2. process department.pdf

8/16/2019 2. Process department.pdf

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2.0 PROCESS DEPARTMENT

2.1

Block Diagram

The flow diagrams of a plant, showing the material and energy streams,

are the most useful documents to visualize the process in its globality

and understand the sequence of the unit operations foreseen in that

plant.

In the block diagram, a complex plant is divided in a sequence of

modules (block), each one representing a unit, a sub-unit or a group of

equipment performing a certain operation or function. The several

blocks are connected by arrows, which represent the main flow ofmaterials.

Normally the feedstocks are entering at the left side of the sheet, and

the succession of the blocks is from left to right. The final products are

normally obtained at the right side of the diagram.

Each block contains a name describing its function, and the lines

between blocks are also identified by the description of the stream and,

eventually, by its flow rate.

In Fig. 01 a typical refinery block diagram is shown. In this diagram

each block refers to a refinery unit, and the streams connecting the

blocks represent the petroleum cuts and the final products.


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F i g .

0 1


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2.2

Process Plant Definition

HOLD-UP TIMES

Level Emergency Span (1) [min]Service Level Normal Span [min] HL - LL

HHL-HL LL-LLL

Feed to Unit 10 2 2

Feed to fractionator 5 2 2

Feed to furnace 5 2 2

Product to storage 2 1 1

Reflux to column (2) 5 1 1

Compressor K.O. Drum (3) 5 2 2

Fuel Gas K.O. Drum 5 (4) 2 2

Steam Flash Drum 5 (4) 1 2

Steam Boiler 2 (5) - -

Hot Oil expansion tank (6) - -

Sour Water Stripper Surge Drum 60

Crude Oil / Gas separator 5 (7) 1 1

Crude Oil / Gas separator

(API 12 J)

Oil gravity < 0.85

0.85 < Oil gravity < 0.934

0.934 < Oil gravity < 1

1

1 – 2

2 – 4

1

1

1

Flare K.O. Drum 10 - 30 (8)

Note:

1. applicable only if HHl or LLL independent alarms /shutdown are foreseen;

2.

for reflux drum consider the gold-up times for both the reflux and the product streams;

3. if no liquid is entering the K.O. Drum, assume a liquid mass flow rate equal to 5% of gas flow

rate;

4. volume shall be at least equal to the volume of 15 m of inlet pipe;

5. volume shall be not greater than 1/3 of Boiler volume;

6. volume (calculated between ambient temp. and max operat. temp.) shall be not greater than 1/2of total volume;

7.

HL not higher than 60 % of diameter;

8. based on maximum liquid discharge to flare system.


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2.2.1

Process Simulation

The first activity to be performed by the process engineer is the

definition of all the unit operations, with the correct sequence, required

to obtain the desired products, starting from a given feedstock.

Normally this activity needs equilibrium calculation of multicomponent

mixtures, which can be more easily performed by a process simulator.

These dedicated softwares enable the users to characterize the

feedstock, selecting the chemical components (or pseudo-components)

or defining a crude assay. Then it is possible to select the appropriate

thermodynamic property package to be applied for flash calculations,

and finally the user will select the unit operations to be performed

sequentially along the plant.

The unit operations taken into account to simulate the plant are all

those which may vary a process parameter (pressure, temperature, flow

rate, composition) of a stream. Typical unit operations are:

L/V separation, corresponding to an equilibrium calculation at T

& P of the stream; in the plant this is performed by a vessel

separator;

heating / cooling of a stream, to obtain an increase / decrease of

its temperature, in conjunction with a pressure decrease; thisoperation can be performed by a heat exchanger, with or without

change of phase;

compression / pumping of a gaseous / liquid stream, to impose

an increase of its pressure; this operation implies also an

increase in temperature;

flow through valve, causing an adiabatic (isoenthalpic)

depressurization;

distillation, to calculate the separation obtained through a

distillation column; this is the most difficult process calculation,

due to the complexity of simultaneous energy and material

balance among the components, stage by stage.


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The way to proceed to carry out the process simulation is the following:

a preliminary scheme with all necessary unit operations isprepared, characterizing all the input streams (feedstock);

process variables have to be adjusted in order to obtain the

optimum operating conditions, necessary to get the desired

product;

if applicable, a sensitivity analysis shall be conducted, by varying

some process variable in order to optimize the equipment sizing;

this shall be done for the distillation column: the required

product separation may be obtained with a low number of trays

and a greater reboiler duty, or alternatively with a greater number

of trays and a reduced reboiler duty; after the preliminary process simulation has been completed,

the corresponding Process Flow Diagram (PFD - see paragraph

2.2) shall be prepared; always during the preparation of the PFD

some modifications to the scheme are necessary: for instance the

temperature at the exit of a cooler shall be increased because the

cooling water temperature has been assumed lower than actual

one, or an operating pressure at one vessel should be increased

because the contribution of some control valves has been

underestimated, etc.

after the modifications of the PFD, the process simulation shall be

finalized incorporating all necessary changes of processparameters.


