Cargo Tank Heating Systems on Tankers

Cargo tank heating keeps liquids that would otherwise solidify or become too viscous to pump at a temperature where they flow freely through the ship’s cargo lines. On a product tanker loading 380-centistoke heavy fuel oil in a cold-weather port, the difference between a working heating system and a failed one can mean the difference between a smooth discharge and a cargo that has to be reheated at the terminal for 24 hours at the charterer’s expense. On a bitumen carrier, the stakes are higher still: bitumen solidifies irreversibly if it drops below around 100 degrees Celsius, and recovering a solidified parcel is not always possible.

The fundamental physics is straightforward. Heat flows out of the cargo and through the tank walls at a rate set by the temperature difference, the insulation, and the exposed surface area. The heating system must supply at least that much heat continuously. What makes cargo tank heating a discipline is the range of cargoes, each with its own temperature requirements, compatibility constraints, and regulatory obligations, and the range of heating media, each with its own engineering trade-offs.

This article covers the full picture: why cargoes need heating, the three heating media (steam, thermal oil, and hot water or glycol), coil arrangement and materials, the heat-loss calculation, regulatory obligations under the IBC Code and MARPOL Annex II, over-temperature cargo-quality damage, the charter-party heating clause, thermal-oil fire risk, and the cargo temperature log. For the companion calculation tools, see the Tanker Cargo Heating Duty Calculator and the Cargo Heating Coil Steam Rate Calculator.

Why cargoes require heating

Liquid cargo temperature matters for two reasons: pumpability and quality.

Pumpability is the immediate concern. Every viscous liquid has a pour point, the temperature below which it stops flowing under gravity, and a viscosity-temperature curve that determines how hard the cargo pumps must work. 380-centistoke HFO has a pour point around 30 degrees Celsius and must be kept above 40 degrees Celsius to stay within the suction-capacity of most cargo pumps. Palm oil has a pour point around 27-33 degrees Celsius and congeals visibly in cold weather. Bitumen’s pour point is above 100 degrees Celsius.

Cargo quality is the second concern. Some cargoes are damaged by heat, not just by cold. Vegetable oils accumulate free fatty acids (FFA) at an accelerating rate above their species-specific threshold temperatures. The Codex Alimentarius standard for crude palm oil sets a maximum FFA of 5 percent by weight; a ship that overheats a palm-oil parcel by 15 degrees Celsius for three weeks can push FFA well above that floor and trigger a full cargo rejection. Wax-bearing crude oils can suffer asphaltene-flocculation or wax precipitation if temperature cycling is too severe. Molten sulphur undergoes polymerisation above 160 degrees Celsius, converting from a mobile yellow liquid into a viscous, dark-red mass that blocks cargo lines.

The IBC Code addresses temperature from both sides. Column “l” in IBC Code Chapter 17 specifies, for each listed product, any special heating or cooling requirement. Products in Category X, Y, and Z (MARPOL Annex II) that need temperature control are required to be carried in tanks certified for that purpose, and the ship’s International Certificate of Fitness (ICoF) must list those tanks and their heating or cooling capability. MARPOL Annex II Regulation 16 requires that cargo tank arrangements for noxious liquid substances include adequate means to control cargo temperature where the substance’s properties require it.

ISGOTT Section 10.7 addresses heated cargo operations on oil tankers, including HFO, crude, and blended fuel oils on product and combination carriers. It sets out minimum temperatures for pumpability, the risks of over-heating, and guidance on the cargo temperature log.

The three heating media

Three heating media account for virtually all cargo tank heating on commercial tankers. The right choice depends on the target temperature range, the cargo’s compatibility requirements, and the ship’s existing utility systems.

