What you need to Know about Fuel Polishing

If you own, manage, design, or install emergency generators you likely have heard about diesel fuel polishing.  Why are you hearing about it now?  The reason is changes to the composition of diesel fuel to meet air emissions regulations.

Most diesel fuel does not require fuel polishing. It is primarily consumed withing a few weeks or months after leaving the refinery.  The storage time in tanks, for example at a truck stop, is relatively short.  For truck fuel, regular tank checks and maintenance to remove tank bottom water will prevent bio issues and tank bottom corrosion.

Why is fuel polishing needed?

Fuel Polishing is needed for diesel fuel in emergency generator tanks.  The reason is that emergency generator fuel is stored for long periods of time, even up to 10 years before it is consumed.  A critical facility may want to store diesel fuel for 8-72 hours operating time, but without power outages only use a small amount for testing.

For example:  A hospital has 4 large generators that consume 160 gallons per hour at full load.  To meet a 72 hour run time regulation at 75% tank fill storage of 61,400 gallons would be required.  If the generators were tested at 25% load for 15 minutes every week the consumption would be about 2000 gallons annually.  So the facility might be storing fuel for 20-30 years.

Smaller generators, including life safety generators may only be designed for 2 hour run times.  But the problem is the same – diesel fuel stored for long periods of time.

Fuel polishing for your diesel generators is needed now because: Diesel Fuel Changed AND Diesel Generators Changed

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Fuel Polishing Basics

Fuel Polishing is very fine filtration of diesel fuel to remove impurities of dirt and water. 

This is accomplished by pumping diesel fuel from a tank, through a series of filters, and back to the tank.  This circulation ideally happens by drawing fuel from one end of a tank, and returning fuel to the opposite side of the tank to avoid dead zones of unfiltered fuel.

Learn More About Earthsafe M30 Fuel Polishers

When Should Fuel be Polished?

Diesel fuel will start to degrade 3-6 months after refining.  So a fuel polishing strategy needs to be thought about.  There are 3 ways that people approach the problem:

  • Annual Fuel Test and Contingency Polishing.  Many facilities are required to have annual testing of their emergency generator fuel.  Then is a test fails, they will order a service for fuel polishing the generator tank.  There is a risk of shutdown or failure with this approach.An improved contingency approach is to track fuel test parameters for degradation and order fuel polishing based on this analysis prior to test failure.


  • Annual Planned Fuel Polishing and Test.  Some facilities will budget for annual fuel polishing and testing service to assure clean fuel quality.  This approach will avoid most problems, but may not be frequent enough if a fuel tank has water or bio issues that can spike upon reaching a critical threshold.


  • Installed Automatic Fuel Polishing Equipment.  Most newer critical facilities have installed systems for fuel polishing to assure continuous fuel quality.  These systems automatically start and cycle fuel in a tank, usually to allow for 1 tank volume turnover per week.

What is a Micron?

Filter are rated in microns as to the size of particle that can be captured.  1-25 Micron rated filters are commonly used, with 1-2 Micron being a common standard for final filters.  In comparison, a human hair is about 70 Micron, and the visible threshold is about 40 micron.

What type of filter is needed? 

Fuel Polishing systems may have a series of filters with the idea that coarse filters upstream will extend the life of fine filters downstream.  But to minimize filter changes, the most common arrangement is a coarse filter/strainer/bag filter followed by a fine filter / water coalescer followed by a final water absorbing filter element.

What is a coalescer filter?

A coalescing filter helps separate water from diesel fuel.  Water (specific gravity = 1.0) and diesel (specific gravity = 0.85 to 0.90) will naturally separate because of the difference in specific gravity.  This separation happens faster if the water particles are larger.  The coalescer is a material, like polypropylene) that water particles like to cling to, and then merge to form larger particles which separate faster.

What is a water absorbing filter?

Fine water particles may pass through a coalescer filter.  A water absorbing filter is a final filtration stage where the fine water particles form a chemical bond with the filter material.  Coalescers may be preferred because they can be drained of accumulated water, but absorbing filters need to be replaced at the end of their useful life.

How is Fuel Polishing Controlled?

Fuel Polishers have an integrated electronic controller for operation and monitoring.  The controller typically has a color touch screen to allow the user to enter cycle schedules and monitor performance.

