ummmm Sorry Maine, what you state is totally unsubstantiated.
Being involved deeply with filtration engineering for well over 30+ years its VASTLY BETTER from a hydrodynamic as well as filtration efficiency standpoint to put any pump on the INLET side of a filter. If there is a proper screen downstream (similar to but with sufficient surface area in comparison to whats normally found on the suction side of the dip tubes (grossly undersized), there is no need as such pumps will handle particulate well into 100µM without difficulty or harm. One must acknowledge that the particles in most diesel fuel systems are 'soft' and deformable with few hard particles such as scale, rust, etc. Its the same with automotive high pressure systems, automotive (truck and locomotive), aircraft, ship and stationary prime mover (generator) systems. The only poor reason to operate a fuel vacuum delivery system is to match the characteristiclyh poor copper tube/compression fitting system (cheap and dirty) so that if there is a leak, the system sucks air and stops instead of filling the bilge with oil. Filtration efficiency is much better with pressure feed (debris deposition considerations), the pump will not stall (single stage pumps will slip at 6" hg. vac.) while being able to deliver upwards of 15 psig in the pressure feed mode, etc. etc. Flow through a filter is based on characteristic pressure (or vacuum) versus flow rate. If a vacuum pump normally stalls at 6 inhg.vac. (approx -3psi motive force) will be about 1/5 (15 psig) the motive force available if the SAME pump would be in pressure feed mode .... which on ANY filter flow vs. differential pressure curve will show that you can have 5 times the maximum flow rate (or approx. five times the service life) in pressure mode versus vacuum mode .... up to the limit at which the pump either stalls or slips. Each filter needs its own gage. Most Racors will handle upwards of 30 psi pressure before the pleats collapse or will exhibit 'breakthrough'. A well and proper designed pressure feed system with matching hose/tube/fittings, etc. is VASTLY superior to a vacuum feed. A pressure feed system that is monitored by a pressure switch that signals sudden drops in pressure (like on an automobile with an installed pressure safety cut off switch) is all thats needed to detect 'leaks' ... so that if there is a leak the electric pump will automatically shut down. The financial problem for 'boat builders' is that a system built of stainless double flared tube plus matching connections, high pressure and rated flex tube, putting a 12vdc pump IN/ON the tank and the proper monitoring system ...... is more expensive. The typical vacuum system is 'silly' from a fuel delivery standpoint. The only saving factor for those with poor maintenance skills and techniques it will fail 'safe' ..... and as your boat drifts toward the rocks because of a poor design fuel system. I include not routinely maintaining tank cleanliness along with poor maintenance skills. Just look at the HIGH PRESSURE side of the injection pump, its well designed and hardly ever leaks, doesnt it. A vacuum motive fuel system is simply 'cheap and dirty'. BTW - the usage of vacuum gages on a vacuum motive system is only valid ..... while reading the gages while the engine is at wide-open-throttle (WOT). If you have ~ 6" hg. vac or -3 psig, the filter is near its full capacity of debris. Since Racors are the most common, go to Parker.com --> marine ---> racor ... and look up the flow versus differential pressure curves for your filter; look up the maximum consumption of fuel for your engine in gph or lph and enter the curve and find the corresponding vacuum or pressure differential .... that will establish the highest vacuum or pressure on the gage so that you are sure of adequate flow to the engine at WOT. Most will change out at 75% of that pressure gage value. ;-)
. sorry about all the questions. The answers may help. No fair, it took me 30 minutes to type this and some of these guys are on the same page. r.w.landau
