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Design considerations of hot oil system

- An essential utility to oil & gas plants

Subhasish Mitra

School of Engineering, University of Newcastle, Callaghan, 2308, NSW, Australia

Email: Subhasish.Mitra@uon.edu.au

Phone: 61-2-4033 9208/61-4-32150723

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Abstract:

Although used as an essential utility extensively in process industries especially in oil and gas

plants, design methodology for hot oil system is not well documented in the open literature. To

meet this gap, a design guideline for this process system is described systematically. Sizing basis

of all the equipment in the system is presented with illustrative calculations. Additionally

essential considerations required for development of the Process & Instrument diagram, control

system along with system protection philosophy and basis for selection of pressure safety valves

are outlined in the article.

Key words: hot oil system, process design, equipment sizing, oil & gas

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Introduction:

Widely used in process industries especially in oil and gas plants as a heating medium,

hot oil is a heat transfer fluid (HTF) capable of transporting heat energy within a specified

temperature range. Use of HTF is attractive since it exchanges heat purely in liquid phase by

sensible heat transfer mode rather than by latent heat transfer mode in condensing vapour phase

which enhances system efficiency. Additionally, unlike steam, HTFs do not require high system

pressure to carry out high temperature operation owing to their low vapour pressure and high

boiling point which simplifies the system design. Some typical hot oil grades used in the

industries are Therminol, Dowtherm (A, G, J, Q, HT), Syltherm, Shell thermia, B.P. Transcal

etc. To achieve optimum fluid life, they need to be used only within the recommended bulk and

film temperature limits specified by the manufacturer. When not subjected to contamination, i.e.,

moisture, air, process materials, etc., and thermal stress beyond the specified limits, HTFs can

give years of service without significant physical or chemical change. A closed loop system

design is often chosen to cater heat duty to the process consumers through a fired heater or waste

heat recovery system. A minor make up although is required to the system as some quantity of

hot oil needs to be discarded from the system due to gradual thermal degradation. Efficient

design of this hot utility system is crucial for satisfactory performance of the respective process.

This article aims at elaborating the major design aspects of hot oil system such as sizing basis of

the equipment in the loop with illustrating calculations, general design considerations, PSV

selection criteria, control philosophy and system protection philosophy.

General description of the HTF system:

A hot oil system in general is a closed loop heating arrangement with a heat source

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typically a fired heater or some kind of waste heat recovery units (WHRU) and heat sinks i.e.

process heat exchangers. Fig.1 illustrates such a system as per Shell design engineering practice

(DEP) [1]. Hot the oil is filled up in the system by a make-up pump through a normally no flow

(NNF) line from the storage tank. To avoid contact with oxygen which eventually deteriorates

hot oil quality; the tank is kept under nitrogen blanket.

The expansion vessel is usually kept at the highest point of the system to vent any trapped

gas. Stable level in the expansion vessel confirms complete filling of the loop. Hot oil is

circulated by the circulation pump through WHRU/heater coils and heat is supplied to all process

consumers. After heat exchange, hot oil is returned to the suction of circulation pump. Supply

temperature of the hot oil is controlled by a temperature controller at the outlet of trim air cooler

which operates on the both main line and bypass line control valve through a split range control

mechanism. Temperatures of the process streams are maintained by controlling the hot oil flow

rates. Process consumers can be completely bypassed through the full flow bypass line during

start up and partially bypassed by sensing the pressure differential through the spill over bypass

line when plant runs under turned down condition. Under these circumstances, WHRU/heater

load is dissipated in the trim cooler on the full bypass line. Volume expansion or contraction of

hot oil system is accommodated in the expansion vessel.

During maintenance of the system or any connected equipment in the loop, hot oil is

drained into the storage tank through the pump out cooler. Fig.2 describes similar process flow

diagram of hot oil system commonly employed in oil and gas plants. This scheme primarily

differs from Fig.1 by introducing a fuel gas fired heater as the heat source and a separate hot oil

draining system. The burner management system (BMS) is an elaborate fuel gas flow control

system to effectively utilize the individual burner of the fired heater and usually supplied by the

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heater manufacturer. During maintenance, hot oil is collected from the low point drains of the

closed loop piping and collected to an underground draining vessel through the dedicated

draining network system. The same vessel can be used for system filling purpose using the drain

pump. Hot oil drums can be emptied into this vessel through a filling connection. For complete

cleaning of this drain vessel, a vacuum truck connection is provided. A basic process control

scheme is presented in Fig.2. Outlet temperature of the fired heater is controlled by a temperature

controller which controls the fuel gas flow and hot oil flow to the heater. In case plant runs under

turndown condition, pressure of the system increases due to reduced demand of hot oil. The

pressure controller senses reduction of flow rate through pressure rise and bypasses the unused

hot oil flow through the hot oil trim cooler. Temperature at the downstream of trim cooler is

controlled by manipulating the motor speed.

