To specifically answer the original posters statement/comment: The purpose of the 'primary stage' (first filter in the series) is to *extend the service life* of the following filter in the series. Typically in (the filtration) industry, most pre-filters are at 5X the µM retention value of the next filter in series. Using a 'prefilter' in this manner 'may' extend the life of the 'next in series' filter by a factor of ~2 to 3X.
Fuel oil filters are rated as a 'nominal' rating ... which in 'filter-speak' means that a 2 or a 10 or 30µM filter only retains approx 90-97% (by weight) of the particle size indicated by the 'rating' ..... the ratings are 'arbitrary' and assigned by each filter manufacturer, etc. A 2µM filter can by definition of 'nominal' ratings, pass a basketball and still be rated at 2µM.
The probably 'most damaging' particle size in a diesel engine is 20µM. That 2µM filter on a 100% absolute retention basis 'may' be passing particles as large as 15µM (to a small weight %).
"Is a smaller retention size filter better? "My OEM system came with a 10µM final stage filter, should I change to a 2µM?" The clear answer is: usually NOT!!!!!
Here's why: It takes 5 times as much 'work' to pass the same amount of fluid through a 2µM filter than a 10µM filter (assuming that the same size/surface area filter cartridge was used). That 'work' will be represented by higher 'differential pressures' across the smaller retention rated filter .... which means that the velocity of fluid 'through' the filter will be about 5 times faster .... driving the debris very deeply into the filter media with the typical result of clogging the filter *5 times faster* than if you kept the OEM 10µM!!!!!! 5 times as much work will apply to your engine's lift pump .... possibly at long term causing the lift pump diaphragm to prematurely rupture/leak, etc.
So, if you want to 'filter down a step .... say from 10µM down to 2µM .... then the CORRECT methodology would be to *****increase the surface area***** of the 2µM to 5 times the surface area (look at the filter manufacturers website for the surface area details.
Each filter of this 'fuel oil type' also will capture particles much smaller than the indicated 'rating'. The particles typically found in fuel oil systems are mostly 'soft and deformable' and will upon higher differential pressures across the filter .... simply extrude through the filter, downstream and into the 'next' filter. (These deformable particles/gels, etc. do not burn very well and are the probable cause of 'coke' / blockage in the exhaust system - buts that's another story'.)
Filters only remove what comes down stream from the tank. If the tank is fouled, dont depend on a filter to do the particle removal job (about ~15-25 grams of 'crud' per typical Racor canister) for very long. The obvious reply is that if you are experiencing 'filter problems' (short filter life, etc.), the answer lies at the end of a long handled scrubbing brush ..... get your arms into and inside the tank and clean the residue of resins produced by funguses (Cladosporium Resinae is the typical) and bacterial slimes from the tank walls. A clean tank wont 'grow' particles (by agglomeration), and a ***clean tank*** rarely needs a fuel filter to be changed as an 'emergency'. If you have a tank that fouls easily, or are or have experienced 'filter problems', then the very best advice I can offer is begin a routine cleaning schedule to mechanically SCRUB the internals of your tank on at least an every 3-4 year interval.
How do I know that I have a fouled tank? Drain/remove some of the fuel from the tank, put it into a clear clean glass and hold the glass with oil between your eyeball and very strong white light; if there is any HAZE noted in the oil ... then you probably have a fouled tank. Polishing the fuel wont clean the tank; the only way is to get inside with a long handled scrub brush or power washer wand and remove the resins and dead bacteria from the tank walls. What you dont want to do is make yourself vulnerable for clogging the filters during an extremely heavy sea state ... like when 'shooting an inlet' etc. and the sloshing tank contents are knocking the crud off the walls and the only way to keep the engine running is to change a filter every 10-15 minutes while you power-puke into the bilge. This will rarely happen if you have a CLEAN tank.
Some hints for a clean tank and rare 'emergency filter change-outs':
Make a schedule (every 3-4 years) to clean out the tank.
Inspect the 'haze quality' of the oil every few months (good to check for 'haze' when taking on fuel from a 'suspect' source).
Keep only the MINIMUM amount of fuel in the tank that you need, plus some reserve.
Do not keep the tank 'topped-off'.
If possible drain ALL the oil from the tank when storing the boat long term (winter, etc.) ... take the fuel home and burn it in your oil furnace.
Buy 'fresh fuel' .... from a 'high turnover' depot that caters to watermen OR jug it in from a 'truckstop' - avoid marinas (that have low fuel 'turnover') whenever possible.