Saturated steam (coil heating)

Steam at 5-7 bar gauge, corresponding to a saturation temperature of 159-170 degrees Celsius at the coil surface, is the standard heating medium on crude tankers, product tankers, and most chemical tankers. The ship’s auxiliary boilers generate saturated steam; it travels along insulated manifolds on the cargo deck, enters each tank’s supply header, passes through the in-tank coils, and returns to the boiler feed system as condensate. The latent heat released during condensation, 2,085-2,165 kJ/kg at these pressures, makes steam an efficient heat-transfer medium.

Steam heating is well matched to cargoes with target temperatures between 40 and 130 degrees Celsius. 380-centistoke HFO is typically carried at 40-65 degrees Celsius. Palm oil is carried at 35-45 degrees Celsius. Crude oil on heating trades (Orinoco belt extra-heavy crude, for example) is kept at 50-75 degrees Celsius. All of these are comfortably below the maximum safe coil-surface temperature that steam at 7 bar can deliver.

The Cargo Heating Coil Steam Rate Calculator computes the steam consumption rate from the heating duty and the steam enthalpy. The Cargo Tank Heating Coil Pressure Calculator determines the steam supply pressure needed to reach the coil-surface temperature required for the cargo setpoint.

Thermal oil (heat-transfer fluid)

Thermal oil systems, also called high-temperature heat-transfer-fluid (HTHF) systems, circulate a synthetic oil (typically an alkylated aromatic or polyalkylene glycol formulation) through a fired heater and then through the in-tank coils. Fluid temperatures at the heater outlet can reach 280-320 degrees Celsius. The circuit operates at or near atmospheric pressure, which is the critical advantage over high-pressure steam at equivalent temperatures. Saturated steam at 300 degrees Celsius requires a boiler pressure of 86 bar; no commercial cargo coil system is designed for that.

Thermal oil is the standard heating medium on bitumen and asphalt tankers, where carriage temperatures of 140-180 degrees Celsius are normal and discharge temperatures can reach 195-200 degrees Celsius. It’s also used on specialised chemical carriers handling products such as crude tall oil, crude sulphate turpentine, and certain resins that require heating above the practical steam range.

The fired heater is a significant piece of equipment. It uses heavy fuel oil or diesel as combustion fuel, includes a temperature-controlled burner management system, and must be fitted with high-temperature alarms and automatic fuel cut-off. DNV class rules and the equivalent requirements of other classification societies require oil-level monitoring, leak detection in the heater’s firebox, and automatic shutdown on low oil flow. IACS UR F30 specifies the minimum requirements for cargo heating systems on tankers, including thermal-oil specific requirements for expansion tank sizing, relief valve setting, and oil sample analysis intervals.

The key fire hazard of thermal oil is that the fluid’s flash point (typically 100-120 degrees Celsius for synthetic HTF oils) is well below the operating temperature of the circuit. A leak from a corroded coil or a failed pump seal will auto-ignite on contact with hot metal surfaces. Classification societies require that thermal-oil heater rooms be equipped with fixed CO2 or equivalent fire-suppression systems and that thermal-oil piping runs in cargo areas be enclosed in double-walled or otherwise drip-proof arrangements. DNV Rules for Classification Part 4 Chapter 6 specifies these containment requirements.

Leak detection in thermal oil systems relies on three mechanisms: oil-level monitoring in the expansion tank (a sustained level drop indicates a circuit leak), high-hydrocarbon-vapour detection in cargo spaces (a coil leak inside a tank will cause a vapour-level rise), and daily checks of cargo samples for thermal-oil contamination, which shows up as a drop in flash point or a change in colour.

Hot water and glycol systems

Hot water or water-glycol circuits circulate at 70-110 degrees Celsius through cargo heating coils, heated via a steam-to-water or electric-element heat exchanger. The maximum coil-surface temperature is limited to the water circuit temperature, which is 30-50 degrees Celsius lower than a direct steam coil at the same steam pressure. This lower surface temperature makes hot water and glycol circuits the preferred choice on chemical tankers where the cargo specification prohibits direct steam contact at elevated temperatures, such as certain ester solvents, food-grade oils, and heat-sensitive resins.

