is called stack loss, is expressed in BTU/
minute by
R[19] = 0.6R[10] (58)
The water volume removed from the emulsion is a function
of the efficiency E [ 2 ] . The water volume remaining in the
emulsion is similarly related to efficiency as:
ur/n - W[2](l - E[2]) ._.,
WL'^J - 1 - WL^JEL^] f^^)
As free water separates from the emulsion fluid in the
heated section, the water accumulates at the bottom of the
vessel and forms a water layer whose volume is denoted V [ 7 ] .
A weighted float detects the top of this water layer and
47
operates a mechanically linked valve. The pressure of the
gas in the top of the dehydrator is used to force free water
out of the vessel when the valve opens. The rate at which
water is forced out of the dehydrator is proportional to the
depth of the water layer and thus is proportional to V[7].
If the volume is sufficiently large, it is possible for free
water to spill over the baffel plates into the grid section.
Since there are two rates at which water leaves the heated
section when the spillover condition occurs, two different
variables are assigned. These variables of water leaving
the bottom of the vessel and water spilling into the grid
section are called R[ll] and R[13], respectively. The vari
ations of these parameters are given for several cases.
Case 1: V[7] ^ 5 7 . 2 - Dump Valve Closed R[ll] = 0 R[13] = 0
(60) (61)
Case 2: 57.2 < V[7] < 60.7 - Dump Valve in Linear Range R[ll] = 0.066(V[7] - 57.2) (62)
R[13] = 0 (63)
Case 3 60.7 < V[7] < 207.1 - Dump Valve Fully Open R[ll] = 0.625 (64)
R[13] = 0 (65)
Case 4: V[7] > 207.1 - Dump Valve Fully Open, Spillover Occurring
R[ll] = 0.625 R[13] = R[7]W[2]E[2] - 0.625
(66) (67)
The volume of the water layer depends on the inlet rate,
the efficiency, and the water removal rates. This relation
ship is
V[7] = V[7] + R[7]W[2]E[2] - R[ll] - R[13] (68)
48
The rate at which emulsion leaves the heated section
depends on the inlet emulsion rate and the rate at which
water leaves the heated section. Although the emulsion accu
mulates as a layer of liquid floating on the water layer,
the volume of this layer is of little interest for the purpose
of this model. It is true that the sum of this and the water
layer volume is used to determine if the volume of liquid
in the heated section is sufficiently large to justify stop
ping the emulsion charge pumps at the inlet surge tank.
However, this action has never occurred in a condition not
generated by maintenance work or electronic failure of the
sensing device. In each case, the invalid indication caused
no abnormalities in the central battery operation because
the condition was rapidly rectified.
Since the volume of emulsion in the heated section is
not considered, the rate at which emulsion liquid leaves the
heated section can be readily shown. This emulsion rate is
denoted R [ 1 2 ] . The relationship for R[12] is given as
R[12] = R[7] - R[ll] - R[13] (69)
Before ending the discussion of the heated section of the
dehydrator, the rate at which the interface fluid, R [ 6 ] ,
passes through the section should be mentioned. The interface
fluid travels the same path as the emulsion fluid because the
two fluids were mixed by the charge pumps. The interface
fluid is not affected in any way by the action of the heated
section because this fluid is a by-product of crude oil
49
processing. On the other hand, the interface fluid has no
effect on the processes occurring in the heated section.
Thus, R[6] is not used in the description of the heated section
but does play a significant role in the grid section.
The grid section uses the processes of electrostatic
field action and chemical reaction to remove water from the
emulsion fluid. Heat is not considered a direct participant
in the process of water separation, but its minor effect is
included in the chemical reaction process.
The fluid entering the grid section from the heated
section is normally emulsion fluid only, R [ 1 2 ] , but free water,
R [ 1 3 ] , can spill into the grid section. The fluids separate
into water, interface fluid, and emulsion layers in the grid
section. Under normal conditions, the top of the interface
fluid is a significant distance below the high voltage active
grid, the lower grid.
The resistance from the active grid to the top of the
interface fluid depends on three factors:
1. The water content of the emulsion
2. The concentration of foreign substances in the emulsi on
3. The distance from the interface layer to the active grid.
These factors have all been considered and are all
included in the following derivation of grid section
efficiency.
When a high concentration of foreign material is calculated,
TEXAS TECH LIBRARY
50
IND[1] shows this abnormal concentration. When this con
dition occurs a dummy volume of this residue-laden emulsion
begins to accumulate in the grid section by the relation
V[15] = V[15] + 0.1R[7] (70)
Again, it is assumed that this emulsion only composes about
10% of the total emulsion inlet stream. If V[15] exceeds
a value of 34.8 barrels about 10% of the anticipated emulsion
layer, an indicator, IND[22], shows abnormal operation.
When IND[1] indicates normal emulsion quality, V[15] begins
to decrease as
V[15] = V[15] - 0.1R[7] (71)
Once IND[22] has indicated abnormal operation, the indication
is not removed until V[15] reaches zero. V[15] is not allowed
to become less than zero.
One of the most critical aspects of the operation is
the thickness of the interface fluid layer. The interface
fluid enters the dehydrator grid section at a rate given by
R[6]. The interface pump removes the interface fluid from
a particular depth in the dehydrator. Since the water layer
is expected to remain at a fairly constant depth, it is pos
sible to remove interface fluid from a selected depth.
If the top of the water layer rises significantly, the inter
face pump is handling water instead of interface fluid, and
interface fluid is not removed from the grid section as it
enters the section. Therefore, the volume of interface fluid
increases and the distance from the top of this layer to the
51
active grid decreases.
The interface fluid cannot be broken down by any appli
cation of heat, chemical, or electrostatic field. In addition,
this fluid is a virtually perfect conductor of electric
current. If this fluid layer ever contacts the active grid,
the grid shorts to the ground potential of the water layer.
Also, if such contact is made, the interface coats the grid
with conductive material and shorts the grid to the grounded
steel walls of the dehydrator. This coating can only be
removed by draining the vessel and manually cleaning the
grid.
Another aspect of the interface fluid activity is the
operation of the interface fluid pump. This pump can be
manually operated at a high and low rate. The manual oper
ation of this pump is simulated by use of a manually entered
indicator, IND[42]. When the pump is to be operated normally,
the interface pump rate, R [ 1 5 ] , takes the value 0.255. When
abnormal operation is required, this value increases to 0.383.
A float-operated dump valve removes free water from the
grid section. This is the same type valve arrangement that
is used to drain the water from the heated section. The volume
of the water layer, V [ 1 0 ] , controls the free water outlet
rate, R [ 1 4 ] , and two indicators, IND[7] and I N D [ 8 ] , that show
if the interface pump is handling water, I N D [ 7 ] , or interface
fluid, I N D [ 8 ] . The rate, R [ 1 4 ] , is related to the volume,
V [ 1 0 ] , as follows.
52
Case 1: Y[10] < 98.4 - Interface Pump Handling Interface Fluid
R[14] = 0 (72)
Case 2: 98.4 < V [ 1 0 ] < 116.7 - Interface Pump Handling Interface Fluid
R[14] = 0.034(V[10] - 98.4) (73)
Case 3: V[10] > 116.7 - Interface Pump Handling Water
R[14] = 0.625 + R[15]. (74)
If the water layer volume is small enough the interface
pump can be handling emulsion instead of interface fluids.
If the volume of the interface layer, V[12], is less than
18.3 barrels and the water layer volume, V[10], is less than
116.7 barrels, the interface pump is removing emulsion from
the grid section at the rate, R[15]. In this case, the rate
at which emulsion enters the grid section becomes
R[12] = R[12] - R[15]. (75)
The volume of the interface fluid layer also depends
on its own volume and that of the water layer. The variation
of V[12] is
Case 1: Interface Pump Handling Interface Fluid V[12] = V[12] + R[6] - R[15] (76)
Case 2: Interface Pump Not Handling Interface Fluid
V[12] = V[12] + R[6]. (77)
The variation of the volume of the water layer depends on
the efficiency of the heated and grid sections as well as
the action of the water dump valve. The free water removed
from the emulsion is R[l2]W[4]E[3]. Then,
V[10] = V[10] + R[12]W[4]E[3] + R[13] - R[14]. (78)
Emulsion completely fills the space from the top of the
dehydrator to the top of the interface fluid. The emulsion
A
53
above the grids has had as much water removed as is possible
under a given set of conditions. Thus the water proportion
of this liquid is the outlet water proportion. The emulsion
between the active and ground grids still contains some water
that will separate before the emulsion rises above the ground
grid. The volume of emulsion above the active grid is fixed
by the geometry of the dehydrator.
The volume of emulsion between the active grid and the
top of the interface layer is a variable. The variation of
this parameter depends only on the volumes of the interface
fluid and water layers.
V[ll] = 468.8 - V[10] - V[12] (79)
The volume, V [ l l ] , is the key parameter in determining
the efficiency of the grid section and determining whether
or not the grids are in operation. It should be noted that
water separation due to electrostatic action occurs in the
region between the active grid and the interface fluid.
Such water separation also occurs to a larger extent between
the grids where close grid spacing creates high intensity
electrostatic fields. The efficiency calculated for the
grid section includes the water separation processes in both
grid areas.
The efficiency of the grid section depends primarily
on the magnitude of the electric field from the active grid
to ground potential planes. This voltage is a linear function
of the water proportion of the inlet emulsion stream. In the
54
model, the efficiency is shown to be a function of this water
proportion.
