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Diesel Fuel Quality Maintenance

Long Term Fuel Storage Concerns for Diesel Engine Generators

A little foresight and planning can go a long way when it comes to ensuring smooth engine performance and increasing the life of fuel tanks and fuel piping systems. Having quality fuel available at all times especially when needed most for diesel engine generator back-up power can be crucial to a Data Center’s operation and can be part of Storm Preparedness planning.

Fuel testing requirements are driven by stored fuel amounts. In general, large fuel storage tanks (about 4000 gallons and larger) should have the fuel tested every six months. For smaller storage systems, testing annually may be sufficient. Many facilities exercise their generators often enough so that stored fuel turnover is thought to be high enough that testing may not be required and the fuel stored basically does not get ‘old’. That thinking may lead to trouble considering what may be lurking inside your fuel and how it got there.

The need for fuel testing is driven by the need to monitor contaminants in the stored fuel which, if left unattended, can lead to corrosion of carbon steel tanks and piping components and eventual leaks. Particulates in the fuel can also cause erosion of the engine fuel injection components. Engine fuel filters, both primary and secondary filters, should be checked and replaced when necessary. Measured pressure drop across filter cartridges is a good indicator for replacement. Inlet and outlet pressure gauges should be installed for this purpose. Most fuel filter manufacturers label their filters with a cartridge replacement pressure drop value directly on the filter housing.

Quality Fuel

First, you should be starting with quality fuel to begin with. When ordering fuel, be sure the fuel supplier knows this application is for a Stationary Diesel Engine Generator. All diesel fuel sold in the United States must comply with requirements of ASTM Standard D975 and contain no more than 15 PPM sulfur. This standard also requires that biodiesel used for blending shall meet the requirements of ASTM D6751 and that no more than 5% by volume biodiesel will be allowed.

For stationary diesel engine generators, fuel specification under ASTM D975 is as follows:

  • ASTM D975 Grade No.2-D S15, 15 PPM MAX Sulfur, 5%MAX biodiesel

The S# signifying the maximum sulfur content of 15 ppm, referred to in the recent past as “Ultra-Low Sulfur Diesel” required by federal regulations for cleaner emissions. The higher sulfur content fuels used in the past provided lubricity to the fuel which is now lost to the ultralow sulfur requirement of S15 grade. In order to restore lubricity, additives are used. Biodiesel is a common additive blended with fuels to improve lubricity. Biodiesel additives such as soybean oil also increase the potential for biological contamination (microbes) and also increase freeze potential. Synthetic additives are recommended in lieu of biodiesel if possible. Check with fuel supplier to find out what lubricity additives are used. Using a reputable fuel supplier who understands your fuel application and requirements significantly reduces this risk. Good fuel suppliers will not allow co-mingling of unlike products in their truck’s fuel delivery tank and will follow the practice of draining down or “emptying out” at the terminal prior to filling with a different fuel type.

Corrosion Effects of Contaminated Fuel

Contaminants in the fuel almost always originate from unwanted water. Sources of water can be moisture entering a tank through vents, condensation and sometimes with the fuel delivery. Water, being heavier than the fuel, will be located at the bottom of the storage tank and will form sludge. For this reason, the fuel suction piping or foot valve inlet is located 6" above the bottom of the tank in order to stay above the sludge layer and draw clean fuel into the piping distribution system supplying the engine.

Microbes will grow at the fuel-water interface layer. Microbiological influenced corrosion (MIC) can form both in the presence of oxygen (aerobic) and without oxygen (anaerobic). MIC corrosion can cause pitting of carbon steel tanks and piping components which can eventually lead to leaks. Microbes can also form from water in contact with biodiesel blends that are added to the fuel for lubricity.

Long Term Stability Additives

Fuel stability is defined as the resistance to change over time. Fuel stability can degrade when stored for long periods of time. Temperature also has an effect on stability. Aboveground tanks for instance that are in direct sunlight should be and are required by some Codes to be painted with a reflective color (white for example) to reduce the radiant heat gain effects. If outdoor tanks are sheltered or enclosed as is the case with a generator enclosure, concern over high temperatures are significantly reduced.

