June 29, 2010

Water Contamination in Diesel Fuel
by David Doyle, CLS, OMA I & II
Vice President & Operations Manager

Water is the most common form of contamination in diesel fuel. There will always be some measure of water in any diesel fuel; however, the total water content is normally less than 200 ppm and—most of the time—measures less than 100 ppm. At around 150 ppm, a slight haze will start to develop in the fuel. However, with adequate filtration and tank maintenance, the occasional presence of slightly hazy fuel should not cause operational problems. When the dissolved water content in fuel repeatedly measures between 200 ppm and 500 ppm, the fuel should be observed and monitored closely.

The primary cause of water contamination in a diesel fuel storage system is condensation. Most water problems occur after long-term accumulation. Fuel tanks will breathe in moisture laden air, and changes in ambient temperature will cause water to accumulate on the inside walls of the tank. Sudden drops in air temperature cause rapid condensation. This condensation is particularly predominant in marine applications. The problem of condensation development in a storage tank can be compounded when breathing vent locations are situated too closely to a moisture-generating source, such as compressors or steam generators. Breathing vents that are placed too closely to the ground or to decking can also promote condensation in fuel storage tanks. Improper tank inlet or vent placement can have as great an effect on the promotion of water contamination as climatic conditions do.

There are also more direct sources of condensation, such as poor seals in caps or leaking fill pipes in marine or land based storage tanks, which allows rainwater to enter a fuel storage system. Underground tanks can collect surface water when recessed caps are situated too closely to ground level. A high water table can also promote water accumulation when combined with storage tank flaws in underground tanks.

Water problems can often occur right after a fuel delivery. The problems may not stem from receiving wet fuel during the delivery but may be attributed to previous water accumulation that settles in the bottom of a tank and is then kicked up and mixed with the new fuel during the delivery process. Samples taken from the delivery hose during the fueling process can provide a customer some rudimentary—but effective—quality control in relation to the product being received. A rule of thumb should be: if it looks bad, it probably is bad.

The effects of high or continuous water contamination can lead to:

  • Fuel degradation
  • Corrosion of certain fuel-wetted components
  • Premature plugging of primary and/or secondary fuel filter units
  • Fouling of injector nozzles
  • Sticking or seizure of critical components within the fuel injector units
  • Microbial growth creating sludge and causing additional corrosion and filter plugging

ULSD can contain a higher wax content. Higher wax content will contribute to the ability of the fuel to hold more water. The refining process of ULSD to remove the sulfur can also increase the paraffin wax content, which may enable the fuel to retain more dissolved water. After time, the suspended water will drop out of the fuel. This increases the likelihood of greater microbial growth in ULSD bulk storage.

The prevention of water contamination and the problems water contamination invites center around proper housekeeping of the fuel storage tanks, proper fuel quality, and tank setup. Periodic inspection of tank bottoms and dispensers every three to six months is recommended. Solutions to unacceptable levels of water contamination in a storage system involve pumping out tank bottoms, filtration, centrifuging, tank cleaning, and the application of biocides.

In summary, good housekeeping and proper tank design are the easiest and most effective ways to combat water contamination in a fuel storage system. When excessive or long-term moisture accumulation persists in a fuel storage tank, the source of water entry should be identified for future prevention.

Return to Homepage