The primary 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 the secondary fuel filter (if installed). Primary fuel filters typically have a nominal rating of 10 – 30 microns. Secondary fuel filters are mounted between the transfer pump and the injectors. The secondary 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. Secondary fuel filters typically have a nominal rating of 2 – 10 microns.
A primary diesel fuel filter is the first filter in a two filter system. It is located before the secondary filter, and usually before any transfer pump in the system. A primary filter is used to remove the bulk of free water and contaminants and thus protect the transfer pump (if present) and secondary filter. Typical primary filter efficiency is 95% to 98% @ =10 to 50 microns. A secondary filter is the second filter in a two filter system. It is located between the primary filter and the fuel injection system, often after a transfer pump. Typical secondary filter efficiency is 95% to 98% @ = 3 to 10 microns. Newer high pressure fuel injection systems require removal of the smallest measurable hard particles. The main advantage of a two filter system over a single filter system is that much of the dissolved asphaltenes present in diesel fuel are caught by the primary filter, leaving the secondary filter relatively clean and ready to remove free water and fine particles. A properly siz