(80) E[3] = 0.95 - 0.31W[4]
The voltage variation is given by
R[49] = 1.68 X 10^ - 1.1 X 10^W[4]. (81)
The transformer which supplies grid current is designed
to furnish constant power to the grids.
R[50] = 1 .2375 X lO' (82)
As the water content increases, the conductivity of the
emulsion liquid also increases as does the grid current. As
the grid current increases, the voltage decreases in (81) due
to the action of the transformer.
As the current reaches abnormally high levels due to a
concentration of residue-laden emulsion, the efficiency drops
to very low levels. If the interface fluid contacts the active
grid, indicated by a zero value of V [ l l ] , an indicator, IND[5],
is set which shows the grid section to be out of service and
which causes the heated section to begin using the high temper
ature thermostat. When this occurs, the efficiency, voltage,
and power parameters take a value of zero.
The emulsion above the ground grid contains volumes of
oil and water. The oil contained in this area is
V[13] = V[13] + R[12](l - W[4]) - R[16](l - W [ 5 ] ) . (83)
The water volume is given by
V[14] - V[14] + R[12]W[4]{1 - E[3]) - R[16]W[5] (84)
where R[16] is the outlet emulsion rate and W[5] is the water
55
proportion of the outlet emulsion stream which is calculated
to be
WL5] = VL13J + 'VrTTT, (85)
The outlet emulsion rate is given by
R[16] = R[12] + R[13] + R[6] - R[15] - R[18], (86)
The emulsion outlet stream can be visualized as the sum
of the outlet oil and outlet water streams. The oil stream is
R[17] = R[16](l - W[5]) (87)
and the water stream is
R[18] = R[16]W[5]. (88)
The rate at which fluids are returned to the inlet surge tank
is given by
R[20] = R[ll] + R[14] + R[15]. (89)
No provision is made to account for emulsion being part of the
rate, R [ 2 0 ] . On some occasions, R[15] may be the rate of an
emulsion stream, but this rate is insignificant and the
water proportion is not calculated.
The requirement imposed on the central battery by the
pipeline operator is that the water proportion of the central
battery outlet stream not exceed 1%. At the time the original
battery was constructed, water content analyzers were installed
to monitor the outlet streams of the electrostatic and recycle
dehydrators. These monitoring devices were never used in the
battery. However, the model is modified to incorporate these
monitors by means of indicators that show the time when the
water content of the dehydrators exceeds 1%.
56
Cool ing , Sales , and Recycle Tanks and LACT Uni ts^
The cooling, sales, and recycle tanks and the LACT uniti
are shown in Figure 8. Fluid from the electrostatic dehydrator
enters the cooling tank as shown. As the warm fluid enters
the tank, the fluid rapidly cools as heat energy is trans
ferred to the atmosphere. A small amount of water is removed
from the emulsion by the action of residual heat and demul
sifier chemical. This water removal is represented by a
fixed efficiency of 10%.
A recycle pump removes water from the bottom of the
tank. However, this pump is operated manually. The cooling
tank has an external visual indication that informs operating
personnel that a significant water level exists. The model
uses an indicator, IND[14], to indicate the presence of a
significant water volume,
V[19] > 95.4.
It is then necessary to manually indicate to the model that
the pump is to operate.
The volume of water varies by
V[19] = V[19] + 0.1R[16]W[5] - R[24] (90)
where [24] takes the values zero or 2.5.
The emulsion in the cooling tank contains volumes of oil
and water which can be described by
Y[20] = V[20] + R[16](l - W[5]) - R[21](l - W[6]) (91)
and
V[21] = V[21] + 0.9R[16]W[5] - R[21]W[6] (92)
\
COOLIKO TANK
57
SALES TANK VOLUME
(BARRELS) VOLUME
(BARRELS)
ACCcnrAaLE
CMULSION
Rf27}*
WATER MOi^lTOR
409.1
^^ | l0 . l r \WE^!ULSION LAYfR, V \ \EMULSIOM LAYER \
^.t^//Ji
-©~" -P.Z2^
UNACCEPlACLE CMULSION
' • V - . • •
I N L E T S
W(9J
Rf25)
RECYCLE PUMPS -^sr^ R : ? 4 1
EMULSION INLET n(iei,wtej
• • ,
RECYCLE TANK
125.9
RECYCLE PUl'.PS •^Riaoi
• F t S l J SOUTH
Figure 8. Cooling, Sales and Recycle Tanks and LACT Units Diagram
58
respectively. Then, the water proportion of the emulsion is
given by
UTAT V[21] ^^^^ - VL20J + VL21J. (93)
The emulsion fluid in the cooling tank builds until the
top of the fluid level reaches the equalizing pipe that connects
the cooling and sales tanks. The volume of the cooling tank
is given by
V[16] = V[16] + R[17] - R[21] - R[24]. (94)
When V[16] exceeds 409 barrels, the emulsion spills into the
sales tank at the same rate emulsion enters the tank. That
is, the rate of emulsion into the sales tank is given by
R[21] = R[16]. (95)
The proportion of water being removed from the cooling tank
by the recycle pump and the equalizer, W[9l and W [ 1 3 ] , respec
tively, depends on the free water volume, V[19]. These pro
portions are given for three cases:
Case 1: V[19] 0 No Free Water W[9] = W[6] W[13] = W[6]
Case 2: 0 V[19] 409 Normal Free Water W[9] = 1 . 0
W[13] = W[6l
Case 3: V[19] 409 Excessive Free Water W[9] = 1.0 W[13] = 1 .0
(96) (97)
(98) (99)
(100) (101)
Provided that the sales tank does not contain excessive
fluid, the cooling tank cannot have excessive fluid as indi
cated in (95). However, if the sales tank contains excessive
59
fluid, (95) becomes
R[21] = 0. (102)
The volume of the cooling tank varies by the relationship in
(94), but if the latter definition of R[21] applies, it is
possible for the cooling tank to reach an excessive volume of
786.8 barrels.
Free water is removed from the emulsion in the sales tank
as was the case with the cooling tank. This water is removed
at a fixed efficiency of 5% because less heat energy is avail
able and less demulsifying chemical is available than was
available in the cooling tank.
Free water accumulates in the sales tank until the free
water volume exceeds 95.4 barrels. The sales tank recycle
pump rate is then entered manually. This rate, R[25], may take
the value of zero or 2.5.
The emulsion and water volumes of the sales tank vary in
a manner similar to these volumes in the cooling tank. The
emulsion layer is assumed to contain oil and water volumes
given by
V[22] = V[22] + R[21](l - W[13]) - (R[22] + R[23])(l - W[7]) (103)
and
V[23] = V[23] + 0.95R[21]W[13] - (R[22] + R[23])W[7] (104)
respectively. The free water volume is given by
V[18] = V[18] + 0.05R[21]W[13] - R[25], (105)
Since the emulsion volume is composed of volumes, V[22] and
V[23], the water proportion of the emulsion is given by
60
There are two purposes for the use of the cooling and
sales tanks. First, they serve as storage vessels as well as
providing a hydrostatic head for the LACT units. Secondly,
the fluid spends a relatively long period in the two uninsulated
tanks, allowing the fluid to cool from 100° F. to 80° F. or
less. The monitoring devices in the LACT units are calibrated
for an ambient temperature of 80° F.
The Lease Automatic Custody Transfer, i,e., LACT, units
contain the following components:
1. A pump to move emulsion through the unit.
2. A capacitance detector and associated electronic package to determine water proportion from the dielectric constant of the emulsion.
3. A diverting valve that sends emulsion containing less than 1% water to the sales meter and emulsion with more than 1% water to the recycle tank.
4. An automatic sampling device that catches minute samples of the emulsion and collects these samples to facilitate analysis of an average emulsion sample at the end of each month.
5. A wery precise positive displacement liquid meter that records all acceptable emulsion which is transferred to transportation faci1ities.
Two such LACT units are connected to the sales tank. The
pumps from both units are operated by the liquid volume in the
sales tank. One unit, called LACT No. 1, is utilized when the
tank volume, V [ 1 7 ] , is between 110.1 and 141,6 barrels. The
61
second unit, LACT No. 2, is utilized in addition when the
volume exceeds 141.6 barrels. The pump rates for the two
pumps are given by
Case 1: V[17] < 110.1 Rr22] = 0 Unit No. 1 R[23] = 0 Unit No. 2
Case 2: 110.1 ^ V[17] < 125.9 R[22] = Previous Value R[23] = 0
Case 3: 125.9 < V[17] < 141 .6 R[22] = 8 R[23] = 0
Case 4: 141.6 < V[17] < 152.4 R[22] = 8 R[23] = Previous Value
Case 5: V[17] > 152.4 R[22] = 8 R[23] = 8.
107) 108)
109) 110)
111) 112)
113) 114)
115) 116)
Both pump control devices have a hysteresis effect. For example,
the pump for unit No. 1 is turned on when V[17] is more than
125.9 barrels, but the pump is not turned off until V[17] is
less than 110.1 barrels. The hysteresis for the two pumps is
indicated by (109) and (114).
The water proportions of the sales tank to the LACT units,
W[14], and the sales tank to the recycle tank, W[8], are given
by
Case 1: V[18] < 0 No Free Water Wr8] = W[7] WL14] = W[7]
Case 2: 0 < V[18] < 95.4 Normal Free Water W[8] = W[7] W[14] = 1.0
Case 3: V[18] > 95.4 Excessive Free Water '^r8] = 1.0
141 = 1.0.
