Causes of Diesel Contamination

Emulsified water, freestanding water, diesel particulate, microbial growths and diesel sludge can cause significant damage to emergency generators including failure. Fuel contamination (fouling) typically occurs from three sources, 1) the natural aging process and chemical nature of the fuel, 2) the storage tanks and fuel intake and return process and 3) via the supply chain.

Natural Contamination

There are three sources of freestanding and emulsified water and microbial growths in ultra low sulphur diesel and bio blend diesel all of which are naturally occurring.


Diesel fuel is highly hygroscopic, meaning it readily absorbs water from moisture in the air. Water contamination in diesel fuel is of particular concern as it can lead to the corrosion of steel components and the promotion of microbial growth.

Note: Increased use of additives and the introduction of biodiesel, for the management of mandated fuel sulphur reductions, have dramatically increased surfactant levels in diesel.

Surfactant levels are the measurement of the tenacity in which the emulsified water and fuel molecules bond together. This change in fuel surfactancy has given rise to consistent failures of congenital separation and coalescence media used to separate water from diesel fuels.

Microbial Growth

Microbial growths that naturally occur in diesel can form a layer of organic debris that adheres to the walls and bottom of the storage container as fuel ages. The growths survive and flourish by living in or around the water line and feeding on the rich hydrocarbons present in the asphaltene layer. Adding fresh fuel to a contaminated fuel supply accelerates the development of these growths. Growths that break away from the sides and float freely in the fuel can unexpectedly clog fuel lines or filters during emergencies.

Repolymerization and Oxidation

These are natural processes by which the diesel molecules lengthen and bond to produce varnishes and insoluble gums. These particles then drop to the bottom of the tank to form asphaltene (also known as diesel sludge).

Tank and System Contamination

By-Pass Fuel

The design of fuel storage tanks and role of diesel in a generator further contribute to the oxidation and repolymerization process adding to the rate of fuel contamination. Diesel fuel serves two purposes to a diesel engine. The first is as a combustible for engine operation and the second is to lubricate and cool critical components during operation. The portion of fuel used to lubricate and cool critical components is commonly referred to as by-pass fuel. By-pass fuel is sent back to the day tank or main tank as heat de-generated fuel. A 900 KW diesel generator has a fuel circulation rate of roughly 348 gallons per hour. Only 51 gallons of this volume will be used in the combustion process. The remaining 297 gallons will be sent back to the day or main tank. This warmer-than-environment fuel can create situations where condensation can form on the inside of storage tanks, further deteriorating the integrity of the fuel.

Tank Current Contamination

Over time, diesel sludge and freestanding water accumulate at the bottom of storage tanks, commonly referred to as diesel sludge, becoming a source of contamination to any new or recently filtered fuel as it enters the tank.

As noted fuel storage systems are designed with an intake line to provide fuel to the generator and a return line to return by-pass fuel back into the tank. When generators enter extended operations, the fuel inside the tank tends to form a natural current. As fuel volume is reduced inside the tank, the current becomes stronger and a situation where contaminations are stirred up from the bottom can occur. The stirred up contaminants then enter the fuel intake lines, overwhelming inline filters or, in the worst-case scenario, damage lift pumps and injectors. Many facilities experience this when a supply of new fuel is needed.

In addition to the contamination of caused by the current created by the intake and return lines diesel, being a naturally hygroscopic fuel, attracts moisture out of the atmosphere and threw vents required on storage tanks further contributing to in-tank contamination and loss of fuel integrity.

Supply Chain Contamination


Like the aforementioned contamination caused by the fuel currents within the tank, the refilling process also causes fuel contamination in much the same manner. With low levels of fuel in the tank, the high volume pumps on refuelling trucks agitate the existing contamination (water and diesel sludge) in the tanks causing immediate contamination of the new fuel.

Delivery Contamination

In addition to the contamination caused by the refuelling process the supply chain from the refinery to the final storage tanks can itself be a source of contamination. During this process fuel can change hands multiple times and each time exposing the fuel to numerous sources of contamination, including dirty transportation tanks and temporary storage tanks and various sources of water contamination.