Glycol addition (typically 20-35 percent monoethylene glycol) prevents freeze damage to the circuit in cold-weather ports when the system is de-pressurised between voyages, and it also raises the circuit’s maximum operating temperature marginally by increasing the boiling point. Chemical tankers fitted with glycol heating circuits often have multiple independent loops serving different tank groups, so that a leak in one loop cannot contaminate a cargo in another tank via shared piping.

The trade-off is heat-transfer rate. Hot water at 90 degrees Celsius delivers a lower log-mean temperature difference to a cargo at 50 degrees Celsius than steam at 5 bar does. For high-heat-loss situations, such as a large tank in cold seawater with thin insulation, a glycol circuit may need a larger coil area than a steam circuit to meet the same heating duty.

Comparison of heating media

In-tank coil arrangement

Cargo heating coils are almost always located in the bottom third of the cargo tank, typically in serpentine (zigzag) runs of 50-80 mm nominal-bore pipe laid horizontally across the tank floor. The logic is that heat rises: hot cargo near the coils becomes less dense and rises toward the cargo surface, setting up a slow natural convection that distributes heat through the tank without mechanical agitation. Coils at the bottom also remain submerged until the tank is nearly empty, extending their useful heating time during discharge.

Suction-well coils are a separate, denser coil arrangement concentrated in and around the cargo suction bell-mouth or stripping well. The suction-well coil heats the last cargo in the tank, which is the coldest, highest-viscosity fraction, to ensure that the stripping pump can clear the tank to the contractual ROB (remaining on board) figure. On bitumen carriers, suction-well coils may be the only coils needed in the lower part of the tank because the entire tank bottom is effectively one large suction well.

Coil spacing is set by the heat-flux requirement. ISGOTT recommends that the coil area provide a heating flux not exceeding 3,500 W/m2 of coil surface for cargoes sensitive to localised over-heating, such as vegetable oils. Denser coil packing increases the average cargo temperature at the coil surface, which can cause FFA formation or protein denaturation in food-grade cargoes even if the bulk cargo temperature is within specification.

Coil supports are steel chairs or brackets welded to the tank floor and lower bulkheads. They hold the coils clear of the tank plating (to prevent galvanic corrosion at contact points), allow drainage during tank cleaning, and are spaced to prevent sagging. Thermal expansion must be accommodated: a 20-metre carbon steel coil run expands by approximately 24 mm when heated from ambient to 130 degrees Celsius, so expansion loops or U-bends are built into each coil bank.

Some chemical tankers and vegetable oil carriers use deck-mounted heat exchangers rather than, or in addition to, in-tank coils. The cargo is drawn from the bottom of the tank through the cargo pump, heated in the shell-and-tube exchanger on deck, and returned to the top of the tank. Deck heating avoids localised hot spots inside the tank and allows the heat exchanger to be inspected and cleaned without entering the cargo space, but it consumes cargo pump capacity and adds piping complexity.

Coil materials and chemical compatibility

The IBC Code’s compatibility requirements govern not just what cargoes may be carried in the same tank, but what materials the tank and its equipment may be made of. Heating coils are internal tank equipment and must satisfy the material requirements of the cargo being carried.

Carbon steel coils are standard on crude and product tankers and on black-product chemical tankers (bitumen, fuel oils, lubricant base stocks). Carbon steel is suitable for most petroleum products and some chemicals at elevated temperatures, but it corrodes in the presence of chloride-containing aqueous cargoes, strong acids, and certain solvents.

316L stainless steel (UNS S31603) is the standard coil material on chemical tankers handling food-grade vegetable oils, fatty acids, alcohols, esters, and similar products. Its 2-3 percent molybdenum content provides resistance to chloride pitting, and its low carbon content (0.03 percent maximum) prevents sensitisation during welding. The IBC Code’s Chapter 17 product table specifies “stainless steel” as a tank and coil material requirement for many of the more reactive chemicals.

