Heavy fuel oil: residual marine fuel grades and handling

Heavy fuel oil is the residual fraction left after atmospheric and vacuum distillation have taken the lighter products out of crude oil. It is too viscous to pump at ambient temperature, carries the crude’s ash, metals and asphaltenes, and must be heated, settled, centrifuged and filtered before a marine diesel engine can burn it. It has powered ocean shipping for a century because its cost per unit of energy is lower than distillate .

The specification that governs it changed substantially in 2024, and the change reaches the purchase order. ISO 8217:2024, the seventh edition, replaced a single residual table with three, renamed every residual grade, deleted five of them, and gave the surviving grades a minimum viscosity for the first time. A charter or bunker specification still naming RMG 380 without a suffix now names a grade that does not exist. See bunkering operations for the delivery process the specification governs.

The 2024 grade tables

ISO 8217:2024 carries four grade tables. Table 1 covers distillate and bio-distillate, which is marine gas oil territory. The three residual tables are:

The -0,5 and -0,1 suffixes carry the sulphur content in the category name itself, an exception the drafters made because shipowners order fuel as “an RMG 380 point five”. The H suffix in Table 4 marks high sulphur , and Clause 10.5 states that Table 4 fuels are for use in conjunction with approved equivalent alternative abatement technologies such as exhaust gas cleaning systems .

RMA 10, RMB 30, RMD 80, RMG 500 and RMG 700 were removed outright. RMG 700 has no successor: the only remaining 700 mm2/s grade is RMK 700H in Table 4, so there is no VLSFO or bio-residual grade above 500 mm2/s at all.

Two placement rules matter. Clause 10.4 sends all residual fuels containing FAME to Table 3, whether they would otherwise be ULSFO, VLSFO or HSFO. And Clause 10.3 makes clear that ULSFO is not a separate product class in ISO 8217: it is the -0,1 variant of a Table 2 grade, or a Table 1 distillate. See ISO 8217:2024 and bunker quality .

The new minimum viscosities

Before 2024 a residual grade had a maximum viscosity and no floor, so a 380 grade delivered at 60 cSt was a commercial complaint rather than a specification breach. That changed:

CIMAC gives two reasons for the 120 mm2/s floor on the 380 grades. The first is commercial: to ensure the ship gets a sufficiently high viscosity fuel when ordering a 380 grade. The second is technical, and more interesting: statistical analysis of testing-agency data showed total sediment can increase as viscosity falls, and 120 mm2/s is the point above which the centrifuge must be set at its maximum permitted temperature, so sedimentation can be controlled by temperature. The 2 mm2/s floor on the 20 grades is described as the technical limit for fuel injection at engine inlet.

CCAI: a limit, in the body of the standard

The Calculated Carbon Aromaticity Index is a specification maximum in all three residual tables: 860 on the 20 mm2/s grades and 870 on every other residual grade. Accounts stating that the standard sets no CCAI limit and that 870 is only an engine maker’s figure have it exactly backwards.

The formula sits at Clause 6.2, not in an annex, and takes three inputs: density at 15 degrees C in kg/m3, kinematic viscosity in mm2/s, and the temperature in degrees C at which that viscosity was measured. The familiar shipboard short form, density minus 141 times the log log of viscosity plus 0.85, minus 81, is that formula with the temperature term collapsed, and it is valid only where the viscosity was measured at 50 degrees C.

What CCAI is for is narrower than its reputation. Annex C.2 states it is included in the tables to avoid residual fuels with uncharacteristic density-viscosity relationships, and CIMAC is blunter: it was incorporated as a means of identifying anomalous fuels rather than as an indicator of ignition performance, and it has never been intended as a go or no-go characteristic. Fuels of similar CCAI can behave very differently in the cylinder, which is why the standard points at IP 541, the constant volume combustion chamber method, for actual ignition assessment. A note adds that CCAI was developed for petroleum-derived fuels and its applicability to bio-residual fuels has not been established.

Sediment, stability and compatibility

Asphaltenes are held in suspension by the aromatic fraction of the fuel. Disturb that balance, by heat or by blending with a paraffinic fuel, and they drop out as sludge that blinds filters and overwhelms separators. The standard measures the tendency three ways , each defined by the ageing applied before filtration:

• TSE, existent total sediment: the sediment present at the time of test, by hot filtration under ISO 10307-1.
• TSP, potential total sediment: after ageing for 24 hours at 100 degrees C under ISO 10307-2 Procedure A. Annex H describes it as representative of the fuel’s resistance to thermal stress.
• TSA, accelerated total sediment: after dilution with a paraffinic solvent then one hour at 100 degrees C under Procedure B. Annex H describes it as representing the fuel’s tolerance for retaining asphaltenes when commingling with a paraffinic fuel under limited thermal stress.

