Join Date: Jul 2000
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No downside at all.....
You can arrange a day tank so that its always filled by the main system with already filtered fuel, etc. Such an arangement is constructed so that the tank never empties as the exit nozzle is higher than the fill nozzle and is always filled but when your lift pump fails or you filters 'choke' .... just manually throw a **** to allow the tank to vent to atmosphere and throw another one to drain the day tank directly into the remaining system. On most 30-40 ft. boats a 3 gallon day tank will allow about 4 hours of operation (at .75 GPM flow. :-)
My personal fuel system is somewhat complicated but simple in concept: recirculation Fuel polishing system is on whenever engine is on ---> Single but LARGE Racor in main delivery piping ---> day tank (locked out until 'needed' but always storing ~3 gallon of clean filtered oil) mounted ABOVE engine so it can gravity feed --->'choke filter on engine, etc. System is pressure feed so all the filters are at better removal efficiency and on-stream service life (electric pump is at the main TANK). Polishing system maintains tank at less than submicronic levels using 5uM depth type filter. Tank is manually cleaned every 2 years. All lines are stainless steel with double flared connections ... as copper is reactive with diesel fuel. I change the Racor about every 3-5 years (based on operating pressure differential vs. engine max rpm) as all the filtration 'work' is accomplished by the polisher leaving the Racor as a 'redundant' or last chance filter ... I track the DP gage to monitor system performance and alway KNOW long before I NEED to change a Racor. My typical resident particle distribution in the tank from constant recirculation through 5uM on board polishing filters is essentially sub-micronic (based on nephalometry (light scattering) coorelation measurements ... ie.: the oil is always CRYSTAL clear.
I visually inspect all fuel before taking it onboard ... if its not 'fresh' or has visual haze when held to bright white light in clear glass container ... I simply go somewhere else. I usually get my fuel from a truck stop and carry it in or only buy from fuel depots that service the commercial marine trade. I put onboard only what I plan to use plus a bit for reserve. I never FILL the tank and use a desiccant vapor absorbing filter on the tank vent .... no water vapor intrusion (water vapor/humidity equilibration). When off the boat for long term I close the tank vent valve (my tank is rated for high vacuum and some pressure) ... I built the tank.
I find that almost all diesel fuel systems on 'boats' are poorly designed (vacuum feed ... which is bad as a filtration application and BAD from a hydrodynamic viewpoint as vacuum motive systems - suck), contain a lot of copper (reactive with #2 fuel) .... and are built that way because thats the absolute cheapest way to do it (cheap and dirty).
I (hope) dont worry about tank slime, bacterial debris from breaking loose from the tank walls in a heavy seaway .... If it does, I always have 4 hours of CLEAN fuel in the daytank ... and dont have to risk power-puking into the bilge because of plugged filters. If all hell breaks loose (broken lift pump, 'particulating' tank, etc. ... , I can wait 3-4 hours before I HAVE to do 'something'. :-)
Last edited by RichH; 01-18-2007 at 12:12 AM.