Design of the system:

Fired heater/WHRU load:

Generally natural draft or mechanical draft (induced or forced) fuel gas fired heater is

used as heat source in the hot oil system. In some cases, fired heater may be required only as

stand by when most of the heat input into the system is obtained from waste heat recovery coils

in flue gas stack of on-site gas turbine generator (GTG). This is essentially applicable for both

onshore and offshore oil and gas plants which are often located in remote areas or in oceans far

away from shoreline and have no access to grid power. In such cases, the power needs to be

produced locally using gas/diesel fuelled power generator. The flue gas as combustion products

leaves GTG stack at a very high temperature (500 600C) and is a rich source of available heat.

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By installing heat exchange coils inside the stack and controlling the stack damper opening, this

heat can be extracted to be made useful in the hot oil loop. To design the loop, first the heat

requirement in the process side needs to be determined. In a typical onshore oil and gas plant, the

hot oil is used primarily in the following process section oil stabilizer (removes the low

molecular weight volatile components especially methane and ethane from the crude oil and

stabilizes the oil by reducing vapour pressure (RVP : 8 10 psia) for long time storage), de-

ethanizer and de-butanizer (separates C1-C4 gases from natural gas liquids (NGL) obtained after

cryogenically cooling the associated gases from oil/gas well and off gases stabilizer column top)

and molecular sieve regeneration (used for removing moisture from gas stream to lower dew

point before entering into cryogenic section). To illustrate the sizing methodology of the loop,

the following hot oil consumers are identified in a typical onshore oil and gas plant and presented

in Table 1. The heat loads presented can be considered as representative figures for a typical 100

mmscfd capacity gas plant which were obtained from solving the heat and mass balance model

of the entire gas plant using HYSYS simulator. To limit the discussion to the hot oil utility

section only, the details of the simulation methods of the gas plant has not been presented in this

study.

Combining all these thermal loads, the total heat duty of the fired heater is found to be

6607 kW. All the designed heat load figures include 10% margin unless otherwise specified. The

heat duty will proportionately increase if there are parallel production trains and may require

separate fired heaters. Fuel gas requirement to fired heaters can be estimated if fuel gas LHV at

the operating conditions is known. A typical low pressure fuel gas available at 5 barg pressure

and 45oC has a LHV of 44,380 kJ/kg. The LHV depends upon the fuel gas compositions and

various operational cases need to be analysed to find out the lowest LHV to be considered for the

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design case. Assuring 85% thermal efficiency of the heater, fuel gas flow requirement is (6607 x

3600)/ (44380 x 0.85) = 630.5 kg/hr.

Hot oil flow rate:

Once the total heat duty from process consumers is known, the major sizing parameter

remains then estimating the total flow rate of hot utility in the system. This requires knowledge

of the physical properties and characteristics of the heat transfer fluid. Table 2 presents

properties of Shell Thermia B, a preferred heat transfer fluid often used in process industries.

Physical properties of heat transfer fluid is very much temperature dependent and the design

process must take into account such property variations with temperature. Table 3 provides the

temperature dependency of the physical parameters essential for design of the system. Supply

temperature of the hot oil is fixed at 260oC (Tsupply), little above the fire point ensuring that

maximum heat transfer is possible without degrading the fluid quality subject to maximum

permitted bulk temperature. Determining return temperatures of the hot oil streams is rather

critical and requires thorough consideration to ensure the specified approach temperature

(usually 15oC) in the design of respective heat exchangers assuring no temperature cross. Table 4

presents the return temperatures of the hot oil streams from the process consumers obtained from

the above considerations.

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Solving the following heat balance equation, mass flow rate of hot oil stream (MHTF) through

each heat exchanger can be found out,

MHTF x Cpavg x (Tsupply Treturn) = QHX (1)

where QHX is the design duty of the respective process heat exchanger.

Average specific heat of hot oil (Cpavg) can be obtained by averaging the heat capacity value of

hot oil at supply and return temperature from Table 3 by linear interpolation.

Hot oil flow rate through each heat exchanger obtained from Eq.1 is listed in Table 5.