TSP at 0.10 percent by mass is the conformance measure, and the reference method in a dispute across all three residual tables. For Tables 2 and 3 only TSP may be used to judge conformance, with TSA and TSE reported; for Table 4 either TSA or TSP may be used. A footnote adds that the limit applies to a 10 g test specimen, and that failure to complete filtration of 10 g within 25 minutes means the fuel does not meet the specification.

TSA was demoted because the relationship it relied on broke. For HSFO, TSA typically exceeds TSP, so a passing TSA implied a passing TSP. Below roughly 200 mm2/s that stopped holding. Clause 6.8.2 records the finding from data gathered since 2020: HSFO is more sensitive to chemical ageing, while VLSFO with a viscosity typically below 200 mm2/s is sensitive to both thermal and chemical ageing.

Annex H, stability of residual fuels , is new in 2024 and it is informative. It names supplementary compatibility methods including ASTM D4740, ASTM D7060, ASTM D7112 and ASTM D7157 alongside ISO/PAS 23263, and it states plainly that mixing fuels which are individually stable can produce a product which is not stable, in which case the fuels are described as incompatible. What it does not do is set a compatibility limit, endorse a spot-test grade threshold or require any compatibility test. That remains a commercial and operational judgement, and it is why commingling different stems in one tank is avoided.

Cat fines: the limit is at the manifold, not the engine

Catalyst fines are aluminium and silicon oxide particles carried over from the refinery’s fluid catalytic cracking unit. They are equal in hardness to corundum and quartz, and they destroy engines by three-body abrasion: trapped between piston ring and liner or between ring and ring groove, and embedded in the graphite lamellae of the liner surface, where they keep working long after the fuel has been changed.

ISO 8217:2024 limits them as aluminium plus silicon, and it is not one number:

Clause 6.15 states what that limit is for, and it is not the engine. The limits restrict catalyst fines to levels at which onboard fuel treatment plants, meaning settling tanks, centrifuges and filters, operated in accordance with good practice and the maker’s procedures, are expected to reduce them to an acceptable level at the engine inlet. It is a bunker-manifold limit predicated on onboard cleaning. The two documents the standard cites for that expectation are CIMAC Recommendation No. 25 and MAN Energy Solutions Service Letter SL2017-638. The reference test method is IP 501. See also microbial contamination of fuel oil , a separate degradation route the standard does not limit.

The engine-inlet figure is therefore an engine maker’s number, and the two majors state it differently. MAN SL2017-638 says a maximum of 15 ppm aluminium plus silicon is acceptable for a short period of time, but the normal level must be kept lower, and its worked examples set the normal target at roughly one fifth of the bunkered figure: 80 ppm bunkered should be cleaned to 15 ppm, 40 ppm to 7 or 8 ppm, 30 ppm to 6 ppm. WinGD’s fuel guideline sets a flat 15 mg/kg absolute maximum at the engine inlet, alongside sodium at 30 mg/kg, water at 0.20 percent by volume and potassium at 30 mg/kg, all tighter than the ISO manifold limits. See cat fines in marine fuel .

The treatment plant, and what each stage actually does

Fuel goes from bunker tank to settling tank , where water and the heaviest solids drop out over time; then through a centrifugal separator; then to the service tank; then through the fine filter and the viscosity controller to the engine, where specific fuel oil consumption is measured. Each stage does one job, and only one of them removes cat fines in quantity.

The separator is the stage that cannot be substituted. MAN SL2017-638 sets the operating conditions: 98 degrees C or higher for HFO, 40 to 50 degrees C for distillates, and a scale for ULSFO by viscosity at 50 degrees C running from 50 degrees C below 20 cSt up to 98 degrees C above 80 cSt. It specifies a PID rather than a proportional-only controller on the preheater, because a P-only controller causes excessive temperature variation. Sizing is stated as approximately 0.23 litres per kWh in relation to the certified flow rate, with the certified flow rate given according to CEN CWA 15375: certified flow rate rather than nameplate capacity, because nameplate throughput assumes a cleanliness the fuel does not have.

Parallel comes before series. MAN recommends operating two separators in parallel where the bunkered cat fines content exceeds 25 ppm aluminium plus silicon, to reduce the flow through each and raise cleaning efficiency, and turns to series operation only if flow reduction is not possible. The frequent claim that modern practice is two purifiers in series inverts the maker’s order of preference. See marine fuel and lube oil purifiers .

The filter is an indicator, not a barrier. MAN specifies a 10 micrometre absolute full-flow automatic back-flushing filter before the engine as standard, then states that such a filter is by no means sufficient to remove all the cat fines but will act as an indicator of insufficient separator operation. A filter back-flushing more often than it used to is reporting that the separators are losing.

