[FAQ] Guidelines for Monitoring Fuel Characteristics & Properties


In a recent joint industry guidance issued by shippers, refiners and suppliers alongwith key major industry forums such as the CIMAC, a detailed report on how and what to do regarding fuel quality has been portrayed. The best way to do this is starting by monitoring fuel characteristics and properties.

Keeping this in mind, we have issued this excerpt from that guidance in order to provide a comprehensive idea about the issue.

Variability of max. 0.50%-sulphur fuels

With the current maximum sulphur limit of 3.50%, all ship systems that could use residual
grade products up to the viscosity norm of 380 cSt at 50˚C (V50) and, in some cases, up to 700 cSt, will generally have been doing so. However, it is fully expected that fuel oils as supplied, meeting the 0.50% sulphur limit, will range from light distillates (DM—distillate marine) through to heavy residual fuel oil (RM—residual marine) with a range of widely differing fuel oil formulations in between.

  • Ships that usually operate outside an ECA will encounter greater variations in the composition and formulations of the fuel oils being supplied than they have previously been familiar with.
  • The increasing demand for very-low-sulphur fuel oil (VLSFO) is leading to an increase in the blending of lighter blend stocks to bring the sulphur content to 0.50%, and many of these are expected to be distillates.
  • Ships will need to give more focus to a proactive management approach to addressing these variations and the accompanying uncertainties relating to handling
    and performance.
  • This can be achieved by making sure that the ship’s crew know the fuel
    characteristics as loaded, and that they are able to respond to the requirements for storage, handling and use of these fuels on board.

Fuel specifications interpretation and application

  • ISO 8217:2017 specifies the requirements for fuels for use in marine diesel engines and boilers prior to conventional on-board treatment, and covers all sulphur content levels, including 0.10%, 0.50% and >0.50%-sulphur fuels.
  • This standard covers the technical boundaries defined by ship machinery installations and is used in most bunker purchase contracts. It is regularly reviewed (typically every three to five years) and updated to reflect the most recent market developments in fuels and machinery design.
  • Previous editions are still being selected as the basis for contractual specifications for purchase agreements between fuel suppliers and the shipowner/operator; however, it is recommended that fuel buyers purchase and use the latest edition of the ISO 8217 Standard (ISO 8217:2017).
  • The latest edition of ISO 8217 was published in March 2017, following which the IMO requested the ISO to consider 0.50%-sulphur fuels in the framework of the Standard.
  • Taking into account that a revision of ISO 8217:2017 was not possible in the available time frame, the development of a Publicly Available Specification (PAS), ISO PAS 23263, Considerations for fuel suppliers and users regarding marine fuel quality in view of the implementation of maximum 0.50% S in 2020, was initiated.
  • The PAS will address the anticipated fuel characteristics and properties of the marine
    fuels that will be placed on the market to meet the 2020 sulphur requirements and will be used in conjunction with ISO 8217:2017
  • Some distillate fuel formulations may have characteristics exceeding the maximum limits ofDM/DF grades and, as such, can be considered within the context of ISO 8217:2017, Table 2.
  • These fuel formulations typically have a higher viscosity and a more waxy nature, which can lead to inferior cold flow properties. When ordering a fuel, the ship operator needs to take into account the ship’s specific technical capabilities and operational pattern, including ambient conditions, in order to select the most appropriate grade of fuel.
  • This should take into account any specific limitation relating to one or more of the fuel characteristics listed below, such as maximum and minimum viscosity and cold flow properties.

Key fuel properties

Cold flow properties

The same principles that apply to present-day fuels with respect to cold flow properties will apply to max. 0.50%-sulphur marine fuels.

1. Fuel purchasers need an awareness of:

  • any limitations that the ship may have in the area of cold flow management on board such as limited fuel heating capability;
  • the intended voyage, and likely ambient temperatures to be encountered while that fuel is on board;
  • the cold flow properties of the fuel being purchased.

2. Significant operational problems can arise if supplied fuels are inappropriate for the ambient conditions, especially if the ship does not have adequate fuel heating capability; where relevant, cold flow properties should be specified in the purchase contract.

3. Wax crystal formation starts at the cloud point (CP) temperature of the fuel and it is recommended that the supplier provides information on cloud point and cold filter plugging point (CFPP) temperatures so that the ship’s crew can ensure that the bunker fuel temperature does not fall below these values.