With the total flow rate obtained from Table 5, following line sizing criteria provided in

Table 6 (fairly standard as per the industrial practice of process design) can be used to decide

various line segments diameters of the circulation loop with 10% design margin. Heat transfer

fluid present in the loop is considered as boiling liquid and stricter sizing criteria are imposed on

the sizing of pump suction line. For gravity driven flow lines such as hot oil drain lines, where

flow occurs due to static head only, designed liquid velocity should be restricted to

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Hot oil start-up pump

Fired heater and waste heat recovery units from gas turbine generator (GTG)

Hot oil run down cooler

Heat exchangers (Consumers)

Hot oil storage tank

Hot oil make up pump

Hot oil drain drum and drain system

Hot oil sump pump

Hot oil expansion vessel:

The expansion vessel allows for thermal expansion of the hot oil. Additionally this vessel

is used for venting low boiling point components generated in the system during normal

operation and purging out inert gas and water vapour during hot oil drying in start-up phase. The

expansion vessel minimizes the consequences of any upsets in the hot oil system operation.

Following are some significant aspects that need to be taken care of while designing this vessel,

accommodating thermal expansion of the hot oil heated from minimum to maximum operating

temperature.

maintaining the NPSHr for the hot oil circulating pumps under all operational circumstances.

venting of possible residual water present in the circuit during start-up.

allowing filling of equipment and during re-commissioning after shut down for maintenance.

The largest volume of the individual equipment that can be maintained while the hot oil system

remains in operation usually determines this inventory. The expansion vessel is connected to the

system return line on the pump suction side. The vessel is elevated so that the normal operating

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level of the hot oil in the vessel is higher than the highest possible hot oil level in the system

(generally it is the fired heater or WHRU coils and typically 15 20 m from datum level). This

will facilitate proper venting and provide sufficient NPSH for the loop circulation pump.

If this requirement is difficult to meet, a lower elevation may be selected but additional

design measures are then required to prevent vapour locking in the high points of the circuit. Hot

oil system pressure needs to be positive at the highest point to avoid any boiling and overflow

into the flare system. The expansion vessel is connected to the flare and equipped with an inert

gas (nitrogen or fuel gas) blanket to serve as a barrier between the hot fluid (usually operating at

a temperature above the flash and fire point of the hot oil) and the flare. The vessels vapour

space is prevented from contacting the atmosphere as it expedites aging of the hot oil and allow

moisture to enter the system during shutdown periods (these might create corrosive acid

compounds and a safety hazard). Only for operation at high temperatures, particularly

approaching or exceeding the boiling point of the hot oil, a positive pressure of at least 1 to 2 bar

above the vapour pressure of the hot oil (at this temperature) should be maintained otherwise a

blanketing gas pressure in the range of 200 to 300 mm wC (water column) needs to be

maintained. The nitrogen blanketing supply can be equipped with a split-range controller or self-

actuating PCVs and a non-return valve, which will regulate the nitrogen supply and its vent to

flare. A dead pressure zone is required between the inert gas supply pressure and the vent-to-

flare set pressure. In this dead zone, the pressure is not controlled and is allowed to float freely

while the nitrogen supply and vent-to-flare valves are both closed. This dead zone will reduce

nitrogen consumption and lower the starting point of venting low boiling point components. The

non-return valve prevents hot oil vapour and nitrogen back-flow into the nitrogen system in the

event of a pressure increase in the vessel.

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A start up line between return line header and expansion vessel top is provided which can

be used to vent out air pockets in the loop during start-up by continuous pump circulation.

During operation, low boiling degradation products are vented on pressure control and routed to

the flare. The expansion vessel is equipped with a pair of safety relief valve capable of protecting

the system against over-pressure caused by events such as fluid degradation, contamination, mal-

operation, and overheating or tube failure in the process heat exchangers. The outlet of safety

relief valve is routed to flare.

If the ambient temperature falls below the freezing point of the HTF, to prevent

possibility of congealing, blanketing gas lines and safety relief lines along with associated valves

are required to be heat traced in order to prevent line plugging.

The expansion vessel serves the combined function of an expansion vessel and a knock-

out drum. It should have sufficient capacity to cater for various operating upsets in the system.

The expansion vessel allows for degassing of the hot oil and therefore should be fitted with a half

open pipe type inlet device. This vessel is designed based on volume expansion (loop hold up

consisting of pipe volume, fired heater/WHRU coil volume and all heat exchanger hold up) of

hot oil system of between maximum and minimum possible operating temperature. Volume

expansion (typically ~ 20%) is considered as difference between specific volume (m3/kg) i.e.

inverse of specific gravity of hot oil at maximum and minimum operating temperature of the

system which is required to be accommodated between Low liquid level and High liquid level of

the expansion drum. An additional 20% is added to cater for various operating upsets in the

systems such as vaporization of residual water in the system and a tube burst.