How to Set Fuel Polishing Schedules

The user will set a start time and cycle time typically for a weekly tank volume turnover.  The cycle time is usually calculated for example:  a 10,000 gallon tank would require a 20 GPM rated fuel polisher to run for 500 Minutes or 8.3 Hours.  User may set the controller to start every Tuesday at 8:00 AM and cycle for 8.5 hours, and the pump will stop at 4:30 PM and restart Tuesday of each week.

How is Fuel Polishing Monitored? 

The fuel polishing controller monitors the equipment status.  The controller may connect to remote building management systems or cloud based monitoring.  Here are the items that are monitored:

  • Dirty Filters.  Filters are monitored by differential pressure from inlet to outlet.  When the rated differential pressure is sensed than an alarm is activated indicating that a filter change is needed.
  • Water in Coalescing Filters:  Water separates from fuel in the coalescer and settles into the bottom of the filter.  As the water accumulates it level rises to the point where a water sensor triggers and alarm.  The water can be drained and the alarm will reset.
  • Pump Flow:  A flow sensor is used to prove that the fuel pump is operating correctly.  If not the controller will indicate a pump alarm to call for troubleshooting.
  • Leak Detection:  It is important that the pump and filter assembly includes a containment base in the event of leaked fuel and to facilitate filter changes.  A leak sensor will initiate a alarm at the control panel and also stop the pump operation until the problem is corrected.
clean oil

What kind of diesel is used for generators? 

Diesel for emergency generators is called #2 Diesel. It may be “winter blended” with some proportion of #1 Diesel, to prevent diesel fuel gelling.

Diesel for trucks is usually gold in color. Diesel for generators is dyed red to indicate that it is for off-road use and that road use taxes have not been applied.

What is Ultra Low Sulfur Diesel ULSD?

Ultra Low Sulfur Diesel has been phased into full use over the last 20 years. The EPA mandated the change to diesel fuel manufacturing to reduce emissions for truck transportation, and now also off-road applications including generators.

While the change has had a huge impact on reducing air emissions, it has caused various problems particularly with the longer term storage scenarios of diesel generators.

What is the problems with Ultra Low Sulphur Diesel for Generators?

ULSD diesel accelerates the issues of diesel quality in long term storage for generators. Generator fuel tanks with ULSD diesel may accumulate water in the bottom of the tank more rapidly. 

This water layer can lead to bio-growth (algae) and accelerate the corrosion of steel tank bottom plates.  In addition the relatively sulfur less fuel is a more favorable environment for bio-growth  (algae ) to thrive.

How did Generator Technology Change with High Pressure Fuel Rails? Why does it Matter?

In order to meet lower emissions requirements, generator manufacturers designed engines with high pressure fuel rails.  This is the parts of the engine that deliver fuel to the engine piston chambers.

This change in the machine design made the parts more susceptible to particulate and water in the fuel as it could cause excessive wear and shortened machine life.

So just as ULSD increased fuel quality problems, the machines are requiring higher quality fuel.

Why Does Diesel Go Bad?

Diesel fuel will typically degrade starting within 6 months of delivery.  It degrades in 4 primary ways: Dirt, Water, and Bio-Growth.

Dirt in Diesel Fuel

Particulates start to fall out of solution as diesel ages.  If you have ever touched years old diesel, you can feel the grit between your fingers.  In a glass jar the diesel will look dirty compared to a clean and bright sample of new fuel.

Dirt is drawn into the engine fuel supply as it operates.   Excessive dirt will clog filters prematurely and cause engines to fail.  Fine dirt particles passing through the fuel filter will cause excessive wear on engine components.  This is particularly acute in newer engines with high pressure fuel rails.

Dirt will eventually form as sediment the bottom of tanks.  There is not a good way to remove this sediment.  Accumulating over time, this sediment will require costly tank cleaning and taking engines out of service.

Fuel Polishing very effectively removes dirt from stored fuel and avoids all of these problems.

Water in Diesel Fuel

How does water get into the fuel tank? Water can enter fuel storage tanks by (a) rainwater leaking through top of tank fittings, (b) water in delivered fuel, and (c) water from moisture in air.

Water from moisture in air is the most common source.  Fuel Tanks are vented.  When fuel is delivered to the tank, air escapes through the vent.  When fuel is withdrawn from the tank, air enters the tank to fill the void space.  Any moisture in the air will form as condensation in the relatively cooler space inside the tank.