117 118
(119) (120)
(121) (122)
62
When the LACT units monitor the water proportion of the
emulsion, they can route fluids depending on the acceptability
of the water content. This action is simulated by calculating
rates of acceptable emulsion, R[26] and R[27], and of unac
ceptable emulsion, R[28] and R[29]. R[26] and R[28] apply to
LACT No. 1, while R[27] and R[29] refer to LACT No. 2. The
values of these four variables are calculated by
Case 1: W[7] < 0.01 Acceptable Emulsion R[26] = R[22] R[27] = R[23]
R[28] = 0 R[29] = 0
Case 2: W[7] > 0.01 Unacceptable Emulsion R[26] = 0 R[27] = 0
R[28] = R[22] R[29] = R[23].
The volume of liquid in the sales tank is a function of
the previous volume and the summation of input and output rates
in time. This volume is given by
V[17] = V[17] + R[21] - R[22] - R[23] - R[25]. (131)
At no time in the history of the central battery have the pumps
which cause R[22] and R[23] been left off. Therefore, no
provision has been included to manually assign a value of zero
to either variable. On rare occasions, excessive fluid volume
accumulated because R[21] was larger than the combined magni
tudes of R[22], R[23], and R[25]. Then, it is possible for
fluid to fail to flow from the cooling to the sales tank even
with all pumps operating. This situation occurs when
V[17] > 409 Barrels.
( 1 2 3 ) ( 1 2 4 ) ( 1 2 5 ) ( 1 2 6 )
( 1 2 7 ) ( 1 2 8 ) ( 1 2 9 ) ( 130 )
63
The recycle tank is merely a storage vessel. By the time
emulsion enters this tank, the fluid is so cool and the demul
sifier so depleted that no separation of water from emulsion
occurs. The free water from the sales and cooling tanks settles
to the bottom of the tank. The emulsions from the two LACT
units mix in the tank. Since it is possible for operating
personnel to fail to turn off the recycle pump when all free
water has been removed from the cooling or sales tank, it is
possible for emulsion instead of free water to enter the recycle
tank.
The water proportion in the emulsion pumped from the tank
differs from the proportion of water in the emulsion layer.
Two centrifugal pumps move fluid out of the tank. Thus, the
free water and emulsion are completely blended into an emulsion.
The emulsion and water layers are treated in the model as if
they composed a single layer with oil and water volumes. The
water volume is defined by V[25] while the tank fluid volume
is given by V[24] .
V[24] = V[24] + (R[24] + R[25] + R[28] - R[30] - R[31])(132)
V[25] = V[25] + R[24]W[9] + R[25]W[8] + (R[28] + R[29])W[14] - (R[30] + R[31])W[10] (133)
The water proportion can be defined by
v r 2 5 i WHO] = VT24T. (1 )
Two pumps send emulsion from the recycle dehydrators. One
called the North Recycle Pump, rate R [ 3 0 ] , handles emulsion
for the North Recycle Dehydrator. The South Recycle Pump, rate
64
R [ 3 1 ] , supplies the South Recycle Dehydrator. Both pumps are
operated by a single float control device that determines the
pump rates as shown below:
Case 1 : V[24] < 49.4 R[30] = 0 R[31] = 0
Case 2: 49.4 < V[24] < 125.4 R[30] = Previous Value R[31] = Previous Value
Case 3: V[24] > 125.4 R[30] = 6 R[31] = 6.
135) 136)
137) 138)
139) 140)
As has been the case with several previous rates, the recycle
pump rates are influenced by the hysteresis of their control
system.
The fluid handled by the recycle pumps is sent to the
dehydrators for further water removal. Additional demulsi
fying chemical is added to the fluid so that it and the emulsion
are thoroughly mixed before reaching the dehydrators. The rate
at which the chemical additive enters the system is never
changed. Thus, the effect of the chemical is included with
the effects of other independent variables determined by the
recycle dehydrators.
Recycle Dehydrators
A schematic diagram of the recycle dehydrators is shown
in Figure 9. These vessels are vertical dehydrators and
formed the backbone of dehydration systems until the perfection
of the electrostatic dehydrators. These vessels are quite
65
NORTH SOUTH
EXHAUST HEAT LOSS
m
HEAT CXO4Atf0£R
.INLET BAFFLE
CKIULSIOK INLET
ftlSO]
WHO]
DUMP VMVe*
V/WATER I A Y E ' R ' ' / /
y///';7///A
1
It ^CURNER CONTROL
jj^LjtiLET rJaai R
EXHAU3T HEAT l o s s
R(391
V'ATCR CUTLET a3?.l
T»r?uLsi6N nasi I?E*TURN OUTLET LINE
Riwr
RfMj
Figure 9. Recycle Dehydrator Schematic Diagram
66
effective in removing water from emulsions, but they have a
much lower limit on the maximum throughput when compared to
electrostatic dehydrators. Often vertical dehydrators are
taken from surplus equipment stocks for use as recycle vessels
in major central batteries. It should be noted that these
vessels use only the effects of heat and chemical action for
water separation.
The two dehydrators are identical to each other. The
relationships for temperature, efficiency, and fluid volumes
are the same for one vessel as for the other. The two vessels
are treated separately because their initial conditions may
vary. The relationships of temperature, efficiency, and
volumes are developed for the North Recycle Dehydrator. The
parameter designations are changed to describe the South
Dehydrator and the relationships are duplicated.
Fluid that enters the recycle dehydrator has already been
processed and insufficient water was removed from the emulsion
as it passed through the electrostatic dehydrator. Thus, it
is assumed that the emulsion causes difficulty in water re
moval. The recycle dehydrator is operated at much higher
temperatures than is the electrostatic dehydrator.
The gas burners are fired when the vessel fluid temperature
is less than 120° F. and the burners are turned off when the
temperature exceeds 150° F. Gas is fed to the burners only
when they are burning. The gas rate to the burners, expressed
in BTU/minute, is given by
67
R[36] = 6.67 X lO"^. (141)
Obviously, the fluid temperature is a function of the
rate at which the gas is burned. When R[36] is
T m 80R[30] - (Rr32l - R[33])T[5]
^[5] = Rr3Qj ^ R[32]'^ R[33] ^ 40 (142)
T[5] reaches and holds a value of 150° F. until the burner
rate, R[36], takes the value of zero. At this time, T[5] varies as
^._ 80R[3Q] + (R[32l + R[33])T[5] '1-5J = RL30] + R[32] + RL33J (143)
The heating efficiency of the dehydrator is assumed to be 40* ..
Then the exhaust heat loss from the dehydrator is given by
R[37] = 0.6R[36]. . (144)
Obviously, R[37] is zero when R[36] is zero.
The efficiency of water separation is a function of both
chemical inlet rate and temperature. However, the demulsifier
inlet rate is constant and can be removed from the definition
of efficiency by a judicious choice of constants in this
definition. The dehydrator efficiency is given by
E[4] = 0.04 + 0.00633T[5]. (145)
As water separates from the emulsion, the water and
emulsion segregate into layers. The height of the top of the
water layer is detected by a float which operates the water
dump valve. The free water is siphoned from the vessel and
dumped into the return line to the inlet surge tank. The
volume of free water is
Y[26] = V[26] + R[30]W[10]E[4] - R[32]. (146)
68
The rate at which free water leaves the vessel is given by
Case 1: VE26] < 71 .6 R[32] = 0
Case 2: 71.6 < V[26] < 107.4 R[32] = Previous Value
Case 3: V[26] > 107.4 R[32] = 6.
(147)
(M8)
(149)
Emulsion fluid accumulates above the free water layer
until the top of the emulsion is at the spillover tube. Emul
sion fills the tube until a hydrostatic device senses that the
tube is full and opens the emulsion dump valve. The emulsion
dump valve operates in discrete steps, but these steps occur
so frequently that the emulsion is considered to be flowing
at the continuous rate of
R[33] = R[30](l - W[10]E[4]) (150)
It was determined that the volume of the emulsion layer
is a constant 44.75 barrels. The volume of water contained
in the emulsion layer is given by
V[27] = V[27] + R[30]W[10](1 - E[4]) - R[33]W[11]. (151)
From this relationship, the water proportion of the emulsion
is determined to be
w[in = im. (152)
Since the two dehydrators have the same relationships for
the various parameters, the above discussion need not be
repeated for the South Recycle Dehydrator. It is sufficient
to name the parameters of this vessel and duplicate the pre
vious relationships.
69
Fuel Inlet Rate R[38] = 6.67 X 10^
Fluid Temperature Tfel = 80RI31] ^ (R[34] - Rr35l)Tr6l ^ .. "• -' R[31J + RL34] + R [ 3 5 ] ~ ^ ^ ^°
TFAT 80R[31] H- (R[34] - Rr35l)Tr6l '1- -' = RL31J + R[34] -f R[35]
Exhaust Heat Loss
Effi ciency
R[39] = 0.6R[38]
E[5] = 0.04 + 0.00633T[6]
Free Water Outlet Rate Case 1 : V[28] :5 71 .6
R[34] = 0
Case 2: 71.6 < V[28] < 107.4 R[34] = Previous Value
Case 3: V[28] > 107.4 R[34] = 6
Free Water Layer Volume V[28] = V[28] + R[31]W[10]Er5] - R[34]
Volume of Water in Emulsion Layer V[29] = V[29] + R[31]W[10](1 - E[5]) - R[34]W[2]
Emulsion Water Proportion uri9i y[29] WL12] = 2^-75"
153)
154)
155)
156)
157)
158)
159)
160)
161 )
162)
163)
The completion of the discussion of the recycle dehydra
tors completes the cycle of emulsion through the central
battery. Emulsion and free water are dumped without mixing
into the return line. This return line also contains free
water, interface fluid, and perhaps emulsion. All these
fluids intermingle but do not mix to form emulsion. The
temperature of the return fluids stabilizes prior to reaching
the inlet surge tank. However, the heat energy carried by
70
the return fluids can be accurately described by assuming
that each individual fluid stream retains the temperature of
the vessel from which it comes.