The inventory between LL and LLLL should be 25% of the vessel volume or 150 mm

whichever is more while HHLL is fixed at 150 mm above HLL. Vessel diameter can be found

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out by setting HHLL at 80 85% of vessel ID assuring that 75% vessel volume gets

accommodated within HLL. The remaining volume of the vessel volume allows for gas-liquid

separation and is filled with inert gas.

A sizing calculation for expansion vessel is illustrated below.

Hot oil expansion drum calculation:

Piping volume = (Dp/2)2 Lp where Dp = pipe ID, Lp = piping length according to plot

plan.

As per P&ID and plot plan, total piping hold up volume: 65.5 m3

Total piping volume with 10% margin = (65.5 x 1.1) = 72 m3

(Margin can be increased up to 30% if major uncertainty persists in the plot plan)

Equipment hold up volume = 15.1 m3 (this comprises of volume of heater coil and heat

exchangers. Heat exchanger volumes are calculated as follows considering HTF flows in the tube

side. Shell volumes need to be considered otherwise for hold up calculation if HTF flows in shell

side),

n (D/2) 2 L where Dt = tube ID, Lt = tube length n = number of tubes.

So, Total system volume = (72 + 15.1) = 87.1 m3

Knowledge of heat transfer fluid density is required to determine the expansion volume which is

reported in Table 7. Expansion volume from cold start up to normal operation is considered as

design case for the vessel. A check case is performed to ensure adequate design margin in case of

process upset.

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Mass of total hold up (density at min. op. temp.) = (87.1 x 973) = 84748.3 kg 84766.7 kg

Volume of oil required based on density at max op temp = (84748.3/868) = 97.6 m3

Expansion volume: (vol at max. op. temp. vol. at min. op. temp.) = (97.6 87.1) = 10.5 m3

With 20% margin on expansion volume = (10.5 x 1.2) = 12.6 m3.

The check case is considered to see adequacy of the given margin. Volume of oil required based

on density at min op temp = (84748.3/868) = 97.6 m3

Volume of oil required based on density at max. op. temp. = (84748.3/854) = 99.2 m3

Expansion volume: (vol. at max. op. temp. vol. at min. op. temp.) = (99.2 - 97.6) = 1.6 m3

Max. expansion of volume including the process upset = (10.5+1.6) = 12.1 m3 can be

accommodated within the 20% margin. So design is adequate.

An expansion vessel of configuration 2.2 m (ID) x 7.6 m (L) for this service ensuring an L/D

ratio of more than 3 is considered in the selection.

The calculated design expansion volume should be accommodated within the operating liquid

levels i.e. HLL and LL. Levels are adjusted within the controllable range to accommodate the

desired liquid volume.

Volumes within the levels are calculated by adding part volume of cylinder and head.

Part area of cylinder (Acylin) between BTL and LLLL can be calculated from the following

equation,

Acylin = D2/8(2-sin 2) (2)

where angle can be calculated as follows,

= cos-1((D/2-LLLL)/ (D/2)) (3)

Part volume of cylinder can finally be found as follows,

Vcylin = D2/8 (2-sin 2) L (4)

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where L = length of cylinder

Part volume of the vessel head (2:1 SE) can be obtained as

Vhead = /2(DH2/2 H3/3) (5)

Total occupied vessel volume is therefore obtained by summing up the volume of cylindrical

body and the head from Eq. 4 and Eq.5. Similarly volume occupied between all the levels can be

calculated which are presented in Table 8.

The above calculations show that between HLL and LLL a volume of 15.26 m3 is

provided which is sufficient for the calculated expansion volume with margin (12.6 m3). Thus

the selected diameter and length of expansion vessel are suitable to meet the design requirement.

Normal level is based on expansion volume for design case since vessel will be operating at

2100C max however it can lie anywhere between HLL and LL preferably at 50% of the range

depending on the operating conditions.