4. Reference should be made to the CIMAC document on the cold flow properties of marine fuel oils.

5. The pour point is an important parameter used to ensure that fuels remain pumpable at low temperatures and to guide fuel storage temperatures. If fuels are held at temperatures close to, or below, the fuel pour point, the fuel may be difficult to pump, and separated wax may block filters and create deposits on heat exchangers.

6. In severe cases, wax will build up in storage tank bottoms and on heating coils, which may restrict the coils from heating the fuels. In these extremes, it may not be possible to dissolve the wax simply by use of heating coils; manual cleaning of tanks or provision of additional temporary steam heating may be the only solutions.

7. When managing the anticipated variability of max. 0.50%-sulphur fuels, both distillate and residual, attention needs to be given to the cold flow properties to ensure that the fuel onboard is maintained at a temperature high enough to avoid the problems described above; fuel should, therefore, be stored at a temperature which is at least 10°C above the pour point; the same recommendation applies to max. 0.50%-sulphur fuels.

8. In order to reduce the risk of plugging and ensure a good circulation, fuel filters should be provided with a trace heating system having sufficient capacity to maintain the temperature inside the filter at a value which is higher than the CFPP of the fuels intended to be used.

9. Compared with residual fuels, distillate fuels have, in the past, generally not required heating. However, the increased use of distillate blending components with higher cloud points for the blending of low-sulphur fuels may necessitate some level of heating for fuel storage, transfer and injection.

10. It is always recommended that distillate fuels are kept at a temperature which is at least 10°C above the pour point in storage, and at least 1°C above the CFPP temperature throughout the processing stages in filters and separators.


The stability of a fuel is defined in terms of its potential to change condition during storage and use. In terms of specification requirements, stability is normally assessed by measuring the total sediment, representing the sum of the insoluble organic and inorganic material separated from the bulk of a fuel sample by filtration through a standard filter under specified conditions. Stability relates primarily to the potential for asphaltenes to precipitate and lead to the formation of sludge.

The filtration tests do not differentiate between the inorganic sediment present in the
fuel and the organic sludge components.

  • While it is known that high levels of sludge can cause filtration and separator problems in the shipboard fuel systems, fuels meeting the sediment requirements specified in ISO 8217:2017 would be expected to be stable and not cause operational problems.
  • However, for max. 0.50%-sulphur blended fuels, the characteristics of the blending component feedstocks, method of production and type of cutter stocks used may be different from those in use today and, consequently, the current stability test methodologies for assessing fuel stability are being reassessed.
  • It is anticipated that PAS 23263 and the CIMAC (International Council on
    Combustion Engines) guidelines will include further advice on methods for stability assessment and recommendations regarding their use. Suppliers will need to continue to supply fuels that are stable and homogeneous at the point of delivery, meeting the testing requirements as specified in ISO 8217:2017, and may take into account additional advice to be given in the forthcoming PAS 23263.
  • Additional concerns have been raised about the potential for tank stratification during storage due to the separation of heavier components in the fuel. Any separation may be more of an issue for max. 0.50%-sulphur blended fuels due to the wider variation in the density and viscosity anticipated between different fuel batches.
  • The extent of any separation will be dependent on the fuel characteristics, storage conditions and storage time. While problems would not be expected to arise for a well-blended, homogeneous fuel oil during normal operation and handling, operational experience yet to be gained through the use of max. 0.50%-sulphur fuels will determine whether stratification is more of an issue with these fuels.
  • In view of this, fuels should preferably be used on a ‘first in/first out’ basis. If stratification is suspected following testing of tank samples, recirculation of tank contents may be used (if possible) to homogenize the tank, while the ship’s crew will need to be aware of the potential resulting variability of the fuel characteristics across the tank.
  • In these circumstances, particular attention will need to be given to the viscosity and density of the fuel. Early indications of stratification may be picked up by the
    response of the viscosity controller to a change in fuel injection temperature.
  • At this point, variability in the sulphur content of the fuel should also be considered.
    With respect to the total sediment determined in the standard filtration specification testing, for present-day fuels greater emphasis is generally given to organic sludge existing in fuels rather than to any inorganic material that might be present.
  • The assumption has been that the levels of inorganic sediment (e.g. extraneous sand, rust scale, catalyst fines, etc.) would be low, and that such material would be removed during normal handling operations (e.g. via centrifugal separators and filtration).
  • While max. 0.50%-sulphur fuels may be produced using different blending component stocks from those used for present-day fuels, no increase in levels of inorganic sediment in the finished fuels is anticipated.


