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19-02-2012, 14:03
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#1
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Registered User
Join Date: Sep 2008
Posts: 152
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Fuel Filters in Series
I have had someone suggest that once you have one 2 micron fuel filter in the line to the engine, adding another 2 micron filter in series will not reduce the flow further. I am an electrical type and not very adept at fluid dynamics but it seems that putting one 2 micron filter in the line would reduce the flow by "X" and that putting a second 2 micron filter of the same type in series with the first would then result in a total flow reduction of "2X". Can someone tell me if I am correct, and point me to the fluid formula governing the topic?
Thanks,
Bob
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19-02-2012, 14:23
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#2
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Moderator
Join Date: Aug 2009
Location: Solent, England
Boat: Moody 31
Posts: 18,423
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Re: Fuel Filters in Series
Bob, it won't reduce the flow but might increase the pressure. However, if the fuel pump is on the engine and the filters between the tank and the engine, then air pressure is used to force fuel through the filters by creating a partial vacuum. The surface area of the filters is huge, doubt its going to be a problem for a small yacht engine.
Pete
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19-02-2012, 14:25
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#3
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Moderator Emeritus
Join Date: Aug 2005
Location: Carlsbad, CA
Boat: 1976 Sabre 28-2
Posts: 7,505
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Re: Fuel Filters in Series
Why would you want filters in series?? If one clogs, the other one will be blocked by the clogged one.
Best to plumb in parallel with valves so you can switch between the two should one become blocked. From experience, it's no fun to be cleaning out and changing a fuel filter while bouncing around in the ocean because it became clogged with algae or detritus from the fuel tank. Way better to just switch a valve to draw from the second filter and soldier on.
It would seem that if there is reduced flow from one filter, adding another in line of the same size will double the restriction.
__________________
Peter O.
'Ae'a, Pearson 35
'Ms American Pie', Sabre 28 Mark II
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19-02-2012, 14:37
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#4
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Registered User
Join Date: Sep 2008
Posts: 152
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Re: Fuel Filters in Series
Quote:
Originally Posted by Pete7
Bob, it won't reduce the flow but might increase the pressure. However, if the fuel pump is on the engine and the filters between the tank and the engine, then air pressure is used to force fuel through the filters by creating a partial vacuum. The surface area of the filters is huge, doubt its going to be a problem for a small yacht engine.
Pete
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Pete,
Thanks for the explanation. The case I am referring to is where the pump is on the engine. If the pressure is kept constant at atmospheric is it still true that two series filters would have exactly the same flow as one? Sorry for the rudimentary questions, but this is not my area of expertise. My thought process was making an analogy to two resistors in series with a constant voltage.
Bob
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19-02-2012, 14:51
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#5
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always in motion is the future
Join Date: Feb 2009
Location: in paradise
Boat: Sundeer 64
Posts: 18,866
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Re: Fuel Filters in Series
Quote:
Originally Posted by rabend
Pete,
Thanks for the explanation. The case I am referring to is where the pump is on the engine. If the pressure is kept constant at atmospheric is it still true that two series filters would have exactly the same flow as one? Sorry for the rudimentary questions, but this is not my area of expertise. My thought process was making an analogy to two resistors in series with a constant voltage.
Bob
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No, adding filters will create additional resistance just like electrical resistors in series. The difference is that the pump might just put more effort into pumping, creating a deeper vacuum, which results in the same flow. This is not good, as it eats away at the margins normally used for filter elements that start to clog.
ciao!
Nick.
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19-02-2012, 15:18
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#6
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Moderator
Join Date: Aug 2009
Location: Solent, England
Boat: Moody 31
Posts: 18,423
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Re: Fuel Filters in Series
Quote:
Originally Posted by s/v Jedi
No, adding filters will create additional resistance just like electrical resistors in series. The difference is that the pump might just put more effort into pumping, creating a deeper vacuum, which results in the same flow. This is not good, as it eats away at the margins normally used for filter elements that start to clog. ciao! Nick.
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Well yes and no. We use 1.5 litres an hour to move a 31ft yacht at 5 knots. If there was an air bubble in the fuel line you would be pushed to see it move the fuel flow is so slow even accepting some is returned unused.
Pete
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19-02-2012, 15:20
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#7
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Registered User
Join Date: Feb 2012
Posts: 19
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Re: Fuel Filters in Series
Quote:
Originally Posted by rabend
I have had someone suggest that once you have one 2 micron fuel filter in the line to the engine, adding another 2 micron filter in series will not reduce the flow further. I am an electrical type and not very adept at fluid dynamics but it seems that putting one 2 micron filter in the line would reduce the flow by "X" and that putting a second 2 micron filter of the same type in series with the first would then result in a total flow reduction of "2X". Can someone tell me if I am correct, and point me to the fluid formula governing the topic?
Thanks,
Bob
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All you need is a large prefilter like Raccor with visible decanter bowl. The filter located near the lifting pump is a polishing filter, You could keep it for two years providing you replace the prefilter as required. Like one member say, having two prefilters in parrallel with proper isolating valve is the best.
Remember: the larger the prefilter, the better (just like a boat), as an example, if my engine was 50HP, I would size the prefilter for 100HP.
