10 Experts: Clean Fuel is Critical for New Low Emission Generators

Whitepaper ESS.WP.7923.10 Experts Clean Diesel Needs for Generators


We reviewed the available technical literature regarding diesel fuel quality, and created a synopsis for the 10 documents that seemed to us to provide comprehensive and unbiased information. Here is our take away from the experts:

1. New Engines need Clean Fuel: New Generators designed to meet air emissions requirements operate at higher pressures and with closure machine tolerances in their fuel systems. Dirt and water contamination in the fuel causes wear on the precision parts, directly affecting engine performance.

2. Your New Fuel Is Dirty: There are new standards for fuel cleanliness defined as ISO 18/16/13 for fuel delivered to your site, but the engines require 12/9/6.

3. And it is Getting Worse: Natural breakdown of fuel in long term storage and high temperatures from the fuel system causes particulates to fall from solution, and water accumulates from natural tank breathing and temperature change

4. You Need Filter Purifiers: To accomplish the final filtration needed for the engine, the generator filter coalescer needs a reliable level of incoming cleanliness, achieved with a circulating filter purifier polisher.

5. You Still need Biocides: Even with the filter polisher, there is still the opportunity for microbial growth along even a thin water film at the bottom of the tanks.

6. Microfiltration and Coalescing Separators do the Job: Filter polishing through microfiltration and coalescing separators are effective in achieving the required cleanliness levels. Centrifugal systems are insufficient and magnetic system ineffective

7. ULSD and Bio-Fuels are a Challenge. Ultra Low Sulfur Diesel (ULSD) and Biofuels present their own challenges to the filtration coalescing separators. So it is advisable to design filter surface areas conservatively.

8. Online Continuous Monitoring is Coming: The technology is getting priced better to address the need for predictability and preventative.

Technical Paper Synopsis ESS.TP.710.FLT00

10 Experts: Clean Fuel is Critical for New Low Emission Generators

Technical Paper Synopsis ESS.TP.711.FLT01
CAT 237-8 Bulk Diesel Fuel Filtration
By Caterpillar: Richard Douglas


Technical Paper Synopsis ESS.TP.712.FLT02
Filter Solutions for HPCR Fuel Systems
By Cummins Filter Barry Verdegan et al


Technical Paper Synopsis ESS.TP.713.FLT03
Diesel Engine Fluids Recommendations
By Caterpillar SEBU6251


Technical Paper Synopsis ESS.TP.714.FLT04
The Dangers of Contamination
By Caterpillar Viewpoint


Technical Paper Synopsis ESS.TP.715.FLT05
Fuel Service Bulletin - Cleanliness
By Cummins


Technical Paper Synopsis ESS.TP.716.FLT06
Fuel Service Bulletin – Microbial
By Cummins


Technical Paper Synopsis ESS.TP.717.FLT07
Diesel Fuel and Your Engine
By Caterpillar SEBD0717-4


Technical Paper Synopsis ESS.TP.718.FLT08
Diesel Fuel Filtration
By Pall Filter


Technical Paper Synopsis ESS.TP.719.FLT09
Guidelines for Diesel Fuel Filtration
By Parker Racor Velcon


Technical Paper Synopsis ESS.TP.720.FLT10
Diesel Fuel Technical Review
By Chevron

Technical Paper Synopsis ESS.TP.711.FLT01

CAT 237-8 Bulk Diesel Fuel Filtration By Caterpillar: Richard Douglas

Ninety years ago, the Caterpillar Operator’s Manual stated that dirt and water causes 90% of all the problems with diesel fuel systems. The same is true today. Some things never change. What has changed over the years is the power density of engines and resultant injection pressures of the fuel systems. Rigid new exhaust emission standards are also causing significant design changes, cost increases and reliability losses in modern engines.

Tier 4 Emission Standards
New Tier 4 Emissions Standards require high pressure fuel systems which simply cannot provide adequate performance and service life without very clean fuel. As fuel system performance degrades, exhaust emissions increase.

Cost of Dirty Fuel
Two factors drive accelerated fuel system wear and failure: Elevated injection pressures and Contaminated fuel. Modern common rail fuel systems must have very clean fuel to provide adequate service life. Putting dirty fuel in the tank causes rapid filter plugging and short component.

