Fuel Oil: ISO 8217 Grades, Specification and Export Supply

When crude passes through an atmospheric column and then a vacuum column, what remains at the bottom is a fraction that will not boil at any economic temperature. That vacuum residue is the backbone of commercial fuel oil. Refineries with conversion units add other heavy streams to the pool: visbreaker bottoms from thermal cracking, and slurry or clarified oil from the fluid catalytic cracker. None of these is pumpable on its own at ambient temperature, so the blender adds cutter stock — gas oil, light cycle oil, kerosene or a heavy distillate — until the blend reaches the viscosity number the buyer ordered.

That is the entire product in one sentence: a residue, cut back to a target viscosity, sold against limits on the things that survive the process. It matters because the specification you are buying against does not describe a molecule. It describes a bulk property (viscosity, density) and a set of ceilings on contaminants that came in with the residue — sulphur, metals, ash, sediment and catalyst fines. Two cargoes can both be legitimate RMG 380 and still behave very differently in an engine.

The names you will see on an offer

• HFO, heavy fuel oil — the generic name for residual fuel burned in marine engines and industrial boilers.
• RFO, residual fuel oil — the same family described by its origin rather than its weight.
• IFO 180 and IFO 380 — intermediate fuel oil, older commercial shorthand in which the number is the maximum kinematic viscosity in mm²/s at 50 °C. In a modern contract these should be written as their ISO 8217 designations: RME 180, RMG 180 or RMG 380.
• CST 180, CST 380 — the same viscosity numbers expressed in centistokes. One mm²/s equals exactly one cSt, so 380 cSt and 380 mm²/s are the identical limit written two ways.
• Bunker C — North American usage for the heaviest marine residual fuel, broadly the territory of ASTM D396 Grade No. 6.
• Mazut, M-100 — CIS terminology, specified to GOST 10585 rather than ISO 8217. The critical difference is the reference temperature: GOST 10585 writes the M-100 kinematic viscosity limit as maximum 118 mm²/s at 80 °C, while ISO 8217 writes every residual limit at 50 °C. The "100" in M-100 is not a 50 °C viscosity and does not convert to one on the face of the document, so a mazut number and an ISO 8217 grade number are not comparable without a viscosity–temperature calculation on the actual batch.
• HSFO, VLSFO, ULSFO — high sulphur fuel oil (above 0.50 % m/m), very low sulphur fuel oil (max 0.50 % m/m) and ultra low sulphur fuel oil (max 0.10 % m/m). These are sulphur classes, not viscosity grades, and every one of them still needs an ISO 8217 grade written next to it.

Marine fuel and industrial fuel are specified by different standards

For fuel supplied to ships, the governing document is ISO 8217, Petroleum products — Fuels (class F) — Specifications of marine fuels. It splits the market into distillate marine grades (the DM table: DMX, DMA, DMZ and DMB) and residual marine grades (the RM table: RMA 10 to RMK 700). Everything on this page below concerns the RM table.

For land-based industrial burners in the Americas and in markets that follow American practice, the reference is ASTM D396, Standard Specification for Fuel Oils. Its heavy grades are defined by kinematic viscosity in the same way, but at different reference temperatures:

• Grade No. 4 (Light) — 1.9 to 5.5 mm²/s at 40 °C, flash point minimum 38 °C.
• Grade No. 4 — 5.5 to 24.0 mm²/s at 40 °C, flash point minimum 55 °C.
• Grade No. 5 (Light) — 5.0 to 8.9 mm²/s at 100 °C, flash point minimum 55 °C.
• Grade No. 5 (Heavy) — 9.0 to 14.9 mm²/s at 100 °C, flash point minimum 55 °C.
• Grade No. 6 — 15.0 to 50.0 mm²/s at 100 °C, flash point minimum 60 °C. This is the grade that needs preheating in every part of the system.

The commercial trap is obvious once stated: a viscosity figure means nothing without its reference temperature. Grade No. 6 at 50 mm²/s measured at 100 °C is a far heavier material than an RMB 30 at 30 mm²/s measured at 50 °C.

Where it is burned

At sea, residual fuel feeds slow-speed two-stroke main engines, medium-speed four-stroke auxiliaries and oil-fired boilers. On land it fires power station and industrial boilers, cement kilns, glass and ceramic furnaces, steel reheat furnaces, brick works, sugar and paper mills, and the burners on hot-mix asphalt plants where gas is not available. In every one of those applications the fuel arrives cold and viscous and must be heated, filtered and, in marine use, centrifuged before it reaches a burner or an injector. The handling system is as much a part of the purchase decision as the specification.

