Article #2

Here is another article I found on the subject, during my recent research.

Dave














fuel filter

Fuel Filter Rules

Guidelines for easy filter installation.
By Jim Daly
Motor Boating Magazine

Diesel engines come with secondary filters mounted just before the fuel injection pump, but they must also have primary filters to remove water and particles that are in the fuel supply. Water in the fuel drops into a see-through bowl at the bottom of the filter and can be removed by opening a drain plug or valve when necessary. Such filters are commonly referred to as fuel/water separation filters.

Installing a new fuel/water separation filter is one of the best investments you can make for the health of your diesel engine. And it can make for trouble-free boating. The installation is straightforward if you remember the cardinal rule of fuel systems: fuel pressure must be maintained. If it isn’t, no matter how beautiful the installation, the engine will be starved. Most diesel engines want a vacuum of no more than 10” of mercury (Hg) or approximately 5 psi. At greater vacuum the engine will either fail to start or will run erratically at high rpm. Fuel pressure can be reduced by the length of the fuel line, the number of fittings, the height of the filter above the fuel tank and the number and angle of bends in the line. Here are the rest of the rules.

1 All fittings should be full-flow National Pipe Thread Fuels (NPTF) type because they cause the least fuel pressure drop (typically 0.1 psi per fitting versus as much as 1.6 psi for common pipe fittings) and they are designed to seal when wrench-tightened. NPTF fittings seal because the crests of the threads are crushed when tightened, whereas conventional fittings have a tiny gap between the crest and root of the thread when tight.

2 Use only fuel-proof thread sealant/lubricant for pipe “dope.” Teflon tape only lubricates the thread (does not seal) and will shred under pressure, potentially sending particles into your fuel system.

3 Try to keep the filter body below the top of the fuel tank to reduce pressure drops and have a light color (or reflective material) behind the bowl so that it can be easily inspected for water accumulation. Don’t forget to leave enough room around (typically above) the filter body to allow access to the filter cap as well as space to remove and replace the filter element.

If you have sufficient reserve in your vacuum calculations after counting the pressure increases from hose lengths and fittings, adding a stopcock or two is helpful. Depending on the level of the filter input fitting in relation to the fuel tank, a stopcock in front of the filter can prevent fuel from draining back into the tank or siphoning in the opposite direction. Similarly, a stopcock on the output side of the filter could keep spills to a minimum. Though the fuel lift pump is typically much lower than the fuel filter fitting (so fuel should stay in the line), a good swell could rock the boat enough to spill a small amount of fuel.

4 Be sure to use a USCG-rated fuel line Type A1 or A2 (capable of passing the 2.5-minute fire test) of adequate diameter. For example, a 3?8-inch inside diameter (ID) hose can supply up to 36 gph, keeping the flow rate at two feet per second or less (common flow rate requirement). A 1?2-inch ID hose doubles that capacity to 72 gph. Also, be aware that every foot of fuel hose adds approximately a 0.1-psi pressure drop to the system.

5 The installation should be plumbed as in the diagram and is really not as complicated as it sounds. Just follow the rules by using components rated for fuel systems and not substituting fittings that simply look like they’ll work just fine.