Copper, brass, bronze are REACTIVE with diesel fuel. Such materials will lessen the 'shelf life' of diesel ... normally 6 months in warm weather without coming in contact to 'copper'. Stainless steel tubing with carefully made/assembled double flared fittings is best. Use the MINIMUM of copper containing parts in your system.
For any recirculation system you want lots of 'volume flow' with little pressure drop across fittings, valves, etc. so the pump doesnt have to 'work so hard' and most of the 'pressure' is used to drive the filter .... use a tubing bender and make long radiused elbows, etc. Keep the amount of connections, valves, etc. to a MINUMUM to keep the pressure losses to a minimum.
For a recirculation system consider: the pump 'pressure feeding' the recirculation filters (pressure feed) as the filters will have better on-stream service life (Its the way that debris deposits on/in filter media; although you must make precise connection so that it doesnt leak. The reason for vacuum feed in fuel systems is: use of cheap compression fittings (always eventually leak) and such a leak will suck air and stop the engine .... instead of filling the bilge with oil. Pressure feed is best but the connections, etc. must be 'perfect' ... steep fines are involved if you inadvertantly pump fuel overboard.
Ultimately you want the polishing system to 'turn-over' and re-filter ALL the fuel many many times in as little time as possible. 10-20 uM 10" long 2.5" dia. depth type 'industrial' filter cartridges are probably the best choice: are economical; and, with many many tank turn-overs will result in the tank will 'equilibrate' down to essentially 'submicronic' particles in very short time. Consider an industrial filter housing made from carbon steel with a cast steel/cast iron 'head' but with a drain valve ... to which you attach clear plastic (tygon, etc.) tubing so that when the filter housing traps free water you can SEE the water in the bottom tube and can easily drain it. Use EPDM gaskets or 0-rings whenever possible as it 'swells less' than the common (and qucikly becoming obsolete - Buna, Neoprene, etc. gaskets, etc.)
Stay away from 'self contained' spin-on filters .... very expensive and sometimes hard to get in 'far away' places. Usage of such 'spin-on' filter 'lock' you into using a single manufacturer. There are literally 'hundreds' of industrial filter manufacturers ... so the availability is much much greater.
Totally scrub-out and clean the fuel tank internals BEFORE you apply a polishing system ... no sense in wasting a lot of filter cartridges for 'naught'. Clean out the tank every few years.
Hope this helps. :-)
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hope this helps.