Diesel Bugs

[Rick D]

Argh. I picked up a bug, probably from filling my tank from auto fuel stations instead of the marina while our fuel dock was being rebuilt. So I was hoping that this season's change of my primary filter would see no more gunk. Wrong! my conclusion is that wishing just didn't work. So, I'm trying a more aggressive tactic. I priced fuel polishing locally. $185 plus $85 for each additional hour. He said three hours for mine. So, $355 for twenty gallons (half full) is $17.75 a gallon. Double argh. So, maybe a DYI first?

So, I Googled for bug treatments and, of course, there is a zillion. I have always used an additive; the last three years I have used StaBil diesel treatment. I suspect that isn't powerful enough to knock this out, so I am going to try Bio Kleen.

I went to Minney's Marine Surplus and picked up a used dual filter, a Fram, big one. Checking the filters, they didn't list the micron spec's. I called them and found they are both nine microns. Why both nine is beyond me, but I suspect it's OK.

So, I have a high capacity electric fuel pump. My plan is to rig some fuel line to maybe three feet of copper line for both the inlet and return. I will have to go through the fuel gauge sending unit hole since there is no room to install an access plate without tearing up half the aft of the boat (much less pull the tank).

I believe I will nail the bug with the additive over night, then fill the tank (per recommendations) and let it sit another night. Then I'll use the intake to put the probe around and under the baffles as best as I can. Then, I'll check it mid-season and try wishing for a good result.

So, what do you think? Disposing of stale or contaminated fuel in SoCal is a nightmare, so I'd like to try repairing this first.

 

[Rich Stidger]

Sounds like you have a plan that would work. I've never had to polish fuel, but it doesn't seem like rocket science. I think I would use the probe into the deck fuel inlet to pull the fuel out to the filter and then use the unused generator port to put the fuel back into the tank.

Since you probably have the same tank as I do the tank has four ports. Two are for the aux and two are for a genset. I was told that the genset pickup doesn't go to the bottom of the tank so that you can't run out of fuel. The engine pickup is lower than the genset pickup. I think the probe sucking out the fuel would have the best chance of vacuuming up the bugs that fall to the bottom of the tank.

FWIW, I use Bio-Bor additive and have never had a problem. But I also burn 175-250 gallons of diesel every year between my aux and genset.

 

[Herrick18911]

I think your plan to suck up the fuel from a opening port is better than the fuel fill as You will be able to get to the bottom and all around the tank better there. It is better to route the return through the fill tube. Also, you are likely to find that there is crud sticking to the walls of the tank. You may need to use something to soak that up or scrub it out. Using the polisher, you will find that there will be turbulence inside the tank which will sir up crud that will enable it you to clean out the fuel better. That's why the longer you use it, the more crud you'll clean and filter out. Be prepared to need to change your filters afterwards more regularly in case you don't get all the crud out.

Good luck! Todd

 

[zeehag]

pumps and filters, and bio-bor makes cali diesel work forever.....circulate your fuel thru pumps and filters and clean your tank by hand. saves money. polishers, per se, make waay too much money off those who dont know how to do this--i watched em clean my fuel and tank 2 different times, and i wont do that again--i will do it myself, as i have done since leaving san diego.. isnt that hard, actually , and isnt magic.... good luck.

 

[shemandr]

Do others in your yard have the same problem - the same fuel dock was closed? If so maybe you can negotiate a lower rate to do a number of fuel tanks.
I was told when I was looking into this that the polishers use the pressure of the returning fuel to clean the sides of the tank. Sounded like a mess to me. I dropped the idea and I haven't had a problem.

 

[zeehag]

they do that, yes, but i make a hand cleaning, as the polishers did miss gunk in tankage, on the sides of tank....

 

[Steve Dion]

Rick:

Have you consider just emptying the tanks into jerry cans, clean out the tank the best you can and refilling through a Baja Filter?

Actually I would have thought that fuel from a truck stop/gas station would be more likely to be clean than many marine stations.