Because fuel used by the diesel engine is also used for cooling engine components such as the turbo cooler, engine fuel pumps will typically flow approximately 2.5 times more fuel than what is consumed by the engine at 100% load. This excess fuel flow returns to the base tank or day tank and is hotter than before. Thus, it is important to have good thermal stability for fuel even though the fuel tank may be enclosed and not in direct sunlight.

Cold Weather Additives

Fuel suppliers will typically add a cold weather additive to reduce the pour point temperature. This helps to prevent fuel from getting “waxy” in cold weather. This is good and will reduce your need to provide more additive. Cold weather or stability additives should never increase sulfur content beyond 15 ppm maximum. Check with your fuel supplier to find out what winter additives are used.

Notes about Biodiesel

Biodiesel, also referred to as feedstocks or blend stocks is a replacement for petroleum diesel fuel and is produced from vegetable oils or animal fats through a special chemical process. Examples of biodiesel include: Soybean Oil; Cooking Oil; Fryer Grease; Oils from Palm, Canola, Coconut or Rapeseed; Animal fats/tallow.

Biodiesel acts as a fertilizer for existing microbiological growth in fuels. Biodiesel has a higher affinity for water than petroleum diesel fuel. Diesel fuel alone can contain up to 60 ppm water whereas biodiesel can contain about 1200 ppm water. The higher water content associated with biodiesel increases the probability of microbiological growth and MIC corrosion which can corrode carbon steel tanks and piping components and even fiberglass can be pitted or ‘etched’ by MIC.

Biodiesel has lower stability than petroleum diesel and degrades faster.

Biodiesel’s attraction to water is stronger than coalescing filter media. The use of coalescing filters are common for primary fuel filtration on diesel engines. The principal of separating water through a coalescing filter media is based on petroleum diesel fuel having less attraction for water than the coalescer filter fibers. Thus water is attracted to the coalescer fibers until they increase in droplet size, condense and fall off to the bottom of the water separator where they are drained off. Water that may have been effectively removed from diesel fuel will not be as effectively removed from biodiesel and can pass through the coalescer unseparated. Large amounts of water in fuel is a leading cause of fuel system engine failure. Water absorbing filters should be considered today in lieu of coalescing filters.

Fuel Polishing

Depending on the level of contamination found, fuel may be salvaged by recirculating the tank contents through a fuel polishing system until fuel is restored to acceptable quality. A fuel polishing system may be mobile (truck mounted) or can be a permanently installed part of the fuel storage system. Fuel polishing systems include at a minimum, 3 stages of filtration, coarse particle filter, water removal filter, and fine particle filter. The coarse stage is typically a wye strainer with 100 mesh screen, equivalent to about 0.006 in. or approximately 150 microns. This may hardly seem ‘coarse’ since by comparison the diameter of a human hair is about 100 microns. Nonetheless, this is coarse relative to contaminant particles in fuel. Many diesel engine manufacturers provide 4 micron engine mounted filtration standard on their engines. Water removal by coalescer media or absorption media filter is a crucial part of polishing and finally, a 5 micron filter is recommended for the fine stage of particulate filtration. Ports for introducing treatment additives can also be a part of polishing systems.


The key to a good fuel quality program is to buy clean fuel and keep storage tanks dry. Check the bottom of your storage tanks for water. If water is found, remove it. Find the source of the water and keep it out if possible. Set up a fuel quality maintenance program and contract with a fuel quality services provider to have fuel samples taken and tested by a qualified lab on a regular basis. The fuel quality testing program should include testing of bottom and middle fuel samples taken on a regular basis. If required, treat fuel as recommended according to the lab analysis results. Common treatment additives that might be recommended are microbicide, a long term stability additive, or a cold weather additive.

Paying attention to fuel quality will increase the useful life of your tanks and fuel piping system. Benefits will extend to smooth engine performance and improve overall reliability of your back-up power system.

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