What Is Clean Fuel?
The question of what is clean fuel has been the subject of debate for many years. Until recently, the definition of clean fuel was “Clear and bright”. What does that mean? How clear? How bright? There is only one acceptable and meaningful way to measure and discuss fuel cleanliness. That is ISO cleanliness level. What do we mean today when we say “Clean fuel”? Caterpillar’s current minimum recommendation for fuel going into the machine fuel tank is for High Pressure Common Rail Systems ISO
16/13/11 or better Free Water 500 ppm or less

Caterpiller Clean Fuel


Bulk fuels are simply not delivered to sites at these cleanliness levels. Further confusion is introduced when sites attempt to determine fuel cleanliness by bottle sampling. The only reliable way to sample fuel is in dynamic flow with a laser particle counter. Contaminated fuels and the associated cost of failures is something which can be controlled or eliminated by the effective use of bulk filtration. However, I leave you with a few thoughts:

  • In many cases, bulk fuel filtration is a far more efficient and effective option to clean fuel than loading up machines with additional filtration capacity.
  • Fuel cleanliness must be clearly defined in terms of ISO cleanliness levels.
  • Measuring of fuel cleanliness must be done dynamically, using laser particle counters

Technical Paper Synopsis ESS.TP.712.FLT02

Filter Solutions for HPCR Fuel Systems
By Cummins Filter Barry Verdegan et al

High Pressure Common Rail (HPCR) fuel injection systems needed to meet diesel emission regulations operate at pressures in excess of 1800 bar and involve clearances between moving parts on the order of 1-5 μm. For these reasons, HPCR tends to be contamination sensitive relative to its predecessors. This presents new challenges to fuel filters. To protect HPCR components, much lower concentrations of fine particles and water should be maintained over a broad range of operating pressure, flow and vibration conditions.

In an effort to improve engine reliability, the Worldwide Fuel Charter sets maximum contamination levels for diesel fuels at an ISO 4406 Solid Contamination Code of 18/16/13 or cleaner for solid contaminants, and at a water content of 200 mg/kg or lower. These limits apply to fuel supplied to a vehicle. HPCR injectors may need to have cleanliness levels approaching 12/9/6, 64 times lower particle concentrations at 4 μm than called for by the Charter.

Solid Contaminant Removal Challenges

Good contamination control practices include, but are not limited to:

  • consistently obtaining in-spec fuel from reliable, dependable sources,
  • the use of filters and fuel water separators for storage and day tanks,
  • routine tank cleaning,
  • steps to ensure that biological growth does not get started,
  • clean fuel transfer operations, and
  • appropriate on-board filtration and water removal,

Water Contamination
Undissolved water is a contaminant to be avoided in fuel for HPCR systems. Water has numerous detrimental effects on fuel injectors: It serves as a media for the growth of biological organisms, corrodes susceptible metal components, concentrates acids and ionic species that accelerate corrosion, freezes at cold temperatures, reacts with certain additives, and reduces fuel lubricity.

Interfacial tension, the force required to rupture the interface between two liquid phases, is one of many parameters that affect fuel water separation. Ultra Low Sulfur Diesel ULSD is mandated as an enabler for emission control technology. The ULSD additives influence interfacial tension and, therefore, fuel water separation.

Protection of the HPCR fuel system on new diesel engines is one of the most demanding contamination control challenges today. Fuel as clean as an ISO Code of 12/9/6 may be required from contamination levels in excess of 24/23/19. The introduction of ULSD and biodiesels make fuel water separation more difficult. To meet these challenges, contamination control programs that use integrated, optimized filtration systems will be required.

Technical Paper Synopsis ESS.TP.713.FLT03

Diesel Engine Fluids Recommendations By Caterpillar SEBU6251

Contamination Control Recommendations for Fuels
Fuels of ISO 18/16/13 cleanliness level or cleaner as dispensed into the engine or machine fuel tank should be used. Reduce power loss, failures, and related downtime of engines will result. This cleanliness level is important for new fuel system designs such as Common Rail injection systems and unit injection systems. Injection system designs utilize higher fuel pressures and tight clearances
between moving parts in order to meet required stringent emissions regulations. Peak injection pressures in current fuel injection systems may exceed 30,000 PSI. Clearance in these systems are less than 5 μm. As a result, particle contaminants as small as 4 μm can cause scoring and scratching of internal pump and injector surfaces and of injector nozzles.