 

[thinwater]

Stabil is not a biocide. I have done a bunch of testing, both in my "real job" and for Practical Sailor. There is a summary here.
http://sail-delmarva.blogspot.com/p/diesel-biocides.html

Bio-bor and Bio-Kleen both tested well. I've used both industrially as well.

I would treat longer than over-night. Give these a week to work; it takes time for them to soak through accumulations of bugs. Also, biocide treatment should be preventative; like the common cold, you can catch it from the air.

The other good preventative is keeping the fuel dry. Try the H2OUT vent filters. They really do work.; I've done both lab and field testing with gasoline, some lab testing with diesel, and am beginning more extensive field and lab testing. Proper installation is critical and the instructions may over-simplify.

Can't the stale fuel go into the used oil tank? If the recycler is Demeno/Kerdoon or a collector that takes to them, they recycle the used oil into marine diesel oil (big ships, not the stuff you burn) and will be happy to have the #2 oil. Evergreen in northern California makes base oil (lube oil building block) and does not want diesel fuel in the mix. Yes, I'm in Virginia, but I have close contacts with both companies.

I've found polishing services (industrial) to be a waste; if they can do the job, you would have been better served improving your filtration with an on-board polishing system, and if they couldn't do it is because the tank was bad and mechanical cleaning was required anyway. If you install on-board polishing, it should be a one-time investment. With preventative treatment and polishing, this will never recur in any serious way.

 

[Rick D]

Couple of things...

... first, thanks for your comments. I have watched fuel polishing before and it doesn't look too complicated, just a matter of filtration for the big volumes in large motor yacht tanks. So:
1. I typically use between 100 and 140 gallons per year of diesel.
2. I was purchasing for two years at a local passenger-car station that had difficult entry. There are large commercial operators in the area, so I never saw any large truck, and only rarely a pick up. I suspect this is where I got the bug and it likely turns over little diesel. It also turned up in close proximity time-wise to my starting to use it.
3. The haz-waste disposal units near me want no diesel, especially mixed with the oil. That's three of them. I could try others.
4. The Bio Kleen instructions say to add the shock treatment amount to the capacity of the tank, not the fuel. Then to fill it up to full capacity so as to let the biocide work on the accumulations on the tank walls. Makes sense to me.
5. I'm the only one with a problem that I know of. Maybe I should say I'm the only one who knows he has a problem.
6. I was thinking of putting the fuel return probe through the same hole to try to use it to agitate the sides of the tank too.
7. The H2O Out vent filter looks pretty slick. I never had a problem for 10 years on this boat and another 15 on two others. Sooo, I was reluctant and worry about the fuel guys who top off. Right now, however, it looks real attractive in retrospect

Anyhow, thanks so much for your comments and suggestions; keep 'em coming. This is new to me.

 

[RichH]

Recirculation polishing by itself will be of NO benefit in cleaning up a fuel system contaminated with common 'kerosene fungus' - Cladosporium Resinae, etc. which is a spore forming fungus and the spores (and other divisible fragments) will be in the range of 2-3 micrometers.
To decontaminate you must reduce the biomass living inside the tank on the tank walls ... usually a thick growing mat of fungus and its (black resinous) products of metabolism - dead fungus, living fungus, fungal spores and 'asphalt-like' cellular, etc. waste.

Two ways to remove the asphaltic waste, etc.:
1. Drain the tank, Open the tank access ports and mechanically scrub the insides. Soak up the 'debris' with paper towels and either burn them (where lawfully permitted) or send to a hazardous waste disposal. No need to be 'anal' in the cleaning out, just get out 'most' of it, a wee bit of chemical tank cleaner and biocide in the future will keep down the growth. A fairly thorough mechanically cleaning exponentially reduces those 'living particles' that grow.

1a. alternate if the tank has no access ports. To the contaminated oil, add the proper amount of a chemical tank cleaner such as Starbrite "Tank Cleaner" (http://shop.sailboatowners.com/prod.php?17903),
add compatible biocide, mix thoroughly and let the tank sit for 1-2 WEEKS. Then agitate the tank contents and drain the tank. Add a few gallons of new oil, slosh around and then drain.
If you have a home oil burner, take the contaminated oil home and use it your oil burner. Otherwise discard the oil, such as taking it to a 'reclaimer', etc.

