Water in Diesel: Where It Comes From and How to Keep It Out of Your Tank

Most diesel problems on a worksite are not diesel problems. They are water problems. Water gets into a tank in ways nobody notices, it sits at the bottom where the suction draws from, and the first anyone hears of it is a machine that will not start on a cold morning or an injector pump that has quietly worn itself out.

This is a practical guide to where the water comes from, what it does, how to tell it is there, and what actually keeps it out.

How water gets into a diesel tank

There are three routes, and a tank in normal use has all three open.

Breathing. A tank pulls air in every time fuel is drawn out, and pushes it back out as the day warms. In a Queensland summer that air carries a lot of moisture, and overnight it condenses on the cooler inside walls and runs down into the fuel. A tank that is only ever half full does more of this, because there is more air above the fuel line to breathe.

Delivery. Water can arrive with the fuel. It should not, and from a reputable supplier with clean tankers it rarely does, but a dirty delivery hose or a tanker that has not been drained can put water in a tank in one fill.

Rain and wash-down. A fill point or vent that is not sealed, a hatch left open, or a pressure washer aimed at the wrong place. Simple, and more common than it should be.

What it does once it is in there

Water is heavier than diesel, so it sinks and collects at the bottom of the tank. From there:

• It gets drawn into the pump and through to the engine, where it corrodes injectors and fuel pumps that are built to very fine tolerances. On a modern common rail engine this is an expensive repair.
• It gives microbial growth somewhere to live. Bacteria and fungi grow at the interface between the water and the fuel, producing a dark sludge that blocks filters and, left long enough, eats into the tank itself.
• It accelerates rust inside a steel tank, which then sheds particles into the fuel.

None of this happens overnight. It happens over a season, which is why it catches people out.

How to tell you have a water problem

• Filters blocking faster than they used to. The most common early sign. A filter with a saturation gauge will show it directly.
• A dark, slimy residue in the filter bowl or on a dipstick. That is the microbial growth.
• Hard starting, rough running or loss of power across several machines that share the same tank. One machine misbehaving is the machine; all of them is the fuel.
• Cloudy diesel in a sample jar, or a visible layer at the bottom if you let it settle.

The cheapest diagnostic is a sample from the bottom of the tank, left to stand in a clear jar for a few minutes. Water separates and sits underneath. If you see a layer, you have your answer.

Keeping it out

Three things, in order of how much difference they make.

1. A desiccant breather on the tank vent

This is the one most tanks do not have, and it addresses the biggest source. A desiccant breather sits on the vent and dries the air before it enters the tank, and filters dust out of it at the same time. The Donaldson TRAP breathers fitted to Fuel It tanks do exactly this: they remove particulate and moisture from air entering the tank during dispensing. When the desiccant is saturated it changes colour, and the element is a simple swap.

2. Particulate and water filtration on the dispensing line

A filter between the tank and the nozzle catches what the breather did not, and catches anything that came in with a delivery. The filters on Fuel It dispensing kits are rated for both particulate and water removal, at 30 micron, with a pressure gauge that shows when the element is loading up. That gauge is worth watching: it is the earliest warning you will get.

For fleets where fuel quality matters more than usual, a two stage kit that cleans and dries the fuel in series before it reaches the nozzle is the next step up.

3. Keep the tank fuller and the fill point sealed

Less air above the fuel means less condensation. A tank on a regular delivery schedule that keeps it topped up breathes less than one run down to the bottom between fills. And check the fill cap, hatch seal and vent after every delivery and every wash-down.

If the water is already there

Draining the water layer off the bottom of the tank is the first step, and on a tank with a drain point it is straightforward. If microbial growth has set in, the tank usually needs to be cleaned out and the fuel either polished or replaced, because the sludge will keep reseeding the new fuel otherwise. That is a job worth getting done properly rather than repeatedly.

Talk to us about what your tank has in it and we will tell you honestly whether it is a drain, a filter change or a clean-out.

The short version

Put a desiccant breather on the vent, keep a water-separating filter on the line and watch its gauge, keep the tank topped up, and keep the lid on. Do those four things and water stops being the reason your machines will not start.

Talk to us

Every Fuel It self bunded tank package is specified with a breather and water-separating filtration as standard, because we would rather not have this conversation after the fact. Call 1300 368 016, email sales@fuelit.com.au, or request a quote. Filtration and breather parts are in the accessories range.