Water in the fuel causes cavitation, corrosion of fuel system parts, and provides an environment where microbial growth in the fuel can flourish. Other sources of fuel contamination are soaps, gels, or other compounds that may result from undesirable chemical interactions in the fuel particularly in Ultra Low Sulfur Diesel (ULSD). Gels and other compounds can also form in biodiesel fuel at low temperatures or if biodiesel is stored for extended periods. The best indication of microbial contamination, fuel additives,or cold temperature gel is rapid filter plugging of bulk fuel filters or machine fuel filters.

Contamination Controls Cummins

In order to reduce downtime due to contamination follow these fuel maintenance guidelines.

  • Fill machine fuel tanks with fuels of ISO 18/16/13 cleanliness level or cleaner, in particular for engines with common rail and unit injection systems. When you refill the machine, filter the fuel through a 4 µm absolute filter (Beta 4 = 75 up to 200) in order to reach the recommended cleanliness level. This filtration should be located at the device that dispenses fuel to the engine or machine fuel tank. In addition filtration at the dispensing point should remove water to ensure that fuel is dispensed at 500 ppm water or less.
  • CAT recommends the use of bulk fuel filter / coalesce units which cleans the fuel of both particulate contamination and water in a single pass.
  • Install and maintain a properly designed bulk filter / coalesce filtration system. Continuous bulk filtration systems may be required to ensure that the dispensed fuel meets the cleanliness target.
  • Centrifugal filters may be used as a pre-filter with fuel that is heavily contaminated with gross amounts of water and/or large particulate contaminants. Centrifugal filters can effectively remove large contaminants, but may not be able to remove the small abrasive particles required to achieve the recommended ISO cleanliness level. Bulk filter / coalescers are necessary as a final filter in order to achieve the recommended cleanliness level.

Technical Paper Synopsis ESS.TP.714.FLT04
The Dangers of Contamination
By Caterpillar Viewpoint

“There’s no question that the use of contaminated fuel in modern fuel systems can cause a host of
problems with machinery,” says David Barker, a product support consultant in Caterpillar’s Global
Mining Division. “But there are things sites can do to control the cleanliness of the fuel they use.”

The two most common contaminants in fuel are excessive dirt and water. These contaminants are
typically introduced into fuel during the transportation, mixing or storage process. Water, the most
common contaminant, may be introduced when warm, moist air condenses on the fuel tank walls and
condensation builds up. Excessive water can reduce the lubricating qualities of the fuel and cause
injector seizure and contingent engine damage. In addition, fungus and bacteria live in water. When
they get into a fuel system, they can plug fuel filters, reducing their life and causing premature injector
wear-out, all of which affect the life of the machine. Particles of dirt, sediment and other solids have a
similar effect on fuel injectors, causing them to wear out prematurely and leading to unnecessary

The two critical factors leading to fuel quality are the cleanliness of the fuel delivered to the site, and the
storage and handling practices used after it arrives. In order to maximize engine life, diesel fuel supplied
to the fuel tank should have a maximum particulate level of ISO 18/16/13 with a maximum water
content level of 0.1 percent (1,000 ppm).

Fuel Contaminants

SUPPLEMENTING WITH FILTRATION The fuel filters on machines are designed to provide final filtration for moderately clean supply fuel. Machine filtration is not intended to clean fuel contaminated with large amounts of dirt and water. If contaminated fuel is used, the capability of the onboard filtration is overwhelmed.

One way to reduce the burden on machine fuel filters is to install a bulk fuel filtration system. Bulk fuel filtration consists of high capacity filters that remove both excess dirt and water from the supply fuel before it is put into the machine.

The self-contained unit is mounted on a skid, and is placed between the fuel storage tank and fueling station. The 4-micron, beta 200, full synthetic particulate filter elements remove dirt in a single pass and effectively hold large volumes of debris. The coalescer unit contains multiple elements capable of removing up to 3 percent water by volume to 1,000 ppm (0.1 percent) or less at the rated flow. Water removed by these elements falls to the bottom of the coalesce unit and may be automatically drained to an external water storage vessel.

Technical Paper Synopsis ESS.TP.715.FLT05
Fuel Service Bulletin - Cleanliness
By Cummins

Fuel Cleanliness
This section explains the importance of fuel cleanliness to the successful operation of Cummins®
Engines. Modern fuel systems have been developed to improve combustion performance and fuel
consumption, and improve engine performance.