Produced and Fresh Water Injection Sv^stjem
Free water which was removed from the inlet surge tank
is pumped to the free water surge tank. The rate of free
water removal was defined in (15). The water accumulates in
the tank. The water volume is given by
V[30] = V[30] + R[9] + R[40] - R[41]. (164)
A float operates the coalescor pump which removes water
and sends it through the coalescor to the coalesced water
tank. The rate at which the water is pumped is given by
Case 1: V[30] < 188.8 R[41] = 0
Case 2: 188.8 < V[30] < 251.8 R[41] = 0.111(V[30] - 188.8)
Case 3: V[30] > 251 .8 R[41] = 7.
(165)
(166)
(167)
The central battery serves one major oil field, but it
also provides a method of disposing of the water produced by
another nearby field. The water from this field enters the
free water surge tank at a constant rate,
R[40] = 0.2.
Water is mixed in the tank and used for injection in the major
field.
The water enters a vessel called a coalescer. This vessel
is not included in the model because its operation is very
71
slow when compared to the other operations in the battery and
this vessel has never caused any operational problem of inter
est in this work. A description of this device is included
in this study to assure that the battery description is complete
The coalescer is an excelsior-packed tank whose purpose
is to remove droplets of oil from the produced water. The
packing causes the velocity of liquid flowing in the vessel
to be sufficiently reduced so that tiny droplets of oil merge
to form larger droplets. As the droplets coalesce, a layer
of nearly water-free oil develops. This oil is drained into -
the recycle tank, but the rate of oil collection is negligible
in comparison to other rates.
The clean water travels into the coalesced water tank
which is merely a storage tank. The volume of water in the
tank accumulates by the relation
V[31] = V[31] + R[41] - R[42]. (168)
Water is removed from the tank by a float-controlled
pump. The rate at which water is removed is given by
Case 1: V[31] < 377 R[42] = 0
Case 2: 377 < V[31] < 504 R[42] = Previous Value
Case 3: V[31] > 504
(169)
(170)
R[42] = 10. (171)
The amount of produced water is not sufficient to satisfy
the water injection system demand at this time in the life
of the oilfield. Therefore, for a few years, it is necessary
72
to produce fresh water from water source wells to obtain the
needed water. The inlet fresh water rate is
R[44] = 41.7.
The fresh water contains about 25 parts oxygen per million
parts water. This oxygen content is sufficient to assure the
survival of aerobic bacteria which then accumulate and plug
control tubes, meters, and small orifices in the injection
system. To assure optimum operation of the system, the bac
teria must be destroyed by removing most of the oxygen from
the fresh water.
Residue gas, which is obtained from a nearby gasoline
plant, is pumped through a glycol scrubber which removed oil
and water from the gas. The gas is then compressed and pumped
into the bottom of two oxygen desorption towers. The water
falls through a series of baffle plates while the gas bubbles
upward through the water. The agitation caused by the baffle
plates and the prolonged contact of gas and water causes the
gas to absorb the dissolved oxygen from the water.
The water, which now has an oxygen content of less than
5 parts per million, is exhausted under pressure to the mixing
vessel. The residue gas, which contains less than 1% oxygen,
is injected into the dehydrator fuel gas system.
The rate at which gas passes through the desorption towers
is
R[43] = 1.4 cubic feet per minute.
The energy content of this gas is not given because the gas
73
is not burned in the desorption towers. Rather, gas is merely
pumped through the towers.
The fresh water and produced water enter the mixing vessel
This unit is a tall, slender tank that contains a series of
baffel plates that assure complete mixing of fresh and pro
duced water. The volume of water in the mixing vessel is
given by
V[32] = V[32] + R[44] + R[42] - R[45]. (172)
If this volume exceeds 161.1 barrels, the fresh water source
well pumps and produced water pumps are stopped.
Two pumps move water from the mixing vessel to the injec
tion station suction tank. Since the outlets of these pumps
are connected, only one rate, R[45], is considered. The two
pumps are controlled by hydrostatic devices which sense the
height of water in the mixing vessel. This height can be
converted to the volume of water in the vessel, V[32]. The
outlet rate, R[45], can be defined in terms of V[32]:
Case 1: V[32] < 53.7 R[45] = 0
Case 2: 53.7 < V[32] < 71.6 R[45] = Previous Value
Case 3: 71.6 < V[32] < 89.5 R[45] = 23
Case 4: 89.5 < V[32] < 107.4 R[45] = 46
Case 5: V[32] > 107.4 R[45] = 46.
(173)
(174)
(175)
(176)
(177)
It should also be noted that R[45] takes the value of zero
when the pumps are stopped by a device sensing an excessive
74
volume in the suction tank.
The water injection station uses a combination of centrif
ugal and positive-displacement pumps to provide a large volume
of water at high pressure. The pumps are operated manually
and their total output rate is
R[46] = 52.
A hydrostatic control unit stops all pumps if the volume of
the suction tank is less than 167.9 barrels.
The required rate from the injection station is not
constant because this demand is determined by field operations
At all times, the demand rate is less than R[ 4 6 ] . A pressure
control system sends part of the output of the injection
pumps back to the suction tank to hold a constant discharge
pressure of 1250 psig. The rate at which water returns to
the suction tank is given by
R[48] = R[46] - R[47] (178)
where R[47] is the rate demanded by the injection system.
The injection system demand will be manually supplied in order
to simulate field operation conditions.
The volume of water in the suction is a continuously
varying parameter. This volume is given by
V[33] = Y[33] + R[45] + R[48] - R [ 4 6 ] . (179)
Prior to entering the suction tank, all water is routed
through a water filter system. This system is used to remove
all particulate material that remains in the mixed water.
This filter system is not included in the model because it
75
does not interfere with the water handling processes of the
battery.
A small volume of water is removed from the suction tank
for use in backwashing the water filters and coalescer. This
water, containing the solids caught by these vessels is pumped
into the water disposal tank. Fluids brought to the battery,
but known to contain foreign agents that cannot be processed,
are also placed in this tank. Liquids and solids from the dis
posal tank are pumped to a water disposal well located five
miles from the central battery. The disposal system is not
included in the model because of its negligible effect on
battery operation.
Interconnection of Vessel s in the Central Battery
As has been detailed in the previous discussion, all of
the vessels are connected to provide a continuous liquid flow
path. The diagram in Figure 4 shows the major connections
between vessels. However, it is not possible to show all
fluid paths between vessels or inside individual vessels due
to the complexity of the battery.
One fact that is apparent from a cursory glance at Figure
4 is that a number of feedback loops exist in the central bat
tery. Since the mathematical derivations indicate that some of
these loops employ positive feedback, that is, the inlet surge
tank temperature variation in equation ( 8 ) , it appears that
multiple conditions exist which would cause the battery to
76
exhibit unstable, oscillatory behavior. This possibility is
investigated in Chapter III.
Model Flowchart Development
The derivation of a mathematical model of the central
battery was completed in this chapter. In order that the model
may be presented in a straightforward manner, it is best to
present the model schematically. One method of presenting
the model is by the use of a flowchart. Since the calculation
of individual parameters would normally be conducted by a
digital computer, the flowchart approach leads directly to the
programming of the computer.
Several flowchart presentations could be made for the
m o d e l , and one is shown in Appendix B. This flowchart utilizes
the concept that all parameters are calculated for a one-minute
period. Then, the model loops back to the starting point and
calculates the parameters for another time period.
The example flowchart is prepared for a program for a
Control Data Corporation Model 1604 computer. The input and
output functions of the model are shown for this computer.
CHAPTER III
ANALYSIS OF MODEL
All mathematical relationships were developed based on the
available operating data from the central battery. The data
for several parameters such as field inlet rate, emulsion
outlet rate, produced and fresh water rates to the injection
station, and injection rate were available on a daily basis
for the full time the battery was operational. Other data
such as vessel temperatures were available on a weekly basis.
The remaining data was available in the form of qualitative
observation and experience. It should be noted that this data
was obtained from experienced personnel who have been trained
to carefully observe the operating features of all vessels.
Therefore, the qualitative data is considered to be quite
accurate and acceptable.
Comparison of Model Operation to Actual Operation
The rate at which liquid enters the inlet surge tank varied
from 17,500 to 18,000 barrels oil and 11,750 to 12,000 barrels
water per day. From 50% to 75% of the water was contained in
an oil-water emulsion. Therefore, the actual inlet rate of
emulsion ranged from 23,375 to 27,000 barrels per day while
free water entered the tank at rates ranging from 3000 to 5875
barrels per day.
The model allows emulsion to enter the tank at rates
77
^ X
ifj^^iei!-^
78
ranging from 0 to 52,416 barrels per day in eight discrete
steps of 7488 barrels per day each. In the range of normal
tank operation the model allows the discrete rates of 22,464
and 29,952 barrels per day. The free water inlet rate is re
quired to be a constant value of 4320 barrels per day. The
model rates shown above are expressed in barrels per day for
ease in comparison to actual data but are exoressed in barrels
per minute when being used in the model.