Hot oil circulating pumps:

Hot oil circulating pumps are centrifugal pumps 1 X 100% typically arranged as 1

working + 1 stand-by unless there is a clear justification for 3 X 50 % capacity to maintain the

closed loop circulation through fired heater or WHRU or in combination of both as per project

requirement. Flow rate of this pump is designed based on heat duty of all the consumers typically

all the reboilers. 10% margin is applied on total calculated flow rate. For line sizing refer Table

8. If continuous filtration is applied via a bypass across the pump (10% of total flow max), the

capacity of the pumps should include this additional flow. In the event of low hot oil pressure,

the spare pump should take over automatically. The stand-by pump should be maintained in a

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pre-heated state in order to avoid thermal shock when starting by providing the bypass across the

discharge check valve. Due to prolonged operation, hot oil may degrade generating some lower

boiling point components which lead to higher vapour pressure of the hot oil in the system than

the pure hot oil as specified by the manufacturer. The rise in vapour pressure lowers the NPSHa.

To determine the NPSHa to the pump, it is assumed that the vapour pressure of the hot oil is

equal to the pressure in the expansion vessel at normal operating temperature. If necessary, the

height of the drum is raised to ensure that there is sufficient NPSHa. While calculating NPSHa, it

is wise to keep 1 meter margin to account for any unforeseen pressure loss. NPSHr is specified

by the pump manufacturer and should be less than NPSHa by at least 1-2 ft margin. Discharge

pressure of the pump is obtained by summing up expansion vessel pressure and all the pressure

drops incurred in the discharge line including line, fittings, equipment and valves. A general

condition applies to all pumps to be capable of cold filling of the system.

Hot oil filters:

Organic HTFs degrade over time due to thermal cracking, oxidation and contamination.

The by-products of degradation are sludge and coke. Contaminates can also include dirt, sand,

dust, mill scale, and slag from piping that accumulate during down-time maintenance or from

installation. Often a Y type or basket type strainer is installed at the pump suction. Typically the

strainer contains 100 mesh size stainless steel woven wires. These are designed to protect the

pump and flow meter.

Installing filter in the loop has following benefits

Removal of particulates that can degrade the oil

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Maintains viscosity of fluid longer by reducing sludge build-up

Maintains thermal efficiency of system longer and reduces energy cost

Extends HTF life

Reduced maintenance costs by protecting pumps and valves from contaminates

The strainer should be cleaned regularly to prevent pump cavitation which can cause mechanical

seal failure. For continuous filtration purpose, hot oil loop generally is provided with 1 X 100%

filter in 1 working + 1 stand by arrangement at a side stream bypass line around circulation pump

discharge. A partial flow rate up to 10% max is routed through the filter to screen thermal

degradation product. A differential pressure indicator across the filters in the bypass line is fitted

to monitor fouling in the system. Filters need to be equipped with 75 m to 100 m elements

during commissioning and initial operation, and subsequently these are replaced with 10 m to

20 m elements unless the hot oil manufacturer of makes more stringent recommendations or

project has a different requirement.

Hot oil start up pump:

1 X 100 centrifugal pump without any stand by is provided in case WHRUs are used as

heat source in the closed loop hot oil system. This pump is supplied power from emergency

diesel generator as it is required to maintain a small circulation flow through WHRU coils before

the GTGs start. This is an essential requirement as WHRU coils are not advisable to run dry

while GTGs are running because of thermal damage possibility. This pump is sized to cater to

5% (max) of total system flow rate in order to maintain a velocity of about 1 m/sec and should

have same discharge pressure to that of circulation pump.

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Hot oil trim cooler:

In order to improve operation and increase the flexibility of the hot oil system, a trim

cooler is installed in the loop. This cooler serves the purpose of rejecting heat during heater start-

up or when consumer duties in the loop suddenly reduce because of decrease in plant throughput

or some inadvertently caused mal-operations. Typically, an air-cooled heat exchanger is selected.

The cooler should be capable of rejecting the minimum heater duty at stable operation (heater

turn-down is ~ 25%, usually specified by the manufacturer) or highest process consumer duty in

the system, whichever is more. Flow rate through cooler can be estimated by the oil temperature

at cooler outlet which is normally fixed at 600C. In addition to 10% margin on flow rate, 10%

margin on thermal duty should also be provided by means of surface area.

Process heat exchangers:

In systems with heat users operating at pressures above that of the hot oil system, the

piping design should take into account of all hazards caused by a tube rupture inside this

equipment. Hot oil distribution headers and piping to consumers are sized for 110 % of the

maximum flow. The spill over lines and control valves are sized for the flow of the largest

consumer to allow for a sudden block-off of the heat user. Manual bypass lines are sized for 100

% flow. The following usually apply except for double-pipe heat exchangers:

If the process pressure exceeds the hot oil system pressure, the preferred arrangement is to ensure

a free flow (no valves) from the consumer (heat exchanger) to the expansion vessel. If valves are

installed, the following alternatives may be applied:

The hot oil system is designed for the higher pressure (2/3rd rule)

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Overpressure protection devices (safety relief valves or rupture disks) are installed at the

outlets of the affected heat exchangers with relief to flare via a liquid separator.

