Floscan Pre-Installation Manual for Diesel Models - Manual (Page 8)

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Diesel engine fuel systems are usually protected by a “Primary” filter plumbed upstream of the engine’s lift or fuel transfer
pump, and a “Secondary” filter plumbed between the lift and injection pumps. Each filter type performs a different function,
and both are needed for complete protection. The Primary Filter, Fuel / Water Separator (FWS) or Sedimentor is your main
defense against water and heavy fuel contamination. It uses gravity and centrifugal force to remove free and emulsified
water, and large contaminants. Engine manufacturers recommend that Sedimentors should also remove smaller particles in
the 25-30 micron range. Many incorporate a 20 to 30-micron particle filter for additional cleaning. Using a primary filter
under 20 microns results in frequent filter changes and a High Vacuum, fuel restricting pressure drop across the forward fuel
line. They should be avoided.
Better quality secondary filters come with a drain port and are called Agglomarators. They also have the ability to trap
suspended water droplets and remove them by coalescence. Fuel traveling at high velocity enters the filter housing where a
swirling motion is created. After centrifugal force removes the larger particles, fuel passes through the secondary filter
element, (Usually 7–10 microns) for its final cleaning.
Filter Element Types – Depth & Surface
Filtration elements in use on today’s Marine Diesel fuel systems fall into two categories, (Depth & Surface). In depth type
filter elements, fuel is forced to travel a circuitous route through multiple layers of material. Contaminants are trapped
because of the intertwining path that the fuel must take.
Surface type elements work by catching contaminants and holding them on its surface.
Filter Element Micron Ratings – Nominal, Absolute & Beta Ratio
A nominally rated depth type filter element doesn’t have one consistent hole, or pore size. Each element is assigned a,
“Nominal Rating” such as 25 Micron, (1 micron = 1 millionth of a meter, or 0.000039”). A 25 micron rating means that the
filter element’s, “Mean flow pore size” is 25 Microns. Initially the filter will remove most contaminants 25 micron & under,
but it will also allow a certain percentage of contaminants over 25 micron to pass through, (Percentages vary between
manufacturers as there is no standard to follow).
Mean flow pore size is that pore size which allows half the fuel to pass through holes of equal or smaller size, while the
remaining fuel passes through larger holes. In a nominally rated 25 micron filter, half the fuel flows through holes 25 microns
or less, while the other half flows through holes larger than 25 microns.
An absolute filter rating gives the largest hole or pore size in the filter element. A filter with an absolute rating of 25 micron
has no hole larger than 0.000975”. In theory, the filter traps all particles larger than 25 microns, and most particles under 25
as well.
Beta Ratio is a method of comparing and rating filter performance. It is the ratio of particles before filtering to the remaining
particles of the same size after filtering. If a filter is rated at Beta 25=75, it means that at the 25 micron particle size, the ratio
of particles upstream, versus downstream of the filter is 75 to 1. At 25 microns this filter is 98.7% efficient.
Filtration Media
Pleated paper is the most common depth type filter media used on fuel systems. They’re manufactured by binding specially
treated wood cellulose fibers with resin. When pleated, this process produces a structurally stiff paper with a high capacity to
catch and hold dirt, a low absolute micron rating, and a greatly enhanced beta ratio performance.
Synthetic depth type elements are produced from several man made fibers including, dacron, fiberglass, and
polyvinylchloride. Compared to cellulose, synthetic fibers have very smooth surfaces. A smooth surface greatly reduces fuel
flow resistance, and allows for a much lower absolute micron rating.
Surface filters are manufactured by weaving steel wires or nylon monofilaments into wire screens or nylon cloth. These
filters have a high or coarse micron rating and are used as fuel system pre-filters. They’re sometimes used on fuel tank pick-
up tubes, or up-stream of the primary filters.