Due to the changes in the way that max. 0.50%-sulphur fuels will be manufactured, it is expected that there may be a wider variation in the viscosity (and density) of fuels received on board ships.  As is the case today, it is recognized that fuels bunkered at different geographical locations, or even those obtained from the same supplier at a given location, could have variable characteristics.

  • The ship’s crew will need to be more aware of the characteristics of the fuel being delivered, so that the correct procedural requirements can be identified and implemented with respect to storage, handling and operation.
  • The ability of the centrifugal separators to remove water and solids from the fuel is dependent on the fuel’s viscosity; the lower the viscosity, the higher the separation efficiency.
  • The recommended separation temperature for fuel oils with a viscosity above 180 cSt measured at 50°C is 98°C. For lighter grades, the equipment manufacturer’s guidance should be followed, where lower temperatures are usually recommended.
  • Fuel in storage tanks will also need to be heated to facilitate pumping; it is recommended that fuels are stored at a temperature which is at least 10˚C above the pour point, and typically around 40˚C
  • Apart from the low-viscosity grades, fuels need to be heated prior to injection into engines (or burners) for combustion, to ensure that the viscosity is within the limits prescribed by the equipment manufacturer, typically in the range of 10 to 20 cSt, to obtain optimal spray patterns.
  • Viscosity exceeding the manufacturer’s specifications at the injectors may lead to poor
    combustion, deposit formation and energy loss; unburnt fuel may impinge on cylinder liner walls, and overpressurizing and overloading of the fuel injection pumps/piping and camshaft may occur.
  • If the viscosity is too low, this may lead to inadequate dynamic lubrication of fuel injection
    equipment and poor distribution of the spray pattern in the combustion space.

Acid number

Acid number is an indication of the presence of acidic compounds in the fuel. Most often, these acidic compounds are weak acids, such as naphthenic acids, that are naturally occurring in the crude feedstocks and which may also originate, to a lesser extent, from fuel degradation during storage. The acid number is primarily linked to the crude source of the derived products.

There is currently no evidence to suggest that the acidity of max. 0.50%-sulphur fuels will be significantly different from today’s fuels, or that it will present increased operational risk. Although it is rare, low levels of strong (inorganic) acids have been found in fuels, sometimes as a result of carryover from refinery processing.

  • Fuels with a high acid number have been known to cause corrosion of metal surfaces, especially in some types of fuel injection equipment; hence, limits for acid number are specified in ISO 8217:2017 for both distillate and residual fuels.
  • ISO 8217:2017 also states that the fuel shall be free from inorganic acids; a fuel in which an inorganic acid species is present, even at very low levels, is likely to be corrosive.
  • This aligns with MARPOL Annex VI, Regulation 18.3 which also states that the fuel shall be free from inorganic acids.
  • ISO 8217:2017 includes an informative annex on acidity which highlights that, while high acid numbers may be indicative of significant amounts of acid compounds and possibly other contaminants, some of which may be corrosive, fuels manufactured from naphthenic crudes can also have acid numbers exceeding the maximum specified but are still acceptable for use.
  • However, acid numbers below the specified limits do not guarantee that the fuel is free from problems associated with the presence of acidic compounds.


Apart from DMX grade fuel used for emergency purposes outside the machinery spaces, the minimum flashpoint defined in ISO 8217:2017 for distillate and residual grades is 60°C, reflecting the SOLAS (Safety of Life at Sea) requirement that all fuels used within machinery spaces onboard the ship must have a minimum flashpoint of 60°C as determined by a closed-cup test method

While there has been speculation that some max. 0.50%-sulphur fuels may exhibit flashpoints below 60˚C because of changes in the way fuels will be manufactured to meet demand (e.g. due to the use of a lower flashpoint blend component), the existing regulatory minimum flashpoint requirement of 60˚C will remain in place for 2020 and beyond.

  • The fuel supplier remains responsible for ensuring that the fuels delivered meet the minimum flashpoint of 60˚C to be compliant with ISO 8217:2017 and the SOLAS legislation.
  • The flashpoint of a fuel oil has no relation to its performance in an engine nor to its auto-ignition qualities.
  • It does provide a useful check on suspected contaminants such as gasoline, since as
    little as 0.5% of gasoline present can markedly lower the flashpoint of the fuel.
  • Ship classification societies also give instructions on the permissible temperatures at which fuels can be stored.
  • Flashpoint is considered to be an indicator of the fire hazard associated with the storage of marine residual fuels.
  • However, it is not a definitive guide to safety, because even if fuels are stored at temperatures below the determined flashpoint, flammable vapours may still develop in
    the tank headspace, sometimes over a period of days before equilibrium is reached.
  • The appropriate safety precautions, in line with legislation and local regulations relating to fuel storage and distribution, are necessary at all times.