The temperature of the field inlet liquid is about 70° F.
for most time. During very cool weather, i.e., atmospheric
temperature at or below 20° F., the inlet fluid temperature
can be as little as 40° F. for several hours. During very warm
weather, i.e., atmospheric temperature at or above 90° F., the
inlet fluid temperature may be as large as 80° F. The model
allows the temperature of the inlet liquid to vary from 40° F.
to 92.5° F. in steps of 7.5° F.
The temperature of the liquid in the inlet surge tank
is dependent on the field inlet temperature and on the outlet
temperatures of the dehydrators. For cool fluid entering the
tank at moderate rates, the temperature of the tank is about
50° F. For warm fluids entering the tank and for high temperature
operation of the dehydrators, the tank temperature reaches
about 90° F. For a rapid change in dehydrator temperatures,
the tank fluid temperature will change by about 5° F. in 10 to
15 minutes. The field fluid temperature cannot change rapidly
because of the heat energy stored in and around the pipeline
79
system. However, the inlet temperature can change by as much
as 5° F. in a period of about 4 hours. When this change occurs.
the tank fluid temperature reaches the same temperature in about
2 hours.
The model approximates this temperature variation. For
a field inlet temperature of 40° F. with inlet rates totaling
29,000 barrels per day, the tank fluid approaches a temperature
of 50° F. When the tank fluid starts at about 60° F. and the
field inlet temperature is 80° F., the temperature increases
at about 0.1° F. per minute until a stable temperature of 90°
F. is reached. The time required for the stable temperature
to be reached is about 5 hours.
When the tank fluid has a stable temperature of 70° F.,
the field inlet temperature is 60° F. and the return fluid
temperature is 100° F. If the return fluid temperature sudden
ly increases to 140° F., the tank temperature increases by
0.4° F. per minute until a stable temperature of 74° F. is
reached. This temperature is reached in about 10 minutes.
However, if the return line temperature is suddenly returned
to 100° F., the tank temperature drops by about 0.01° F. per
minute to the original temperature of 70° F. in about 7 hours.
The predicted temperature variation with time and fluid
rates very closely approximates actual conditions. Only when
the dehydrator temperatures suddenly drop, does the model
depart from actual operation. In the case where the model
took about 7 hours to restabilize with reduced return fluid
80
temperature, the actual tank temperature would stabilize in
about 4 hours. This departure is not considered to be a serious
problem becuase the time predicted is within the order of
magnitude of the actual time. Also this is an unusual set of
conditions that would rarely occur.
The volumes of liquids within the tank are monitored by
observing exterior level indicators. The floats that are used
to control the outlet rates are set such that the depth of the
water layer varies from 3' to 3' 6". The depth of fluid in
the tank varies between 5' 3" and 5' 11". Operating personnel
note that the depths of fluids vary at fractions of an inch
in several minutes.
Samples are occasionally taken from the emulsion outlet
stream and analyzed for water content. When the field inlet
temperature is near 80° F. and the tank temperature is near
90° F., the water proportion is 1% or less. When the tank
temperature is low, i.e., 50° F., the water proportion may be
as large as 10%.
The model allows relatively small changes in tank fluid
volumes. When the water level is 3' 4", i.e., water layer
volume of 2093 barrels, the inlet rate of 4.6 barrels per
minute exactly matches the outlet rate of 4.6 barrels per
minute. If the inlet rate suddenly increases to 7 barrels
per minute, the water layer would rise about 0.35 inch while
the outlet rate would increase 5 barrels per minute in a period
of 10 minutes. In about 50 minutes, the level would rise about
81
2 inches and the outlet rate would increase to 7 barrels per
minute.
The height of the top of the emulsion layer varies in a
similar manner. If the inlet rate is 14 barrels per minute
and the height of the top of the emulsion layer is 5' 9", the
outlet rate is also 14 barrels per minute. If the inlet rate
were suddenly increased to 18 barrels per minute, the depth
of tank fluid would begin to increase by less than 0.1 inch
per minute.
The efficiency of water removal can be compared to the
water proportions observed in the inlet and outlet streams.
For an inlet water proportion of 25% and tank temperature of
50° F., i.e., low ambient temperature condition, the model
predicts that the efficiency will stabilize at a value of 60%.
This efficiency results in a stable outlet water proportion
of 11.7%. Under these conditions, the demulsifier chemical
inlet rate would be increased to its maximum value and the
efficiency would increase to 81%. Then the water proportion
would decrease to 5.9%.
On the other hand, if the inlet water proportion is 20%
and the tank temperature is 70° F., the efficiency of the tank
is calculated to be 82%. In this case, the outlet water pro
portion would drop to about 4%.
In the case of high atmospheric temperature, the inlet
water proportion is about 10% and the tank temperature is
about 80° F. In this case, the efficiency is 93% and the
32
outlet water proportion is 0.8%.
The model's predictions of volumes, outlet rates, and water
proportions are consistent with field operation. The very small
magnitude of volume and rate fluctuation approximates the slow
ly varying continuous operations. The water proportion varies
with temperature and inlet water proportion as in the actual
facility. This indicates the calculated efficiency adequately
predicts the rate at which water is removed from the emulsion.
As was stated in earlier discussions, little was known
about the continuous operation of the electrostatic dehydrator
prior to this study. It was observed that when it became ne
cessary to use the high temperature thermostat, the outlet
temperature would reach 140° F. after about 30 minutes. If
the low temperature thermostat was activated, the outlet tem
perature dropped to 100° F. within about 2 minutes.
The water proportion of the emulsion entering the electro
static dehydrator is essentially the same as the proportion
leaving the inlet surge tank. About half of the water entering
the dehydrator is separated in the heated section. The lowest
water proportion possible at the outlet is about 0.5%.
When normally composed emulsion enters the dehydrator,
the rate at which water is removed from the emulsion is pri
marily dependent on the temperature and chemical inlet rate.
When emulsion which contains water in the proportion of 5% to
3 0 % enters the dehydrator, the dehydrator is capable of reduc
ing the water proportion to 1% or less at the outlet stream.
83
The water removal rate in the grid section decreases somewhat
with increased water proportion.
When emulsion which contains an excessive proportion of
residual materials from field operations enters the dehydrator,
the efficiency of the dehydrator is reduced significantly. If
this condition is allowed to progress without intervention,
the water proportion at the outlet rises to levels above the
acceptable level of 1%.
If the interface fluid in the grid section ever touches
the active grid, the fluid leaves a semisolid coating on the
grid which creates a \/ery low resistance connection to ground
potential. If the liquid layer is lowered, the coating remains
on the grid. This coating can be removed only by alternately
washing the grids with emulsion and water by mechanically
varying the liquid heights in the vessel. It is also possible
to drain and vent the vessel and manually wash the grid, but
this is an undesirable method of cleaning the grid.
If the active grid is shorted to ground, the high temper
ature thermostat assumes burner control. When the fluid temper
ature is increased to 140° F., the heat action alone will
remove about 70% of the water in the emulsion.
The model treats the electrostatic dehydrator as two
operational units. By considering the cascaded efficiencies
of the heated and grid sections and by summing emulsion and
water outlet rates, the values calculated by the model can be
compared to the actual vessel's operation.
84
For a normal emulsion rate of 19 barrels per minute,
inlet water proportion of 5%, and outlet temperature of 100^
F., the efficiency of the heated section is 50'".. The efficiency
of the grid section is dependent on the water proportion of
the emulsion entering the grid section. For these conditions,
the overall vessel efficiency is 90% and the outlet water
proportion is 0.5%. This value provides excellent agreement
with field data.
For an inlet rate of nineteen barrels per minute composed
of 25% water and 75% oil, the efficiency of the dehydrator
drops to about 52% and the outlet water proportion increases
to the unacceptable level of 14%. In this case, the inlet
chemical rate to the inlet surge tank would be increased to
reduce the inlet water proportion to the dehydrator. Provided
the water proportion to the dehydrator is less than 8%, the
outlet water proportion is 1% or less without resorting to
manual intervention. Again, this agrees well with field
observed conditions.
In the model, the interface fluid layer begins to approach
the active grids only when the interface pump removes a fluid
other than the interface fluid. Within about 2 hours the model
allows the interface fluid to contact the grid. At this time,
an indication is given to use the high temperature thermostat
and to notify the model operator that the grids have been
disabled. This condition can be corrected only by stopping
and initializing the model. This procedure corresponds to
85
draining and cleaning the vessel.
Another condition which can cause the use of the high
temperature thermostat occurs when the water proportion of
the emulsion between the grids is so high that the normally
nonconductive emulsion begins to assume finite values of
resistance. This condition is modeled by calculating the
volume of water contained in this emulsion. If this volume
exceeds a maximum value, the grids stop operation and the
high temperature thermostat is used. This condition is self-
correcting if the water volume decreases.
When the high temperature thermostat is simulated, the
overall efficiency may decrease because .the grid section makes
no contribution. For the case of 19 barrels per minute of
emulsion containing 25% water, the temperature reaches a limit
of 140° F. about 2 hours after the high temperature thermostat
was activated. At this time, the efficiency of the dehydrator
reaches a maximum value of 49% and the outlet water proportion
reaches a stable level of 14.5%. It should be noted that this
is the same proportion as shown previously when the grid section
was active. However, this water proportion in an active grid
section would decrease the efficiency of the grids until the
high temperature thermostat became active. It should also
be noted that this behavior is consistent with actual operation
under abnormal conditions.