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A printout of a process simulation scheme is shown in Fig. 02.

Fig. 02


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2.3 Process Flow Diagram

The Process Flow Diagram (PFD) is always used by the process engineerin the design work and in process study.

The PFD represents the heart of the process design, because in this

diagram it is possible to see:

the correct sequence of unit operations (pumps, heat exchangers,

L/V separators, distillation columns, etc.) needed to get the plant

performance;

the routing of the main process streams, with their flow rates;

all the operating conditions (pressure & temperature); all process controls foreseen in the plant;

eventual information regarding energy streams (power of

machinery, duties of furnaces and heat exchangers, etc.);

the stream numbers, from which it is possible to find a complete

stream characterization in the document "Heat and Material

Balance" (see paragraph 2.4);

It is quite obvious that the finalization of the PFD is obtained only after

the optimization of the process scheme, with all relevant process

calculations (flash, distillations, heating/cooling, phase change, etc.).

Each modification to material balance, operating conditions and

equipment change (addition or deleting) has to be reflected into PFD.

Some simple rules are used when preparing PFD:

all equipment (having an item) shall be represented in the PFD.

This means that all items are shown on PFD;

all engines (compressors and pumps) and eventual tanks shall be

drawn in the lower side of the sheet, over an hypothetic grade line

placed at one quarter of the sheet height from the lower edge;vessels, column and heaters will be drawn in the superior side,

leaving the middle space free for exchangers and connecting lines;

column condensers and reboilers will be drawn beside their

pertinence equipment; air condensers will be represented on the

horizontal trace of the overhead line, while the water condensers

will be placed upon or below the receiver;


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spare equipment do not need to be represented separately from

the main equipment: for istance the pumps P-1 A/S (A is the

main and S is the spare pump) will be represented as a single

pump, but showing the item P-1 A/S;

the incoming streams shall be located preferably on the left side

of the sheet, while the outgoing streams should leave the sheet

from the right side;

lines connecting the equipment shall be vertical or horizontal;

oblique lines are not allowed; in case of intersection of two lines,

the less important one will be interrupted. The order of

importance is:

-

main process line- secondary process line

- utility line

- instrument line

In case of intersection of two lines of equal importance, the vertical one

will be interrupted.

starting from the second issue of the diagram, every modificationmade on the previous revision shall be evidenced by a revision

cloud around the modified elements; this will help the reader to

identify the modifications made, without analyze the whole

drawing.

A typical PFD is shown in Fig. 03.


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Fig. 03


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2.4 Heat and Material Balance

After the finalization of the PFD and the process simulation, it isnecessary to prepare the "Heat and Material Balance", containing a

detailed characterization of each stream.

This document is mainly constituted by a table, where for each stream

the following physical properties and characteristics are shown:

stream label (as shown on PFD)

temperature & pressure

flow rates, volumetric and mass

vapour fraction

molecular weight

physical properties (density, specific heat, thermal conductivity,

viscosity, surface tension)

stream enthalpy

stream composition

These data are used to fill the data sheets of equipment and to make all

subsequent process calculations. These information are available from

the printout of the process simulation, if it has been done, or shall becalculated, case by case, if the particular system does not require a

simulation.

In Fig. 04 an extract of a typical material balance is shown.


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Fig. 04


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2.5 Data Validation

It is important to check and validate the quality and the correctness ofthe documents described in this chapter, i.e. PFD, Process Simulation,

Heat and Material Balance, because any mistake done at this first stage

of the project may have tremendous impact on plant performance. Any

change in operating conditions will cause changes in flow rates of the

relevant streams, and a re-sizing of the connected equipment could be

needed. If the changes happen at a late stage of the project, when all

equipment have been already supplied, the economic consequences will

be significant.

2.6 Piping and Instrumentation Diagram

2.6.1

General

The Piping & Instrumentation Diagrams (P&ID) have the scope to show

all equipment, all pipe elements and all the instrumentation existing in

a plant. P&ID is a schematic drawing, but the sequences in which all

the plant elements are installed must be faithfully represented;

2.6.2

Graphic Representation in P&ID

While preparing a P&ID it is necessary to consider that during the

subsequent engineering phases additional elements may be required.

For this reason in the first draft of the drawing it is opportune to leave

sufficient free space in the drawing area.

