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Diesel Fuel Polishing

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Fuel Polishing is defined as the removal of water, sediment, non-combustible particulate matter and microbial contamination below levels stated in ASTM D975 (Standard Specification for Diesel Fuel Oils) while resuspending combustible particulate matter to maintain ASTM standards for BTU value, lubricity, and Cetane. 

Fuel polishing uses centrifuges, coalescers, fuel conditioners, and – yes – filters to remove non-combustible particulate matter (sand, dust, cigarette wrappers, lady bugs, etc.) from fuel.  In short, everything that wasn’t really supposed to be there in the first place.  Because water is heavier than fuel, the centrifuges and coalescers do a good job of removing that, too.  We then use water blocking filters to pick up the last little bit of suspended – or “entrained” – water available.

The primary difference between fuel polishing and fuel filtration is that fuel polishing acknowledges that fuel itself can degrade and cause solids.  Fuel filters will simply pull the solids out because they plug up filters whereas fuel polishers break down the combustible solids so that the fuel stays within the industryspecifications.  

With today’s engine packages, this is more important because the solids that fuel – particularly diesel, kerosene, home heating oil, and some of the JP (jet propulsion) fuel, creates are exactly the aspects of fuel that add lubricity and BTU value to it.  If you cycle the fuel through filters time after time after time after time to remove all of those solids, you’ll eventually knock the fuel out of spec!  It simply won’t have the power or lubricity that your diesel engine will require!  

A majority of the engine failures are the result of poor fuel quality. Plugged fuel filters, premature fuel pump failures, damaged injectors, lost time, and lost revenue can all be directly attributed to a diesel engine trying to burn fuel that it should not.  Generators, trucks, locomotives, ships, and heavy equipment have all suffered, at one time or another, from such an issue.


It usually isn’t the problem of your fuel distributor, your maintenance staff (hired or in-house), or the engine manufacturer.  Diesel fuel has its own dirty little secret:  it can degrade all by itself.  And it will, if it is stored long enough without any maintenance.  Add to that the strong potential of water in the bottom of a tank and we have the beginnings of a dramatic situation.  The good news is that we know how to fix it!


Two big things have happened since Y2K in the world of diesel fuel consumption:  1. the widespread use of the common rail fuel injection system, and 2. the federal mandate to manufacture ultra-low sulfur (15 ppm, known as S15) diesel fuel.  One could argue if either one is actually good for the end user or not, but the fact is that we have them both, and they are here to stay.


Simply put, a diesel engine demands high quality fuel (or food, if you will) or it simply isn’t going to be happy.  Primary and secondary filtration on a new engine is often starting at 5 microns and ending at 2 microns!  Injectors that used to pour fuel into an engine at a “loosely” regulated rate now have nozzles so small that you can’t see the opening with the naked eye, and the rail head that supplies fuel to those nozzles is holding the fuel under 20,000/30,000 psi!  There just isn’t much room for error in this new engine world.


At the same time, refineries have had to alter the process that makes diesel fuel so that it meets the government mandate for ultra-low sulfur.  The cracking and refining process that is necessary artificially ages the fuel and badly affects the stability (meaning storage life) of the fuel.  Long term storage of fuel is now considered 8-12 months.  Anything not consumed in that time is going to create quite a problem for the “Tier” rated engines.


All is not lost!  Fuel is a major investment and should be treated just like your engine is.  Put it on a preventive maintenance schedule.  You already check the air filter, the coolant, and the lube oil, so why not check out the bottom of your fuel tank, too?  A visual sample is fine if you are NOT mission critical.  If you are, have the fuel quality tested, too.  If the fuel fails to meet ASTM D975 (the specification for diesel fuel), then have it polished.


Fuel polishing is not fuel filtration.  There’s a big difference, and an important reason to make sure you know what you are getting.  Filtration is generally defined as the process of separating suspended particles from a fluid by flowing both through a porous material in which the fluid can pass while the suspended particles are retained.


Will filtration remove a fluid from a fluid (like WATER from DIESEL)?  Well, some filter media (the stuff the filter is actually made from) will absorb water – like paper, for instance.  Not always, though, so keep an eye on what kind of filters you buy!  Water blocking filters are more expensive than particulate only filters, but to Diesel Fuel Doctor’s eyes almost always worth it.

So, to recap, filtration removes anything from a fuel that won’t fit through a certain size hole (the micron size rating of the filter).  And, the water absorbing tendency of the filter media may or may not remove free and entrained water in the fuel.

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