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Ive been deeply involved in 'filtration engineering' for over 40 years, including indirect work with and for the 'parent' of Racor.
The acknowledged most damaging particle in a marine diesel is ~20ÁM.
Typical set up for marine diesel engines (unless otherwise specified by the engine mfgr.) is tank --> 30ÁM --> 10ÁM --> 15-17ÁM for the (very small) engine 'guard' filter --> engine.
It takes a lot of WORK to make such filters operate. A 2ÁM (of equal surface area) will have 1/5th the flow rate capacity (vs. differential pressure across the filter) of a 10ÁM. To operate at the designed (WOT) flow rate of a 10ÁM a 2ÁM will have 5 TIMES the pressure drop across it; OR, must have 5 TIMES the surface area to equate to the resistance or on stream service life of the 10ÁM.
High differential pressure across a 2ÁM can lead to 'exponentially earlier' plugging with debris and significant work load on the lift pump and may cause premature lift pump diaphragm FAILURE. There are exponentially MORE particles at the 'finer' ÁM distribution size range ... .
Simple speak: putting a 2ÁM (nominal) filter in place of a normally used 10ÁM will cause premature plugging (approximately 10 times 'faster' or at ~1/10 of the total amount of fuel filtered before plugging), undue load on the lift pump, etc. etc.
The small engine mounted guard filter should be LARGER in ÁM retention size the next 'upstream' filter in the sequence. Its there 'only' to stop debris if the upstream filters FAIL... a 'last chance' filter.
If you somehow erroneously feel that using a 2ÁM is required for 'your' system, it should be preceded by a 10ÁM, the 10ÁM should be preceded by a 30ÁM .... all these 'prefilters' are to extend the onstream service life of the immediate downstream filter. All filters add to the overall workload on the engine's lift pump.
Modification of the 'typical' system should only be based on system operational history of total gallons filtered before plugging (in accordance to recorded 'history' of total gallons filtered vs. WOT volumetric flow in gallons per hour vs. differential (gage) pressure across each filter).
On most marine diesel engines a 2ÁM filter HAS NO TECHNICAL REASON to be in the filter train.
Maintain and clean the internals of the tank on a regular basis, buy only 'fresh' fuel from high turnover supply sources .... and you wont be 'challenging' the filters with a lot of debris.
Last edited by RichH; 06-02-2011 at 05:01 PM.