The graphic representation shall follow these general principles:

a. Equipment distribution in the drawing area should be uniform in

order to avoid areas with excessive concentration of drawing

elements. If the drawing is particularly empty, it is not advisable

to spread drawing elements over all available drawing area,especially if it leads to separate elements which functionally

operate close one to the other. The placement of the objects on

the drawing shall be as much as possible rational and

“aesthetically pleasant”, to facilitate the reading and the

interpretation of the drawing.The way of the lines shall be chosen

in order to reduce the number of curves and intersections with

other lines.


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b.

Equipment dimensions shall be selected to maintain, as much as

possible, the real proportionality scale: a pump should be smaller

than a tank and bigger than an instrument symbol.

c.

Main drains, sample connections, all the manoeuvring valves,

insulated / jacketed / steam traced lines, slop lines, start-up

lines, drainage systems, etc., shall always be indicated.

d.

All the machines (compressors and pumps) and eventual tanks

shall be drawn in the bottom side of the drawing area,

horizontally aligned on a hypothetic ground line placed more or

less at a quarter of the drawing height. Vessels, columns and

heaters will be drawn in the upper side, leaving the middle space

available for the interconnecting lines and exchangers. It is

obvious that the condensers and the reboilers will be represented

beside their pertinence equipment.e.

The air condensers will be represented on the horizontal run of

the overhead line while the water condensers will be placed on the

vertical run upon or below the receiver, depending on their

effective installation.

f.

Minimum elevation above grade shall always be indicated for each

vessel. The elevation shall refer to the bottom tangent line for

vertical equipment and to the bottom line for horizontal

equipment. Moreover if some elevation difference between

equipment and/or piping runs is mandatory to guarantee a

correct plant operation, it shall be clearly shown on P&ID.

g.

Whenever a complex equipment requires the representation of

many components / instruments (for instance a furnace with

several coils, skin thermocouples, fuel to burners, snuffing steam

and soot blowers), it would be opportune to use multiple sheets to

represent all details required. In this case on the first diagram

where the relevant equipment is drawn, it must be written the

number of drawings where these details are shown.

h. If the diagram is made of several sheets, all the pipes going to

consecutive P&ID, or coming from the previous P&ID, shall be

extended at left or right side of drawing, and the height shall be

correspondent to the height of the same line on linked P&ID; sothat two sequential drawings can be overlapped and all the lines

pass from one to the other.


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i.

If any pipe is not connected to any equipment in one or more

sheet, it shall be represented only in the drawing where the

equipment is shown. This means that it is not necessary to draw

a line sequentially along all sheets, if this is not specificallyrequired to represent some elements (fitting or instrument) on

these sheets. Pipes incoming and outgoing from the sheet shall

show the fluid indication, the destination (From\To), the origin /

destination drawing number, on the panel in which they are

connected.

j. In the pipes incoming (feed) and outgoing (products) from the

plant, the battery limits shall be clearly represented, with the

eventual block valves and/or spectacle blind. Limits relevant to

the scope of supply (for packages, parts of plant, equipment, etc.)

shall be clearly represented, with indication of each responsible

for the supply (for instance: Client, package’s vendor, EPC

contractor, etc.) at the transition point. Supply limit shall be

shown also for instrument (for instance between a local

transmitter, by package vendor, and DCS controller, by

Contractor).

k. Generally the equipment item shall be placed under the machines

and storage tanks, while for vessels, heaters, columns etc., it

shall be placed upon.

The equipment service shall be indicated under its item.

Two different solutions could be actuated for the graphic location of

the equipment item and service:

In the drawing area, next to the equipment the item only shall be

indicated, and in the lowest/highest side of the sheet, vertically

aligned with the equipment, it is represented the item, the service

and any other information relevant to that equipment; these

information shall be located in the bottom side if referring to

machines (pumps and compressors), in the upper side for all

other equipment.

Item and service of equipment are indicated next to the same

equipment in the drawing area.

Solution 1 is normally the preferred one.

The equipment items normally used in P&ID are listed in paragraph

2.6.3.


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l.

The connection lines of the equipment shall be horizontal or

vertical while the oblique lines, unless particular exceptions, shallbe avoided.

m.

In case of interception of two lines, vertical line is interrupted if

both lines have the same importance, otherwise the less

important line will be interrupted. The line importance level is:

- Main process lines

- Secondary process lines

- Utility lines

- Instrument lines

n.

Starting from the second issue, any modification of the elements

of the drawing shall be identified by revision clouds, with a

revision index.

o.

Line numbers or pipe diameters shall be written:

- in horizontal position upon the line, if it is horizontal;

- in vertical position (bottom-up) on the left of the line ifit is vertical.