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19-02-2012, 15:38
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#8
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Registered User
Join Date: Sep 2008
Posts: 152
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Re: Fuel Filters in Series
Quote:
Originally Posted by Pete7
Well yes and no. We use 1.5 litres an hour to move a 31ft yacht at 5 knots. If there was an air bubble in the fuel line you would be pushed to see it move the fuel flow is so slow even accepting some is returned unused.
Pete
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Pete,
Are you taking into consideration that the fuel flow return to the tank is a multiple of the fuel actually burned, that the injector pump is cooled and lubricated by the fuel flow, and that additional restriction to flow may put a greater load on the lift pump? I am not sure I have that all calibrated.
Thanks,
Bob
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19-02-2012, 15:52
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#9
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Registered User
Join Date: May 2008
Location: Nova Scotia until Spring 2021
Boat: Custom 41' Steel Pilothouse Cutter
Posts: 4,976
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Re: Fuel Filters in Series
I have opted for the Racors in parallel. If you have the ability to a) filter fuel down the deck fill to get the egregious muck and bug parts out, and b) you have a rig to polish the fuel into a daytank, into which the return line can be routed, you are guaranteeing that you have at any given time at least a day tank's worth of clean fuel you could put straight to the lift pump.
More than one Racor is, depending on its height above the tanks, going to make the lift pump work harder, perhaps to the point of affecting flow, if I am understanding your question.
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19-02-2012, 16:12
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#10
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Registered User
Join Date: Sep 2008
Posts: 152
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Re: Fuel Filters in Series
Quote:
Originally Posted by S/V Alchemy
I have opted for the Racors in parallel. If you have the ability to a) filter fuel down the deck fill to get the egregious muck and bug parts out, and b) you have a rig to polish the fuel into a daytank, into which the return line can be routed, you are guaranteeing that you have at any given time at least a day tank's worth of clean fuel you could put straight to the lift pump.
More than one Racor is, depending on its height above the tanks, going to make the lift pump work harder, perhaps to the point of affecting flow, if I am understanding your question.
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Thanks. Excellent advice! I agree that having two filters in parallel is the way to go. Unfortunately, due to the configuration of my boat, I don't have room for the second filter. I also like the idea of being able to switch filters quickly in an emergency.
Bob
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20-02-2012, 05:31
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#11
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Moderator
Join Date: Aug 2009
Location: Solent, England
Boat: Moody 31
Posts: 18,423
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Re: Fuel Filters in Series
Quote:
Originally Posted by rabend
Pete,
Are you taking into consideration that the fuel flow return to the tank is a multiple of the fuel actually burned, that the injector pump is cooled and lubricated by the fuel flow, and that additional restriction to flow may put a greater load on the lift pump? I am not sure I have that all calibrated.
Thanks, Bob
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Bob, yes but even so the amount of fuel flowing through the fuel line is tiny, well on a small yacht engine it is. A previous V8 5.7L petrol volvo was a completely different matter.
So will the resistance go up with two filters in sereis, yes, will the flow rate drop, nah.
If you can't manage two filters in parallel then sit a simple CAV water seperator in somewhere to catch water, larger lumps and any dirt. This is our set up and whilst Europe has good clean fuel we haven't had a problem an the water seperator catches any build up before it can reach the engine.
Pete
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20-02-2012, 10:04
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#12
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Registered User
Join Date: May 2008
Location: Nova Scotia until Spring 2021
Boat: Custom 41' Steel Pilothouse Cutter
Posts: 4,976
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Re: Fuel Filters in Series
The CAV with a Fram or similar filter is a good idea, too. So is rerouting the fuel vents from the topsides to the cabin top, but that's more ambitious.
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20-02-2012, 10:07
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#13
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cat herder, extreme blacksheep
Join Date: Jul 2008
Location: furycame alley , tropics, mexico for now
Boat: 1976 FORMOSA yankee clipper 41
Posts: 18,967
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Re: Fuel Filters in Series
i have an electric assist pump to prime and i have small inline filters prior to that with a racor after the pump. works great. none are 2 micron--i believe they are 30 and 10 micron with the closest to engine being the finest filtration.
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20-02-2012, 10:10
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#14
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always in motion is the future
Join Date: Feb 2009
Location: in paradise
Boat: Sundeer 64
Posts: 18,866
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Quote:
Originally Posted by S/V Alchemy
The CAV with a Fram or similar filter is a good idea, too. So is rerouting the fuel vents from the topsides to the cabin top, but that's more ambitious.
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The fuel vents on a monohull should cross, i.e. the vent for the port tank shoul be at starboard and vice versa.
ciao!
Nick.
__________________
“It’s a trap!” - Admiral Ackbar.
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20-02-2012, 10:33
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#15
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Registered User
Join Date: Aug 2006
Location: Skagit City, WA
Posts: 25,483
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Re: Fuel Filters in Series
I have used two racors in series before using a 10 micron in the first and a 5 or 2 in the second. My theory was that if i was having clogging problems, the 2 micron would clog real fast, so I protected it with the 10. On an older boat I would say parallel is best though, then you just switch the valve if you have clogging problems. On a newer boat or new tank, one Racor and the engine filter should be fine.
__________________
"I spent most of my money on Booze, Broads and Boats. The rest I wasted" - Elmore Leonard
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