CST 180 Fuel Oil: Specification, Heating Data and Export Supply

CST is the abbreviation for centistokes, the traditional unit of kinematic viscosity. One centistoke is one square millimetre per second, so 1 cSt and 1 mm²/s are the same quantity written two ways — modern standards print mm²/s, the trade still writes cSt, and nothing changes between them. This page uses both, deliberately, because certificates and offers do the same. Kinematic viscosity is measured by timing the flow of a temperature-controlled sample through a calibrated glass capillary viscometer under ISO 3104, with the viscometer itself calibrated to ISO 3105. One further unit pairing is worth stating plainly because it appears on every certificate: a contaminant limit written in mg/kg is numerically identical to the same figure written as ppm by mass, so a 50 mg/kg catalyst fines limit and a 50 ppm catalyst fines limit are the same limit.

The 180 is a maximum. It is not a target and it is not a range. A parcel testing at 162 mm²/s is a perfectly good CST 180; a parcel testing at 196 mm²/s is not, whatever else the certificate says. That is the reverse of the convention a buyer brings over from penetration-grade bitumen, where 60/70 is a two-sided band with a floor as well as a ceiling. The habit travels badly. A CST 180 offer tells you what the fuel will not exceed; it tells you nothing about where inside the range the batch will land, and the difference between a 175 cSt parcel and a 120 cSt parcel is a real difference in heater duty at the receiving end.

The reference temperature is the part people forget

A viscosity without a temperature is not a specification, it is a number. In the ISO 8217 residual system the reference temperature is always 50 °C. The same fuel that measures 180 mm²/s at 50 °C measures roughly 23 mm²/s at 100 °C and of the order of 1,560 mm²/s at 20 °C. All three numbers describe one product and none of them is wrong. An offer that says only 180 cSt without naming the temperature is either careless or is quoting on a basis you have not agreed to, and the gap between 180 at 50 °C and 180 at 100 °C is the gap between an ordinary intermediate bunker fuel and an extremely heavy residue that most burners on the market will not handle. Write the temperature into the contract line itself.

What the number does not tell you

Viscosity is one line of an ISO 8217 residual specification that runs to sixteen properties. On its own, CST 180 says nothing about:

• Sulphur. ISO 8217 sets no sulphur limit at all — it defers to the statutory requirement in force where the fuel is used. A CST 180 enquiry with no sulphur cap is incomplete, because 0.10 %, 0.50 % and 3.50 % m/m products all trade under the same viscosity name at very different prices.
• Density. Two 180 cSt fuels can differ by 30 kg/m³, which changes the tonnes you receive for the cubic metres you measured and changes the separator settings needed to take water out.
• Carbon residue and ash. Combustion and fouling behaviour lives in these lines, not in the viscosity line.
• Catalyst fines. The most destructive contaminant in residual fuel is completely invisible to a viscosity test.
• Pour point. A fuel can sit comfortably inside the viscosity limit and still be a solid block in an unheated shore tank in January.

The standard grade names behind the commercial name

The internationally recognised specification for residual marine fuels is ISO 8217. Within it, exactly two grades cap kinematic viscosity at 180 mm²/s at 50 °C: RME 180 and RMG 180. The letters read as R for residual, M for marine, and a third letter that indicates how tight the contaminant limits are — E is the cleaner set, G the looser one. So the phrase CST 180, on its own, describes two different products that share a viscosity ceiling and differ materially on carbon residue, ash, CCAI, vanadium, sodium and catalyst fines. A contract that specifies only CST 180 is satisfied by the looser of the two.

ISO 8217 has been revised several times and the edition matters. The 2017 edition is the one most commonly written into fuel oil contracts and is the basis of the table below. A 2024 edition has since been published; it restructures the tables and introduces bio-residual grades alongside the conventional ones. Name the edition in the contract. A clause reading only to ISO 8217 leaves the acceptance limits open to argument at the exact moment you least want one.

Two general requirements of the standard are also worth knowing because they are enforceable and are almost never quoted in an offer. The fuel must be free from any material that jeopardises the safety of the ship or adversely affects machinery, and free from added substances or chemical waste. Residual grades must also be free from bio-derived material other than de minimis levels of fatty acid methyl ester. Those clauses are what a buyer relies on when a cargo passes every numeric line and still behaves badly.

Viscosity at 50 °C — ISO 3104

This line sets the heating duty for the whole installation. Transfer pump sizing, heater capacity, separator throughput, burner or injector performance are all designed around a viscosity target at the point of use, and that viscosity is reached by temperature. A parcel delivered at 178 mm²/s and a parcel delivered at 120 mm²/s both pass as CST 180, but they need different tank and heater temperatures to arrive at the same viscosity at the nozzle. Ask for the actual measured value on the batch, not the grade limit, and set the heater controller from it.

