ENGINE FAILURES THROUGH FUEL CONTAMINATION

An MAIB investigation into the grounding of a fishing vessel found that fuel contamination by water was the most likely cause. The main and auxiliary engines had stopped twice previously, the first time having been a couple of days before.   Fuel filters were changed and bled, but no trace of water was noticed in the fuel.

Finally the main and auxiliary engines stopped again when the vessel was about one mile from land.  Attempts to restart the main engine were unsuccessful and the vessel grounded on rocks.

Fuel Systems

Fishing vessels are often fitted with storage tanks from which fuel is transferred (or purified) into a service tank, situated above the engine level.  The transfer may be carried out manually, but some systems do it automatically.

Fuel systems should be regularly checked for water by using the self-closing drain valve fitted at the bottom of the service tank.  The neglect of such regular checks can be a problem with automatic transfer systems.

Larger vessels are fitted with settling tanks, allowing water to settle and be drained out of the fuel before transfer (or purification) to the service tank takes place.  If significant amounts of water are shown draining from the tank, another tank should be used.  It is very important to find the source of the water contamination, which may be entering either the storage or service tank.

Possible sources of fuel contamination by water

Water contamination in a fuel system is most likely from the air vent pipe fitted on exposed decks.  Vent pipes must be fitted with an efficient means of closing them weathertight, but at the same time, they need to release any pressure build up inside the tank.  One device for this is a ball non-return valve.

It is very important that vents pipes work correctly.  The closing mechanism and exposed pipe should be checked regularly and any defects should be repaired (temporarily, if necessary) as soon as possible.

Fuel filling lines may also be a source of water contamination.  Caps or flanges on these pipes must always be fully secured after fuel is taken on board.

Occasionally, contaminated fuel is supplied from ashore.  It is rare, however, for diesel fuel to be supplied containing significant amounts of water.

On smaller vessels, fresh water tanks sometimes adjoin fuel tanks.  This arrangement may be the cause of fresh water contamination of the fuel (and vice-versa!)

Purifiers and Coalescer Filters

Fuel purifiers and coalescer filters assist the removal (and examination) of water from fuel systems.  However, one should not solely rely upon these. The service tank self-closing drain valve should still be checked regularly for water contamination.