The function of the cooling and sales tanks is simply to
store emulsion. The volume of the liquids remains relatively
86
constant because liquid simply flows from this tank to the
sales tank. The water layer volume increases at a rate of about
1/2 barrel per hour when acceptable emulsion is being processed.
Under this condition, the water layer increases to a level
where drainage is required in about 70 hours. When abnormally
large quantities of water are being processed, only about 40
hours are required.
The emulsion volume of the sales tank varies with time.
The LACT units remove emulsion at rates based on the tank
volume. LACT unit No. 1 is operational at all times. The
second operates about 75% of the time. The model indicates
that the tank volume varies between 139 and 155 barrels and the
first pump runs continuously while the second runs about 70%
of the time. Only under unusual conditions does the model
indicate unacceptable emulsion at the outlet of the sales tank,
and the operation represents actual operation.
The recycle tank seldom holds significant volumes of liquid
The liquid normally contained in the recycle tank is only the
water that is removed from the bottom of the sales and cooling
tanks. The model accurately predicts that unless the recycle
pumps are disabled, the tank has a fluid volume that is slowly
variable in time.
The recycle dehydrators can operate for several days with
no liquid throughput. The rate of flow depends on the fluid
accumulation in the recycle tanks with no fluid throughput.
The temperature of the vessels is relatively constant. With
87
fluid flowing through the vessels, the temperature varies on
the order of several degrees per minute, because of the rapidly
changing volumes in the vessel. The burners of the dehydrators
are active for most time that fluid moves.
The temperature of the modeled dehydrator varies by sev-ral
degrees per minute. The variations are, of course, in the form
of discrete steps in temperature.
According to operational data, emulsion with a water
proportion of 5% to 20% has 75% to 90% of this water removed
by the time the emulsion leaves the vessel. The calculated
efficiency of the modeled dehydrator is 80% to 90?^ depending
on temperature.
The model does not take into account operational problems
in the water system such as the presence of thin layers of oil
on top of the water in each tank, the inefficiency in various
filters caused by occasional volumes of oil in the water system,
or occasional failures to remove the maximum volume of free
oxygen because of inadequate gas flow. These problems occur
so infrequently that their inclusion is not justified. When
these problems are not considered, the model gives results of
rate and volume data that is virtually identical to the actual
data. This accuracy is expected since the strictly volumetric
relationships of the water system were developed based on
measured operation of rather simple operations.
The previous discussions conclude the numerical analysis
of the model. In addition to the numerical analysis, it is
88
desirable to consider the actual continuous operation of the
central battery so that a stability analysis can be made of
the battery.
Stability Analysis of the Continuous System
As was stated in the second chapter of this work, it was
not considered feasible to derive continuous relationships for
all parameters. These relationships, even in the s-domain,
are quite complex, and solutions for these relationships may
not exist as real functions of time.
It is desired to make an estimate of the continuous
relationships in order to analyze the stability of the model
and the actual facility. It is possible to simplify some of
the battery operations in order to make such an analysis even
though such a simplification would not be adequate for the actual
model. Furthermore, field operations have indicated that most
unstable behavior occurs in the inlet surge tank and the
electrostatic dehydrator. Therefore, the central battery is
reduced to only the inlet surge tank and the electrostatic
dehydrator. This simplified process is shown in Figure 10 as
a continuous system.
In the following discussion, some of the variables from
Chapter II are used. Since this derivation deals with contin
uous time and frequency functions, it is necessary to differ
entiate between the independent variables using standard
notation, that is f(t) and F ( s ) . In order to avoid confusing
'..ijiiJUilfii!-
X Ufff l! )
89
FIELO IWLET LINE -> -Rl Wl T l
I M F T
TANK
R» F.r.ror?N • « ' • LINE
R7 V/2
R IZ T4
ELCCTFX^^T/LTIC OCHYl^SAi OR
EMULSION OUTLLT
-*~ RI6 W6
Figure 10. Simplified Dehydration Syste m
90
the independent variable notation and the variable identifi
cation, the bracketed numbers are denoted by variable numbers
only. For example, if T[3] is to be considered as a function
of time, it is denoted
T3 ( t ) .
The variation of temperature of the liquid was derived in
Chapter II. The relationship in the s-domain is
TQ/c^ H Rin(s)Tin(s) T3(s) = Rout(s) (180)
s + V(s) .
In the simplified case, the only two inlet streams are
1 . Field Inlet Stream
2. Free Water Return from Dehydrator.
For the purpose of this analysis, the field inlet rate and
temperature can be assumed to be constant since both parameters
change by small increments in short time periods of 2 or 3 hours. Then
RlTl
T3(s) = s +
R20(s)T4(s)
R7TIT R8(s) + V(s)
(181)
Furthermore
'out(t) = J (Rl + R20 - R8 - R7)dt (182)
where R20 is n e g l i g i l b e compared to R l , R7, and R8. Then
/ ( R l - R8 - R7)dt = / R l d t - / R8dt - / R 7 d t (183)
or
t l / ( R l - R8 - R7)dt] = ^ 4 ^ R7(s )
s R8 (s )
s ( 1 8 4 )
Then
91
T3(s) =
RlTl — T " ^ R20(s)T4(s)
Ry(s) - R8(s"7
(185)
s + Rl(s) - R8(s) - R7(sT s
or
T3(s) = s + Rl (186)
It is interesting to note that for short intervals of time,
the fluid temperature does not depend on the outlet rates from
the tank at all. The solution for T3(t) is some constant term
superimposed on the time function that is the Green's function
of R20(s)T4(s).
The efficiency of the inlet surge tank is still determined
by temperature and by the rate at which demulsifying chemical
is added. For short periods of time, this rate can be assumed
to be constant. Then the tank efficiency is
El(s) = .05 + .011T3(s). (187)
Wl is also assumed to be constant for short periods of
time. Therefore, the fluid volumes become
„./ X R2 + RIWIEI - R8(s) V3(s) = ^ (188)
and
/ > Rl(1 - Wl) - R7(s) Vl(s) = — ^ 1 '-^ (189)
since both terms are integrals of their associated rates.
However, R8 is related to V3 and R7 is related to V3 + VI. Then
.../ N R2 •»• RIWIEI - [V3(s) - 1884],022 V3(s) = ^ 3 ' ^ (190)
and
V3(s) = R2 + RIWIEI + 41.4
s + .022
The emulsion layer volume becomes
Vi(s) = R H l - Wl) (VI - V3 - 3297) .045
92
(191)
(192)
or
1 / N Rl(l - Wl ) -i- 148 - .045V3(s) '^^^ " s + .045 (193)
Substituting the value for V3
V l ( s ) =
R l ( l - W l ) ( s + . 0 2 2 ) + 148(s + . 0 2 2 ) - [R2 - RIWIEI ( s ) + 41 . 4 ] .045
(s + . 0 4 5 ) ( s + . 0 2 2 ) (194)
Knowing values for VI and V3, a value for R7 is
R7(s) = [Vl(s) + V3(s) - 3297].045 (195)
or
(Rl - RlWl + 1 4 8 ) ( s + . 0 2 2 ) + [R2 + R I W I E I ( s ) + 4 1 . 4 ] ( s + . 0 4 5 )
- 3 2 9 7 ( s . + . 0 2 2 ) ( s + . 0 4 5 ) - . 045 [R2 + R I W I E I ( s )
, , •»• 41 . 4 ] R 7 ( s ) = . 0 4 5 (s + . 0 4 5 ) ( s + . 0 2 2 ) ( 1 9 6 )
The outlet water proportion was given in Chapter II as a some
what complex relationship. In this simplified development,
the outlet water proportion is
W2(s) = Wl(s)[l - El(s)] (197)
or
W2(s) = Wl(s)[.05 + .011T3(s)]. (198)
In line with the derivation of a continuous system, it is
assumed that the heated section of the dehydrator maintains a
constant temperature of 100° F. In this case, as discussed
in Chapter II, the heated section efficiency is constant in
93
time and has the value 0.5. Then the water layer volume from
the heated section is given by
V7(t) = . 5 / R7W2dt - .Oee/ (V7 - 57.2)dt (199)
or
w,/ X _ .5R7(s)W2(s) .066V7 3.77 (200)
and
,5R7(s)W2(s) 3.77 V7(s) = s + .066
Then the free water outlet rate is
Rll(s) = [V7(s) - 57.2].066
or
(201)
(202)
The water proportion from the heated section to the grid
section is given by
The rate of emulsion flowing into the grid section is given by
R12(s) = R7(s) - Rll(s). (205)
The efficiency of the grid section is dependent only upon
the inlet water proportion. This relation is
E3(s) = .95 - 6.31W4(s) (206)
or
c . .95 - 2.68W2(s) "(^5 = 1 + .5W2(s) . (207)
The free water volume of the grid section is given by
VlO(t) = /(R12W4E3 - R14)dt, (208)
94
However R14 varies as a function of VIO. Then
VlO(t) = / ( R 1 2 W 4 E 3 - .034V10 + 3.3)dt
or
(209)
VlO(s) = R12W4E3 - 3.3
(210)
and
DiAf \ _.R12W4E3 - 98.4s R14(s) = .034 s + .034 (211)
The outlet emulsion rate is given as the summation of the
inlet and outlet streams of the grid section. Then
R16(s) = R12(s) - R14(s). (212)
This parameter is of primary importance. However, its variation
depends on all the previously discussed parameters.