If designed and operated properly, hot oil systems can be considered to be non-fouling, so U-

tube type heat exchangers may be applied if the hot oil flows inside the tubes. This is cheaper

than floating head type heat exchangers and significantly reduces the risk of leakage and,

consequently, contamination of the hot oil or process fluid. For the design specification of hot oil

systems a fouling resistance of 0.00017 m2/kW is taken. Effects of leakage of hot oil into the

process or vice-versa are reviewed and double welded tube-to-tube sheet connections are

specified, if required. All heat exchangers are equipped with hard piped drains and vents to allow

the hot oil to be drained into the drain drum. To speed up the evacuation, a nitrogen purge point

is installed to allow a hose connection from a nearby utility station.

Hot oil storage tank:

The hot oil storage tank is sized to have a working volume equal to the full inventory of

the system (pipe volume as per plant lay out, fired heater/WHRU coil volume and heat

exchanger hold up), plus an additional 10 % volume to accommodate make-up of losses caused

by venting and mechanical leaks. On plants with multiple parallel trains it may be justified to

reduce the storage tank capacity to hold the inventory of a single train only unless it is feasible

that these trains must be drained at the same time.

The minimum fluid level in the tank is set to ensure sufficient NPSH for the make-up

pump. If the ambient temperature falls below the hot oil minimum pumpability temperature, it

may congeal and plug the pipelines. Special design considerations need to be applied for such

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congealing service. In this circumstance, the tank is heated, preferably electrically, and the

suction line to the pump is heat-traced. The storage tank is equipped with inert (nitrogen/fuel

gas) gas blanketing with self-actuating PCVs connected to flare or vent to atmosphere at safe

location to serve as a barrier between the fluid and the atmosphere to limit aging (oxidation) and

moisture ingress.

During shipment, air bubbles can be entrained in the fluid. If the cold fluid is

immediately pumped into the system, the air bubbles can cause pump cavitation. It is advisable

that the fluid should be near room temperature prior to charging the system. The drums may be

stored in a warm room to bring the fluid up to room temperature. The warmer the fluid, the more

easily it can be pumped into the system. A complete spare hot oil inventory should be made

available to replace a total loss of hot oil from the system due to leakage or contamination by a

process fluid.

Hot oil make-up pump:

1 X 100% centrifugal pump without any stand by is provided for hot oil make up service.

This pump should fill up the entire hot oil system from hot oil storage tank. The pump is sized

for complete fill up of the system within 8 hours to 24 hours (max). In case, hot oil storage tank

is not in the scope of the project then hot oil sump pump should act as make up pump. Discharge

head of the pump is estimated based on the elevation of the expansion vessel.

Hot oil drain drum and drain system:

A hard piped dedicated closed drain system for maintenance purposes is provided. The

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purpose of a hot oil drain system is to collect hot oil inventory in a controlled manner from

piping and equipment prior to maintenance so that it can be returned later to the system for re-

use or controlled disposal, as required. Since drainage of hot oil in hot condition to the drain

drum is not envisaged, it is cooled prior to entering the storage drum by the run down cooler or

via the process. The hot oil inventory can be cooled by alternative means such as with a column

reboiler on hot oil with the column operating on total reflux and thus using the overhead

condenser as indirect means of cooling the hot oil in the system. In systems with a fired heater,

the combustion air fans can be used to cool down the furnace while hot oil is circulated through

the heater tubes.

A drain system is intended to reduce spillage of hot oil, which could lead to HSE

incidents. The drain piping should be installed underground and be free flowing to a closed

collection vessel. Because the installation of drain piping is underground, the drain system is

solely for the collection and draining of cooled down hot oil. The drain header is routed as close

as possible to the drain points to reduce the length of small bore drain piping. Where a free flow

of drained hot oil is not feasible, then an above ground nitrogen purge assisted drain line may be

considered. A suitably sized vent is made on the collecting drum to vent the nitrogen to safe

location at atmosphere or to flare. The collection drum is normally inert gas (nitrogen/fuel gas)

purged to avoid ingress of air and/or moisture from the flare, and be located in a (dry) pit for

secondary containment. The collection drum is sized to receive the hot oil volume from the

largest consumer or group of consumers in the loop that can be taken out of service at the same

time with margin (25% max). The collection vessel is provided with a pump for returning the hot

oil to the hot oil storage tank or into main system itself in case hot oil storage tank is not in

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projects scope. A connection is provided for vacuum truck to empty out the drum for hot oil

disposal. If the collection vessel is also used for make-up of fresh hot oil into the system from

storage drums, a filling connection is made available for connecting a portable drum unloading

barrel pump. This connection may be combined with vacuum truck connection.