These high-pressure fuel systems operate at pressures approaching 2600 bar [37,700 psi], and with
component match clearances typically from 2- to 5-microns for injectors. At these pressures, very small,
hard particles are potential sources of a fuel system malfunction.

Excessive contamination of diesel fuel can cause premature clogging of diesel fuel filters and/or
premature wear of critical fuel injection system parts. Depending on the size and nature of the particles,
this can lead to reduced component life, component malfunction, fuel system and/or engine

Determining fuel cleanliness requires measuring both the size and number of particles per size class in
the fuel, i.e. the particle size distribution. The ISO has developed a protocol for expressing the level of
contamination by coding the size distribution called ISO 4406. ISO 4406 cleanliness codes are expressed
as a series of three numbers (x/x/x), which correspond respectively to the number of particles greater
than 4-, 6-, and 14-microns.

For example, the numbers in the ISO 4406 rating of 18/16/13 translate to:

  • 18 - Up to 2500 particles larger than 4μm (per mL of fuel)
  • 16 - Up to 640 particles larger than 6μm (per mL of fuel)
  • 13 - Up to 80 particles larger than 14μm (per mL of fuel).
ISO Fuel Cleanliness Standards

Engine builders and fuel injection equipment manufacturers have found that the particles greater than
4- microns and greater than 6-microns are particularly critical to the durability of the fuel injection
system. They also recognize that the fuel systems must tolerate hard particles smaller than 4-microns
that are difficult to filter out, even with the finest filtration.

To maximize the efficiency and effectiveness of filtration, Cummins Inc. has adopted the
recommendation of the World Wide Fuel Charter that fuel supplied to engines meet the ISO 4406 code
of 18/16/13 maximum for respectively 4-, 6-, and 14-micron particle sizes.

Cummins Inc. recommends that if the fuel does not meet the ISO cleanliness code of 18/16/13 when
supplied to the engine, additional filtration be applied before the fuel is delivered to the equipment's
fuel tank. A Cummins® Distributor or Cummins Filtration™ representative can supply hardware and
additional filtration guidance and can recommend countermeasures such as improved fuel quality from the fuel supplier, and/or better fuel handling, storage, dispensing, and fuel tank cleaning techniques.

Technical Paper Synopsis ESS.TP.716.FLT06
Fuel Service Bulletin – Microbial
By Cummins

Microbial Contamination
This section covers the recognition of and solutions to microbial contamination of diesel fuel. 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.

There are three general classes of biocides: water-soluble, fuel-soluble, and universally soluble.

Fuel-soluble biocides are best suited for treating fuels which are to pass through several storage steps in the distribution process. A fuel-soluble biocide injected into the fuel early in the distribution system is carried with the fuel through the entire downstream system, effectively sterilizing the fuel until usage. Fuel-soluble biocides are easier to add to the fuel system since the exact amount needed to treat a volume of fuel is easily determined and they have a low toxicity to human and other life forms.

Water-soluble biocides are insoluble in fuel, stay where they are placed until the water bottoms are pumped from the tank. There are a number of disadvantages to water soluble biocides. Water bottoms containing a water-soluble biocide must not be placed in a sanitary sewer system because the biocide can destroy the bacteria used by sewage treatment plants. These water bottoms must be treated as an acidic, industrial oily waste.

Universally soluble biocides are soluble in both water and diesel fuel. They allow you to treat the entire downstream system. However, each subsequent load of fuel does not need to be treated. The biocide will remain in any water that has collected at the bottom of the storage tank and continue to inhibit microbial growth.

Technical Paper Synopsis ESS.TP.717.FLT07
Diesel Fuel and Your Engine
By Caterpillar SEBD0717-4

Fuel Contaminants

Water can become a contaminant if the water is introduced into the fuel during shipment or is the result
of condensation during storage. There are three types of moisture in fuel: dissolved moisture (moisture
in solution), free and dispersed moisture in the fuel, and free and settled moisture at the bottom of the
tank. Most diesel fuels have some dissolved moisture. Just as the moisture in air, the fuel can only
contain a specific maximum amount of moisture at any one temperature. As the temperature drops, the
amount of moisture that can be in the fuel will be lower. For example, a fuel could contain 100 ppm
(0.010 percent) of water in solution at 18°C (65°F). The same fuel can possibly hold only 30 ppm (0.003
percent) at 4°C (40°F).