3. Blow out the entire fuel delivery hosing/tubing all the way up to and including your 15µM engine mounted 'guard filter', (discard & change your 'racors', engine guard filter, etc.)

4. THEN partly refill the tank with NEW oil and THEN recirculate polish the NEW oil (plus residual fragments from 'cleaning') until the oil (sample) when held up in a clear glass container in front of strong white light shows absolutely NO 'haze'.
Recircualtion polishing should be to less than 1-2µM (the spores and other 'divisible reproductive fragments' will be approximately 3µM). The recirculation polishing rate should be in the neighborhood of 3 gallons per MINUTE per 100 gallons of tank volume through your 9µM (???) filters for approx 6 hours MINIMUM. or a total of ~1000 total gallons passed through the 'filters' (per 100 gallon tank) to obtain a residual level of approx. 1-2µM particles in the tank. Doing this with a fuel 'delivery' pump (1-3 gallons per HOUR) will take you ~40 days of constant pumping at 1 gallon per hour to pass ~1000 gallons filtered!!!!
(FWIW - Id seek out a 'filter' that has published or listed filtration efficiency such as 5µM 90% efficiency, rather than blindly and wishfully guessing what that 'nominal' 9µM is actually ... if these 9µM filter were obtained from an auto store or auto supply and were NOT made in China, my educated SWA-guess would that they are 9µM 80% efficiency ... or about 30µM at 95% efficiency. There's lots of filter jargon only speakable by filter geeks. Racors, fairly high quality automotive filters are only efficient to about 95-97% removal at their 'µM rating')

Note - you will not be able to recirculation polish the 'old fuel' down to 'visual clarity' (no haze) due to the (black) dissolved & suspension of submicronic fungal fragments and asphaltic components. Plus being unable to reach 'visual clarity' you will be 'unsure' of the efficacy of recirculation polishing due to the 'black'. If you 'must' use the old oil ..... Only long term (6 hours, etc.) recirculation polish the old fuel and return it to the tank if getting rid of it will be costly or a royal pain in the butt. If you must return the old fuel to the tank then consider to greatly extend the 6 hour recirculation time ... and use your engine as much as possible to get rid of the old fuel. DO NOT top off the old fuel, use it up as quickly as possible and only fill the tank with new when youre nearly 'empty' of old fuel. Take extra 'racors' along with you if you added Chemical Cleaners to the tank as large 'chunks' of accumulated 'softened' (and undissolved by the tank cleaner) bio-mass can break loose from the tank walls.

 

[Solstice]

Stabil is not a biocide. I have done a bunch of testing, both in my "real job" and for Practical Sailor. There is a summary here.
http://sail-delmarva.blogspot.com/p/diesel-biocides.html

Bio-bor and Bio-Kleen both tested well. I've used both industrially as well.

I would treat longer than over-night. Give these a week to work; it takes time for them to soak through accumulations of bugs. Also, biocide treatment should be preventative; like the common cold, you can catch it from the air.

The other good preventative is keeping the fuel dry. Try the H2OUT vent filters. They really do work.; I've done both lab and field testing with gasoline, some lab testing with diesel, and am beginning more extensive field and lab testing. Proper installation is critical and the instructions may over-simplify.

Can't the stale fuel go into the used oil tank? If the recycler is Demeno/Kerdoon or a collector that takes to them, they recycle the used oil into marine diesel oil (big ships, not the stuff you burn) and will be happy to have the #2 oil. Evergreen in northern California makes base oil (lube oil building block) and does not want diesel fuel in the mix. Yes, I'm in Virginia, but I have close contacts with both companies.

I've found polishing services (industrial) to be a waste; if they can do the job, you would have been better served improving your filtration with an on-board polishing system, and if they couldn't do it is because the tank was bad and mechanical cleaning was required anyway. If you install on-board polishing, it should be a one-time investment. With preventative treatment and polishing, this will never recur in any serious way.

I don't mean to hijack this thread, but I have been intrigued with the H20 OUT product and read your excellent article in PS recently. As I recall, you suggest using something like the Racor fuel/air separator downstream. Is this what you mean regarding proper installation? I am also leary of dealing with removing for drying out on occassion, but the concept seems to be perfect for eliminating one of the major places for water ingress.

