Bacteria, fungi and yeasts
These are collectively known as ‘HUM’ (Hydrocarbon
Utilising Micro-organisms) and all can thrive in diesel
tanks under the right conditions. HUM are often
misnamed as ‘algae’ (algae need sunlight to live
and grow, which is not available in a tank). The
microbes live in the fuel/water interface and feed
on the carbon in diesel, producing sludge, acids and
other by-products. They can grow very fast in warm
conditions and single cells weighing 1/1,000,000 of a
gram can multiply to 10kg within 12 hours under ideal
conditions. They can also grow up to 10 times faster in
biodiesel than hydrocarbon-based diesel.
HUM forms long strings or mats of slime on the
walls of the tank and in the fuel. The smell of rotten
eggs (hydrogen sulphide) is confirmation of a badly
contaminated fuel tank. Low pH value of fuel (e.
g. pH4, acidic) is another indication of HUM as the
normal pH of diesel is 6 to 8.
Biocides kill HUM and prevent their growth, but dead
microbes can still clog filters. HUM can be treated or
prevented by using a diesel biocide.Grit, Sand & Dirt
Keeping fuel lines and filter elements and assemblies
clear is important because any particles can block
the nozzle of a fuel injector, causing rough running
and other problems. Always wipe away dirt before
servicing, and cover disconnected fuel hose ends or
opened filter housing when servicing.EIGHT TIPS FOR RELIABLE FUEL
- Buy clean fuel
Diesel in Australia is Ultra Low Sulphur Diesel (ULSD)
with just 10 parts per million, down from 500 ppm
permitted in 2002. This affects the amount of sulfuric
acid circulating inside the engine and emitted from
the exhaust.
It’s possible to get clean fuel from contaminated
tanks, but it’s a good idea to avoid buying diesel
within three hours of a fuel delivery. This allows any
water and dirt to re-settle to the bottom of the tank.
In many remote places, diesel is sold out of steel
barrels. These are often left exposed in all weathers and
are prone to contamination from rain water. Problems
can be avoided if the fuel is left standing overnight in
jerry cans before adding it to the vessel’s fuel tanks. - Keep the deck fill tightly closed
Also check the O-ring periodically. This screw-top fitting
is the first defence to keep water out of the fuel tank.
Ensuring the diesel deck fill O-ring is in place and in
good condition is cheap insurance. Always double check
that the deck fill is closed securely to prevent water
entering the fuel tank. Take extra care if the fuel fill is
located in the cockpit floor or the side deck is likely to
be washed with sea water or flooded by rainwater.
3. Add biocide to the tank
Prevention is much easier and cheaper than cure.
Best practice is to add a maintenance dose of biocide
each time the fuel tank is re-filled. Different brands of
biocide are formulated differently to kill a spectrum of
HUM, so occasionally switching brands makes sense.
If a treatment dose is required to kill an infestation,
the debris of dead HUM can still clog a filter and will
need to be cleaned from the tank, hoses and filters. In
this case, diapers work well. If an empty tank cannot
be accessed for cleaning, it should at least be flushed
with clean diesel.
4. Drain the tank
If possible, drain all water and dirt from the bottom of
the tank. This provides a sample of any contamination
Secondary fuel
filters are either
‘spin-on’ or have
a replaceable
filter element.“Bacteria,
fungi and
yeasts can
grow up to
10 times
faster in
biodiesel than
hydrocarbon-
based diesel.”
Air must be bled from the fuel system any
time work has been done on it, because the
engine will not run with air in the fuel line. You
can bleed the fuel line from the tank to the
secondary filter using the lift/transfer pump.52 facebook.com/BlueWatermagazine
A reliable diesel supply