After the fuel has absorbed the maximum possible amount of water, the additional water will be free
and dispersed. Free and dispersed moisture is fine droplets of water suspended in the fuel. Since the
water is heavier than the fuel, the water will slowly become free and settled at the bottom of the tank.
In the above example, when the fuel temperature was lowered from 18°C (65°F) to 4°C (40°F), 70 ppm
of water became free and dispersed in the fuel.

Engine Effects

Excessive water in fuel will cause fuel pump and injector damage in most modern fuel systems which
use fuel to lubricate the pump or unit injectors. Excess water breaks down the protective film strength
of the fuel and leads to plunger scuffing and seizure failures. Removing excess water is extremely
important when using low sulfur fuels.

Water Contamination

Water Separation
Water separators are critical to fuel treatment. Any excess water in the fuel can cause a failure due to corrosion or scuffing from a lack of lubrication. Separators must be used on engines with unit injectors and sleeve metering fuel systems where fuel lubricates the fuel pump or injectors.

There are two types of water separators. The sediment type is used when the water separator is installed ahead of the fuel pump. The coalescing type of separator must be used if the water in the fuel is mixed or broken into small particles which do not settle. A coalescing type of separator is used if particles are so fine they make the fuel cloudy. A coalescing type separator will separate all water from fuel. This type separator can be put anywhere in the fuel line, such as next to the components needing the most protection from water.

Microorganisms in fuel
All water and fuel offer a medium for bacterial growth. These simple life forms live in the water and feed on fuel. Microorganisms or fungi in fuel cause corrosion and filter plugging. Bacteria may be any color, but are usually black, green, or brown. Bacteria grow in long strings and have a slimy appearance. A biocide added to the fuel will kill the growth and/or slow its formation. Filtering the fuel or proper disposal after using the biocide is required to eliminate filter plugging.


Technical Paper Synopsis ESS.TP.718.FLT08
Diesel Fuel Filtration
By Pall Filter

With increasing operating costs and strict environmental legislation industry faces difficult challenges
when running large diesel powered engines. To ensure optimum engine performance and reliability,
particulate and water contamination in diesel fuel must be controlled to acceptable limits. Standard onboard
fuel filters are not designed to handle high levels of contamination. Therefore, to prevent
premature plugging and frequent change-outs of on-board filters, fuel cleanliness should be controlled
throughout the entire fuel supply chain.

The Impact of Particulate Contamination on Diesel Powered Engines
As engine fuel injection technology has progressed, so has its sensitivity to contamination.

Electronically Controlled Unit Injectors (EUIs)
The injector nozzle openings of EUIs are 6-7 μm in diameter. These openings can become
blocked or suffer erosion from particulate contamination as diesel fuel is passed through them
at high pressures. Nozzle shape can be changed or spray patterns altered, adversely impacting
engine performance in the form of reduced power output and poor fuel economy.

High Pressure Common Rail System (HPCR)
HPCR system technology offers improved power and fuel efficiency and lower exhaust
emissions. To achieve these results, HPCR systems operate at pressures in excess of 2000 bar
and have injector nozzle openings in the 2-3 μm diameter range. This requires diesel fuel 30
times cleaner than what is acceptable for standard EUIs and over 100 times cleaner than what is
typically dispensed at the pump. This level of cleanliness cannot be attained with on-board
filtration alone; supplementary bulk and point of filling filtration is required.

ISO Fuel Standards

Technical Paper Synopsis ESS.TP.719.FLT09
Guidelines for Diesel Fuel Filtration
By Parker Racor Velcon

Today’s strict emissions regulations cause diesel engine and turbine manufacturers to introduce sophisticated technology into the fuel system in order to increase efficiency. These new technology fuel systems usually include higher injection pressures through smaller orifices, the ability to burn low sulfur and bio-diesel fuels, and recirculating systems with high return flows. The mean time between equipment failures dramatically increases without the use of substantially cleaner fuels. This requires tighter control of fuel quality and continuous fuel monitoring. As a result, improved fuel storage design and handling as well as higher efficiency filtration is required.

Dirt Contaminants
Generally, new fuel is delivered meeting current ASTM/EN specifications, and this may have an average cleanliness in terms of particulate of ISO 22/21/18. In a word: dirty. Even when fuels appear visually clean, filtration and laboratory analysis can reveal heavy contamination. To provide some sense of fuel cleanliness, the pictures below show a diesel fuel sample with a particulate content defined by an ISO Code of 22/20/18. The fuel is clear and bright and free of visible particulate, but this fuel contains high levels of sub-40 micron particulate.

