Recomendations for a good fuel/water separator

[never monday]

Always glad to help where I can.

[ronbo1]

Gordo,

Right you are...as usual.

Ronbo

[GordMay]

If you want to add a Secondary filtre prior to the on engine Primary filtre (between the tank and engine) you should use at least a 10 micron filtre. If you use a filtre that is known to be of good quality, then 10 micron will likely be sufficient. If you are at all suspicious about fuel quality, you could go to larger 15 or 30 micron elements.

Depending on engine make and model, the final filtre (primary), mounted on the engine just before the injection pump, will have a single-pass retention (pore size) rating of 2 to 10 microns*1. This is the size that most*2 engine manufacturers figure will protect injector pumps and nozzle tips from damage.

*1: A micron is a thousandth of a millimeter. A grain of salt is roughly 100 microns in size, a human hair about 70 microns, and a single grain of talcum powder about 10 microns.

*2: I understand that Westerbeke recommends 25 micron filtres.
Racor filtre elements have a code number (2010, 2020) followed by a letter or letters: PM is for primary, which is 30 microns; SM is for secondary, which is 2 microns; and TM is 10 microns.

The Secondary fuel filter must offer low restriction because it is mounted on the suction side of the fuel pump, where normally a suction pressure of only 5-6 pounds per square inch is available. This filter has the job of protecting the transfer pump and lightening the load of theprimary fuel filter (if installed). Secondary fuel filters typically have a nominal rating of 10 - 30 microns.

Primary fuel filters are mounted between the transfer pump and the injectors. The primary fuel filter is designed to offer full protection to the fuel injectors. Since these filters are mounted after the transfer pump they tend to see much higher pressures than primary filters. Primary fuel filters typically have a nominal rating of 2 - 10 microns.

The Primary filtre is ALWAYS finer (smaller micron size) than the Secondary filtre.

Some other tips:

Filter units are also sized by flow rate, expressed in gallons per minute. A handy rule of thumb is to multiply peak engine fuel consumption by 3.5 or 4, then divide by 60 to get the fuel flow rate in gpm for a four-cycle diesel. Another is horsepower × 0.18 = gpm.

Asphaltenes can be formed, when diesel fuel is heated and pumped. Asphaltene sludge can be mistaken for bacterial contamination, but it obviously will not be eliminated by a biocide. Accordingly, if you're experiencing filter plugging, you should test to see what is causing the problem.
ie: Dieselcraft "Fuel Bug" Kit ($10): Oil and Diesel Fuel Purification Systems for all diesel engines

Ideally, you should refuel at the end of day or run because today's diesel engines (depending on make) recirculate as much as 75-85% of the fuel delivered to the engine. Each "trip" to the engine and back increases fuel temperature, and sources tell me that in-tank fuel can reach up to 100 degrees F. Hot fuel, in a mostly empty tank, leaves hot air, which cools and condenses, forming water droplets that are added to the fuel. Refueling, before shutting down, is desirable.