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
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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 – 600°C) 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), deethanizer 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 o
and 45 C 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 o
temperature of the hot oil is fixed at 260 C (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 o
(usually 15 C) 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 (M HTF) 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 (C pavg) 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 <0.5 m/sec to avoid excessive pressure drop in the downstream.
Sizing of equipment:
Once the hot flow rate though the system is determined, all the process equipment in the loop can be sized. In this section, a general description of the equipment is provided and sizing basis of individual equipment is described. The hot oil loop in general comprises following equipment (Fig. 1 and Fig. 2) •
Hot oil expansion vessel
•
Hot oil circulation pump
•
Hot oil filter
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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 vessel’s 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 selfactuating 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-toflare 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, maloperation, 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 knockout 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 3
expansion (typically ~ 20%) is considered as difference between specific volume (m /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: 2
Piping volume = (Dp /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 m Total piping volume with 10% margin = (65.5 x 1.1) = 72 m
3
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(Margin can be increased up to 30% if major uncertainty persists in the plot plan) 3
Equipment hold up volume = 15.1 m (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), 2
n (D/2) L where D t = tube ID, Lt = tube length n = number of tubes. So, Total system volume = (72 + 15.1) = 87.1 m
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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 m
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Expansion volume: (vol at max. op. temp. – vol. at min. op. temp.) = (97.6 – 87.1) = 10.5 m 3 3
With 20% margin on expansion expansion volume = (10.5 x 1.2) = 12.6 m . 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 m
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Volume of oil required based on density at max. op. temp. = (84748.3/854) = 99.2 m
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Expansion volume: (vol. at max. op. temp. – vol. at min. op. temp.) = (99.2 - 97.6) = 1.6 m Max. expansion of volume including the process upset = (10.5+1.6) = 12.1 m
3
3
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 (A cylin) between BTL and LLLL can be calculated from the following equation, 2
Acylin = D /8(2-sin 2)
(2)
where angle can be calculated as follows, =
-1
cos ((D/2-LLLL)/ ((D/2-LLLL)/ (D/2))
(3)
Part volume of cylinder can finally be found as follows, 2
Vcylin = D /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 m
3
is
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provided which is sufficient for the calculated expansion volume with margin (12.6 m ). 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 0
210 C 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 startup 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 0
at cooler outlet which is normally fixed at 60 C. 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: •
rd
The hot oil system is designed for the higher pressure (2/3 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 Utube 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 2
systems a fouling resistance of 0.00017 m /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 reuse 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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project’s 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:
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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 250°C. 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
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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 hightemperature oil. Water rapidly flashes to steam and damages the system by over-pressurization. over-pressurizat ion. 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. IPS–E-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 521,4
th
edition, March 1997. 4. Sizing, selection and installation of pressure-relieving devices in refineries, part- I- sizing and th
selection, API recommended practice, 520, 7 edition, January 2000.
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