Ignition quality

Ignition and combustion performance are important aspects of engine operation. Although both are dependent on the fuel characteristics, there is a wide range of other influencing factors including engine design, condition and settings, applied load, ambient conditions and fuel pretreatment. Determining the ignition and combustion characteristics of a residual fuel oil in a simple and reliable manner has proven difficult.

The Calculated Carbon Aromaticity Index (CCAI) was developed as an indicator of the ignition performance of residual fuels in diesel engine applications and is calculated from the measured density and viscosity values. CCAI values typically range from 820 to 870; the higher the CCAI value, the worse the ignition quality.

Limits for CCAI values were first included in ISO 8217:2010 as a guide to avoiding the use of fuels with uncharacteristic density-viscosity relationships (e.g. high density and low viscosity) which tend to exhibit poor ignition quality.

  • It is anticipated that max. 0.50%-sulphur marine fuels could exhibit a wider range of density and viscosity than currently found in the market, which means that there will be greater variation in observed CCAI values.
  • The CCAI will continue to be of value in identifying and precluding the use
    of fuels with unusual viscosity/density relationships.
  • While the CCAI provides a readily available indication of the possible ignition performance of a fuel, the chemistry and characteristics of residual fuels have changed since its development in the 1980s; this means that some fuels available today, which have similar densities and viscosities and similar CCAI values, can have significantly different ignition characteristics.
  • Some fuels with an acceptable CCAI value may exhibit poor ignition characteristics in some engines. Also, some fuels may exhibit poor ignition properties but acceptable combustion properties, and vice versa.
  • To address both the ignition and combustion characteristics of a residual fuel, a standard test method, IP 541, was developed in which fuel is sprayed into a pressurized constant volume combustion chamber at elevated temperature and pressure. The technique is useful for fuel characterization, especially when in-service combustion problems have been experienced.
  • Some engine manufacturers specify CCAI and IP 541 limits for their engines, depending on engine type and application; details may be found in the CIMAC report, Fuel Quality Guide—Ignition and Combustion.

Catalyst fines

Catalyst fines originate from fuel blending components derived from the refinery fluid catalyst cracking unit. There has been speculation that such components will find increased use in the blending of max. 0.50%-sulphur marine fuels; however, the requirement to meet the existing ISO 8217:2017 aluminium and silicon (Al+Si) content limit of 60 mg/kg maximum will remain in force and suppliers will need to continue to supply compliant fuels.

While there is currently no evidence to indicate that max. 0.50%-sulphur marketed fuels will see significantly increased levels of Al+Si content, any changing trends will become apparent as experience is gained in the use of these fuels.

  • Excessive levels of catalyst fines can cause accelerated abrasive wear of fuel pumps, injectors, piston rings and cylinder liners.
  • It is essential that residual fuel is pre-treated through a combination of settling and centrifuging prior to combustion to reduce the level of catalyst fines to a tolerable level, thereby avoiding potentially excessive damage. In this respect, information on
    the key fuel properties will assist the receiving ship in the handling and
    treatment of the fuel on board and should be available from the supplier as appropriate.
  • For max. 0.50%-sulphur fuels exhibiting lower density and/or viscosity, separation of extraneous materials from fuels during settling and centrifuging will be correspondingly enhanced.
  • If it is suspected that an engine is operating on fuel with an elevated level of catalyst fines, it is recommended that operation of the centrifugal separators is fully optimized and monitored closely.
  • Samples taken before and after the fuel is passed through the separators should be
    tested to determine the removal efficiency of the separators.
  • Monitoring the frequency of backflushing and pressure drop through the automatic self-cleaning filter (main filter) can indicate a change of fuel quality or cleaning efficiency.
  • It is also advisable to carry out more frequent inspections of susceptible engine parts to provide an early warning of any accelerated wear.
  • Advice from the manufacturer of the onboard separator machinery should be sought for remedial measures to be adopted, such as separation on low throughput, parallel passes through separator arrangements, etc.

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Source: CIMAC


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