If next to the line there is no space for writing its number, this shall be

placed in close proximity to it. An arrow shall be used to connect the

line number / diameter to the interested line.

The list of the equipment represented in the P&ID shall be indicated

over the template. This list shall be readable with the drawing folded in

A4.


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2.6.3

Item of Equipment

All equipment is identified with an "item". Equipment item is assigned

by the process manager as soon as it is inserted into the process

scheme. It may also be necessary, in order to meet the Client needs, to

rename the equipment previously “itemized” with other items names.

Equipment item will be assigned according to the following criteria

(unless some modifications requested by the Client).

The item is structured as:

TTT-UUNN (X/…/Z) to include the following information: TTT: equipment typology

UU: plant/unit of the equipment

NN: progressive number (within typology and unit)

X/…/Z: functional class – if required (parallel, main or spare)

The equipment typology is identified by means of 3 letters maximum in

accordance with the following Table 1:


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Table 1

Equipment TypeItem Equipment Type

A Basins – Pits

B Fans – Blowers

BC Battery Charger

C Columns

CB Power Factor Improvement Groups

CF Centrifuges

CT Cooling towers

D (as second letter) Diesel Engine

DE Diesel Emergency Generators

DH Deaerators

DR Dryers

DS Desuperheaters

E Heat exchangers

EA Air CoolersEG Electric Generators

F Filters

FL Flares

GE (as 2nd and 3rd letter) Machinery Gas Engines

H Fired Heaters - Furnaces

I Equipment for Solid Handling (belts, lifting equipment, etc.)

J Ejectors

K Compressors

LA Loading Arms

LP Local Panels

M (as 2nd letter) Machinery Electric Motor

MCC Motor Control Center

MX Mixers (both agitator and static mixers)

P Pumps

PC Low Voltage PanelsR Reactors

SG Boilers

SL Silos

SC Steam trap

SWG Medium Voltage Panel

T (as 2nd letter) Machinery Turbine

TG Gas Turbine

TR Transformers

TK Tanks

TS Steam Turbine (generator)

U Various no process Equipment (balances, bridge cranes, etc.)

UPS Uninterrupted Power Supply

V Vessels

W "Package" Unit

X Miscellaneous

Z Special Equipment


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The equipment of the same typology shall be numbered with 4 digits,

where the first and the second will represent the unit and the other two

the progressive number.

For instance a Pumps of Unit 02 will be numbered as: P-0201, P-0202,

P-0203, etc.

When the same service is provided by two or more equipment, the

functional class indicates if the equipment are working in parallel or if

the other one is spare.

The working equipment shall be identified with alphabetic letters

starting from A. The spare equipment will be identified with the letter S.

For instance the item P-0203 A/B/S identifies the 3rd pump of the unit

02, and it indicates that 3 pumps are installed (A, B, S), where two (A,

B) are working in parallel, and one (S) is kept as spare.When an equipment belongs to a "package" or to a complex equipment,

such as a compressor or a gas turbine, its item shall be written after the

main equipment item.

For instance the item W-1202-P-01 S identifies the pump P-01 S

(therefore spare) related to the package W-1202.

2.6.4

Fluid Identification

Each fluid moving along the Plant is identified by a code, made of

maximum 3 alphabetic characters. Fluid can be categorized as process

fluid or utility fluid.

When a project starts, the Fluid List is prepared by the Process

Department, in order to allow the selection of suitable Piping Classes.

The process fluids can be generically identified with a “P” letter.

Different code can be adopted if it is required by the Client.

The utility fluids codes will be in accordance with the Table 2, unless

otherwise requested by Client.


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Table 2

Codes for Utility FluidsItem Fluid

WATER

HW Hot Water

CHW Chilled Water

CW (S) Cooling Water (Supply)

CW (R) Cooling Water (Return)

BW Boiler Feed Water

HBW High Pressure Boiler Feed Water

LBW Low Pressure Boiler Feed Water

SW Sea Water

DM Demineralised Water

PW Process Water

DW Drinking Water

RW Raw Water

FW Fire WaterSTEAM & CONDENSATE

HHS Very High Pressure Steam

HS High Pressure Steam

MS Medium Pressure Steam

IS Intermediate Pressure Steam

LS Low Pressure Steam

LLS Very Low Pressure Steam

HPC High Pressure Condensate

MPC Medium Pressure Condensate

LPC Low Pressure Condensate

SC Suspect Condensate

COMPRESSED GASES

PA Plant Air

IA Instrument Air

NI Nitrogen

IG Innert Gas

HNI High Pressure Nitrogen

OX Oxygen

FUELS

FG Fuel Gas

NG Natural Gas

FO Fuel Oil

CHEMICAL

CA Caustic Solution

CAS Spent Caustic

NH Ammonia

SA Sulphuric Acid

HO Hot Oil

FLARE & BLOWDOWNBD Blow Down

ABD Acid Blow Down

AF Acid Flare

HF Hydrocarbon Flare

SEWERS

CSR Chemical Sewer

OSR Oily Sewer

SSR Sanitary Sewer

WSR Water Sewer

CD Closed Drain


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2.6.5 Line Classification

Each line shall be classified to permit the procurement of the adequate

instrument and piping bulk material.