Hot oil sump pump:

Hot oil sump pump is a vertical submersible 1 X 100% centrifugal pump placed inside

the hot oil drain drum. In case, hot oil storage tank is not in the scope of the project, the sump

pump can be utilized as the make-up pump and will follow the same sizing basis.

Control philosophy:

Hot oil system control scheme, in case to case basis may look little different based on

project requirement however the control objective of the system is to allow stable operation at

continuous turndown of heat demand from design heat duty to zero. Control scheme of

individual equipment is discussed below.

Expansion vessel is provided with inert gas blanketing. Depending on project

requirement this can be fuel gas or nitrogen. Blanketing gas pressure inside the vessel can be

controlled by a split range pressure control arrangement which contains a pressure controller on

the vessel and control valves on the incoming and outgoing blanket gas. Incoming gas stream

pressure control valve receives 0 50% output of the controller while outgoing gas stream

pressure control valve receives 50 100%. A cheaper option of self-actuated PCV arrangement

instead of pressure control valve may also be used.

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Circulation pump is provided with a discharge flow (if performance curve is flat) or

pressure controller (if performance curve is drooping) on bypass loop which protects the pump

from running at shut-off condition when ESD or manual valves at downstream of hot oil supply

line get closed due to some interlock or inadvertent operation.

Fired heaters or WHRUs or a combination of both are provided with individual flow

controller on hot oil inlet line. Outlet streams of are provided with temperature controller which

senses any rise in temperature (due to decrease in heat duty of consumers) and controls fuel gas

firing rate (for fired heater), damper position to control flue gas flow (for WHRUs) and flow

through rundown cooler to maintain temperature of hot oil return line.

Proper control of hot oil run down cooler serves a critical purpose when heat demand

from, and hot oil flow to the heat consumers decreases. Cooler may have different control

scheme if fan blade pitch control is available. A temperature controller at downstream of cooler

senses the temperature and controls the blade pitch to vary rpm in order to maintain the return

line temperature. The same control can be achieved with variable frequency drive subject to cost

implication of the project. The most cost effective control however is through a bypass line flow

control through cooler when air cooler fans run at a fixed rpm.

Hot oil storage tank and drain vessel are provided with inert gas blanketing similar to

expansion vessel hence similar control scheme applies. The drum should have separate level

transmitters for control and trip action of the sump pump. Hot oil sump pump can be provided

with auto start option to start at high level and stop at low liquid level. This would prevent the

possibility of over filling the drum while draining from multiple equipment in the loop.

System protection:

23

For safe operation, some protection measures are employed for intrinsic safety of the

system which may vary from project to project as per specific requirement or philosophy. For

ultimate safety of the system, supply of both hot oil and LP fuel gas are cut off. For WHRU,

dampers are shut off. Expansion vessel is provided with LLLL and HHLL trip. At HHLL, the

make-up pump trips while at LLLL, the entire hot oil system triggers a shutdown. Hot oil

circulation pumps are provided with LL suction pressure trip which triggers a system shutdown.

Additionally, all pumps should trip on LL seal pressure and HH current. Hot oil heater/WHRU

inlet and outlet line are provided with ESD valve which closes when HH temperature or LL flow

is sensed on the outlet hot oil stream from heater/WHRU causing system shutdown. For fired

heater, additional safety interlocks i.e. HH flue gas temperature, HH fire box pressure etc. are

advised by the manufacturer. Hot oil run down cooler is provided with HH vibration trip. If

automatic louvers are provided then upon loss of instrument air signal, louvers should remain in

the last position prior to loss of signal. The storage tank is provided with LLLL and HHLL trip.

At HHLL, hot oil sump pump trips while at LLLL, the make-up pump trips. Hot oil make-up

pump trips on LLLL of hot oil storage tank and LL suction pressure. The pump also trips on

HHLL of expansion vessel. Hot oil sump pump trips on LLLL of hot oil drain drum and on

HHLL of expansion vessel in case it is used as make up pump. Pressure safety valves are

installed in the loop as safety measures as deemed necessary. Table 9 summarizes various relief

scenarios commonly encountered in the hot oil circulation loop. Sizing of the PSVs are done as

per API guidelines [3, 4].