 

[thinwater]

I don't mean to hijack this thread, but I have been intrigued with the H20 OUT product and read your excellent article in PS recently. As I recall, you suggest using something like the Racor fuel/air separator downstream. Is this what you mean regarding proper installation? I am also leary of dealing with removing for drying out on occassion, but the concept seems to be perfect for eliminating one of the major places for water ingress.

Proper installation guides have been written by the AYBC for the new carbon canisters and they are a good guide for this as well.

• The unit should be secured, not just spliced into the hose.
• High spot.
• Must protect from both fuel and water contact.
• Should locate so that resin falls out vent rather than into tank if dusting occurs.
• For gasoline installation it should not be in the engine room if possible, or it must be given heat protection.

As for drying, replacement every few years with resin from and air compressor vendor is economical and should take only minutes. I would not try to dry the resin. Curiously, even when spent the resin still reduces water intake by capturing fog and dew and re-evaporating it through the afternoon.

There will be a follow-up article in 6 months, as the first was cut for length and some testing continues.

This article on ULSD corrosion, sponsored by the API, is thought provoking. Though I think they were reaching a few places, the data is interesting. I am sure of one thing; the ethanol people are hopping mad about it. Have fun watching the kettle and the skillet call each other black.
http://www.clean-diesel.org/pdf/ULSDStoringSystemCorrosion.pdf
http://www.ethanolrfa.org/exchange/...to-blame-for-diesel-fuel-corrosion-incidents/


On of my favorite fuel maintenance tips is to burn more fuel. While it is possible to preserve fuel for long periods, it is more sensible to run the engine enough to burn it through a few times each year. Heck, we change to engine oil each year, no matter how little we run, and lube oil is far more stable.

 

[Rick D]

Rich: this sounds like "abandon all hope..."Recirculation polishing by itself will be of NO benefit in cleaning up a fuel system contaminated with common 'kerosene fungus' - Cladosporium Resinae, etc. which is a spore forming fungus and the spores (and other divisible fragments) will be in the range of 2-3 micrometers.
To decontaminate you must reduce the biomass living inside the tank on the tank walls ... usually a thick growing mat of fungus and its (black resinous) products of metabolism - dead fungus, living fungus, fungal spores and 'asphalt-like' cellular, etc. waste.

Two ways to remove the asphaltic waste, etc.:
1. Drain the tank, Open the tank access ports and mechanically scrub the insides. Soak up the 'debris' with paper towels and either burn them (where lawfully permitted) or send to a hazardous waste disposal. No need to be 'anal' in the cleaning out, just get out 'most' of it, a wee bit of chemical tank cleaner and biocide in the future will keep down the growth. A fairly thorough mechanically cleaning exponentially reduces those 'living particles' that grow. As mentioned, this isn't an option without some serious surgery.

1a. alternate if the tank has no access ports. To the contaminated oil, add the proper amount of a chemical tank cleaner such as Starbrite "Tank Cleaner" (http://shop.sailboatowners.com/prod.php?17903),
add compatible biocide, mix thoroughly and let the tank sit for 1-2 WEEKS. Then agitate the tank contents and drain the tank. Add a few gallons of new oil, slosh around and then drain. Here's what Bio Kleen instructs: SHOCK TREATMENT OF DIESEL FUEL BULK-STORAGE OR EQUIPMENT TANKS: Treat total capacity of each tank – not the amount of fuel in the tank at the time of treatment. Add 1 ounce of Bio Kleen to each 20 gallons of equipment fuel tank capacity (see treatment ratio chart on the other side). After treatment has been added, fill the tank with diesel fuel to maximum capacity so that the rising level of diesel fuel treats the entire interior walls of the tank. And: SHOCK TREATMENT AND MAINTENANCE TREATMENT: Recirculate fuel in tank for 30 minutes following treatment, if possible. Let diesel fuel set for eight hours before using to achieve maximum effectiveness. Treatment may result in additional accumulations of water and dead microbes at the bottom of the tank. If so, pump or drain tank bottoms to complete tank clean-up. So, that's why I was going to fill the tank to capacity, to get the walls with the biocide. I don't trust the return spray from my pump to handle that in the operation I planned.
If you have a home oil burner, take the contaminated oil home and use it your oil burner. Otherwise discard the oil, such as taking it to a 'reclaimer', etc. This is a serious problem. I know of three Haz-mat sources and none will take diesel fuel. Ten years ago, I tried to get rid of about the same amount of stale gasoline, which is easier. One source would accept it one day per month, but only if you also gave them the containers. So, even if I could find some one to take it, at thirty bucks a piece, it would be a bit pricey to get rid of it. Still, I'll work on that further since I agree dumping the fuel and starting with fresh sounds smart.

