The “Cappuccino Effect”

Recently the west of England P&I released a Loss Prevention Bulletin on the “Cappuccino Effect” due to a recent bunker dispute involving a vessel and a bunker supplier. The vessel, a bulk carrier, arrived in Singapore and began to bunker 900 tonnes of high sulphur fuel oil. The crew was alerted to the fact that something was wrong by the rattling of the float valves situated inside the fuel tank vent head bonnets. It was found that air was escaping from the vents at a greater rate than would normally be expected. In addition, the bunker supply hose lying on deck was seen to be jerking violently.

When tank soundings were subsequently taken, the sounding tape and brass bob were found to be covered in bubbles of fuel oil. All of the foregoing symptoms indicated that air had been introduced into the fuel oil resulting in the development of froth and foam on the surface – the so called “cappuccino effect”. This malpractice increases the volume of the fuel and results in artificially high volumetric flow meter readings.

When postbunkering soundings are taken it also gives the impression that the amount of fuel in the bunker tank is more than it actually is. As the air bubbles gradually dissipate, the fuel oil soundings decrease. If a large quantity of bunkers has been stemmed and subjected to the “cappuccino effect”, the consequential shortfall may be significant.

The vessel in question stopped bunkering immediately and tank soundings were taken. Some time later the tanks were sounded again when it was found that the level had dropped by 15 cms. A visual inspection of the contents of the
tanks revealed clear evidence of froth and foam on the surface of the fuel.

Upon completion of bunkering the difference between the vessel’s figures and those of the bunker barge amounted to 46 tonnes. Although this resulted in a heated dispute, the Barge Master eventually accepted the vessel’s figures and signed the bunker receipt.

The presence of one or more of the following may indicate that fuel supplied to the vessel contains an excessive amount of air:

  • foam and/or frothing on the surface of the fuel oil on the barge prior to bunkering
  • check the bunker barge supply pump and supply pipework prior to pumping and look for any suspect connections
  • unusual gurgling noises from the supply line or at the manifold may also indicate the presence of air
  • variations in line pressure at the manifold may be an indication that air has been introduced into the line.

For more information on what causes the “Cappuccino Effect”, what precautionary measures to take and actions following signs of the “Cappuccino Effect” you can also refer to:


  1. The Cappuccino effect is usually understood to be a dispersion of bubbles in the fuel. This is a relatively homogeneous distribution and hoses should not be seen to jerk around. This is more characteristic of churn flow. That is to say, there is so much air in the fuel that there is both bubble flow and the flow of free air. Pockets of air that can flow in either direction but also where the flow rates of air and liquid (cappuccino effect liquid) can be at different rates.
    Of all the liquid air possibilities, the Cappuccino effect is primarily only bubble flow.

    In a 380cst fuel it may amount to between 2-5% typically or upto around 10%.
    Usually, the air has been introduced into the fuel within the last 6-12hours and will dissipate in 2 days.
    That a brief pause allowed such a significant drop in level suggests that the air had only just been introduced because such a rapid drop really indicates that it is the fee air that is escaping the fuel and not the bubbles.

    This at least, is my understanding.
    The key fact is that air is introduced. It is not a normal part of fuels. It can be prevented.

    You may find that the plots of density and viscosity measured during bunkering are most helpful as these will indicate both the true cappuccino effect and the transit of air pockets, each instantly identifiable. Examples scan be found here: and RMI has now produced a new discussion on how to use the digital viscometer to help detect and prove at dispute resolution, what has been happening.

    One of the other concerns is that cappuccino effect prevents many quality measurements either inline or off line. This makes it easy to use the cappuccino effect to mask off spec fuels and supply false samples and thus confusing any subsequent dispute or arbitration proceedings. It is therefore a concern that coriolis meters with entrained gas capability are suggested. IT will encourage toleration of the cappuccino effect and shift the focus of fraud to quality and complicate the dispute process. Far better to reject any such fuel the moment the effect is discovered.

    Some operators suggest that even without the free air pockets, the cappuccino effect should be evident when inspecting the drip sample.

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