Kinematic viscosity at 50 °C — ISO 3104

This is the grade-defining property and the one that sizes the heating system. Viscosity is measured by timing the gravity flow of the fuel through a calibrated glass capillary in a controlled bath. The 50 °C reference temperature is a convention for residual marine fuel; distillates are measured at 40 °C and land-based heavy grades under ASTM D396 at 100 °C. If the reference temperature is not stated next to the number, the number is not a specification.

Practically, viscosity determines how hot the fuel must be to pump, to separate and to atomise. It does not determine ignition quality, combustion behaviour, or how much damage the fuel will do — those live in other lines.

Density at 15 °C — ISO 3675 or ISO 12185

Density does two jobs. Commercially, it converts the volume measured in the barge or shore tank into the mass you are invoiced for, so a disagreement over density or over the temperature at which volume was gauged is a disagreement over the invoice. Technically, the 991.0 kg/m³ ceiling on RMG grades exists because conventional gravity separators need a density difference between the fuel and the water to throw water and solids outwards. Above roughly 991 kg/m³ that margin collapses. Separators certified for 1010 kg/m³ are the reason the RMK grades exist at all, and buying RMK for a plant with conventional purifiers is a specification error, not a bargain. ISO 3675 is the hydrometer method; ISO 12185 uses an oscillating U-tube and is the routine laboratory method.

CCAI — a calculated ignition quality index

Residual fuel cannot be given a cetane number: the test engine used for cetane rating cannot burn it. In its place the industry uses the Calculated Carbon Aromaticity Index, which estimates ignition quality from two properties that are always measured anyway:

CCAI = D − 140.7 × log₁₀[log₁₀(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. An RMG 380 sitting at the ceiling on both properties — 991.0 kg/m³ and 380.0 mm²/s — returns a CCAI of roughly 852, comfortably inside the 870 limit for the grade. A lighter, more paraffinic blend at 920 kg/m³ and 180 mm²/s returns roughly 790.

The logic is that for a given viscosity, higher density means a more aromatic molecule, and aromatics resist ignition. A high CCAI therefore signals ignition delay: fuel accumulates in the cylinder before it lights, then burns too fast, producing a steep pressure rise, high thermal load, and in severe cases liner and piston damage. The ISO 8217 limits are 850 for RMA 10, 860 for RMB 30, RMD 80 and RME 180, and 870 for the RMG and RMK grades.

Two cautions. CCAI is calculated, so it can be satisfied on paper by a blend that behaves badly in practice; it is a screening index, not a combustion test. And it says nothing about what happens after ignition — carbon residue and cat fines govern that.

Sulphur — ISO 8754 or ISO 14596

Sulphur is now the line that decides whether a cargo is legal, not merely whether it is good. ISO 8217 names two methods: ISO 8754 determines sulphur by energy-dispersive X-ray fluorescence, ISO 14596 by wavelength-dispersive X-ray fluorescence. They are not interchangeable in a dispute, so name the one that governs in the contract rather than writing "ISO 8754 / ISO 14596" and leaving the choice open. Beyond legality, sulphur drives cold-end corrosion: sulphur trioxide in the flue gas condenses as sulphuric acid below the acid dew point, attacking economisers, air heaters and stacks in boilers, and cylinder liners in engines. In engines the countermeasure is cylinder oil base number — high-BN oils for high-sulphur fuel, low-BN for 0.50 % fuel — and switching fuel sulphur without switching cylinder oil is a recognised cause of damage in both directions.

Flash point — ISO 2719

Minimum 60.0 °C by Pensky-Martens closed cup for every RM grade. This is not a quality preference, it is a safety floor written into SOLAS for fuel used on board ships. A certificate showing a flash point below 60 °C is grounds for rejection rather than for negotiation.

Pour point — ISO 3016

ISO 8217 sets the upper pour point separately for winter and summer quality. Only the two lightest grades are held tight — RMA 10 at 6 °C summer and 0 °C winter, RMB 30 at 24 °C in both seasons. From RMD 80 upwards the limit is 30 °C in both, which means a fully compliant cargo can be a solid on a cool morning. Storage must be held above the pour point on the certificate, with margin, and the certificate value is the one that governs — not the grade limit. This has become sharper since 2020 because low-sulphur blends often use paraffinic cutter stock and can wax at temperatures that surprise operators used to aromatic HSFO.