Each line shall have its own number. A unique line number can be

assigned to contiguous runs of piping having the same piping class, the

same operating temperature and the same design conditions.

In order to assign a specific piping class to a line, it is necessary to

select, within the piping classes having the material and the corrosion

allowance specified for the specific fluid, the one with the flange rating

suitable to the design conditions of that line.

Material and corrosion allowance to be applied for a certain fluid are

normally defined in the “Material Selection Diagram”, document

prepared by the process team before the development of the P&IDs.

If the change of the piping class (and line number) is operated between

two contiguous lines, it is necessary to show the exact point of the

change. In this case a class transition segment (spec-break) with the

indication of the two contiguous line numbers shall be inserted. If the

change of the line number is due to the necessity to change the material

of the line (i.e. the piping class), at the transition point it shall be shown

the change in piping class instead of the change of line number.

The lines numbering shall be carried out in accordance with the Client

Standards. In absence of specific requirements by the Client the here

below described APS standards shall be used. The line number is a text cell which contains the following character

groups:

DDDD-FFF- NNNNN-CCCC-L

DDDD: nominal pipe size, in inches or, whenever required by the

Client, in millimetres; maximum of 4 digits;

FFF: fluid code; maximum of 3 digits;

NNNNN: line number, eventually combined with the unit number;5 digits maximum;

CCCC: piping class; 4 digits maximum

L: insulation code; 2 digits maximum;

Example: 12"-LS-25523-AB01-H


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Line number shall be made of the unit number followed by a

progressive number of 3 digits (from 001 to 999).

Process lines shall use progressive numbers from 001 to 499 (withineach unit). The numbers from 500 to 999 shall be used for the utility

lines.

As soon as the project starts it shall be decided the units numeration

and the numbers range to be used for each utility in the Plant (for

example: LS from 501 to 550, MS from 551 to 600, CW from 601 to 650,

etc.).

Table 3Insulation Codes

ItemInsulation

AAnti Sweetening Insulation

NNot Insulation and Not Painting

VPainting

CCold Insulation

HHeat Conservation

P Personnel Protection

SSteam Tracing

EElectrical Tracing

UUnderground Line

XAcoustical Insulation

W Jacketing

Note:

1 - Those codes may be incremented to meet specific project needs. Whenever the lines need morethan one insulation type (for example for heat conservation and acoustical insulation) it will bepossible to use the double code.


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2.7 Process Calculation

In a previous paragraph has been described the process simulation, to

be performed during the development of the Process Flow Diagrams,

which finalize the flows of the streams within the plant and the

operating variable. After that, before the preparation of the process data

sheets of equipment and instrumentation, the process engineer is called

to calculate, from a process point of view, most of the plant

components. It is important to note that the complete design of a plant

component (like a vessel for instance), requires the contribution of

several engineering specialties: the process engineer, who identifies

some basic data, the mechanical engineer, who calculate the

mechanical details and identifies the exact materials to be used for eachpart of the equipment, the piping engineer who identifies the orientation

of nozzles, etc.. In the next paragraphs it is described the main

calculation to be carried out by the process engineer, necessary for the

subsequent preparation of the process data sheets.

2.7.1 Line Sizing

This calculation is required to identify the diameter of each pipe of the

plant. The calculation is based on the following input data:

type of fluid (liquid, vapour, mixed phase)

flow rate

fluid characteristics (density, viscosity, etc.)

expected pipe length

available pressure drop

At the beginning of the project, when the routing and the components of

the pipes has not yet finalized, the pipe length is simply estimated on

the basis of the distance of the connected equipment.


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Using dedicated software, or graphs collection, it is possible to obtain,

for each line, the following two characteristic parameters:

fluid velocity

specific pressure drop

On the basis of extensive experience, the correct size of the pipe is

chosen in order to keep these two parameters within a range of

acceptable value.

In the following tables 4-1 A-C are indicated the acceptable range for

velocity and specific pressure drop, which could be used for pipe size

selection.


