I thought I could put this subject to bed, but after reading psneeld's
post, it got me thinking that maybe the 'fuel flow vs. filter capacity vs. filter effeciency equation' needed more investigation. I decided to go back to the original Racor - Parker Hannifin documentation I used for my research a couple of years ago to see if I had overlooked anything. Lo and behold, maybe EVERYONE IS RIGHT!! 2-Micron, 10-Micron or 30-Micron - you are all correct!
Racor clearly states that "If the secondary filter has a rating of two (2) microns, the primary filter should have a rating of (10) microns. If the secondary filter is rated at 10 microns, the primary should be 30."
But later in the document they state that in a marine environment, "If the installation can allow the use of a filter large enough, then a two micron filter can serve in a system as the only filter in that system."
Interestingly, throughout the document, there are distinctions made between marine, over-the-road and agricultural usage relating to fuel quality, cleanliness and filtration.
From this I think it might be safe to conclude that adequately sized
, 2-Micron filters of the proper type can be used as Primary filters. There should be no adverse effects on the lift pumps, and the filters should have a reasonable service life.
Below are excerpts from the document where I have highlighted in red the two paragraphs that at first, seem to be contradictory, but ultimately seem to support both sides of this issue.
RACOR Fuel Filtration
PRIMARY FUEL FILTRATION
Primary fuel filters play a very important role in the diesel fuel system. Their primary purpose is to protect the transfer pump and remove water contamination. Another purpose is to protect the secondary filter from much of the asphaltene carried by the fuel (Fig. 37).
The finer the level of filtration that the secondary filter is designed for, the more quickly it will choke from the asphaltene in the fuel. A properly specified primary filter will share the choking effect of asphaltene, thus allowing the secondary to be replaced less frequently.
If the secondary filter has a rating of two (2) microns, the primary filter should have a rating of (10) microns. If the secondary filter is rated at 10 microns, the primary should be 30. It is recommended that filters which combine water separation with the task of primary filtration be used instead of a simple primary filter.
It is desirable to choose a primary filter that will provide the lowest pressure drop or negative pressure at the outlet of the filter at rated flow when the filter element is new. No more than two inches of mercury (2”/hg) negative pressure should ever be allowed when a new primary filter is first installed.
Recently (1998), many engine and fuel injection system companies have begun to recommend that the fuel injection system be protected by a water separator. In fact, suppliers of common rail fuel injection systems demand that high efficiency water removal be part of the primary fuel system. The best value in meeting this demand is to install a primary filter that is designed to separate water on the suction side of the fuel system. Water separators are discussed in detail in Chapters 9 and 10.
Fuel filters are supplied for various applications and, therefore, there is a need for different levels of filtration efficiency in the removal or retention of particulates. The hydraulic industry uses a rating method that uses the term "Beta Ratio" to describe a filtration efficiency level. The diesel fuel filtration industry generally uses simple efficiency as the method of rating a fuel filter. Since there is no such thing as a fuel filter that provides absolute filtration of the particle sizes that are cause for concern, the
industry uses terms like 96% @ 5 microns. This term means, that when tested to SAE or ISO test methods the filter will retain 96% of all 5 micron size and larger particles.
Racor makes filters with various filtration efficiencies, but its standards for non-OEM (Original Equipment Manufacturer) are 2, 10, and 30 micron filter elements. The actual efficiency ratings for these are 98%, 95%, 90%, respectively. Racor also makes extensive use of a 7 and 20 micron filter medium which are used to meet certain engine manufacturer's requirements for a final filter and a primary filter.
Racor’s two micron filter medium should only be used in final or secondary filters where the fuel is first filtered by a primary filter. The primary filter for a two micron final filter should use a 10 micron medium. The exception in using two micron filter in a primary filter is to obtain high-efficiency water separation, and is usually used in marine applications where the fuel supply may be cleaner but also many contain water more often. If the installation can allow the use of a filter large enough, then a two micron filter can serve in a system as the only filter in that system.
The new high pressure common rail fuel injection systems require high efficiency in removal of small particles. The requirement is 95% for three micron particles. Racor fuel filters have a medium designed for these applications. Replacement elements should state, "For Use With Common Rail Fuel Injection Systems."
Dirt levels in fuel also direct the level of efficiency required. Since the filter removes a percentage of dirt particles, it follows that when a much greater amount of dirt is present in the fuel, a greater number of particles will pass through the filter. Diesel engines used in earth moving or agriculture should use fuel filters that have higher efficiency than those for over-the-road or marine use.
This publication is 68 pages long and is worth downloading and reading, or at least keeping as a resource.
Parker Brochure No. 7550
OK, now I don't know what filter element I will use next time I change my own filters. 56hp, how many gph, yada yada yada . . . .
I'll worry about this another day. I'm going boating now!! Have a great day all.