The Importance of Fuel Maintenance for Emergency Standby Generators

Fuel degradation is an inevitable, natural process. And unless an adequate fuel sampling, testing, monitoring and fuel maintenance program is implemented on your standby generator, fuel breakdown will continue to be a potentially expensive liability and a major contributor to your overall operating costs.

Diesel fuel samplesDiesel fuel is a very complex fluid. It is not homogenous and no two batches will ever be identical. Fuel deterioration, filterability and shelf life depend on a variety of factors; fuel breakdown is also dramatically accelerated by changes in temperature, water, microbial contamination and exposure to heat and pressure from the engine’s injection system.

Fuel shelf life and filter clogging tendencies depend on a series of factors such as transportation and storage, natural oxidation and breakdown, the source of the crude oil, use of production additives and the addition of biofuel. Fuel breakdown is also accelerated by hot fuel that the engines return to the day tanks. Hot return fuel and condensation are important contributors to bad fuel in emergency generator tanks.

Suffice it to say that all engine and generator manufacturers agree that a generator maintenance and fuel testing program is recommended. We could provide abstracts from a variety of manufacturers all stating the same thing, “Sample and test your diesel fuel on a regularly scheduled basis.” NFPA 110 -2010 Edition also makes that very same statement in appendix A. In addition, there is a separate section pertaining to fuel systems that recommends that the fuel provided to the genset shall be clean and tanks should be sized accordingly so that the fuel is consumed within its’ storage life.

It has been stated that 80% of emergency generator engine failures are fuel related. Dirty fuel not only causes engine failure, it also aids in the release of Green House gases by releasing more soot and smoke into the atmosphere.

So let’s talk about a diesel fuel maintenance program.

  • What are some of the considerations in regards to tank location and sizing?
  • How often should the tank be tested?
  • What tests should be performed?
  • What remediation is available if my fuel does not meet ASTM D975 standards?

Tank location:

  • Underground: Tanks underground are prone to water leakage at the fuel supply cover assembly. Improper seating and locking of the cover will lead to water contamination. Tanks underground also will have a difference in temperature between the bottom and top of the tank. The upper part of the tank will fluctuate accordingly with the ground temperature. This difference in temperature will lead to condensation forming on the tank walls and dropping to the tank bottom. Overtime, this water will accumulate, providing a growth medium for fungal and microbial growth.
  • Aboveground: The heat from the sun will cause the fuel inside the tank to expand and contract on a daily basis. This daily contraction will also lead to condensation forming on the walls of the tank and settling to the bottom and again providing a growth medium.

Tank sizing:

  • All too often, end users will want a larger than needed tank to ensure there is sufficient fuel for an extended run during a power failure. However, diesel fuel is a hydrocarbon. It inherently has water in it. After time, the fuel will begin to separate and oxidize. The separation will cause water to accumulate in the bottom of the tank. The oxidation will lead to the formation of tank sediments and solids, all of which will settle to the bottom of the tank.  Over sizing the tank will only lead to fuel degradation issues down the road. Fuel manufacturers state that diesel fuel should not be stored for more than 6-9 months.


  • A fuel sample should be taken from the very first tank filling to provide a bench mark of the quality of the fuel. Annual samples should then be taken to determine the rate of deterioration of the fuel. If the fuel will be subject to extreme temperature fluctuations due to geographical location or other outside factors, semi-annual or quarterly testing would be recommended.

Tests performed:

  • ASTM D975 provides a comprehensive listing of tests that can be performed on diesel fuel. Some of these tests are designed for the refineries and are much more complex than what is needed by the end user. However, the following listing is recommended as a minimum to ensure fuel quality and conformity with federal, state and local requirements:
    1. Cetane Index  (D-976)
    2. Water by Karl Fischer  (D-6304)
    3. Water and sediment  (D-1796)
    4. API gravity  (D-287)
    5. Distillation  (D-86)
    6. Micro-organisms  (D-6469)
    7. Total sulfur (D-5453)
    8. ISO Cleanliness (ISO-4406)

You will note that there are two different tests for water content. One detects free water, dissolved water and emulsified water. The other detects free water and sediment. The sulfur test is to ensure that your diesel fuel is no more than 15 ppm as mandated by EPA standards. The ISO Cleanliness test is recommended by the engine manufacturers to measure particulate. Today’s diesel engines have much more stringent tolerances and even the slightest water and sediment contamination can lead to engine failure.


  • First and foremost, remove the water from the tank. The microbial and fungal growth occurs in the water, not the diesel fuel. The highest concentration of growth activity is at the fuel water line. No water equals no growth medium. This can be accomplished by:
    • Coalescing – Gravity drags water droplets out of the slow flowing fuel.
    • Stripping – A silicon based medium that inhibits the passage of water but allows the fuel to flow.
    • Absorption – A filter medium that has a high affinity for water and a low affinity for fuel.

Particulate filtration is the process of passing the fuel through a specified micron rated filter. As the fuel is filtered, the filters should sequentially have lower micron ratings to trap the smaller micron contaminates.ASNE technician performing diesel fuel polishing

Fuel polishing is defined as the removal of water, sediment and non-combustible particulate matter and microbial contamination below levels stated in ASTM D-975 while reversing the oxidation process and re-suspending combustible particulate matter to maintain ASTM standards for BTU, lubricity and cetane. Besides the removal of water and sediment, a full spectrum catalyst needs to be added to aid in meeting these requirements.

As you can see, a comprehensive standby generator diesel fuel maintenance program will make sure that your fuel is stored, tested and maintained to ensure the highest reliability of the system. It will aid in lowering your carbon footprint by lowering emissions and will also aid in lowering the maintenance costs of your generator’s engine.

To learn more about the fuel maintenance program, please contact our resident generator fuel maintenance expert Adam Sargent at

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