At the engine, WinGD recommends 13 to 17 mm2/s at the inlet, with 10 to 20 mm2/s mandatory for heated fuel and 3 to 17 mm2/s for non-heated fuel; MAN sets a floor, that viscosity at engine inlet should always be kept above 2 cSt. Those are maker requirements. ISO 8217 specifies viscosity at 50 degrees C at the manifold and says nothing about injection temperature. See marine fuel oil systems and marine engine fuel injection systems .

Tank practice matters as much as the machinery. MAN’s list is sloped tank bottoms, draining settling and service tanks at regular intervals, and leading the service tank overflow pipe all the way to the bottom of the tank with a return to the top of the settling tank, or a separate recirculating line and pump. In heavy weather it recommends running the standby separator simultaneously with reduced flow, and taking suction from a high suction line: rolling stirs settled cat fines back into suspension, so the period when the plant is least able to cope is exactly when the fuel reaching it is dirtiest. Sampling should run a full set (bunker, before separator, after separator, engine inlet) every four months, with a before-and-after separator efficiency check whenever the bunkered figure exceeds 25 ppm.

Ash, metals and cold flow

Vanadium is limited to 150 mg/kg on RMA 20 and RME 180H, 350 mg/kg on the 180 and 380 grades and 450 mg/kg on the 500 and 700 grades. Sodium is 50 mg/kg on RMA 20 and RME 180H and 100 mg/kg on the rest. Both are measured by IP 501 or IP 470.

The ratio between them is what decides the damage. Annex G records that a sodium to vanadium ratio of 1 to 3 is generally claimed to yield the lowest ash-melting temperature, and that this assumes increasing importance as vanadium rises above about 150 mg/kg. Once a deposit reaches its sticking temperature it attacks piston crowns , exhaust valves and turbocharger blade surfaces, and in boilers the waterwall, superheater and reheater tubes, by hot corrosion.

On cold flow, Annex F is unusually direct. The paraffinic and waxy nature of residual VLSFO precludes the use of cloud point and cold filter plugging point; cold flow behaviour is best characterised by pour point; and in storage and transfer these fuels should be heated and kept at a temperature at least 10 degrees C above the pour point. Pour point maxima are 6 degrees C on the 20 mm2/s grades and 30 degrees C on every other residual grade, so there is no single pour point figure for residual fuel. See cold flow properties of marine fuel .

Two further 2024 provisions bear directly on contamination. Clauses 5.3 and 6.17 require the fuel to be free of organic chlorides, defined as total organic halogen as chlorine not exceeding 50 mg/kg by EN 14077, which is new in this edition. Clause 6.16 requires the fuel to be free of unrefined used lubricating oil, judged by calcium above 30 mg/kg together with either zinc above 15 mg/kg or phosphorus above 15 mg/kg.

Calorific value is calculated rather than specified: there is no such row in any grade table. Annex J gives the net specific energy formula from density at 15 degrees C and the water, ash and sulphur mass percentages, which returns about 39.9 MJ/kg at a density of 991 kg/m3 and about 40.5 MJ/kg at 960 kg/m3. The carbon dioxide emission factor applied to residual fuel in the IMO framework is a separate figure; see carbon conversion factors for marine fuels and the well-to-wake HFO pathway .

The sulphur regime

MARPOL Annex VI regulation 14 sets the limits: 0.50 percent m/m globally since 1 January 2020, and 0.10 percent inside an emission control area. The current text is the 2021 revised Annex VI, adopted by IMO resolution MEPC.328(76) and applying from 1 November 2022. The carriage ban, effective 1 March 2020, prohibits carrying fuel oil above the applicable limit for use as fuel on a ship without an approved equivalent, which is what makes a non-compliant sample in a service tank a violation regardless of what the engine is burning at the time.

The SOx emission control areas are the Baltic Sea and North Sea areas, the North American area, the United States Caribbean Sea area, and the Mediterranean Sea area from 1 May 2025, with further areas adopted for the Canadian Arctic, the Norwegian Sea and the North-East Atlantic. China’s domestic emission control areas are a national measure and not a MARPOL ECA, a distinction worth preserving. See emission control areas , MARPOL Annex VI regulation 14 and the Baltic SECA and NECA .

Two mechanisms sit around the limit. A ship may exceed it only under regulation 4, the equivalents provision, under which an administration may allow an alternative arrangement at least as effective in emissions reduction terms; in practice that means an approved exhaust gas cleaning system. And where compliant fuel could not be obtained despite best efforts, the ship submits a fuel oil non-availability report under regulation 18.2, in the format at appendix 1 to IMO resolution MEPC.320(74). A FONAR is a report, not an exemption: it evidences the ship’s efforts for the flag and port State to weigh, and it does not authorise burning non-compliant fuel. The bunker delivery note that records what was supplied is governed by regulation 18 . See FONAR and BDN documentation and regulation 4 on equivalent arrangements .