Aboveground tanks also breath.  As the temperature increases during the day, the fuel expands and pushes air our of the tank.  As the temperature drops at night, the fuel contracts and air is drawn into the tank.  This is called diurnal breathing.  It can be a significant cause of water entering the tank.

Water is more dense than fuel so inside the tank it will slowly settle to the bottom of the tank, forming a water layer.  This water layer causes important problems.

  • The water fuel interface is an environment where bio-growth can begin and thrive.  This can cause quality test failures, and in the extreme algae may grow and clog engine filters causing failure.
  • The water layer can be a corrosive environment that attacks steel tank bottoms.  Steel tanks now include specific warnings about this problem.
  • Water in fuel can be drawn into the fuel system where it can cause excessive wear and failure.

Fuel Polishing very effectively removes water from stored fuel and avoids all of these problems.

Bio Growth “Algae” in Diesel Fuel

“Algae” in diesel fuel is a misnomer and bio-growth in fuel is more accurately micro-organisms.

All hydrocarbon fuels are essentially sterilized by the high temperatures encountered in the refining process; however, they can become contaminated soon after leaving the refinery by micro-organisms. These micro-organisms, primarily bacteria and fungi, exist rather harmlessly in moisture-free fuel, passing through fuel systems without having any negative effects.

However, in the presence of water, these micro-organisms begin to grow and reproduce. The growth of a large colony of micro-organisms in a fuel system can cause several issues. The first and usually most obvious is fuel filter plugging with a greenish-black or brown slime, frequently accompanied by a foul odor. The second issue these micro-organisms can cause is corrosion due to the acid by-products some of them produce.

Some indicators of microbial contamination are slime deposits on tank walls, piping, or other surfaces which are exposed to fuel. These deposits are usually greenish-black or brown and are slick to the touch. A more conclusive approach is to routinely check the fuel by means of one of the several available test kits. These can detect micro-organisms long before there is any visible evidence of contamination.

When it has been established that microbial contamination is present and action must be taken, there are several approaches. Since all metabolic processes of an organism are conducted in water, denying the microorganism access to water will prevent growth, thus preventing the development of large, troublesome colonies. Therefore, the first and most important step in prevention is to keep fuel systems dry. Keeping a fuel system entirely dry is impossible. In cases where microbial contamination is a recurring issue, a microbicide can be used to chemically treat the fuel or the water.


fuel test

Diesel Fuel Testing

In the last few years, requirements for generator fuel testing have become part of life safety codes.  Local fire and building code officials are also requiring test documents as part of their inspecions.

 NFPA 110 “Standard for Emergency and Standby Power Systems” is the standard for people who own and operate buildings with emergency generators.  It’s the starting point of what a building owner should consider as good practice, and may go further toward a legal requirement since it is referenced in building codes.

Here is what NFPA 110 has to say about fuel testing: “A fuel quality test shall be performed at least annually using appropriate ASTM standards. Limited fuel quality testing performed annually using appropriate ASTM test methods is recommended as a means to determine that existing fuel inventories are suitable for continued long term storage.  Special attention should be paid to sampling the bottom of the storage tank to verify that the stored fuel is as clean and dry as practicable and that water, sediment, or microbial growth on the tank bottom is minimized.  ASTM D975 Standard Specification for Diesel Fuel Oils, contains test methods for existing diesel fuel.”

The  most common method of diesel fuel testing is to have your generator maintenance company perform the sampling, then send the sample to a lab for testing.  This will likely cost $100 - $500 annually, however more extensive testing can run the cost over $1000.

What is tested?  The ASTM D975 Standard describes 13 different tests.  However many of these tests are for newly refined fuel, would be available on a test certificate from your fuel supplier, and the characteristics are not subject to degradation.  Here are the recommended annual tests on fuel characteristics that are subject to degradation:

  • Water: Max 500 PPM (by KFM)
  • Microbial : Pass/Fail
  • Particle Count: Max 18/16/13 (by ISO 4406)

Regular fuel polishing will maintain fuel quality within the accepted range.  You never need to fail a test.

Learn more from our Whitepapers:

Clean Fuel for Generators: Drilldown of NFPA 110 Rules

Testing Fuel for Generators: Drilldown of NFPA 110 / ASTM 975 Rules