14 Critical Points to Know When You Co-mingle (blend) Two Fuels



When it comes to internal oil transfer or bunkering of fresh fuel, it is inadvertent that some mixing of two different fuels will happen.  If the vessel’s endurance is not adequate, at times ship’s staffs are forced to co-mingle two different fuels in order to maximize bunker in-take.

Thus, this technical write-up would help to understand critical points to consider before co-mingling two different fuels.

  • Stability is the propensity of the fuel to stay stable under long storage conditions.  Typically fuel oils can be stored for at least a year without significant changes.  
  • ISO 8217 specifically states that “The fuel shall be a homogeneous blend of hydrocarbons derived from petroleum refining.”  This means that if the fuel is not stable, it is not meeting the specification.
  • A simple ASTM D4740 test gives a good idea both for compatibility between two fuels to be blended and also the stability of an already blended fuel.  These tests can be carried out onboard.  You can also get a compatibility test kit to keep onboard.  This costs around €600.  Ship staff can use this to assess the compatibility.
  • Lack of stability occurs when there is not enough aromatics to support the asphaltenes in the fuel.  This throws the asphaltenes out of solution and the precipitation of this substance leads to filter and purifier choking with possible risk of machinery coming to a halt.
  • From the supplier’s side, the co-mingling or blending of bunker fuels is a must.  The problem occurs if he uses a low quality and low cost cutter stock.  Problems also occur when the supplier tries to do line blending when delivering the fuel to the ship.  This can lead to wrong sampling, stratification and other problems.
  • From the ship’s point of view, it is ideal if the storage tank is empty to receive fresh bunkers.  Often the necessity to blend with an existing fuel in the tank leads to problems particularly if the two fuels are not compatible.  
  • Blending onboard is not advisable.  If it is unavoidable, first check for compatibility.   Secondly, the blending has to be carried out in a certain order such as a lower density fuel should be pumped on top of a higher density fuel, circulate the fuel as much as possible, heat the fuel to achieve mingling through thermal convection currents.  Keep in mind that two perfectly stable fuels can become unstable when blended.
  • The stability of the fuel can be categorized into blending stability (which is already covered above) and secondly, storage stability.  When the fuel is heated for a long time, degradation of stability may take place with thickening and asphaltene precipitation.
  • Please keep in mind that waste lube oil, contaminants in the fuel, excessive quantities of low quality cutter stock such as shale oil etc.  will contribute to instability in the fuel.
  • Two other tests that can be employed to assess the potential of the fuel to become unstable are Xylene Equivalent and Reserve Stability Number.  It is possible to identify fuels which are fully stable or which are on the verge of becoming unstable.
  • Adding good fuel to an unstable fuel which has already precipitated asphaltene may not help.  It is possible that some additives may help.
  • When switching from HFO to distillate fuel while approaching port or similar conditions, there is a risk of incompatibility between the two fuels.  The asphaltenes could precipitate leading to filter clogging.
  • In addition to compatibility test which can be carried out onboard, a mini blending test can be carried out with half to one liter of fuel samples.  In an emergency, the best way is to take the settling tank level to the bottom and then put the second fuel into the settling tank.  A certain amount of incompatibility may occur but it will be restricted to the small quantity in the settling tank.
  • Please keep in mind that all stability/compatibility tests are carried out in a 50:50 ratio of the two fuels.  This is the worst condition.  If they are compatible at this ratio, they will be compatible at all ratios such as 80:20, 65:35 etc.

If you have any further queries or questions regarding  Compatibility/stability or co-mingling two different fuels, please comment below.


  1. Hello sir.a DG is running on HFO. When i have to stop for 2 hrs , or 6 hrs, can i mix diesel with HFO in engine and stop DG ? Does it bas any negetive effect of mixing diesel with HFO ? Or i have to fully flush the fuel system with diesel which is also a economical loss also as diesel prices r getting high day by day . Thanx in advance..

  2. Hello VISWA Group:

    What are the effects of comingled fuel on the fuel injection pumps and fuel injector of a marine diesel?
    What point of a change over operation would a problem arise if any (entering a SECA; leaving a SECA)?

  3. The issue of blending fuel onboard is becoming serious with the 2020 Sulphur content reduction of HFO taking off. Blending onboard is not advisable but unavoidable. What advice will you give ship personnel in the process of changing to new HFO and how effective has additives been in resolving the problem of incompatibility?

    • Thank you for reading and commenting.
      Keep an eye on the issues discussed in our articles.
      It will help you on the key issues.

      Thanks & Regards,
      Team Mfame

  4. Hi sir.
    I have a problem with the auto filter. it was always triggering DP high but when opened the fuel is like a foam or frothing i would say. Any thoughts? Thanks

  5. Hi, what will happen if we mix two lsmdo but different density (green and red color), could we expect some problem or for diesel is not so crititical as for fuel oil.

  6. can we do commingling test of LSMGO on board /
    mix 500 ml each of on board LSMGO and 500 ml of to be supplied LSMGO – will it require any heating.

    • Dear Mr. Alok Kumar,
      Thank you for your comment.
      Firstly a “clear & bright” LSMGO usually does not pose any compatibility issue. Thus co-mingling can be done if both the LSMGO are “CLEAR & BRIGHT” in visual appearance.
      If Any of the LSMGO – is not CLEAR & BRIGHT in visual appearance, then it could contain traces of residual fuel. In this case, a compatibility test has to be performed on board.
      We can very well perform a compatibility test on board for LSMGO, however, the samples need not be heated as we do it in case of HFO.

      We have been informed that Viswa Lab has designed “STATE-OF-ART” on board compatibility/Stability test kit which can be used to perform the test on board. We will contact Viswa Lab and provide more information about the same.

  7. Most of the old tonnage are built with only two storage tanks for Diesel oil and the capacity of these tanks is not good enough for vessels trading more in ECA areas. Hence one of the fuel oil storage tank is cleaned and used for storing LSMGO. Due to limited number of tanks, very often it is inevitable to co-mingle low sulphur gas oil. Understand Commingling of diesel oil will not cause any trouble as there are no asphaltenes present. Please confirm my understanding is correct?

    • Dear Mr. Prasad,

      Thank you for the comments.
      Mixing between two DMA grades or two distillate fuels which are “clear and bright” (Both the fuels being co-mingled should be clear and bright in visual appearance) should not pose any problem. If either of these are not clear and bright, it means the darker one can contain traces of residual fuel and there is risk of incompatibility with subsequent asphaltene precipitation.

      To answer your question, if you intend to co-mingle two different LS MGOs, please ensure that both the LS MGO’s are “CLEAR & BRIGHT” in Visual Appearance. If both the LS MGOs are Clear & Bright, then co-mingling them should not usually pose a problem.
      The co-mingling of two residual fuels and a distillate-residual fuel needs a compatibility test to be performed prior co-mingling. Hope we answered your question!.

Comments are closed.