Density at 15 °C and CCAI

Density is measured by hydrometer under ISO 3675 or, more precisely and more commonly today, by oscillating U-tube under ISO 12185. It does two jobs at once. Commercially it converts the volume you measured into the mass you paid for, using the volume correction factors of the ISO 91 petroleum measurement tables. Technically it feeds the Calculated Carbon Aromaticity Index, an estimate of ignition quality derived from density and viscosity:

CCAI = D − 140.7 × log[log(V + 0.85)] − 80.6, where D is density at 15 °C in kg/m³ and V is kinematic viscosity at 50 °C in mm²/s.

Work it through for a 180 cSt fuel at 985 kg/m³ and the answer is about 855, comfortably inside the RME 180 limit of 860. Work the same 180 cSt viscosity at the density ceiling of 991.0 kg/m³ and the answer is about 861, already outside it. In other words an RME 180 cannot sit at both its density ceiling and its viscosity ceiling at the same time, which is a useful arithmetic check when a certificate looks a little too convenient. A high CCAI signals a fuel that is slow to ignite. It matters most in medium-speed four-stroke engines, where ignition delay shows up as rough running and late burning, and least in a large boiler where residence time is generous.

Micro carbon residue — ISO 10370

The mass fraction of carbonaceous material left after the sample is pyrolysed under controlled conditions. It is the best single predictor of how heavily a fuel will foul: deposits on injector tips and burner nozzles, on exhaust valves, in the economiser and across the turbocharger. RME 180 caps it at 15.00 % m/m; RMG 180 allows 18.00 % m/m at the identical viscosity. On equipment with tight nozzle geometry and limited soot-blowing capacity, that three-point gap is the whole reason to specify RME rather than RMG and to accept the price difference that goes with it.

Ash, vanadium and sodium

Ash is the incombustible mineral fraction, measured by ISO 6245 — 0.070 % m/m maximum for RME 180 against 0.100 % m/m for RMG 180. Within the ash, vanadium and sodium are the two elements that drive high-temperature corrosion. Vanadium oxides and sodium sulphate form low-melting eutectics that deposit on and then dissolve the protective oxide layer on exhaust valve seats, turbine blades and superheater tubes. The damage is worst when the two appear together, so a fuel carrying modest vanadium but elevated sodium can be more aggressive than one carrying high vanadium and almost no sodium. Elevated sodium is also the classic fingerprint of seawater ingress: read it against the water result before accepting any other explanation.

Aluminium plus silicon — catalyst fines

The single most destructive contaminant in residual fuel. Aluminium silicate particles carried over from refinery catalytic cracking survive combustion, are harder than the metal they meet, and abrade cylinder liners, piston rings, fuel pump plungers and injector needles. RME 180 caps them at 50 mg/kg and RMG 180 at 60 mg/kg, but the delivered limit is not the working limit: engine builders commonly ask for no more than about 15 mg/kg at the engine inlet, which means the fuel has to be settled and centrifugally separated on board or on site, not merely strained through a filter. Cat fines are dense, they concentrate in tank bottoms and at the water interface, and stripping the last of a tank is precisely how a concentrated slug reaches an engine that has run happily on the same cargo for a week.

Water and total sediment

Water is limited to 0.50 % V/V under ISO 3733, and it is not a trivial line. Water carries sodium and cat fines with it, it is cargo mass you paid for and cannot burn, it steals heat in the pre-heater, and it is the direct cause of tank boil-over when a cold wet bottom is heated quickly. Total sediment aged, under ISO 10307-2, is limited to 0.10 % m/m and answers a different question: how much solid material will drop out of this fuel over time and after thermal ageing. That is the stability test, and it is the line most often absent from a thin certificate.

Flash point, hydrogen sulphide and acid number

Flash point minimum 60.0 °C by ISO 2719 Pensky-Martens closed cup is a safety floor written into international carriage rules, not a performance property. Hydrogen sulphide is capped at 2.00 mg/kg by IP 570, and note carefully that this is a liquid-phase limit which says nothing about what accumulates in a warm tank vapour space. Acid number, maximum 2.5 mg KOH/g by ASTM D664, was added to catch acidic contamination capable of attacking fuel pumps and bearings.

The used lubricating oil check

ISO 8217 requires residual fuel to be free of used lubricating oil and defines the test arithmetically. The fuel is deemed to contain used lubricating oil if calcium exceeds 30 mg/kg together with zinc above 15 mg/kg, or calcium exceeds 30 mg/kg together with phosphorus above 15 mg/kg. Ask for those three elements on the certificate. They are cheap to run, they are a direct indicator that waste streams have been blended into the cargo, and their absence from a certificate is itself a piece of information.