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Table 4-1 A

Liquid Service

Service Pressure Drop

[bar/km]

Velocity

[m/s]

1. Pump suction:

- C1,C2, C3 at boiling point 0.4 – 0.7 0.5 max

- liquid at boiling point 0.6 – 0.9

- subcooled by 25 °C 2.3 – 3.5

1.0 max (Ø ≤ 18")

1.5 max (Ø > 18")

2. Pump discharge 3.0 – 5.0 3.0 max

3. Natural circulation (reboiler inlet) 0.2 – 0.4 - - -

4. Column side extraction

- Ø ≤ 8" 0.7 max

- 8" < Ø ≤ 16" 0.9 max

- 16" < Ø 1.1 max

5. Inlet to L/L separator 1.0 max

6. Cooling Water

- branches to users 2.3 – 3.5 2.5 max

- interconnecting headers 0.6 – 1.0 2.5 max

7. Utility Water 3.0 – 4.5 1.5 – 3.0

8. Boiler Feed Water

- P ≤ 50 bar 3.0 – 4.5 1.5 – 3.0

- P > 50 bar 7.0 – 9.0

9. Steam condensate 0.3 – 0.6 1.0 max

10. Sea water 2.0 – 2.5

11. Hot Oil 1.0 min

12. Gravity Lines 0.2 – 0.5 1.0 max

13. Drain Lines 1.0 max (Ø ≤ 2")

Table 4-1 B


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Gas Service

ServicePressure Drop

[bar/km]

Velocity

[m/s]

1. Process Gas

- general 18 – 24

- with CO2 > 1 % vol. 15 max

2. Compressors suction / discharge

- centrifugal max allowed:

{- 2.2608X3 + 9.2286X2

– 21.332X + 28.931}

with X=Log10 (Density)

expressed as kg/m3

- reciprocating max allowed:

{- 2.186X3 + 7.6539X2 –

12.444X + 15.618}

with X=Log10 (Density)

expressed as kg/m3

3. Vacuum 4 % Abs. Press. max

4. Steam to users

P ≤ 10 bar 0.5 – 2.0 30 max

10 < P ≤ 30 bar 1.0 – 2.5 45 max

30 bar < P 1.0 – 2.5 50 max

5. Steam to headers

P ≤ 10 bar 0.1 – 0.2 30 max

10 < P ≤ 30 bar 0.2 – 1.0 45 max

30 bar < P 0.2 – 1.0 50 max

6. Column overhead 0.2 – 0.6

7. Plant Air 30 max

8. Instrument Air 30 max

9. Fuel Gas 40 max

10. Flare header 0.7 Mach max

11. Safety Valve discharge 0.5 Mach max

ρv2 < 60'000 kg/ms2


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Table 4-1 C

Mixed Phase

ServicePressure Drop

[bar/km]

Velocity

[m/s]

1. Preliminary Sizing 5'000


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2.7.2 Vessel Calculation

2.7.2.1

Liquid / Gas Separator

These vessels shall be sized in order to keep vapor velocity sufficiently

low and facilitate the separation of the two phases.

Whenever suspended droplets of liquid must be removed from the vapor

to the maximum extent (e.g. process compressor suction separators),

mist eliminators shall be installed.

The L/V inlet nozzle shall be always equipped with internal distributor

promoting the separation of the phases.

Vessels with vapor flow only, such as compressor KO drum, shall bedesigned as for liquid-vapor separators, assuming that the

characteristics of liquid are those of the liquid in equilibrium with the

vapors.

The recommended hold – up times shall be in accordance with the Table

here enclosed.

The installation of a high liquid level alarm shall be foreseen (and the

corresponding hold-up shall be considered) on the following services:

compressor suction separators

unit surge drum

The installation of a low liquid level alarm shall be foreseen (and the

corresponding hold-up shall be considered) on the following services:

critical pump suction vessels

L/V separator feeding liquid to a downstream vessel by pressure

difference.

The min. liquid level in L/V separators (horizontal or vertical) shall be at

least 250 mm.In horizontal L/V separators, the vapor space, or space above the

maximum liquid level, shall not be less than 20% of diameter.


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For vertical separators, the following geometric constraints shall be

satisfied:

the height between the bottom of inlet nozzle and the max liquid

level shall be not less than 300 mm or inlet nozzle diameter,

whichever is greater;

the height between the top of inlet nozzle and the top tangent line

of the vessel (without demister) shall be not less than 50 % of

vessel diameter, with a minimum of 500 mm and a maximum of

900 mm;

the height between the top of inlet nozzle and the bottom of the

demister shall be not less than 250 mm plus the 25 % of vessel

diameter, with a minimum of 500 mm; the height between the top of the demister and the top tangent

line of the separator shall be not less than 250 mm plus the 7.5

% of vessel diameter, with a minimum of 450 mm;

Vessel height or length should be within the range of 2-4 times vessel

diameter.

