I regard that engine mounted filter as only a 'guard filter' ... just
there to 'choke up and clog quickly' if all hell breaks loose
upstream/prior of this filter. Filters do break and can extrude
'soft'/deformable particles when operaing at high pressure across the
filters. These filters are made from a 'resinated' cellulose and the
water in the oil (free water and water in emulsion form) can 'digest'
or sofften the cellulose if the filter is left in such system for long
periods. For most applications the 'guard' filter should be preceded
with a filter of the exact same 'retention rating' ..... unless the
'guard' filter has much much larger surface area than the preceding
filter in the series.

Most diesel injector rebuild shops seem to be of the opinion that a
20µM particle is the most damaging (small) particle for injectors and
high pressure pumps, etc.

Common boat fuel Filters are only 'rated' at a 'nominal' rating -
jargon of the filter industry that means that a lot of particles
'larger' than the rating can pass through (typically by about 3-7% by
weight basis). Therefore a filter should be sized about 3 times
'finer' than the target retention. Therefore, to protect against 20µM
particles you need somewhere about 7µM. Racor only makes 30, 10 and
2µM (all probably @ 97% 'nominal' retention) .... so choose a 10µM as
the 'final' (sometimes called the 'secondary') .... AND also guard
filter.

Why 10 and not 2? A 10µM will have 5 times the flow capacity (with
'size' or surface area of the filters being the same) of a 2µM and
will capture a goodly % of 2µM particles. What this also means is that
it takes 5 times the WORK to pump through a 2µM filter than a 10µM
filter .... and the potential to break your diaphragm lift pump is much
higher the smaller you go in retention size. So, if you simply change
from the 'OEM design' at 10µM down to 2µM you need to increase the
'surface area' of the filters to keep the same work load on the lift
pump; or, you can expect premature failure of the pump due to the
increased 'work load'.

Also in nature the smaller the particle size the exponentially MORE of
them will be there .... so I guess the real question becomes if the
most damaging particle is ~20µM and the smaller the 'retention' the
more work you will have do while needing larger surface area filter to
do the same amount of 'work' and the smaller retention will capture
MORE particles thus you need to de even more work, etc. .... why do
this 'finer' retention when it probably isnt necessary, more costly?
..... and will probably cause premature failure of the lift pump
diaphragm?

A serious filtration system on a boat has probably an independent
recirculation polishing system (to constantly remove the particles as
they 'form' by agglomeration IN the tank) somewhere at 3GPM 'turnover
rate' constantly pressure feeding a standard industrial configuration
filter of 10" X 2.5" dia., 5uM 'depth' type filter. Constantly
reciruclating fuel through a 5µM will result that the fuel IN the tank
will have essentially only 'submicronic' particles. You can hard wire
a polisher to be on any time the engine is on. The main filtered
deliver system then simply ---- 10µM ---- 2µM ----- small 'day tank'
---- 2µM guard filter, with the main delivery system hardly ever being
challenged by particles as essentially most/all are constantly removed
by the polishing system. The purpose of the (gravity feed) day tank is
such that if catastrophy happens such as broken lift pump, clogged
filters, etc. etc. you can simply open the vent on the day tank and
have about 2-3 gallons of fuel to keep on going for about 2 - 3 hours
until you can 'sort things out later on' .... beats hell out of
power-puking into a bilge when changing clogged filters when a rough
sea is running.
All these filters are 'pressure feed' (using stainless steel tubing
with double flared connectors, instead of the cheap-and-dirty 'vacuum
feed' system --- and filters monitored with gages so you can monitor
the system and know WHEN to change out, and long before the filters
choke. The recirculation system uses very 'cheap' filters, does 99% of
the work of filtration, keeps the particles from 'growing' IN the tank
because they are constantly removed, keeps the tank cleaner, and if you
get crud - will quickly return the tank down to a 'background' of
essentially submicronic particle very quickly. etc. Of course you
need to start such a system with a CLEAN tank. Most tanks need to be
cleaned out on a regular basis - saves a hell of a lot of filter cost
and hassle at the wrong time.

hope this helps.





In article , Wayne.B
wrote:

On Sat, 08 Dec 2007 04:50:48 GMT, Rich Hampel
wrote:

Ive been deeply involved in filtration engineering for the past 30
years so I just thought Id remove a lot of the common misperceptions
especially about 'boat filters' with these posts.

Rich, while you are here, perhaps you could comment on the question of
what micron rating to use in Racor filters mounted ahead of a 2 micron
engine mounted filter. The trawler groups have thrashed this one to
death, and we've touched on it here a couple of times.

In most cases the Racor is much easier to change than the engine
mounted filter leading some to believe that the Racor should be 2
microns also. I've come full circle after reading some of these
discussions and have started using 10 micron Racors and changing them
when they begin to show a significant vacuum guage reading. I've had
no issues with clogging the 2 micron secondaries on the engine and
just replace them once a year as routine maintenance.