The primary issue that needs to be understood is that visible particulate is not the primary concern, as dirt contaminants that are large enough to see will most likely settle out and are easily filtered. It is the sub 40 micron materials that are of particular concern and especially those in the range below 15 microns that tend not to settle during storage as both time periods are insufficient and thermal convection prevents it. Particulate in this size range will cause fuel system wear and disruption in diesel power generation and transport equipment.

The most common method for reporting diesel fuel cleanliness is through the use of ISO4406 which consists of three numbers relating to the total number of particles per milliliter greater than 4μm, 6μm and 14μm sizes. We believe that continual testing looking for trends in fuel quality is as or more important than the actual number recorded in time, and the recommendation for real-time online reporting is strongly encouraged when possible, or frequent on-site testing at a minimum in order to assure fuel quality is continually met.

If fuel is generally manufactured by distillation of crude oil, how does fuel get dirty? Sources of contamination may occur from many sources:

  • Simple rust, airborne dirt and water,
  • Temperature variation causing precipitation of dissolved water,
  • Additive issues from ULSD lubricity, low temperature, and combustion cetane additives.
  • Biodiesel compatibility, ageing, and purity issues
  • Microbiological spoilage

Water Contaminants

It is important to note that water can accumulate in diesel within a tank even if the fuel was dry to begin with. As a tank is emptied, the space in the tank is taken by air coming in through the vents. This
incoming air can contain considerable moisture depending on the environment. The air above the fuel will cool overnight much more quickly than the fuel. This cooling may produce condensation inside the tank. High humidity environments and environments subject to large temperature changes from day to night are especially vulnerable.

Filtration Options
There are simple dirt microfilters, coalescing filters to remove free water and dirt, and water absorbing filters to provide a higher level of water removal.

These may be conventional pleated cellulose blends, polymeric fiber, or glass fiber systems with replaceable cartridges. In terms of particle capture efficiency, the replaceable cartridge filters are available in a range of particle sizes (0.5 to 100μm) and in high dirt systems, these provide the most cost effective approach. Selecting the right type of media for application should be done with care.

Cyclone type filters may also be appropriate to remove gross contamination. Cyclone system users should be aware that the turbulence in a cyclone can chop up agglomerations creating many more smaller particles. The cyclone type filter is generally most effective with larger particle sizes (>30μm). Pleated products offer the best solution for fine particle removal.

Coalescing Filter

Coalescing Filters
Coalescing filtration is primarily designed to allow the removal of dirt and free water from flowing fuel streams. Stable hazes contain many small water droplets and due to the small size, the water may take days to settle to the bottle of a tank or vessel. The coalescing filter contains a media that coalesces the small droplets into increasingly larger and larger droplets that due to their specific gravity easily settle to the bottom of the housing to be removed by daily flushing of the sump.

In high dirt and water environments, using the coalescing media to control both the dirt and water contaminant may not be the optimal economic approach. In order to deploy the most cost effective solution, a properly sized microfilter should always be installed upstream of filter water separator.

The coalescing/separating necessarily contains hydrophilic (water attracting) and hydrophobic (water repelling) elements. The surface properties of the media can be susceptible to deactivation by certain types of aggressive surfactants (fuel additives) and care is needed in monitoring the filter to ensure that
this is identified and appropriate action taken. Poor quality or high concentration (>B10) biofuel can also act to disarm coalescing elements over time.

Water Absorbing Filters
Water absorbing filters contain a polymeric media that is designed to adsorb water permanently from flowing fuel streams. Unlike the coalescing media, this type of filter is not susceptible to surfactant deactivation, but they are a relatively costly filter type to remove bulk water and should only be used for final polishing of the fuel to remove any small quantities of dirt and water immediately prior to the delivery to the power generation or diesel transport system.