Material Selection:

Generally, carbon steel is used in most case. Aluminium, brass and bronze should not be

24

used however copper and copper alloy can be adopted in the place of no air contact. Austenite

stainless steel should not be used if chlorinated contamination is envisaged.

System insulation:

The entire system requires insulation to prevent heat loss however selection of insulation

needs special caution. Due to low surface tension and low viscosity at operating temperatures,

hot oils penetrate through joints, gaskets and seals. This results in leaks that can lead to

accumulation of fluid inside insulation. Insulation materials such as mineral wool or similar,

when saturated with organic hot oils, can cause slow exothermic oxidation starting at

temperatures above 250C. The large internal surface area, poor heat dissipation and the possible

catalytic activity of the insulation material may cause significant temperature build-up within the

insulation mass. Such slow reaction may progress undetected and may lead to unsafe situations

such as sudden fires when cladding is damaged or opened for maintenance. Non-absorbent

insulation (e.g. foam glass) or no insulation at all is used at potential fluid creep locations

(instrument connections, valve stems, flanges and joints).

Periodic sampling:

Hot oil is subject to thermal degradation due to continuous operation at elevated

temperature. To ensure that physical properties of the hot oil remain stable or if the system

requires fresh make up, periodic analysis hot oil samples can helps in monitoring health of the

system. Contaminants can also catalyse fluid degradation and result in severe operating and

equipment problems. The most common contaminant in HTFs is water which can be determined

by the Karl Fischer test. The test data collected over time can be used along with the operating

25

history to obtain a complete system analysis. This allows corrective action to be implemented

before the fluid life or equipment efficiency is compromised. The sample must be taken from a

live part of the system, preferably at the heat exchanger or the circulating pump and not from

some stagnant parts like expansion or drain vessel. Also, it is important that the sample is put

directly into the sample container to avoid any contact with air or moisture.

System cleaning:

Irrespective of whether the system is new or old, there are many contaminants that can

find their way into heat transfer systems. Hard contaminants such as weld slag, spatter and mill

scale can damage pump bearings, seals and control valves. The mill scales can promote fluid

oxidation. "Soft" contaminants such as protective lacquers and coatings, oils and welding flux

are thermally unstable and can cause degradation of the fluid. Minute presence of water in the

system can cause pump cavitation and corrosion and if trapped in a "dead leg" and hit by high-

temperature oil. Water rapidly flashes to steam and damages the system by over-pressurization.

The system is cleaned before the new hot oil is introduced. Provisions are made for blowing out

the system with nitrogen to ensure the system is dry prior to start-up.

Conclusion:

A systematic guideline for design of hot oil utility system is described in the present

work. The sizing of the system depends on the heat load requirement in the main process heat

exchangers and needs to be designed to perform satisfactorily in the entire range of plant

turndown capacity. Process design basis for sizing of the major equipment are outlined of which

most critical is the design of expansion vessel. The heat requirement in the loop can be met by

26

either an oil/gas fired furnace or waste heat recovered from onsite gas turbine generator flue gas

stack, the latter one being economical when there is no grid power source. The basic control

scheme is discussed to run the system effectively. Finally, as means of protection, relief

scenarios of the pressure safety valves are identified which are critical for safe operation of the

system.

Acknowledgement:

Author would like to gratefully acknowledge contributions of process department of Petrofac

Engineering (India) Ltd. and Petrofac E&C (Sharjah) through valuable discussions as input to

this work.

27

Nomenclature:

BMS: burner management system

BTL: bottom tangent level

ESD: emergency safe shutdown

FG: fuel gas

GTG: gas turbine generator

HH: high high

HHLL: high high liquid level

HLL: high liquid level

HTF: heat transfer fluid

HSE: health safety environment

LL: low low

LLL: low liquid level

LLLL: low low liquid level

NLL: normal liquid level

LP: low pressure

NPSHa: net positive suction head available

NPSHr: net positive suction head required

PCV: pressure control valve

S: standby

W: working

WHRU: waste heat recovery unit

28

References:

1. Shell Design Engineering Practice, DEP 20.05.10-GEN.

2. IPSE-PR-410, Engineering standard for process design of hot oil and tempered water circuits,

original edition, March 1996.

3. Guide for pressure - relieving and depressuring systems, API recommended practice 521,4th

edition, March 1997.

4. Sizing, selection and installation of pressure-relieving devices in refineries, part- I- sizing and

selection, API recommended practice, 520, 7th edition, January 2000.