Micro carbon residue — ISO 10370

A weighed sample is pyrolysed under inert gas and the carbonaceous residue is measured. It quantifies the fuel's tendency to form deposits: piston crown and ring groove deposits, fouled turbocharger nozzle rings, sooted exhaust gas boilers and rising exhaust temperatures. At 18.00 % m/m for RMG grades and 20.00 % m/m for RMK, the allowance is generous, and a plant designed for distillate will not tolerate a fuel anywhere near it.

Water and ash — ISO 3733 and ISO 6245

Water at maximum 0.50 % V/V is determined by distillation. You pay for water by mass, it loads the separators, and free water carrying sodium accelerates high-temperature corrosion downstream. Ash is the incombustible mineral fraction left after controlled burning; it is where the vanadium, sodium, aluminium and silicon end up, and it is what deposits on valve seats and turbine blading.

How the pool split

The 0.50 % cap removed the largest single outlet for high-sulphur residue. Two products emerged in its place. HSFO continued as a real market, but only for ships fitted with exhaust gas cleaning systems and for land-based users in jurisdictions that permit it. VLSFO, capped at 0.50 %, became the default marine fuel, and ULSFO at 0.10 % serves emission control areas alongside marine gas oil.

The critical point for a buyer is that VLSFO is a sulphur class, not a grade. Most VLSFO is still sold against ISO 8217 residual limits — usually RMG 380 or RME 180 — and the ISO 8217 grade must still be named in the contract. "VLSFO 0.5 %" on its own specifies the one property the regulator cares about and none of the properties the engine cares about.

Why low-sulphur blends behave differently

High-sulphur residual fuel was overwhelmingly aromatic and reasonably predictable. Reaching 0.50 % sulphur usually means blending low-sulphur residue with low-sulphur distillate or paraffinic cutter stock, and that changes several things at once:

• Cold flow. Paraffinic cutter waxes. Pour points can sit uncomfortably close to normal storage temperature, and wax appearance can start well above the pour point. Always work from the pour point on the batch certificate, not the 30 °C grade ceiling.
• Viscosity spread. VLSFO parcels vary widely in viscosity between suppliers and ports, which matters for a heating system tuned to one number.
• Stability. Adding a paraffinic stream to an asphaltenic residue is precisely the recipe for asphaltene precipitation, so stability reserve is thinner than it was.
• Cat fines. A lower sulphur specification does not imply lower catalyst fines. Treat Al + Si as an independent line and check it on every certificate.

Compatibility: two good fuels can make one bad tank

Asphaltenes are held in colloidal suspension by the resins and aromatics around them. Dilute that surrounding phase with a paraffinic fuel and the asphaltenes flocculate and drop out. The result is sludge in tank bottoms, blocked filters, separators running to their limit and, in documented cases, loss of propulsion. Both fuels can be individually stable and fully on specification. Instability is a property of the mixture.

The practical controls are unglamorous and effective:

• Segregate by stem. Do not commingle a new delivery with an existing parcel from a different supplier or port if a separate tank is available.
• Test before you transfer, not after. Where commingling is unavoidable, run a compatibility spot test to ASTM D4740 on a laboratory blend at the intended ratio. The method rates the spot from 1 to 5 against reference standards: 1 and 2 are acceptable, 3 is a warning, and 4 or 5 means the blend is incompatible.
• Test more than one ratio. The worst instability commonly appears at intermediate ratios rather than at the extremes, so a single 50:50 check is not a clearance.
• Use the aged sediment test as the acceptance line. ISO 10307-2 total sediment aged at maximum 0.10 % m/m is the number that belongs in the contract. Instrumental methods give an earlier and far more sensitive read where the stakes justify them: ASTM D7061 measures n-heptane induced phase separation as a separability number by optical scanning device, ASTM D7112 determines stability and compatibility on a heavy fuel oil stability analyser by optical detection, and ASTM D7157 returns the intrinsic stability S-value, which is a measure of how much stress a fuel will absorb before its asphaltenes come out.
• Empty a tank before refilling it where operations allow. Heel from a previous stem is the most common uncontrolled blend on any ship.

The ISO 8217 edition question in contracts

ISO 8217 has been revised repeatedly — 1996, 2005, 2010, 2012, 2017 and again in 2024 — and the editions do not carry identical tables. The 2010 edition rebuilt the residual table outright: it added RMA 10, deleted the RMF and RMH categories, added hydrogen sulphide, acid number and sodium as specified characteristics, replaced total sediment potential with total sediment aged, and dropped the numeric sulphur caps in favour of a reference to the purchaser's statutory obligation. The 2017 edition left the residual limits broadly intact while reworking the distillate table and adding the DF grades that may contain FAME. The 2024 edition is the disruptive one for residual fuel: it splits the residual specification across three tables — residual fuels at 0.50 % m/m sulphur and below, bio-residual blends containing FAME, and residual fuels above 0.50 % m/m — so the words "Table 2" do not point at the same set of limits in a 2024 contract that they did in a 2017 one. A contract that says only "ISO 8217" therefore leaves acceptance criteria undefined, and in a dispute the undefined term is decided by whoever has the better lawyers rather than the better fuel.