3. Blow out the entire fuel delivery hosing/tubing all the way up to and including your 15µM engine mounted 'guard filter', (discard & change your 'racors', engine guard filter, etc.)

4. THEN partly refill the tank with NEW oil and THEN recirculate polish the NEW oil (plus residual fragments from 'cleaning') until the oil (sample) when held up in a clear glass container in front of strong white light shows absolutely NO 'haze'. The good news is that the fuel I removed from the secondary filter looked pristine.
Recircualtion polishing should be to less than 1-2µM (the spores and other 'divisible reproductive fragments' will be approximately 3µM). The recirculation polishing rate should be in the neighborhood of 3 gallons per MINUTE per 100 gallons of tank volume through your 9µM (???) filters for approx 6 hours MINIMUM. or a total of ~1000 total gallons passed through the 'filters' (per 100 gallon tank) to obtain a residual level of approx. 1-2µM particles in the tank. Doing this with a fuel 'delivery' pump (1-3 gallons per HOUR) will take you ~40 days of constant pumping at 1 gallon per hour to pass ~1000 gallons filtered!!!! I'll have to think on this. I can certainly rig something along the lines of what Rich S. suggested to circulate the fuel continuously for whatever time is necessary.
(FWIW - Id seek out a 'filter' that has published or listed filtration efficiency such as 5µM 90% efficiency, rather than blindly and wishfully guessing what that 'nominal' 9µM is actually ... if these 9µM filter were obtained from an auto store or auto supply and were NOT made in China, my educated SWA-guess would that they are 9µM 80% efficiency ... or about 30µM at 95% efficiency. There's lots of filter jargon only speakable by filter geeks. Racors, fairly high quality automotive filters are only efficient to about 95-97% removal at their 'µM rating') The filter I have is a large, high capacity marine dual cannister filter. I am going to use Baldwin filters which are made both in the USA and in China, but within their own factory, not outsourced. I got the micron specs directly from their engineering department. Baldwin PF942 & PF943. I really don't know what else to do.

Note - you will not be able to recirculation polish the 'old fuel' down to 'visual clarity' (no haze) due to the (black) dissolved & suspension of submicronic fungal fragments and asphaltic components. Plus being unable to reach 'visual clarity' you will be 'unsure' of the efficacy of recirculation polishing due to the 'black'. If you 'must' use the old oil ..... Only long term (6 hours, etc.) recirculation polish the old fuel and return it to the tank if getting rid of it will be costly or a royal pain in the butt. If you must return the old fuel to the tank then consider to greatly extend the 6 hour recirculation time ... and use your engine as much as possible to get rid of the old fuel. DO NOT top off the old fuel, use it up as quickly as possible and only fill the tank with new when youre nearly 'empty' of old fuel. Take extra 'racors' along with you if you added Chemical Cleaners to the tank as large 'chunks' of accumulated 'softened' (and undissolved by the tank cleaner) bio-mass can break loose from the tank walls. See my previous note re: Bio Kleen's instruction to fill the tank up. It does look to me like I may have to do this a couple of times. Fortunately, the fuel itself looks clear ad red downstream of the filters, so hopefully, I'll make progress.
Rich, thanks so much for your comments. It may turn out that I have to do all this several times and maybe it won't work either. But, I have to start someplace, and this looks like a rational approach.