Storage

Two rules set the minimum storage temperature and you take whichever is higher. First, keep the bulk at least 10 °C above the batch pour point reported under ISO 3016. With an upper pour point limit of 30 °C, a worst-case batch needs 40 °C simply to remain liquid. Second, keep viscosity low enough for the transfer pump that has to move it. In practice those two rules put routine bulk storage for a CST 180 in the region of 40–50 °C, and higher where the batch pour point sits at the top of the range. Storage at unnecessarily high temperature is not free: it accelerates ageing, drives off light ends and increases sediment, so the target is the lowest temperature that satisfies both rules rather than the highest the coils can reach.

Pumping and transfer

Positive-displacement screw and gear pumps handle residual fuel far better than centrifugal pumps, which lose head rapidly as viscosity climbs. As a working figure, keep viscosity below roughly 1,000 mm²/s at the suction of a positive-displacement pump and considerably lower for a centrifugal one. For a CST 180 that means staying above about 30 °C as an absolute floor, and in practice 45–60 °C for a transfer line expected to move volume at a sensible rate. Trace-heat and lag every line, valve, strainer and dead leg that can sit static. An unheated dead leg is where a transfer system fails first, and it fails when the line is already full.

Settling and separation

Settling tanks are normally run at around 50–70 °C to let water and coarse solids drop out under gravity. Centrifugal separation, which is what actually removes catalyst fines and free water, is conventionally carried out at a separator inlet temperature of about 98 °C. That temperature is not arbitrary: separation efficiency depends on the density difference between fuel and water and on the viscosity of the fuel, and both improve steeply with heat. Running a separator cold, or running it above rated throughput to save time on a tight turnaround, is the most common reason a fuel that met 50 mg/kg on the certificate still arrives at the engine carrying an abrasive load.

Final heating to the burner or injector

The final heater exists to hit a viscosity target, not a temperature target, which is why a well-designed installation controls on a viscometer rather than a thermometer. Typical targets for a CST 180:

• Diesel engine injection: roughly 10–15 mm²/s at the engine inlet, which corresponds to approximately 115–125 °C.
• Pressure-jet and steam-atomising boiler burners: commonly 15–25 mm²/s, which corresponds to approximately 95–115 °C.
• Rotary-cup burners: generally tolerate a higher viscosity again, and therefore a lower final temperature, but the burner manufacturer figure governs.

Atomisation is the entire point of the exercise. Fuel supplied too viscous leaves the nozzle as coarse droplets that burn incompletely, produce soot and unburnt carbon, foul the nozzle and the heat transfer surfaces and, in an engine, wash the liner. Fuel supplied too hot risks vapour lock in the fuel pump and, in any part of the system open to atmosphere, a vapour hazard.

Where the caveat sits

Every range above is typical for the grade, not for your batch and not for your equipment. The controlling documents are the supplier technical data sheet and the Certificate of Quality for the parcel, together with the manufacturer manual for the burner, engine or separator. Where the two disagree, the equipment manual governs the machine and the fuel certificate governs the fuel.

Safety: the controlling rule on temperature

The fuel must be hot enough to pump and atomise — roughly 40–50 °C in storage, 45–60 °C in transfer, about 98 °C at a separator inlet and 115–125 °C at an engine injector — and at the same time no open tank with a vapour space may be taken within 10 °C of the batch flash point shown on the Certificate of Quality. Heating above the flash point happens only in a closed pre-heater downstream of the service tank. If those two requirements collide on your installation, the installation is wrong, not the rule.