Duplex stainless (UNS S32205, UNS S31803) and super-duplex (UNS S32750) coils appear on specialised chemical carriers handling phosphoric acid, chlorinated solvents, and hypochlorite solutions where austenitic grades are susceptible to stress corrosion cracking. These alloys cost roughly 40-60 percent more per tonne than 316L, so their use is limited to tanks explicitly certified for the relevant products.

Titanium coils are specified for a small number of very aggressive cargoes including wet chlorine and fuming nitric acid. Titanium’s capital cost is around 5-7 times that of 316L stainless, so it appears only where no alternative alloy is safe.

Heating coil pressure testing is required by classification societies at each five-year special survey and whenever a coil is repaired or replaced. DNV requires a test at 1.5 times the maximum allowable working pressure (MAWP), with the test witnessed by a surveyor. Coils that leak into the cargo during the voyage contaminate the parcel and breach the IBC Code fitness requirements.

Heat-loss calculation and heating duty

The steady-state heat that must be supplied to hold cargo at its carriage temperature is given by:

where $Q$ is the thermal duty in watts, $U$ is the overall heat-transfer coefficient of the tank boundary in W/(m²·K), $A$ is the wetted surface area of the tank in m², and $\Delta T$ is the temperature difference between the cargo setpoint and the ambient medium (sea water for the bottom and sides, air for the top). Use the Tanker Cargo Heating Duty Calculator to run this directly for a given tank geometry.

Practical $U$-values depend on insulation thickness and quality. An uninsulated steel tank bottom in contact with sea water has a $U$-value around 10-15 W/(m²·K). Adding 100 mm of mineral-wool insulation drops this to approximately 0.40-0.50 W/(m²·K). Adding 150 mm of polyurethane foam drops it further to 0.20-0.30 W/(m²·K). Modern bitumen carriers targeting $\Delta T = 160$ K (cargo at 160 degrees C, sea water at 0 degrees C) aim for $U < 0.25$ W/(m²·K) to keep the installed thermal-oil heater to a manageable size.

The warming rate, which controls how long it takes to bring cargo from loading temperature up to carriage setpoint, adds a transient term:

where $m$ is the cargo mass in kg, $c_p$ is the specific heat capacity in J/(kg·K), $\Delta T_{warm}$ is the temperature rise needed in K, and $\Delta t$ is the allowed warming time in seconds. For heavy crude oil with $c_p \approx 1,900$ J/(kg·K), raising 30,000 tonnes by 15 degrees Celsius over 24 hours requires approximately 9.9 MW of installed heating capacity, far above the steady-state maintenance duty. Most tankers do not have heating capacity for rapid warming on large tanks; the practical expectation is that cargo is loaded as close to carriage temperature as possible, with the heating system making up gradual losses only.

The steam mass-flow rate needed to deliver a given thermal duty is:

where $h_s$ is the specific enthalpy of saturated steam at the supply pressure in kJ/kg and $h_{fw}$ is the specific enthalpy of the returned condensate (feed water) in kJ/kg. At 6 bar gauge (saturation temperature 165 degrees C), $h_s \approx 2,756$ kJ/kg; $h_{fw}$ at 90 degrees C $\approx$ 377 kJ/kg, giving a latent-heat yield of 2,379 kJ/kg. Delivering 500 kW to a cargo tank therefore requires approximately 0.21 kg/s of steam. The Cargo Heating Coil Steam Rate Calculator computes this directly and also checks whether the required steam rate is within the boiler’s capacity.

IBC Code temperature-control requirements

IBC Code Chapter 15 covers ship survival capability and location of cargo tanks. Chapter 16 covers special requirements by substance type, including a section on temperature control. The operative text is in Chapter 17’s product table, where column “l” specifies whether a product requires heating to remain within its viscosity or pour-point specification, and where the footnotes require that the heating arrangement be listed on the ICoF.