 

[RichH]

• Filling the tank before adding Chem cleaners will definitely 'submerge' the bio-mass for better enzymatic action.
• Disposal ... simply find a place that accepts 'used motor oil' and dispose by 'small amounts' at a time ... i.e.: 5 gallons, etc. Just dont do this ofter, or they'll become 'suspicious'. Diesel #2 is 'oil'.
• Baldwin .... Since you describe 'downstream' is somewhat 'clear' ... simply recirculate polish until it can pass the strong light/haze 'test' -- NO or little noticeable haze when 'backlighted'; and then maximize your biocide dosage.

Be patient, as such contaminations dont happen overnight, and sometimes the remedy is a long one.

Best way to prevent all this:
Consider a small but 'high capacity' independent onboard fuel polisher (5µM) thats always on whenever the engine is on. ... removes the spores and fragments as they 'form'. Such particles in fuel tanks 'agglomerate' ... become larger and larger using smaller particles to make larger particles; constant recirc. polishing removes the 'seed particles' (nucleation sites). Walbro makes such continuous duty 'transfer pumps' 1 - 3 gallons per MINUTE. Suggest the Use of 'standard industrial' (steel) filter housings that use 2.5 / 2.75 inch dia. X 10 inch long 'depth' filters ... 5µM (nominal rating) or 'graded pore density' 5-20µM 'depth filters' is probably the best for such service to yield approaching the removal of essentially 'all' submicronic particles via constant recirculation.
Just locate an 'ethical' industrial filter distributor ... who handles Parker Process Filtration, Pall Corp, Filterite, Cuno/GE, Strainrite ...., etc. Stay AWAY from the 'crap' sold in Home Depot, etc.
Only buy your fuel from a 'high turnover' source to insure its 'fresh'.
Only take onboard what you NEED plus some reserve, no need to let fuel needlessly go 'sour'/'stale' etc. in your fuel tank ... use a stabilizer and biocide.

 

[Rick D]

Update On DYI Fuel Polishing

OK, I left the biocide (Bio Kleen) in a full tank for a week. Then I put new filters in my (used from Minney's) dual Fram large capacity filter, hooked up lines and the pump and proceeded to pump. I ran into a problem right away. The pump would only produce a dribble. Figuring the lift was too high, I purchased some additional aluminum tubing and placed both the filter and pump on the swim step. Still nothing. Pondering that, I took apart the pump and found a big gob of gunk clogging the passages. You see, I had put the pump ahead of the filter the first time I tried it and that was all it took. Duh. Anyhow, I pumped for about twelve hours. The pump is rated for 60+ GPH, so I have likely filtered the fuel several times over. I plan to do it another twelve hours. I'll pull the filters out and inspect or replace them and let you know what I find; I can guarantee it will be interesting!





The next thing I need to figure out is if it's advisable to continue using the biocide for the balance of the season, and whether to continue with shock dosage or move to a maintenance level. I will mail the manufacturer for that information.

 

[Rick D]

Follow Up Call

"The next thing I need to figure out is if it's advisable to continue using the biocide for the balance of the season, and whether to continue with shock dosage or move to a maintenance level. I will mail the manufacturer for that information."

So, the manufacturer called back (!) and said I should use the biocide twice a year, based upon my description of the issue, and use a cetane, detergency and lubricity additive (their product of course, Diesel Kleen) regularly and that it is compatible with the biocide, so no worry there.

 

[robertsapp]

So here's my twenty-seven cents worth of advice (inflation). You can polish your fuel till the cows come home, but you won't be doing much about the scaly bio stalagtites stuck to the interior walls of the tank, which will happily keep spawning new growth in the clean fuel. To do the job right, you need to drain the tank and actually scrape the stuck on crap out, and then thoroughly scrub it clean. If you don't have an access port, order one from Seabuilt and install it so that you can get inside the tank.

If like most people your tank is aluminum, and if like most people you've never opened it up and scraped it out, you'll be amazed when you see what those clumps of stuck on algae are doing to your tank. They basically eat aluminum (they don't actuallly eat it, but they produce chemicals that attack it). When I opened the tank on our then 16 year old boat, I found so many areas on the bottom and lower inch of the side walls that were etched to within a paper thickness of perforation that I yanked the tank and took it to a welder to cut out the bottom and weld in new aluminum. No amount of fuel polishing is going to repair internal tank corrosion.