Write the clause properly: name the standard, the edition year, the table number that belongs to that edition, the grade, the maximum sulphur as a number, and the sampling and testing arrangement. For example, an RMG 380 stem might be specified as ISO 8217:2017 Table 2, grade RMG 380, sulphur maximum 0.50 % m/m by ISO 8754, samples drawn continuously throughout transfer to ISO 13739 with sealed retained samples held by both parties. If you cite the 2024 edition instead, check which of its three residual tables your grade and sulphur class actually sit in before you write a table number. Everything after that is arithmetic.

Hydrogen sulphide in the tank headspace

Residual fuel oil can evolve hydrogen sulphide. ISO 8217 caps the liquid phase at 2.00 mg/kg to IP 570, but the liquid limit is not a headspace guarantee — H₂S partitions into the vapour space of a closed, warm tank and accumulates there. The gas is heavier than air, so it collects in the bottom of tanks, cofferdams, pump rooms and sample stations. Its warning smell disappears at concentrations well below the lethal range because it paralyses the olfactory nerve, so the absence of odour proves nothing. Gas test before opening any ullage port or sample cock, wear a personal monitor when working near tank openings, and treat a sample drawn from a hot tank as a source of the gas in its own right.

Thermal burns and engine room fire

The product is handled between roughly 40 °C in storage and 150 °C at a heater outlet. Contact with hot fuel, steam-traced pipework, heater bodies or separator casings produces immediate deep burns, and hot fuel oil sticks to skin rather than running off. The more serious risk is fire: a pinhole leak in a high-pressure fuel line sprays atomised oil that ignites readily on a hot exhaust surface. This is why high-pressure fuel lines are jacketed with leakage detection and why hot surfaces above 220 °C must be insulated and shielded from oil ingress. Insulation soaked with leaked fuel oil is itself a fire load and must be replaced, not dried.

Flash point is a legal floor, not a preference

The minimum 60.0 °C closed cup flash point (ISO 2719) for marine fuel is a SOLAS requirement for fuel used on board, with a narrowly drawn allowance for emergency generator fuel down to 43 °C. Heating in storage tanks is restricted relative to the flash point under SOLAS chapter II-2, and what is permitted depends on the tank's venting, temperature monitoring and alarms — the ship's approved arrangement governs. A delivery whose certificate shows a flash point below 60 °C is a rejection, not a discount.

Confined space entry

A tank that has held residual fuel is oxygen-deficient, may hold hydrogen sulphide and hydrocarbon vapour, and typically carries pyrophoric iron sulphide scale on its internal surfaces. That scale can self-heat and ignite when it dries out in air, which is why tanks are kept wetted during cleaning. Entry requires a permit to work, isolation and blanking of all fuel and steam lines, forced ventilation, continuous atmospheric monitoring rather than a single pre-entry reading, a standby attendant and a rehearsed rescue plan.

Water bottoms and boilover

Free water settles under hot fuel. If that water is heated through 100 °C, or if hot fuel is dropped into a tank containing standing water, the flash to steam is violent and can eject burning oil from the tank. Drain tank bottoms before applying heat, drain them on a schedule during storage, and never assume a tank is dry because the last certificate said 0.30 % water.

Long-term health exposure

Residual fuel oils contain polycyclic aromatic hydrocarbons, and repeated or prolonged skin contact is a recognised health hazard rather than a nuisance. Use impermeable gloves and eye protection when sampling or draining, never use fuel oil or its sludge as a hand cleaner or parts washer, launder contaminated clothing separately from personal clothing, and handle oily rags and used absorbents as hazardous waste. The Safety Data Sheet should travel with the cargo and be available at the point of handling, not filed in an office.

Spill and transport classification

Residual fuel oil is persistent, emulsifies with seawater and is expensive to recover; carriage and discharge as an oil cargo fall under MARPOL Annex I. Packaged fuel oil in drums or IBCs is commonly consigned as UN 3082, environmentally hazardous substance liquid n.o.s., Class 9, where it meets the environmentally hazardous criteria — confirm the classification and the packing group with your shipping line before booking, because the answer varies with the specific product and the destination.