IBC Code 15.19 requires that where a cargo listed in Chapter 17 specifies temperature control, the arrangement must ensure that the cargo can be maintained within the specified temperature range throughout the voyage and during loading and discharge operations. It explicitly requires that the heating or cooling capacity be assessed relative to the worst-case voyage conditions (the lowest sea-water temperature and the longest expected transit time).

IBC Code Chapter 16 includes specific sections on inhibitor requirements, temperature-controlled cargoes (16.6), and cargoes that must be carried under pressure. For temperature-controlled substances, 16.6 requires the ship to carry a record of the cargo temperature throughout the voyage and to make that record available to port-state control on request.

MARPOL Annex II Regulation 16.4 requires that tankers carrying Category X or Y noxious liquid substances that are temperature-controlled must be fitted with instrumentation to monitor and record cargo temperature continuously. This is the regulatory basis for the cargo temperature log.

Some products have both a minimum carriage temperature (to prevent solidification) and a maximum (to prevent quality deterioration or to stay below the cargo’s flash point). Styrene monomer, for example, must not be allowed to polymerise during carriage; IBC Code 15.12 requires inhibitor addition and temperature monitoring. Acrylonitrile requires similar treatment. For these cargoes, the heating system is part of the overall cargo-care framework that includes chemical inhibitors, inert gas blanketing, and crew monitoring.

Cargo-specific temperature targets

The following table summarises carriage and discharge temperature targets for representative heated cargoes. These are operational references; the legally binding requirements are the product entries in IBC Code Chapter 17 and the cargo’s Safety Data Sheet.

Molten sulphur presents an unusually narrow safe window. The sulphur is mobile between 119 and 159 degrees Celsius. Above 159 degrees Celsius, viscosity increases sharply as sulphur polymerises. At 190 degrees Celsius, the cargo becomes a near-solid plastic mass. The Tanker Cargo Heating Sulphur Calculator handles the duty calculation for molten-sulphur voyages.

HFO carried as cargo (bunker barge consolidation voyages, refinery-to-refinery transfers) uses the same steam coil arrangement as ships’ fuel oil service tanks. ISGOTT Section 10.7 requires that the maximum steam supply temperature to HFO heating coils not exceed 85 degrees Celsius to avoid local cracking and deposit formation, which is why HFO heating coils are often fed from a throttled supply line rather than directly from the boiler header.

The Tanker Cargo Heating HFO Calculator provides the duty and steam-rate calculation for HFO cargoes. The Tanker Cargo Heating Asphalt Calculator covers the bitumen and asphalt case with the thermal-oil circuit. The Tanker Cargo Heating Wax Calculator addresses wax-bearing crudes and paraffin wax cargoes.

Temperature monitoring and the cargo temperature log

Classification society rules and MARPOL Annex II require that each cargo tank on a heated-cargo ship be fitted with at least one, and in practice three to five, temperature sensors distributed vertically: one near the suction bell-mouth (bottom), one at mid-depth, and one near the cargo surface. Large tanks (above 3,000 m3) typically carry five or more sensors, with at least two at different radial positions to detect stratification.

Sensors on modern tankers are 4-20 mA Pt-100 resistance temperature detectors connected to the cargo-monitoring panel and logged at 30-minute intervals or better. The resulting cargo temperature log is an official ship’s document. MARPOL Annex II Regulation 16.4 requires it to be retained on board for at least three years and produced on demand to port-state control inspectors. A missing or incomplete cargo temperature log is a deficiency under Paris MOU and Tokyo MOU port-state control regimes.

The temperature log also serves as evidence in cargo-quality disputes. If a charterer claims that a parcel of palm oil was overheated during transit, raising FFA from 3.0 to 5.8 percent, the cargo temperature log showing a continuous record of 38-43 degrees Celsius throughout the voyage is the owner’s primary defence. If the log is incomplete, or if there are unexplained periods at 60 degrees Celsius, the owner’s negotiating position deteriorates sharply.