The parameter, R20, is the sum of Rll and R14. When these
two parameters are expressed in terms of T3, the expression
for T3(s) becomes
335s3 + RiTls^ + .IRlTls + .0022R1T1 T3{s) = (213) Rls(s + .034)(s + .066) .
With an expression for T3(s), it is possible to derive an
expression for R16(s). This expression is
s'' + .195s^ + .191s^ + 94.Is^
+ .025R1W1T1S 898s
R16(s) = - .168T1S - .005T1
s(s + .034)2(s + .066)2(s + .045)(s + .022)
(214)
It is not within the scope of this study to further reduce this
equation and the resulting time function solution is also
beyond the scope of this study. The complexity of this
95
simplified relation verifies the assumption in Chapter II
that the description of the central battery in terms of con
tinuous functions is not feasible.
Even without a precise solution of R 1 6 ( s ) , it is possible
to qualitatively analyze the stability of the parameter. First,
it should be noted that six of the seven poles are in the left
half of the s-plane. The seventh pole, however, is at the
origin. This might cause no stability problem if all eigen
values were in the left half of the s-plane. The negative
signs in the numerator indicate that at least one zero exists
in the right half of the s-plane. Then at least one path on
the root locus plot crosses into the right half of the s-plane.
Therefore, there are some perturbation frequencies that cause
unstable operation. These are low frequencies because of the
pole located at s = 0.
The parameter T3 also has a pole at the origin of the
s-plane. However, all zeroes are located in the left half of
the s-plane. It would be a reasonable assumption that all
eigen values are also in this half of the plane. Because heat
energy is always being removed from the inlet surge tank, it
stands to reason that even under the worst upset conditions,
the tank's fluid temperature would be a damped function for all
conditions. Thus the temperature of the inlet surge tank is
stable.
A qualitative analysis of the remaining system at the
central battery indicates that for some perturbations, the
96
entire facility is unstable. The outlet emulsion rate of the
dehydrator exhibiting instability is an indication that the
behavior of the interface fluid, the water proportions, and
various water outlet rates are also unstable. The fact that on
occasion the actual central battery can be completely upset
verifies this analysis of instability.
Discrete System Behav ioral Analysis
The model describes the central battery as a discrete time
system. It is obvious that a discrete model must be somewhat
different than the actual system. It is desirable to analyze
the effect of describing the processes of the battery as dis
crete time relationships.
A satisfactory analysis of the discrete behavior of the
battery can be obtained by analyzing the simplification used
in the analysis of stability. The most crucial operations of
the battery occur in the inlet surge tank and in the electro
static dehydrator. Although other parameters and vessels are
affected by the operation of this tank and dehydrator, the
effect of other vessels on the overall battery operation can
be assumed to be negligible.
The continuous system described by the relations derived
previously is shown in Figure 11. The description of the
central battery as a continuous system is necessary for the
analysis of discrete behavior.
The first step in the analysis of the discrete system is
97
T4/«j,
T l H l / S >
PlU^.iil>-^' +^L-»—I'IRIWI";
i/s .0S3>>-'>-
- I
P'^-w SJ2,
_/v.
5 > • > - « ^ > •
r<^3*^
>
HO<-(4-
Figure 11. Continuous Analogy of Simplified Dehydration Syst em
98
to approximate this system with a sampled data system. This
approximation is made by inserting a sampler and a holding
circuit at each point in the system where the discrete system
determines the value of a parameter. These insertions are
shown in Figure 12.
It is assumed that the rate of change of each sampled
parameter is small in the one minute sampling period. It is
possible, then, to assume that the parameter variations are
linear within the sampling periods. Since these two assumptions
are valid, it can be stated that each sampler inserts an error
of 50% of the change of the parameter per sampling period.
According to Kuo [ 6 ] , this behavior is the result of a stair
step function being generated by sampling a ramp function.
When the error values have been inserted, a value of the
maximum error of the system can be calculated. Using repre
sentative values of Rl, R2, Tl, Wl, and T4, the maximum error
was calculated to be 5.6%.
The maximum error caused by describing the continuous
processes of the entire central battery in terms of discrete
relationships is not necessarily identical to the value cal
culated above. However, the simplified system which was
analyzed for error insertion is representative of most of the
independently varying parameters in the battery. Therefore,
it can be assumed that the maximum error of the entire model
is approximately 6%.
It was determined that no purpose would be served by
99
Figure 12. Discrete Analogy of Simplified Dehydration System
100
reducing the continuous system to a state diagram of discrete
variables. A value for the maximum error was determined for
the discrete system. This error indicates that the discrete
model is a good approximation of the continuous system. By
intuition it can be determined that the stability of the dis
crete system must be nearly identical to that of the continuous
system. Therefore a detailed analysis of the stability of the
discrete system would produce few new insights into the operation
of the model.
CHAPTER IV
CONCLUSION
The study of modeling the operation of a central battery
leads to several conclusions. The most important is that an
analysis of the type used in this study should be used at any
time a major battery is designed. The study indicated several
feedback loops between vessels that would not be noted without
a detailed analysis.
It was shown that the model was quite accurate in pre
dicting the response of the battery. Theoretically, the
maximum error of the model from the actual facility is 6%,
which is acceptable for most purposes, especially in light of
the fact that it is possible that the actual error may be of
a lower magnitude.
The battery is unstable for certain operational conditions.
This conclusion is indicated by theoretical analysis and is
supported by field operating data. Because the model accurately
predicts the actual battery operation, the model is also un
stable for some conditions. The unstable operation of the
model and of the actual battery takes the form of rapidly
increasing values of parameters such as water proportions until
upper bounds are reached.
There are several aspects of this work which deserve
attention in further studies. For example, a real time solu
tion of all parameters in the battery is justified. These
101
102
solutions would make possible the description of the battery
in terms of Laplace or z-transformations.
Another area of interest is the stability analysis of the
central battery. The analysis in this work is understandably
simplified. However, a complete solution of the stability of
the system could lead to minor equipment modifications which
would eliminate the unstable operation of the battery.
Within the last few years, it has been found that the
efficiency of dehydration and providing water for injection
projects can be improved by designing large, complex facilities
such as the central battery. However, the design of such
facilities is a complicated task that can result in a facility
that may be unstable. This study provides a means whereby
even a complex battery may be studied in detail before final
design plans are made. A model such as the one studied will
accurately predict the response of the facility under a variety
of conditions. Also, the model will predict the stability of
the facility and allow the substitution of equipment that can
improve the stability. Therefore, it may be concluded that
this study has provided a design system whereby facilities
such as the central battery may be designed and constructed
for optimum operation and stability.
LIST OF REFERENCES
1.
2.
3.
4.
5.
6.
Clark, Norman J., Elements of Petroleum Reservoirs, Dallas, Texas: E. J. Storm Printing Company, 1960.
Muskat, M., The Flow of Homogeneous Fluids Through Porous Medj_a, Ann Arbor, Michigan: J. W. Edwards, Inc., 1946.
Case, L. C. , Water Problems in Oi 1 Production - An^ Operator's MajlMl.. Tulsa, Oklahoma: The Petroleum Publishing Co., 1970.
Treating Oil Field Emulsions, Austin, Texas: Petroleum Extension Service of the University of Texas - Division of Extension and Texas Education Agency - Trade and Industrial Service, 1955.
Reynolds, William C , Thermodynamics, New York: McGraw-Hill Book Company, 1965.
Kuo, B. C , Discrete-Data Control Systems, Englewood Cliffs, New Jersey: Prentice-Hall, Inc., 1970.
