bRobins recent post got me thinking. In a recirculating diesel fuel system the fuel is constantly getting run through a filter to remove water and contaminants. So given that the fuel is already being polished what does "polishing" do that the system doesn't already provide? The onlt thing I could see that would make sense is total fuel removal to a tank to get everything out of the bottom of the tank, Clean out the tank if accessible, then run it, the fuel, through a water separator filter to put it back in. Am I missing something in my thinking?
What you are missing is that the normal fuel return back to a tank is extremely low flow. Such low flow (of filtered return fuel) has virtually zero effect on the reduction of tank particles ... by dilution with the small amount of 'return' flow.
Recirculation polishing depends on the 'newtonian exponential laws of decay /dilution' ... using LARGE amounts of fuel passing though LARGE µM filters.
Your 'return system' has virtually NO flow - typically a few cc's per minute. How long will it take for a 50 gallon tank fouled tank (@ ~1000's of 1 to 20µM particles per cc) to be completely filtered when the 'filtered' return is at 25 cc/min?
Answer: INFINITE TIME... 'nutherwords, it wont do the job.
Reason: the small amount of 'return' fuel is DILUTING the almost INFINITE amount of Particles at an INFINITELY SMALL rate.
Properly designed recirculation polishers, to be efficient, are HIGH FLOW devices (typically 1-3 gallon per MINUTE per 100 gallon tank) and use filters which have at least 5-10 times LARGER µM retention than the smallest retention of the 'fuel delivery filter set'.
Recirculation polishers depend almost entirely on TANK VOLUME **TURNOVER** - rapidly filtering through 'crude' very large retention µM and returning LARGE AMOUNTS of fuel. Filters made for fuel oil are not very 'precise' in their retention.
A 2µM 'nominally rated' filter will typically pass 5-10% of 15-20µM particles and will capture probably 5% of 0,1 to 0,2µM particles.
The most damaging particle in a diesel system is probably in the range of 20µM. Most engine manufacturers place a 'last chance' or "guard" filter on the engine - rated at 2µM 'nominal' rating which is about 15-20µM at 100% retention.
At constant delivery pressure, A 15-20µM rated filter will allow TEN TIMES the flow rate of a 2µM filter. A 15-20µM filter will have probably 10% of its 'pores' capturing at 2µM.
Recycle the SAME fuel 10 times through the SAME 15-20µM filter (removing 2µM particles by 10-15% at each 'pass') and the fuel RAPIDLY becomes 'very clean' because of the 'dilution rate' in the tank. Continue to recycle the same fuel at the high rate, and even smaller and smaller particles become retained in the filter and the fuel in the tank eventually becomes 'crystal clear'.
If you depend on the 'return' (filtered at say 2µM (nominal rating)) to clean the fuel ... and that 'return' is only a few (25-50) cc's flow rate ... not a very large DILUTION rate of particles going on INSIDE the tank.
So, well designed recirculation polishers, depend on HIGH FLOW RATE, using very 'open' µM filters to ****rapidly**** DILUTE the particles IN the tank. BTW very 'open' higher µM rated filters are CHEAP.
A recirculation polisher WILL NOT clean a tank !!!! Only regular maintenance and physical scrubbing will clean a tank. A recirculation polisher will help to keep a tank clean and particle free by constantly removing the 'nucleation sites' upon which these particles GROW.
Fungus contamination of the oil is the PRIME reason for most of 'boat' fuel system problems. Boat tankage is small, warm, accessible to humid air. Fungals (Cladosporium Resinae, etc.) in diesel oil, aspirated in from the ambient air during tank draw-down, etc., are RESIN FORMERS using the fuel oil as their nutrient source and which deposit the resins and thick microscopic 'filamentous' mats (mycelium) on the tank walls. As the mycelium filaments accumulate and then later die they breakdown into soft/deformable particles and 'gels'. These small particles continue to 'agglomerate' - small particles coming together forming larger and larger particles (seemingly to GROW into larger size). Water (free water, emulsified water & water vapor) accelerates the growth. Such growth is exponential, the more the contamination the faster the growth. A clean(ed) tank will have minimal spores and growth on which to form more and more particles; once a tank forms the contamination deposits on the walls - you can easily generate a 'particle storm' inside the tank.
With respect to the PRIMARY/SECONDARY/ENGINE GUARD FILTER: Such 'soft/deformable' particles are ROYAL BITCH to filter, as they easily 'slime over' or form a 'slimey bridge' at the upstream surfaces of filter media, blocking the flow INTO the media. Such particles, under increasing differential pressure across the filter, easily are extruded through the filter media ... only to be captured by the next (and at smaller µM retention) filter in the series. The agglomeration process continues at the face of each next filter(s) in the series and the 'problem' gets worse the further downstream in the filter 'train' as you go from larger retention to smaller retention. The higher the differential pressure across the filters the worse the problem becomes. The Rx with respect to filtration efficiency is to use the LARGEST SURFACE AREA filter available and the LARGEST µM filter retention rating to do the job !!!! If one somehow thinks that smaller µM retention filtration is 'better' ... it will 'blind off' exponentially FASTER in a contaminated system.
A recirculation polisher removes such particles BEFORE they enter the 'fuel delivery system filters', thus greatly lessening their chance of becoming fouled.
Attachment is a typical system with a high flow recirculation polisher. 100 gallon system is 'bomb proof' in that it contains a constantly filled emergency 'day tank' that can be switched to 'gravity feed' if necessary and will allow flow to the engine for several HOURS. Its also a PRESSURE FEED system which is vastly more efficient (filter performance and hydrodynamics) but requires double flared tubing connections, etc. etc. - so you if you have a leak you dont fill the bilge or the harbor with oil. Attachment is copyrighted with all rights reserved.