Alarm thresholds are set 5-10 degrees Celsius above and below the carriage setpoint. A sustained high-temperature alarm on a vegetable-oil tank should trigger immediate reduction in steam supply and cargo sampling. A sustained low-temperature alarm on a bitumen tank in cold weather should trigger increased thermal-oil heater output and an assessment of whether the cargo near the suction well is still liquid.

Charter-party heating clauses

Charter parties for heated-cargo voyages routinely include a specific heating clause obligating the shipowner to maintain cargo at or above a minimum temperature throughout the laden voyage. A typical clause for a palm-oil shipment reads: “Owner warrants that the vessel’s heating system is capable of maintaining cargo temperature at not less than 35 degrees Celsius throughout the laden voyage. Should cargo arrive below the warranted temperature due to failure of the vessel’s heating equipment, Owner shall bear the cost of shore-based reheating at the discharge port.”

Three aspects of heating clauses produce recurring disputes:

The first is the difference between a warranty to maintain temperature and a warranty that the heating system is capable of maintaining it. A warranty of capability is defeated if the owner can show that a one-in-a-hundred-year cold spell drove sea water to 0 degrees Celsius when the system was designed for 5 degrees Celsius. A warranty to maintain, in contrast, is a strict obligation regardless of ambient conditions.

The second is the allocation of cost when cargo arrives cold because of a heating system failure during the voyage. The owner’s obligation under standard charter-party forms (SHELLVOY 6, ASBATANKVOY) is to exercise due diligence to maintain the cargo at temperature. A mechanical failure that the crew could not foresee or prevent is often treated as a force-majeure event, but a failure to check steam trap operation or to maintain boiler load is operational negligence and sits with the owner.

The third is discharge temperature. Some charter parties specify a minimum temperature at first drop of cargo to allow efficient discharge from the terminal’s shore tanks. If the cargo is above the minimum carriage temperature but still too viscous to reach the specified discharge temperature within the terminal’s allotted laytime, demurrage disputes follow. The Voyage Cargo Heating Cost Calculator models the fuel consumption and heating cost for a given voyage, which is useful for negotiating heating clauses and costing voyages before fixing.

Cargo over-temperature damage

Over-heating is as damaging as under-heating for many cargoes, and the damage is often irreversible.

Free fatty acid (FFA) rise in vegetable oils. Hydrolysis of triglycerides produces free fatty acids at a rate that roughly doubles for every 10 degrees Celsius increase above a threshold, which varies by oil type. For crude palm oil, the Codex Alimentarius maximum is 5 percent FFA. At 35 degrees Celsius the hydrolysis rate is slow enough that a 30-day voyage produces negligible FFA rise. At 55 degrees Celsius for 30 days, a parcel loaded at 3.0 percent FFA can easily arrive above 5.0 percent. The cargo is then off-spec for edible-oil refiners and may be downgraded to biodiesel feedstock at a significant price discount. Over-heating also accelerates oxidation, producing peroxides and secondary oxidation products (aldehydes, ketones) that affect the oil’s flavour and shelf life.

Asphaltene flocculation and wax deposition in crude oil. Heavy crudes with high asphaltene content (Venezuelan Merey 16, Doba blend from Chad) can experience asphaltene flocculation if heated well above their natural pour point and then cooled rapidly. Wax-bearing crudes (Escravos, Duri) that are cycled through temperature too rapidly can deposit waxy crusts on the coil surfaces that are difficult to remove during tank cleaning and reduce the coil’s heat-transfer effectiveness on the next cargo.

Sulphur polymerisation. The transition from mobile alpha-sulphur to plastic alpha-sulphur begins above 159 degrees Celsius. An undetected failure of the thermal-oil circuit temperature controller that allows the sulphur to reach 170 degrees Celsius for several hours produces a partial polymerisation that dramatically raises viscosity and makes the cargo difficult to discharge. Confirmed cases of fully polymerised sulphur parcels requiring manual break-out have been documented in IMO guidance on the carriage of molten sulphur (CCC circular notes associated with IBC Code Chapter 17 entry for sulphur, molten).