Safety: seven hazards specific to this product

• Hydrogen sulphide in the vapour space. The ISO 8217 limit of 2.00 mg/kg by IP 570 is a liquid-phase figure. H2S partitions into the vapour space of a warm tank and can reach concentrations far above anything the liquid result suggests. H2S is lethal, and olfactory fatigue sets in well below the dangerous range, so smell is not a warning system. Gas-test before ullaging, sampling or entering, and wear a personal H2S monitor for those tasks.
• Boil-over from water at the tank bottom. Water trapped under a hot oil layer flashes to steam and ejects burning-hot fuel through the hatch. Drain and check tank bottoms with water-finding paste before applying heat, raise temperature gradually, and never take a wet tank rapidly through 100 °C.
• Flash point is a batch number, not 60 °C. The standard sets only a 60.0 °C floor and a real batch usually flashes well above it. Setting tank limits from the standard rather than from the batch certificate either throws away margin you were given or, on a batch flashing near the floor, invents margin you never had. Take the figure from the Certificate of Quality for the parcel in the tank, and reset it whenever the tank is topped with a different parcel.
• Thermal burns. Fuel at 120 °C causes immediate full-thickness burns. Treat every line, filter and strainer downstream of the final heater as live: depressurise, drain and cool before breaking a joint, and use a face shield and heat-resistant gloves for the work.
• Carcinogenic potential. Residual fuel oils carry polycyclic aromatic hydrocarbons and are classified in the EU CLP inventory as category 1B carcinogens. Avoid skin contact, do not carry oil-soaked rags in clothing, wash contaminated skin promptly rather than at the end of the shift, and launder workwear separately from domestic laundry.
• Pyrophoric iron sulphide. Tanks, strainers and filters that have held high-sulphur residual fuel accumulate iron sulphide scale. When a tank is opened and the scale dries in air it can self-heat and ignite without any external source. Keep scale wet throughout tank entry and cleaning, and treat sludge removal as hot-work-adjacent.
• Confined space entry. Rusting steel and hydrocarbon vapour together deplete oxygen in a closed fuel tank. Entry requires a permit, forced ventilation, continuous monitoring for oxygen, flammable gas and H2S, a standby person at the manhole and a rescue plan agreed before anyone goes in.

Stability and compatibility are different questions

Heavy fuel oil holds its asphaltenes suspended in a lighter maltene phase, kept peptised by the resins the fuel happens to contain. Stability is a single fuel's ability to keep that suspension intact through storage and heating. Compatibility is whether two fuels can be mixed without the asphaltenes of one flocculating in the diluted, less aromatic medium of the other. The usual trigger is mixing an aromatic, asphaltene-rich residue with a paraffinic stream — which is exactly what happens when a fuel cut back with a paraffinic cutter meets one cut back with an aromatic one. Both certificates can be entirely clean and the mixture can still drop out.

The blend ratio is not intuitive

Buyers often assume that adding a small quantity of a different fuel to a large tank is the cautious move. It is frequently the opposite. Incompatibility tends to peak at intermediate ratios, commonly somewhere between about 10 % and 30 % of one fuel in the other, rather than at 50/50. Topping up a nearly full tank with a modest parcel of unknown origin places you squarely inside that band. The safe practice is segregation: keep parcels of different origin in separate tanks and burn them down one at a time.

How to test before you commingle

• ASTM D4740 spot test. A drop of the fuel, or of a laboratory blend, is spotted onto filter paper, oven-conditioned and rated 1 to 5 against reference spots. Ratings 1 and 2 are acceptable, 3 is a warning, 4 and 5 mean do not mix. It is quick, cheap, and can be run on board or in a terminal laboratory rather than sent away.
• Test a blend series, not a single ratio. Run the two neat fuels and blends at several ratios — typically 90/10, 50/50 and 10/90 — because a clean 50/50 result does not clear a 20/80 mixture, and 20/80 is the mixture you are actually about to create.
• ISO 10307-2, total sediment aged. This is the sediment line ISO 8217 actually limits, at 0.10 % m/m, and it is the one that predicts what will drop out after the fuel has spent time in a heated tank. Its companion method ISO 10307-1 measures total sediment existent, the solids present at the moment of test; ISO 8217 sets no limit on existent sediment for residual grades, so treat that result as a diagnostic on the current condition of the parcel and not as a pass or fail against the standard.
• ASTM D7112 determines stability and compatibility with a heavy fuel oil stability analyser using optical detection, and ASTM D7061 gives a separability number by optical scanning. Either one puts a number on a borderline spot-test result instead of leaving it to interpretation.

Tank cleanliness before loading

Ask for the receiving tank to be stripped, drained and inspected before a parcel goes in, and write that requirement into the contract rather than assuming it. Three specific concerns:

• Water at the bottom. It brings sodium, it degrades separation, it is a boil-over risk and it is cargo mass you did not buy. Check with water-finding paste and record the sounding before loading starts.
• Previous cargo residue. A heel of a chemically different residue — a paraffinic stream, a bio-blend component, a vegetable or animal fat — is enough to destabilise an incoming fuel that would otherwise have been fine. Ask what the tank last held and get the answer in writing.
• Settled catalyst fines and sludge. Cat fines are dense and accumulate at the bottom over successive cargoes. A tank that has never been properly cleaned hands the concentrated layer to whoever strips it last, and that layer can assay at many times the certificate value.

The commercial protection is simple and inexpensive: sample the tank heel before loading, retain the sample sealed, and make loading conditional on a compatibility check wherever the fuel is going onto an existing heel rather than into an empty tank.