The flash point comes from a different convention entirely. ISO 8217:2024 Clause 6.4 points at SOLAS, not MARPOL, and SOLAS regulation II-2/4.2.1.1 prohibits oil fuel with a flashpoint below 60 degrees C except as that paragraph permits. Every residual grade carries a 60 degrees C minimum, tested by ISO 2719 Procedure B, and the same 60 degrees C rule reaches the bunker delivery note . MARPOL Annex VI has no flash point rule. See flash point of marine fuel .

Where heavy fuel oil is prohibited outright

Two polar prohibitions sit in MARPOL Annex I, not Annex VI, which is a frequent misattribution. Regulation 43 prohibits the carriage and use of heavy grade oil in the Antarctic area. Regulation 43A, adopted by IMO resolution MEPC.329(76) and in force 1 November 2022, prohibits the use and carriage for use as fuel of heavy fuel oil in Arctic waters from 1 July 2024, with a deferral to 1 July 2029 for ships meeting Annex I regulation 12A on oil fuel tank protection or Polar Code part II-A paragraph 1.2.1. See the Antarctic special area and the Polar Code , black carbon in Arctic shipping and regulation 12A .

The market: HSFO collapsed, then came back

The most useful commercial fact about heavy fuel oil is that its demand did not disappear in 2020. It collapsed, and then recovered, and the recovery is a scrubber story.

Singapore’s bunker sales, the largest published series, show it plainly. High sulphur fuel oil fell 73 percent in a single year, from 38.10 million tonnes in 2019 to 10.65 million tonnes in 2020. It then rose in every one of the following five years:

HSFO in 2025 was more than double the 2020 trough, and over the first five months of 2026 its share reached the highest level since 2019. VLSFO peaked in its first year and has fallen every year since. Singapore’s total sales set a record of 56.77 million tonnes in 2025.

What made that possible is the installed scrubber fleet, and the alternatives competing with it are set out in alternative marine fuels . The German Environment Agency, citing IMO GISIS data downloaded in June 2025, records that at the beginning of 2025 well over 5,000 scrubbers were installed on oceangoing ships, around one third of the global merchant fleet by deadweight, split roughly 85 percent open loop, 15 percent hybrid and under 1 percent closed loop.

The variable that decides the investment is the scrubber spread, the VLSFO minus HSFO differential that Ship and Bunker publishes under that name. On 3 September 2026 the global average was 874.50 US dollars per tonne for VLSFO against 667.50 for IFO380, a spread of 207.00 dollars; the MGO to HSFO differential the same day was 839.00 dollars, which is the number that decides an ECA-heavy trade. See bunker price indices and benchmarks and scrubber sludge management .

Limitations

This article states ISO 8217:2024 as read from the standard, and the MARPOL and SOLAS positions from the instruments. Three classes of figure are deliberately excluded.

No scrubber payback period or capital cost is given. Payback is a function of the spread, the ship’s consumption and the installed cost, all of which move, and no figure was verified. The spread is given with its date and its published index instead, which is the input a reader can update.

No engine overhaul intervals, no wear limits and no port-by-port price figures are given. The first two are maker’s recommendations that vary by engine type and cylinder oil regime; the third moves daily.

Grade tables are summarised, not reproduced in full. The figures quoted are the ones a superintendent or bunker buyer uses, and the standard itself is the operative document: it carries footnotes, test methods and precision statements that decide marginal cases, and a dispute is settled on the standard rather than on a summary of it.

Two cautions on interpretation. The ISO limits are bunker-manifold limits, and Clause 6.15 says so for cat fines explicitly; the engine-inlet figures quoted here are maker’s requirements from MAN SL2017-638 and the WinGD fuel guideline, and they are tighter. And the sulphur limit that applies is MARPOL’s, not the standard’s: Tables 1, 3 and 4 set no sulphur limit at all and defer to the statutory requirement, while Table 2 reads 0.50 percent or the statutory requirement, whichever is lower.

Related Articles

• ISO 8217:2024
• Bunker quality and ISO 8217
• Marine gas oil
• Very low sulphur fuel oil
• Ultra low sulphur fuel oil
• Cat fines in marine fuel
• Marine fuel oil systems
• Marine fuel and lube oil purifiers
• Exhaust gas cleaning system
• MARPOL Annex VI regulation 14: the sulphur cap
• Emission control areas
• FONAR and BDN documentation
• Flash point of marine fuel
• Cold flow properties of marine fuel
• Bunkering operations
• Fuel switching operations