Thermal-oil system fire risk

The fire record for thermal-oil systems in cargo spaces is notably worse than for steam systems. The mechanism is consistent: a small leak from a corroded coil joint, a failed pump-shaft seal, or a cracked expansion joint allows hot oil at 200-320 degrees Celsius to contact hot metal surfaces. The oil’s flash point (typically 100-130 degrees Celsius for synthetic HTF oils) is well below the circuit operating temperature, so the leaked oil ignites immediately.

IMO MSC circular MSC/Circ.1272 and the associated class-society requirements (DNV Part 4 Chapter 6, LR ShipRight procedures) mandate:

• Expansion tank fitted with a level alarm and automatic heater shutdown on low level
• Double-walled piping or drip-trays under all thermal-oil flanges and valve bodies in enclosed spaces
• Automatic CO2 or equivalent fire suppression in the thermal-oil heater room
• Cargo-space thermal-oil piping run in closed cofferdam sections where practicable
• Monthly oil analysis (viscosity, flash point, acid number, soot content) to detect degradation before carbonisation occurs in the heater

Oil degradation is the leading precursor to heater fires. Carbonised deposits from over-heated oil block the heater tubes, cause local hot spots, and eventually crack the tube wall. Monthly oil samples analysed by a specialist laboratory cost around $150-200 per sample; they are inexpensive insurance against a heater fire that can cost$500,000-2,000,000 in damage and off-hire.

IACS UR F30.4 requires that thermal-oil heaters on tankers be fitted with independent high-temperature cutoffs (set at 10-15 degrees Celsius above the normal operating temperature) in addition to the primary temperature controller. Both the primary controller and the independent cutoff must be tested at each annual survey.

Boiler capacity and cargo heating interaction

On steam-heated tankers, cargo heating competes with hotel load, boiler feed heating, and steam-heating of fuel oil service tanks for the boiler’s total output. A typical product tanker with three cargo tanks each requiring 150 kW of maintenance heating (450 kW total) plus 200 kW of fuel oil service tank heating plus 100 kW of hotel steam load runs its auxiliary boiler at 750 kW, which is within the capacity of a standard 1,200 kW package boiler. But if the ship is pre-heating cargo for discharge at the same time as it’s heating bunkers during bunkering operations, the combined demand can spike to 1,800-2,200 kW and exceed the single boiler’s capacity.

Most tankers have two boilers (one operational, one on standby) and can bring both online for peak demand. The chief engineer’s cargo-planning role includes estimating heating demand by voyage phase and ensuring that boiler capacity is available when needed. The Marine Boiler Fuel Consumption Calculator can be used to estimate the fuel cost of sustained cargo heating across a voyage.

Exhaust gas economisers on the main engine provide free heat during sea passage, reducing auxiliary boiler load. A typical VLCC running its main engine at 80 percent MCR generates 1,000-1,500 kW of recoverable exhaust heat through the economiser, which can supply the entire cargo maintenance-heating load during the laden passage and reduce auxiliary boiler fuel consumption to near zero for that phase.

Maintenance and inspection of heating coils

Heating coil condition is assessed primarily during dry-docking, when the cargo tanks are gas-freed, cleaned, and opened for internal inspection. The inspector looks for:

• Coil corrosion, particularly at support chair contact points and at weld seams
• Coil distortion from thermal cycling or cargo pressure during heavy-weather loading
• Erosion at coil bends where condensate flow velocity is highest
• Scale deposits on the coil external surface (reduces heat transfer)
• Gasket and flange condition at coil header connections

Coil pressure tests at 1.5 times MAWP, witnessed by the class surveyor, confirm integrity before returning the tank to service. Coils that fail pressure tests are patched by welding or, for extensively corroded sections, replaced. Carbon steel coils have a typical service life of 15-20 years; stainless steel coils can last 25-30 years if the cargo compatibility is maintained.