The maximum allowable vapor velocity may be calculated following

several methods, here below described.

Critical velocity method

The “critical velocity” of the vapor phase is defined as follows:

VC = 0.048 * [(ρL/ρV) - 1]0.5

where:

VC = critical velocity, m/sec.

ρL = liquid density at cond., kg/m3

ρV = vapor density at cond., kg/m3

The area used for calculating critical velocity is the one available for

vapor flowing: in a horizontal vessel is the cross sectional area above

the max liquid level, and in a vertical vessel the total cross sectional

area.


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The L/V separators shall be sized for a maximum allowable vapor

velocity as specified on Table 4-2 A.

Table 4-2 A

Max Allowable Vapor Velocity (% of Critical Velocity)

Service without Demister with Demister

Vertical Horiz. Vertical Horiz.

Separator (P < 35 Bar g) 125 220 220 220

Separator (P ≥ 35 Bar g) 100 165 165 165

Compressor K.O. Drum

(P < 35 Bar g)

100 - - - 175 - - -

Compressor K.O. Drum

(P ≥ 35 Bar g)

80 - - - 133 - - -

Steam Flash Drum 125 - - - 220 - - -

Vacuum Service

(P < 0.17 Bar A)

- - - - - - 110 110

Vacuum Service

(0.17 ≤ P < 1.013 Bar A)

- - - - - - 180 180

Flare K.O. Drum - - - 275 - - - - - -

API RP 521 Practice

Following this practice, for a liquid drop of a fixed diameter dispersed in

the gaseous stream it is possible to calculate the dropout velocity and

subsequently the time required to travel vertically down to the liquid

level. If the residence time of the gas phase inside the separator is

greater than the descent time of the liquid particle, the particles of such

particular diameter will be separated.

The minimum diameter of particles to be separated are defined in Table

4-2 B as function of the service.


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Table 4-2 B

Diameter of Liquid Drop to be Separated

Service Diameter [μm]

Flare K.O. Drum 300 - 600

API 12 J Specification

Following this practice, specifically relevant to gas / oil separators, theL/V separation is considered adequate if the actual gas velocity is not

greater than the maximum allowable superficial velocity VA:

VA = K * [(ρL/ρV) - 1]0.5

where ρL and ρV have the same meaning as in the critical velocity

expression, and K is a constant depending upon design and operating

conditions, which should be not greater than:

0.5 * (L/10) 0.56 where L is the separator length, in feet.

2.7.2.2 Liquid – Liquid Separators

These vessels shall be sized in such a way that the settling time for each

liquid phase from the other is less than its residence in the vessel itself.

Settling velocity for the dispersed droplets is calculated using Stokes,

Newton’s or an intermediate law, according to field of application. The

dimensions to be assumed for the droplets are defined in Table 4-2 C

Table 4-2 C

Diameter of Dispersed Droplets

Light phase density (@ 15 °C) Heavy phase Drop Diameter [mm]

850 kg/m3 or lower Water or caustic solution 0.12

over 850 kg/m3 Water or caustic solution 0.08


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A maximum settling velocity of 250 mm/minute shall be assumed for

light hydrocarbons.

It should also be verified that the hold-up time necessary for any phasefor settling satisfies process hold-up requirements.

In Fig.05 is shown a typical calculation sheet for an horizontal V/L/L

separator.


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Fig. 05


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2.7.2.3

K.O. Drum on Reciprocating Compressor

The analogic study of reciprocating compressor circuit will be extended

up to the suction KO drum, when the minimum volume of the drum

itself is calculated as follows:

V = (10*C*A)/(2π*n/60)

where:

V = vessel volume [m3 ]

C = gas sonic velocity [m/sec]

A = cross section of piping between KO drum and compressor [m2 ]n = Compressor rpm. Assume 300 during design phase and verify after

compressor definition.

2.7.3

Pump Calculation

Overdesign

Pump design flow rates shall include a minimum of 5% margin on max.

operating flow rate for pump with rated power higher than 150 kW and

10% for pump with lower rated power.

For reflux and pumparound pumps, design flow rate shall include a

margin of 20% on max. operating flow rate.

Pump differential head indicated in the specification sheet shall be

calculated at design flow-rate.

NPSH Calculation

Suction line losses shall be calculated at design flow rate of the pump.

Pressure drop through the suction strainer shall be taken into account.

Pressure drop across the strainer will be assumed equal to 0.5 m of

liquid column.

