I realize reading it, that my secondary or guard filter is probably 10uM
since it is a Yamaha
(Yamaha and Yanmar ... and Universal and Volvo, etc. dont make
filters .... they buy filters and have someone pait their name on the
filter .... then sell it at 5 times the market price)
part intended for the filter that came with the engine.
I assumed that these were all 2uM from reading previous semi informed
threads.
In real life and without doing an intensive particle distribution
analysis typically any 'terminal/final' filter should be protected by
a pre-filter that is approx. 5 times the retention size of the filter
doing the most-important-work. For best economy, what you want is
that all the sequential filters ----- FAIL all at the same time; and,
for most normal particle distributions in nature the 5:1 sizing will
usually hit the mark the first time out. The next way in 'real life'
is to simply record the differential pressure (gage) across each
filter VERSUS actual gallons/liters filtered, then you can adjust
filter AREA to balance the flows and service life. A fuel oil system
isnt a 'constant state system' so the best is probably just go with
equal surface areas in each and 5X larger retention in the prefilter.
Usually the way filtration of pleated filters works is that (for
constant conditions) if you double the surface area you get about 3
(2.7) times the service life.

PROBABLY A PUMP PROBLEM
probably the beginnings of a pin hole in the diaphragm or a check
valve isnt closing all the way.
I would appreciate your specific answer to the question I've posed which
will apply to many small sailboats. Since the smallest Racor I can buy is
rated at over 15 times the fuel flow of my 20 hp diesel with return line to
the top of the guard filter instead of the tank, does it not make sense to
run a 2uM element in my primary?
NO, not ever. Use a 10µM in FRONT of the 2µM. If the final is 10µM
then use a 30µM RAcor in front of it (Racor doesnt make 50µM) The
10µM will capture a LOT of 2µM particles - the 10µM Racor is probably
60% efficient in capture at 2µM (guesstimate) ... and this will
prolong the life of the final 2µM.

. I don't think my engine fuel pump is
running very hard. I had a lift pump because the filter and fuel pump were
mounted high above the engine.
PUMP should be as LOW as possible; BUT, once the delivery system is
filled it really doesnt matter as the 'siphon effect' will take over -
vertical measured difference between the **maximum/current** fuel
level in the tank versus the vertical height that the pump is
located. Whats really important with (any) pumps is to have some
(siphon) pressure filling the system AND ****the height of fuel****
versus the height of the pump. In your below mentioned 'problem' I'll
bet the farm that this was during the tank being fairly empty... if
the tank was totally filled it would pump MORE (due to this 'siphon
effect' from the tank - also called 'net positive suction head').

PLUS boat engines fuel systems are hooked up ALL WRONG versus the
'hydraulics'. If that lift pump was located back at the tank (like
modern trucks and automobiles are) the pump could possibly develop
upwards of 15 psi pressure, enough to push oil through a partly
clogged filter (they usually stop flowing at 15-20 psi **pressure**
differential. In vacuum mode the best a single stage pump can attain
is about 6" vacuum (about 1/6th of atmospheric pressure - 14.7 psi or
30 inches of vacuum.), so the max. a pump located DOWNSTREAM of a
filter can do is about 2.5 psid (6"vac) if that diaphragm pump is
'pulling' a vacuum, it may be 'stalling' because of the 'vacuum'.
PLUS a diaphragm pump isnt perfect as it pumps a small amount
backwards each time the diaphragm moves down .... until the poppet
check valve fully closes. That 'jiggles' the crud in the filter and
'compresses' the deposited dirt making it additionally harder to pump
through the filter. A small constant displacement gear or vane pump
(with 'blow off valve' to the return line) would be much better.
Pumps dont like to 'pull' they are better at 'pushing' .... filters
will last longer in a pressure feed system than in a vacuum feed
system (has to do with the way the debris is depositied IN the filter
media). If you need to 'protect' the pump use a fine mesh strainer,
NOT a filter.

When I moved the filter and pump to below
the engine, I got 1000 RPM more WOT with the pump turned off and fuel
flowing by gravity than I used to with the lift pump running all the time.
You're adding 'static head' (weight of the column of fluid) to the
inlet side of the pump .... But probably this means that the pump is
faulty ..... Id remove it, put it on a bench and check it to see if
it HOLDS pressure .. just blind off the pump into a small gage
(either end, sequentially) and see when you give it a few strokes (vac
or pressure) that the gage STAYS at the pressure and NOT leaks back to
zero quickly ..... bad pump valves, pin hole in the diaphragm, hole or
tear developing in the diaphragm etc. If fits a valve its probably
the 'inlet' valve.

I like the idea of the constant fuel polishing but I don't think there is
space or weight capacity for it in my boat. The fuel quality is pretty good
up here and the cool climate of Maine seems to help.
I use a 3 GPM 'turnover' rate for a 100 gallon fuel tank. .... start
with a CLEAN tank, do all the filter work in cheapy filters
***offline*** from the main distribution system. Dont even THINK
about using the engine return line for recirculation-polishing
purposes.

I've currently got a debate on similar issues with differing "informed"
viewpoints being presented in my professional life. If you would like to
get paid for an hour of work to offer another viewpoint, please email me
privately.
If its filtration Im partly 'contracted' to more than a few
manufacturers listed above; but, I can help with the 'proof of
concept' and prelim estimates, etc.; then, I can get you in touch with
the correct application engineers, etc. of the most suitable companies
or their technical distributors. For any filtration application I
will need all the 'technicals' on the 'nature of the fluid', whats the
industry, the purpose of filtration, the approximate % of solids(or
deformables) to be removed, the particle 'distribution' of the fluid,
MOST IMPORTANT the absolute viscosity in centipoise, the absolute or
nominal retention desired (and why), the Temps, Pressures, the
'accuracy' of filtration, specific gravity, explosion hazzard,
materials for the housings, batch or continuous, bio-hazard, haz-mat,
etc., sterilization or Titre reduction if biopharm, .... total
chemicals and physicals plus the flow rates on the fluid, etc.
RhmpL33ATattDOTnet.

Thanks again,

--
Roger Long