103
APPENDIX
A. Parameter Designations
B. Central Battery Model Flowchart
104
105
APPENDIX A: PARAMETER DESIGNATIONS
Designation Parameter Description
R[l] Field Inlet Emulsion Rate to Inlet Surge Tank
Rr2] Field Inlet Free Water Rate to Inlet Surge Tank
R[3] Return Emulsion Inlet Rate to Inlet Surge Tank
R[4] Return Water Inlet Rate to Inlet Surge Tank
RFS] Interface Fluid Inlet Rate to Inlet Surge Tank
R[6] Interface Fluid Outlet Rate from Inlet Surge Tank
R[7] Outlet Emulsion Rate from Inlet Surge Tank
RFB] Outlet Free Water Rate from Inlet Surge Tank
R[9] Demulsifying Chemical Inlet Rate to Battery
RflO] Fuel Gas Inlet Rate to Electrostatic Dehydrator (BTU/minute)
11] Water Outlet Rate from Heated Section of Electrostatic Dehydrator
12] Emulsion Outlet Rate from Heated Section of Electrostatic Dehydrator
13] Water Overflow Rate from Heated Section of Electrostatic Dehydrator
14] Water Outlet Rate from Grid Section of Electrostatic Dehydrator
15] Interface Pump Outlet Rate from Electrostatic Dehydrator
16] Emulsion Fluid Outlet Rate from Grid Section of Electrostatic Dehydrator
17] Oil Outlet Rate from Grid Section of Electrostatic Dehydrator
18] Water Outlet Rate from Grid Section of Electrostatic Dehydrator
19] Heat Loss from Electrostatic Dehydrator Exhaust (BTU/minute)
106
R[20] Return Fluid Rate from Electrostatic Dehydrator
R[21] Emulsion Inlet Rate to Sales Tank
R[22] Emulsion Inlet Rate to LACT 1
R[23] Emulsion Inlet Rate to LACT 2
R[24] Recycle Pump Rate from Cooling Tank
R[25] Recycle Pump Rate from Sales Tank
R[26] Acceptable Emulsion Outlet Rate from LACT 1
R[27] Acceptable Emulsion Outlet Rate from LACT 2
R[28] Unacceptable Emulsion Outlet Rate from LACT 1
R[29] Unacceptable Emulsion Outlet Rate from LACT 2
R[30] North Recycle Pump Outlet Rate
R[31] South Recycle Pump Outlet Rate
R[32] Water Outlet Rate from North Recycle Dehydrator
R[33] Emulsion Outlet Rate from North Recycle Dehydrator
R[34] Water Outlet Rate from South Recycle Dehydrator
R[35] Emulsion Outlet Rate from South Recycle Dehydrator
R[36] Fuel Gas Inlet Rate to North Recycle Dehydrator (BTU/minute)
R[37] Fuel Gas Inlet Rate to South Recycle Dehydrator (BTU/minute)
R[38] Heat Loss from North Recycle Dehydrator Exhaust (BTU/minute)
R[39] Heat Loss from South Recycle Dehydrator Exhaust (BTU/minute)
R[40] Water Inlet Rate from Outlying Oilfields
R[41] Water Outlet Rate from Produced Water Tank
R[42] Water Outlet Rate from Coalesced Water Tank
107
R[43] Residue Gas Inlet Rate to Oxygen Desorption Towers (103 feet3/minute)
R[44] Fresh Water Inlet Rate to Oxygen Desorption Towers
R[45] Mixed Water Inlet Rate to Suction Tank
R[46] Injection Pump Outlet Rate
R[47] Injection System Demand Rate
R[48] Bypass Rate to Suction Tank
R[49] Voltage Difference Across Electrostatic Dehydrator Grids (Volt)
R[50] Power Delivered to Electrostatic Dehydrator Grids (Watt)
R[51] Total Fuel Gas Inlet Rate to Battery (BTU/minute)
R[52] Total Heat Loss from Dehydrator Exhausts (BTU/minute)
V[l] Emulsion Layer Volume in Inlet Surge Tank
V[2] Interface Layer Volume in Inlet Surge Tank
V[3] Water Layer Volume in Inlet Surge Tank
V[4] Foreign Emulsion Volume in Inlet Surge Tank
V[5] Water Volume in Emulsion Layer of Inlet Surge Tank
V[6] Oil Volume in Emulsion Layer of Inlet Surge Tank
V[7] Water Layer in Heated Section of Electrostatic Dehydrator
V[8] Liquid Volume in Heated Section of Electrostatic Dehydrator
V[9] Volume Below Top of Interface Layer of Grid Section
Y[10] Water Layer Volume in Grid Section of Electrostatic Dehydrator
V [ n ] Volume from Active Grid to Top of Interface Layer of Grid Section
V[12] Volume of Interface Layer of Grid Section
108
V[13] Oil Volume betv,een Grids of Electrostatic Dehydrator
V[14] Water Volume b&tween Grids of Electrostatic Dehydrator
V[15] Foreign EmulsiC)n Volume in Grid Section of Electrostatic Dehydrator
V[16] Liquid in Cooling Tank
V[17] Liquid in Sale^ Jank
V[18] Free Water in ^aies Tank
V[19] Free Water in tooling Tank
V[20] Oil in Emulsiori Layer of Cooling Tank
V[21] Water in Emulsion Layer of Cooling Tank
V[22] Oil in Emulsion Layer of Sales Tank
V[23] Water in Emulsion Layer of Sales Tank
V[24] Liquid in Recycle Tank
V[25] Water in Recyc'ie Tank
V[26] Free Water in ^^orth Recycle Dehydrator
V[27] Water in Emulsion Layer of North Recycle Dehydrator
V[28] Free Water in Jsouth Recycle Dehydrator
V[29] Water in Emulsion Layer of South Recycle Dehydrator
V[30] Water in Produced Water Tank
V[31] Water in Coalesced Water Tank
V[32] Water in Mixing Vessel
Y[33] Water in Sucti^)n Tank
V|[i] Water Proportitjp of Field Emulsion Inlet
W[2] Water Proporti^jp of Inlet Surge Tank Emulsion Outlet
W[3] Water Proporticjp of Inlet Surge Tank Emulsion Layer
W[4] Water Proportic^p of Heated Section Emulsion Outlet of Electrostatic Dehydrator
109
W
W |
W |
W |
u|
W |
u|
:6]
:7]
:8]
[9]
[10]
[11]
w
w
w
w
T
T
T
T
T
T
E
E
E
E
E
5]
12]
13]
14]
15]
1
2
3
4
5
6
1
2
3
4
5
IND
IND
Water Proporti Electrostatic
Water Proporti
Water Proporti
Water Proport
Water Proporti
Water Proport
Water Proport Dehydrator
Water Proport Dehydrator
Water Proport
Water Proport
on of Grid Section Emulsion Outlet of Dehydrator
on of Cooling Tank Emulsion Layer
on of Sales Tank Emulsion Layer
on of Sales Tank Recycle Pump Outlet
on of Cooling Tank Recycle Pump Outlet
on of Liquid in Recycle Tank
on of Emulsion Outlet of North Recycle
on of Emulsion Outlet of South Recycle
on of Sales Tank Inlet Emulsion
on of Sales Tank Outlet Emulsion
1]
2]
Water Proportion of Electrostatic Dehydrator Return Emulsi on
Field Inlet Fluid Temperature
Return Inlet Fluid Temperature
Inlet Surge Tank Internal Fluid Temperature
Electrostatic Dehydrator Fluid Temperature
North Recycle Dehydrator Fluid Temperature
South Recycle Dehydrator Fluid Temperature
Inlet Surge Tank Efficiency
Electrostatic Dehydrator Heated Section Efficiency
Electrostatic Dehydrator Grid Section Efficiency
North Recycle Dehydrator Efficiency
South Recycle Dehydrator Efficiency
Excessive Foreign Emulsion Volume in Inlet Surge Tank
Excessive Volume in Electrostatic Dehydrator
no
IND[
INDI
IND[
INDI
INDI
INDI
INDI
:3]
:4]
[5]
[6]
[7]
[8]
[9]
IND
IND
10]
IND[
INDI
IND[
INDI
INDI
INDI
INDI
INDI
INDI
INDj
INDI
INDI
INDj
INDj
: i i ]
:i2]
:i3]
:i4]
:i5]
:i6]
:i7]
:i8]
: i9]
[20]
[21]
[22]
[23]
[24]
Excessive Volume in Produced Water Tank
Excessive Volume in Cooling and Sales Tank
Electrostatic Dehydrator Using High Temperature Thermostat
Electrostatic Dehydrator Voltage at Low Value
Interface Pump Handling Water
Interface Pump Handling Emulsion
Electrostatic Dehydrator Power Consumption at Low Level
Unacceptable Water Proportion in Electrostatic Emulsion Outlet
Electrostatic Grids in Normally Uncoated Condition
Interface Pump Operating in High Capacity Mode
Excessive Volume in Electrostatic Grid Section
Excessive Water Volume in Cooling Tank
Excessive Liquid Volume in Cooling Tank
Excessive Water Volume in Sales Tank
Excessive Liquid Volume i
Excessive Liquid Volume i
Excessive Liquid Volume in Coalesced Water Tank
Excessive Liquid Volume i
n Sales Tank
n Produced Water Tank
n Mixing Vessel
25]
Excessive Liquid Volume in Suction Tank
Excessive Foreign Emulsion Volume in Electrostatic Dehydrator
Manual Data Input - No Change in Parameters or P = 0
Manual Data Input - Change Foreign Emulsion Inlet or P = 1
Manual Data Input - Change Inlet Water Proportion or P = 2
Ill
IND
IND
IND
IND
IND
IND
IND
IND
IND
IND
IND
IND
IND
IND
IND
IND
IND
IND
IND
IND
IND
26] Manua P = 3
27] Manua P = 4
28] Manua P = 5
29] Manua P = 6
30] Manua P = 7
31]
32]
33]
34]
35]
36]
37]
38]
39]
40]
41]
42]
43]
44]
45]
46]
Manua
Manua
Manua
Manua
Manua
Manua
Manua
Manua
Data
Data
Data
Data
Data
Data
Data
Data
Data
Data
Data
Data
Data
nput - Change Inlet Emulsion Rate or
nput - Change Inlet Temperature or
nput - Change Status Indicators or
nput - Change Injection Demand Rate or
nput - Change Inlet Chemical Rate or
nput - Data Input Change Required
nput - Change Foreign Emulsion Inlet
nput - Change Inlet Water Proportion
nput - Change Inlet Emulsion Rate
nput - Change Inlet Temperature
nput - Change Status Indicators
nput - Change Injection Demand Rate
nput - Change Inlet Chemical Rate
Inlet Surge Tank Excessive Volume
Inlet Surge Tank Emulsion Pumps Disabled
Sales Tank Recycle Pump Disabled
Interface Pump from Electrostatic Dehydrator at Maximum Rate
Cooling Tank Recycle Pump Disabled
Detailed Analysis Required
End of Program
Inlet Surge Tank Free Water Pumps Disabled
APPENDIX B: CENTRAL BATTERY MODEL FLOWCHART
112
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