For subcooled liquids, the source pressure shall be the minimum

normal operating pressure and the vapor pressure shall be at specified

pumping temperature.


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Static suction head shall be measured from the minim liquid level to the

centreline of a horizontal centrifugal or rotary pump, or the suction

nozzle of a vertical centrifugal pump or reciprocating pump.

Static suction head for pumps connected to storage tanks shall be

calculated at the lowest specified liquid level in the tank a which design

pump flow rate is required.

If vortex breaker is required, the pressure drop through it shall be

considered.

For horizontal centrifugal pumps, the elevation of the pump centreline

shall normally be 0.6 m minimum above grade, unless the actual

elevation is know.

Available NPSH values not higher than 7 m shall be shown on pump

specifications, even if actually available. Maximum value of NPSH = 7shall be shown on pump specification in case of a higher value will be

calculated.

In Fig. 06 is shown a typical calculation sheet for pumps.


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Fig. 06


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2.7.4 Column and Tray Calculation

Column diameter will be calculated using an over design of 10% of

normal flow rates.

In general, valve or sieve trays shall be used, together with rain deck

trays (or equivalent) and packing (unless otherwise required).

Sieve trays may be used in fouling service.

Valve tray columns will be specified with the following max. flooding

factors:

77% for Vacuum Tower

82% for other services

70% for column diameter under 900 mm

Tray hydraulic calculations and hence column diameter confirmation

shall be performed by the tray vendor. The following sizing criteria shall

be recommended to tray vendors.

Required tray flexibility shall be 50 – 110% unless otherwise specified.

The following values are minimum recommended for tray spacing of

valve tray towers:


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Table 4-4 A

Tray Spacing

Fouling Service Tower

Diameter (ID)

Clean Service

1 pass 2 or more passes

[mm] [mm] [mm] [mm]

ID ≤ 1500 300 450 - - -

1500 < ID ≤2300 300 525 450

2300 < ID ≤3100 375 600 525

3100 < ID ≤6000 450 675 600

6000 < ID 525 750 675

Note:

1) If a manhole is present, minimum tray spacing shall be 600 mm or 150 mm morethan manhole diameter, whichever is greater;

Indicative pressure drop values per tray are listed below according to

operating pressure:

Table 4-4 B

Tray Pressure Drop

Column Pressure

[Bar abs]

Pressure Drop

[Bar] / tray

0.05 0.004

1.013 (ATM) 0.005 – 0.008

30 0.01

Vapor load for a column with high liquid load can be increased using

multi-pass trays.

Being more expensive, however, this option is only justified by an

effective saving in total column costs (by utilising smaller column

diameters).


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Downcomer clearance, vertical distance between the tray and the

downcomer bottom, is generally not less than 25mm, 40 mm for dirty

liquid.

Downcomer clearance velocity should be lower than 0.3 m/sec.

Weir height value is usually 50 mm, with a total lenght not lower than

60% of column diameter.

Weir loading, to avoid blowing, should be 0.01 lt/min cm.

Downcomer back-up shall not be higher than 50% of tray spacing plus

weir height.

A lower value (40%) should be used for services with high or moderate

foaming tendencies, or with a tray spacing


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Fig. 07


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2.8 Process Data Sheets

The process data sheets are formats (one type for each class ofequipment) with fixed cells relevant to representative parameters for

that class of equipment, to be filled in by the process engineer.

Once the process data sheet has been compiled, it can be passed to the

engineering section for further design development.

The process parameters which need to be specified on all data sheets

are typically:

operating conditions (pressure and temperature)

design conditions (pressure and temperature) construction material (for istance "carbon steel" or "stainless

steel") and corrosion allowance

connection nozzles size and rating

For pumps, compressors, filters, etc., in general for all the equipment

which are sized taking into account the flow rate and quality of the fluid

handled in the equipment, it will be necessary to specify:

nature of the fluid

flow rate

physical properties of the fluid (at operating conditions)

presence of corrosive / toxic materials

In addition to these data, each class of equipment is identified by

additional peculiar parameters, as shown below.


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Vessel / Column Data Sheets

diameter

height / length

liquid level (high, low) inside the equipment

elevation of some particular nozzle

need for insulation

Pump Data Sheet

differential height

NPSH available

Compressor Data Sheet

differential pressure

gas composition / molecular weight

gas Cp/Cv

Filter Data Sheets

degree of filtration required

efficiency of filtration

Heat Exchanger Data Sheets

Duty exchanged

Physical properties of both cold / hot fluids

Vaporization / condensation enthalpy curve (in case of change of

phase)

The following Fig. 08 - 10 show some typical process data sheet for

vessel, pump and heat exchanger.


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