Technical Paper Synopsis ESS.TP.720.FLT10
Diesel Fuel Technical Review
By Chevron

nadequate lubricity is not the only cause of wear in diesel engine fuel systems. Diesel fuel can cause
abrasive wear of the fuel system and the piston rings if it is contaminated with abrasive inorganic
particles. Fuel injectors and fuel injection pumps are particularly susceptible to wear because the high
liquid pressures they generate require extremely close tolerances between parts moving relative to one

ASTM D 975 limits the ash content of most diesel fuels to a maximum of 100 ppm. (Inorganic particles
and oil-soluble, metallo-organic compounds both contribute to the ash content; but, only inorganic
particles will cause wear.) The U.S. government has a tighter specification of 10 mg/L (approximately 12
ppm) for all particulate matter. However, neither specification addresses particle size. While most fuel
filters recommended by engine manufacturers have a nominal pore size of 10 microns, 6 studies by the
Southwest Research Institute reveal that the critical particle size for initiating significant abrasive wear in
rotary injection fuel pumps and in high-pressure fuel injection systems is from six to seven microns.

However, as engine designs to reduce emissions result in higher fuel rail and injector pressures, the
tighter clearances will have less tolerance for solids and impurities in the fuel. Consequently, some
engine manufacturers are now specifying filters with pore size as low as two microns.

ISO Standard Fuel Cleanliness

Fuel Stability Filter Life
Unstable diesel fuels can form soluble gums or insoluble organic particulates. Both gums and particulates may contribute to injector deposits, and particulates can clog fuel filters. The formation of gums and particulates may occur gradually during long-term storage or quickly during fuel system recirculation caused by fuel heating.

Storage stability of diesel fuel has been studied extensively because of governmental and military interest in fuel reserves. However, long-term (at ambient temperatures) storage stability is of little concern to the average user, because most diesel fuel is consumed within a few weeks of manufacture. Thermal (high-temperature) stability, on the other hand, is a necessary requirement for diesel fuel to function effectively as a heat transfer fluid. Thermal stability may become more important because diesel engine manufacturers expect future injector designs to employ higher pressures to achieve better combustion and lower emissions. The change will subject the fuel to higher temperatures and/or longer injector residence times.

Contaminant Control - Biocides
The high temperatures involved in refinery processing effectively sterilize diesel fuel. However, the fuel may quickly become contaminated if exposed to microorganisms present in air or water. These microorganisms include bacteria and fungi (yeasts and molds). Because most microorganisms need free water to grow, biogrowth is usually concentrated at the fuel-water interface, when one exists. In addition to the fuel and water, they also need certain elemental nutrients in order to grow. Of these nutrients, phosphorous is the only one whose concentration might be low enough in a fuel system to limit biogrowth. Higher ambient temperatures also favor growth. Some organisms need air to grow (aerobic), while others only grow in the absence of air (anaerobic).

The time available for growth is also important. A few, or even a few thousand, organisms don’t pose a problem. Only when the colony has had time to grow much larger will it have produced enough acidic byproducts to accelerate tank corrosion or enough biomass (microbial slime) to plug filters. Although growth can occur in working fuel tanks, static tanks, where fuel is being stored for an extended period of time, are a much better growth environment when water is present.

Biocides can be used when microorganisms reach problem levels. The best choice is an additive that dissolves in both fuel and water to attack the microbes in both phases. Biocides typically are used in the concentration range from 200 to 600 ppm. A biocide may not work if a heavy biofilm has accumulated on the surface of the tank or other equipment, because it may not be able to penetrate to the organisms living deep within the film. In such cases, the tank must be drained and mechanically cleaned. Even if the biocide effectively stops biogrowth, it still may be necessary to remove the accumulated biomass to avoid filter plugging. Any water bottoms that contain biocides must be disposed of appropriately because biocides are toxic.

What special precautions need to be taken with diesel fuel that must be stored for a long period of time?
While storage stability should not be a concern for the majority of diesel fuel users, those who store diesel fuel for a prolonged period, i.e., one year or longer, can take steps to maintain fuel integrity. The steps below provide increasing levels of protection:


  1. Purchase clean, dry fuel from a reputable supplier and keep the stored fuel cool and dry. The presence of free water encourages the corrosion of metal storage tanks and provides the medium for microbiological growth.
  2. Add an appropriate stabilizer that contains an antioxidant, biocide, and corrosion inhibitor.
  3. Use a fuel quality management service to regularly test the fuel, and, as necessary, polish it – by filtration through portable filters – and add fresh stabilizer. This is common practice for nuclear power plants with backup diesel-powered generators.
  4. Install a dedicated fuel quality management system that automatically tests and purifies the fuel.