Reasons for fuel polishing .....
Diesel fuel has a relatively short 'shelf life', typicaly 6 months at summer temps and about 1 year in colder temps. Diesel degrades in the following manner- the lighter fractions volitize out through the vent and the remaining fuel begins to essentially polymerize... dropping the heavy weight alkenes, waxes, tars, etc. to the bottom and onto the tank walls. The alkenes begin as very small 'particles' which become the nucleation sites for larger 'particles' to grow ... many of these particles are 'soft / deformable' and easily pass though fine filter material (by extrustion). These soft particles dont burn very well in the combustion chamber, will pass through and will adhere to the downstream exhaust passages in the form of thick 'coke' deposits. Eventually all this 'particulation' becomes a sludge adhering to the walls, etc. So, essentially old fuel will begin to 'drop-out' the heavy polymerizing materials, tars, etc. .... and a lot of this will simply stick to the walls - ready to be knocked loose during heavy weather .... to plug and overwhelm filter material, etc. This decomposition process is accelerated by contact with copper, etc. .... all cheap boat fuel systems are made from soft copper tube !!!!! The key symptom of degraded fuel is 'cloudiness' --- put some of the tank oil in a clean glass, hold it in front of strong white light ... if its 'cloudy' it has begun to 'go to the dark side'. Polishing wont remove the particles from the tank walls, only mechanical scrubbing, etc. will loosen the accumulated crud. In a refinery, etc. storage tanks are usually cleaned every ~5 years to remove the crud from the inside surfaces. On a boat, simply open the inspection port, get in there with a long handled stiff brush, knock all the crap to the bottom, soak up with paper towels, etc. and then simply burn the rags, etc. Polishing will lengthen the shelf life .... as it continually removes the fine particles which grow into larger particles. Recircualtion polishing is a "high speed" / low pressure filtration at comparatively large retention values (uM) in the filter. The 'secret' of polishing is "high turnover" in which the fuel is passed through the large retention value filters continually / many many times. For a boat you can very quickly reduce the particle distribution in the tank to essentially submicronic levels by 'turning over' the fuel through a 15uM filter many times. A ~15uM filter has little resistance to flow (in comparison to a 2 uM) so the recirc. pump will push a LOT of fuel. A 15uM filter will have retention sites (but to a low % efficiency) much smaller than the 15uM .... say 5, 1, 0,5uM but at low capture efficiency. Since a high speed recirculation constantly puts the same fluid though the filter many many times the capture efficiency ultimately increases by 'statistics' and since the fuel gets 'turned over' quickly the residual 'background' of particles in the tank ***rapidly*** (exponentially) decreases. I recirc filter at 15-20uM to keep my tank particle levels at essentially 'submicronic' levels (vastly beter than 'crystal clear'). 15-20uM filters are a lot cheaper than smaller ( 1, 2 5uM) filter retentions. An on-board polishing recirculation system is VERY fast to recover if tank wall scum does break loose, reduces the challenge load of particles to the fuel line filters, etc. For my personal systems I only put onboard the amount of fuel I *need* plus some reserve. If possible I never buy my fuel from a marina where the fuel sits forever before its get pumped into my tank - only from 'high turnover' depots such as those that sell fuel to the 'watermen' or commercial folks or simply buy it at a truckstop and carry it in. I NEVER let the tank sit full as I use a vapor trap (desiccant adsorbant filter) on the vent line to prevent any moisture uptake - I have a valve on the vent which I close when Im not on the boat (my humongous black iron tank is 'somewhat' vacuum rated - yours probably isnt). For long lay-ups (over winter, etc.) I siphon out all the old fuel and take it home to burn in my oil burner. My recirculation polishing filter set is hard wired to my engine control panel ... when the engine is ON, so is the polisher. ...... and after all that I still have to clean out my tank about every 4-5 years. I dont have to change primary - secondary filters at the worst possible time and I only change those about every 3-4 years (and I get my filters free of charge) as I closely monitor their performance with pressure gages. I also have a ~3 gallon day tank that gets constantly 'turned-over' so in the event that all hell breaks loose (broken diaphragm on a lift pump, plugged filters, etc.) I simply open the bottom of the day tank that feeds 3 gallons of fuel to the engine by gravity .... at WOT that gives me ~4 hours to 'think and sort out' the needed remedy. All my line filters are 'pressure feed' (12v electric pump AT the tank) as pressure filtration is vastly more efficient than in vacuum feed mode ... but I have stainless fuel lines with double flared connections .... not the cheapy compression fittings that are used on soft copper tube and that **always** leak and suck air. BTW - when I do buy from a marina or other questionable fuel source, I first pump a small bit into a clear glass bottle, let the air bubbles gravitate upwards / clear, then hold it up to strong light .... If the fuel is cloudy or has a haze .... I stop, close up, and simply move on to the next fuel source. I dont have 'fuel problems'. Hope this helps.
