Fuel filtering and polishing

Fuel Degradation and Organic Contamination

Diesel fuel is used daily by millions of people throughout the world to power their vehicles. In road vehicles the fuel is used quickly and the small storage tanks are re-filled regularly. However when oil based fuel (diesel, gas oil and light fuel oil) sits in storage tanks for sustained periods of time problems can occur, rendering the fuel unusable (sometimes in a matter of months) if left untreated.

Issued by BP:


Under normal storage conditions diesel fuel can be expected to stay in a usable condition for:
▪ 12 months or longer at an ambient of 20ºC.
▪ 6-12 months at an ambient temperature higher than 30ºC.”

As diesel gets older a fine sediment and gum forms in the diesel brought about by the reaction of diesel components with oxygen from the air. The fine sediment and gum will block fuel filters, leading to fuel starvation and the engine stopping. Frequent filter changes are then required to keep the engine going. The gums and sediments do not burn in the engine very well and can lead to carbon and soot deposits on injectors and other combustion surfaces.

The expected life of a diesel fuel is indicated by the oxidation stability test ASTM D2276. The test measures how much gum and sediment will be deposited after keeping the fuel at 120°C in the presence of oxygen for 16 hours. It roughly corresponds to one year storage at 25°C. A result of less than 20mg/L of sediment and gum after the test is considered acceptable for normal diesel.

Accelerated Ageing

The ageing process can be accelerated by the following conditions:-

  • Contact with zinc, copper or metal alloys containing them. These metals will quickly react with diesel fuel to form unstable compounds.
  • The presence of water. Water allows the growth of fungus and bacteria, these produce natural by-products such as organic acids which make the fuel unstable.
  • Exposure to high temperatures.
  • Exposure to dust and dirt which contain trace elements that can destabilise the fuel, such as copper and zinc.
  • Fuel composition. Some components in diesel fuel naturally age quickly.”
  • Treatment

The extent of the wear or contamination in the fuel will dictate what treatment the site should receive, if any.  The options are:

1. That the fuel be disposed of and replaced with new fuel
2. It may be possible to filter the existing fuel to bring it back into use.

The filtration process removes potentially damaging, particulate contaminants and water from the fuel. By removing the water and its resulting corrosive acidity, as well as abrasive contaminants, the filtration process can keep the diesel oil within its operating specification almost indefinitely (subject to regular filtration).

Bio Diesel

Bio diesel has three distinct problems when being used on fuel systems that have used ‘normal’ diesel fuel for some time.

Issues with bio diesel

1. Solvent Properties

The first issue is that biodiesel has excellent solvent properties.  Hence, any deposits in the filters and in the delivery systems may be dissolved by biodiesel and result in need for replacement of the filters.  The solvent property of biodiesel could also cause other fuel-system problems.

Biodiesel may be incompatible with the seals used in the fuel systems of older vehicles and machinery, necessitating the replacement of those parts if biodiesel blends are used.

2. Microbiological Infection

The second is that bio fuels will, 9 out of 10 times, have a microbiological ‘infection’ when delivered.  These bacteria can take a variety of types both aerobic and anaerobic.  They then produce organic acids, including sulphurous and sulphuric.  These highly corrosive chemicals will cause severe degradation of the fuel.  They also attack and corrode the storage tank and ancillary equipment (particularly pumping systems).  If left untreated it will breakdown the tank’s coatings, corrode metal (pipe and tank surfaces) and destroy injection equipment.

The effect of the microbiological contaminants on fuel injection equipment is to quickly impair performance and dramatically increase wear rates.  This brings about poor combustion conditions, loss of engine power and reduced service life of components.  The presence of microbiological bacteria causes fuel filter blockage through the growth of bacteria farms in the fuel filter.  The bacteria colonies passing from the contaminated fuel tank become trapped by the filter element and create a perfect environment for bacterial growth within the fuel.  The growth accelerates due to the colony’s ability to attract more bacteria passing from the tank, together with the ability to utilise the nutrient and moisture contained in the fuel.  This results in an increase in flow resistance and pressure drop across the filter, producing fuel deprivation and rapid loss of engine performance.

3. Water Suspension

The third problem is the amount of water suspended in the biofuels.  ‘Normal’ diesel contains about 50-70ppm (parts per million) of water whilst biodiesel is around 1,500ppm.  Most engines will begin to struggle if the water rises above 2,000ppm and will fail at 2,500-3,000ppm.  Aircraft fuel has to be less than 2ppm!  If the biodiesel is mixed with normal fuel the water becomes very hard to separate out.

How to Maintain Usable Fuel

There are three options FSS can employ to keep the diesel usable.
1. Remove any solids or bacteria and filter the fuel
2. Remove all water from the fuel
3. Condition the fuel with a magnetic charge


1) In Line Water, Particulate and Magnetic Filtration

This is the simplest and most cost effective method of filtering fuel.  A 10” filter and a magnetic filter unit are placed in the fuel line between the tank and pump, and the generator and boiler.

2) 24/7 Fuel Polishing and Filtering System

Plumbed directly into the bulk oil storage tank, this system filters the fuel 24 hours a day.  This approach is more suited to higher volume, bulk tanks where the fuel may stand for months, or even years, without being used.

3) Annual Filtering

An annual visit by FSS with a dedicated fuel polishing system.  The system circulates all fuel on site at least three times (E.g. 20,000lts of fuel on site would mean we would filter at least 60,000lts through the rig).