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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: [email protected]
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
5
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
15
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
19
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
21
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.
22
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:
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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
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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
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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.
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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
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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.