And because yes, I am a pirate, (200 years too late) I first offered the approximately 15 gallons of old fuel (tank was half full when I drained it) to my dockmates, who wisely declined. Then, not feeling like going through the bother of hauling it to the county dump as hazardous waste, I burned it up in my patio tiki torches over the course of the year. It works just as well as the nice smelling patio torch fuel you buy in the quart bottles at Walmart, just instead of citronella, it smells like you're motoring to charge batteries at anchor.


Cheers

 

[RichH]

You see, I had put the pump ahead of the filter the first time I tried it and that was all it took. Duh. Anyhow, I pumped for about twelve hours. The pump is rated for 60+ GPH, so I have likely filtered the fuel several times over. I plan to do it another twelve hours. I'll pull the filters out and inspect or replace them and let you know what I find; I can guarantee it will be interesting!
[/COLOR]

Suggest that you put the pump back to be BEFORE the filter .... in vacuum mode a single stage pump can only 'pull' about 6"hg. vacuum (~3psi) but in 'pressure mode' (pump delivering TO the filter) it will develop it full 'head' potential (probably about 15 psi ???). The pump 'should' have an integral screen to take care of larger crud to protect itself.
The difference in oil flow between pressure mode vs. vacuum mode will be about 5 times MORE flow rate in pressure mode than in vacuum mode .... recirculation polishing depends on 'turning over' LARGE amounts of fluid. Large amounts of 'fuel turnover' is needed to continually 'dilute' the residual amount of particles in the tank (- Newtonian 'laws of exponential decay' if you want to consider 'the mathematics' of recirc. polishing)
In vacuum mode the filter will also clog abut 5 to 10 times faster than in (GPM vs. time) pressure mode ... as the flow rate and total flow of the filter is based on 'total volume filtered vs. differential pressure across the filter' - GPM vs. ∆P. Filtering in 'pressure mode' will accomplish the filtering 'faster', will filter a much greater 'volumetric amount' of fuel ..... and the result will be 'less residual particles in the tank'.

How to determine when to stop filtering? ... if the fuel is not 'black' or 'dark', occasionally check the progress of the recirculation by sampling the oil by putting a small amount (4-6 oz.) into a CLEAR glass container, then hold it up to STRONG white light ... the sample container directly between your eyeball and the light ... keep filtering 'over and over and over' until the fuel has NO discernable 'haze' .... if no discernable haze, the fuel is now @ less than 1-2µM 'resident' particle contamination (even when filtering with much larger retention rated filters) and the fuel will now appear to be 'crystal clear' .... but only if the fuel is not 'dark' or 'black' due to dissolved asphaltenes from the bio-contamination .

 

[Maine Sail]

Simple, cheap (less than your pro and you'll own it) and effective...

Still this is only polsihing the fuel not cleaning the tank. NO fuel polishing, no matter how much you pay them, can clean the walls of the tank effectively.

 

[Rick D]

Update: Diesel Bug

I finished recirculating my fuel through a large two-chambered filter, the first being a filter-water separator and the second a straight filter, both nine microns. In total, I ran the pumps about thirty hours. The set up was much like the one Maine posted. While I did have the pump on the suction side of the filter, I did continually monitor the flow and it was OK, although, as Rich suggests, I'm sure it wasn't optimum.

So, I drained the bottom of the first filter before I started the second 15 hour stint. There was some contamination but no water. I should have changed the filter regardless, but I didn't. Anyhow, here's how the filter looked when I took it out at home and then a photo of the fuel I drained from the secondary filter.

Anyhow, I am satisfied with this result. I am really surprised I didn't have water in the bottom. I certainly hope the biocide worked as promised. They do stuff for big commercial users and storage facilities, so I gotta believe they have a clue. At this point I don't care much about residual crap on the tank walls if the fungus is dead. The fuel looks clear and clean, so hopefully, I got it before it was too bad! We'll see.