During the voyage, coil integrity can be inferred from the condensate system. A sustained increase in condensate return volume from one tank, without a corresponding increase in steam supply, indicates a coil leak letting seawater backflow or cargo intrusion into the condensate. Modern condensate monitoring panels track individual tank returns. A coil that develops a pinhole leak can allow cargo contamination of the condensate, which poisons the boiler feed water and can require an emergency boiler blowdown, an expensive and disruptive event.

Steam traps deserve specific attention. A failed-open steam trap passes live steam through to the condensate system, wasting heat and raising condensate pressure. A failed-closed steam trap blocks condensate return, allowing condensate to flood the coil and reduce the effective heating surface. Both failure modes are common and both reduce heating efficiency. Steam trap inspection logs on well-managed tankers record individual trap status at monthly intervals.

Integration with the marine boilers and steam systems article

Cargo heating is the largest consumer of steam on a heated-cargo tanker during the laden voyage. The steam generation side, including auxiliary boiler design, exhaust gas economiser integration, feed water quality, and boiler safety valve settings, is covered in the companion article on marine boilers and steam systems. That article also covers the feed-water treatment chemistry that protects the condensate return from contamination by heating coil leaks.

The cargo pump side of the cargo-heating interaction, including the effects of viscosity on pump performance, stripping efficiency, and suction-well heating, is covered in marine cargo pumps and piping. Cargo heating and cargo pumping are operationally linked: the chief officer’s pre-discharge heating plan must account for the pump’s minimum inlet-temperature requirement as well as the cargo’s target discharge temperature.

Limitations

This article addresses commercial tanker cargo heating in the context of MARPOL Annex I/II and IBC Code requirements. It does not cover:

• Cargo refrigeration or cooling systems used on LPG, LNG, and some chemical carriers, which are a separate technical discipline
• Gas carrier cargo conditioning (reliquefaction, boil-off management) or LNG carrier BOG handling
• Cargo heating in jacketed ISO tank containers carried on container ships
• The specific design standards for type A, B, and C independent tanks, which have their own structural and thermal requirements under IBC Code Chapters 4-6
• Regulatory requirements for vessels not covered by MARPOL Annex II (national-flag coastal tankers below the MARPOL GT threshold)

The $Q = U \cdot A \cdot \Delta T$ formula gives the steady-state maintenance duty only. It does not account for the transient heating demand during cargo warm-up, which can be 5-10 times larger, or for the effect of cargo stratification on local coil surface temperatures in large tanks. For detailed cargo heating calculations, use the suite of calculators linked throughout this article and verify results against the vessel’s as-built heating data.

The thermal-oil fire risk discussion above reflects general class-society requirements. Specific vessels may have additional requirements from their flag state, insurance underwriters (particularly for older vessels with pre-2000 thermal-oil installations), or terminal operators at discharge ports. Always consult the vessel’s Safety Management System (SMS) procedures and the terminal’s pre-arrival heating requirements before modifying thermal-oil circuit operating temperatures.

See also

Calculators:

• Tanker Cargo Heating Duty Calculator
• Cargo Heating Coil Steam Rate Calculator
• Cargo Tank Heating Coil Pressure Calculator
• Tanker Cargo Heating HFO Calculator
• Tanker Cargo Heating Asphalt Calculator
• Tanker Cargo Heating Sulphur Calculator
• Tanker Cargo Heating Wax Calculator
• Cargo Steam Heating Time Calculator
• Cargo Pump Heating Coils Calculator
• Voyage Cargo Heating Cost Calculator
• Marine Boiler Fuel Consumption Calculator

Related wiki articles:

• Marine Boilers and Steam Systems
• Marine Cargo Pumps and Piping
• Marine Fuel Oil Systems
• Marine Inert Gas Systems
• Oil Tanker
• Chemical Tanker
• IBC Code
• MARPOL Annex II
• SIRE Tanker Inspections
• Marine Tank Gauging Systems