Any benefit to using two fuel filters?

[Olas]

It's probably a bit late to ask this as I'm already doing it, but, I want to know if there are any potential pros or cons to using two fuel filters inline?

The filter for my application is a Mahle KL15, the larger of the two in this picture - http://www.ktmspain.com/attachments/zona...na-imag0158.jpg

I currently have them plumbed in series, one after the other - my reasoning was that if any contaminants got past the first filter then I'd have another chance of stopping them before they got into the carb and blocked up my jets..but it occured to me that if the first filter got plugged there'd be no flow through thte second filter and no fuel delivery.
Does it make sense to use a couple of T pieces and run the filters in parralel with each other rather than in series? would this have the potential for greater or lesser filtration?
They're cheap enough that I replace them both at every 6-8k OCI but that doesn't mean I dont have an unnecessarily anal and OCD tendency to stay up and think about these things all night when I should be sleeping!

 

[The motor guy]

Yeah I think they do that on the bigger diesels. It's good cause if one is defective then you still have another to filter. Cheap insurance when the fuel system costs 10 grand on these things.

 

[Trav]

No problem using 2. I would probably use a larger one closer to the tank, run them in series, running them parallel doesn't increase the efficiency only the capacity.

 

[RF Overlord]

Originally Posted By: Trav
running them parallel doesn't increase the efficiency only the capacity.
This. If either filter allows a bit of dirt to pass, the other one won't catch it, so there's no real point to doing it this way. Go serial or go home.

 

[Papa Bear]

If something gets through filter 1 why would you think that filter 2 would catch it ??

2 in series would be of little benefit re: catching missed particles

2 in parallel would double capacity of debris before clogging.

 

[Danno]

Originally Posted By: Papa Bear
If something gets through filter 1 why would you think that filter 2 would catch it ??

2 in series would be of little benefit re: catching missed particles

2 in parallel would double capacity of debris before clogging.


Unless you can run a "coarse" filter first and a "fine" filter second.

 

[NHGUY]

If not for rusty gas tanks,I wonder how many fuel filters are actually replaced for being plugged,vs the owners conception of what "plugged" means.I believe gas engine fuel filters are replaced needlessly and too often.One exception..the screw on GM inline filters (33481 Wix),replace them before the threads rust up and you cant get it apart.

 

[Eddie]

Puts more load on the fuel pump if they are seriesed.

 

[ram_man]

Could cause more harm than good.

 

[Olas]

Originally Posted By: Eddie
Puts more load on the fuel pump if they are seriesed.


I'm not arguing this point, just asking for clarification..

If the fuel pump has to draw fuel through two fuel filters, surely there is the same amount of restriction wether they are series or parallel?

Or does a liquid in a pipe behave the same way as current flowing down a core, where resistance in series is expressed as;
*Req = R1 + R2 + .... Rn*

and resistance in parallel is expressed as;
*Req = 1/{(1/R1)+(1/R2)+(1/R3)..+(1/Rn)}*??

If its a stupid question, forgive me. I know nothing about fluid dynamics...

 

[Olas]

Originally Posted By: ram_man
Could cause more harm than good.


Care to elaborate on that with ideas or referenced studies?

 

[Shannow]

Originally Posted By: Danno
Originally Posted By: Papa Bear
If something gets through filter 1 why would you think that filter 2 would catch it ??

2 in series would be of little benefit re: catching missed particles

2 in parallel would double capacity of debris before clogging.


Unless you can run a "coarse" filter first and a "fine" filter second.


I tried that on my Nissan Diesel (there's a thread and a photo somewhere around here)...the injector pump circulates and "polishes" the diesel, and the filters are $60 each for a cheapy.

So I installed a CAV style upstream of the main filter, hoping to catch the rocks, and most importantly the water before it got to the main.

It was around 15um IIRC, so would have done most of the work, and been $10 a change rather than $60.

Problem was that there was too much restriction, and the IP starved at throttle on hills...and they are $4,800, which tempered my experimental bent on the machine.

 

[Shannow]

Originally Posted By: Olas
Originally Posted By: Eddie
Puts more load on the fuel pump if they are seriesed.


I'm not arguing this point, just asking for clarification..

If the fuel pump has to draw fuel through two fuel filters, surely there is the same amount of restriction wether they are series or parallel?

Or does a liquid in a pipe behave the same way as current flowing down a core, where resistance in series is expressed as;
*Req = R1 + R2 + .... Rn*

and resistance in parallel is expressed as;
*Req = 1/{(1/R1)+(1/R2)+(1/R3)..+(1/Rn)}*??

If its a stupid question, forgive me. I know nothing about fluid dynamics...


Not a stupid question...

Fluid differential pressure is proportional to flow squared, not like electrical current.

Two filters in series will have double the pressure drop.

Two filters in parallel will have 1/4 the pressure drop...why bigger filters make more sense if you can uses them.

From a reliability engineering standpoint...

Say that a filter has a 10% chance of failing during an OCI and contaminating your system....it's a made-up number, for the exercise...then you have a 90% chance of making it through the OCI without a filter failure ruining your day

If you put two filters in parallel, you double your chances of a filter failure...90%x90% = 81% chance of making it through the OCI without a filter failure wrecking your day...only one needs to fail, and if the other is still good, it doesn't matter.

If you put them in series, then each one backs the reliability of the other, and you work on the other side of the failure analysis.

With one filter, you have a 10% chance of a filter failure ruining your day.

with 2 filters in series, it's 10%x10%...1% chance of a filter failure getting to your engine.

Reliability wise, two in series is expensive, but makes good sense...as long as the problem that I described in the last post doesn't happen, and the stacked filter differential pressure doesn't cause another expensive problem.





Given the filter pic, it looks like they are on the suck side of a diaphragm fuel pump...which is OK.

What you need to do is find a filter that doesn't have a two hose size end connection, as most of the restriction is going to be the (IIRC) 3/8" hose fitting, and the hole sized for that versus the 1/4" (or whatever the numbers are)...I used to drill out the suction side on mine (plastic swarf captured by the filter) to at least halve the problem.

 

[Shannow]

http://www.bobistheoilguy.com/forums/ubbthreads.php/ubb/showflat/Number/2679520/Searchpage/3/Main/178969/Words/filter/Search/true/Re:_CAV_296_"pre-filt#Post2679520

That's the CAV install...get one with a petrol rated element, and it might suit you rather than the littlies.

 

[Olas]

well reasoned logic and science behind these explanations, which is why I'm spending more and more time on this board.

A lot of people on the internet seem to use the explanation
'because thats what I've always done',
or
'its never been a problem when I've done it'

but I feel like I'm getting valid information from the guys on this board.

Thanks, from England

 

[Shannow]

No probs...but I messed up this bit

Originally Posted By: Shannow
If you put two filters in parallel, you double your chances of a filter failure...90%x90% = 81% chance of making it through the OCI without a filter failure wrecking your day...only one needs to fail, and if the other is still good, it doesn't matter.


81% is the chance of not having a bad day...

So a filter that gave you a 10% possibility of failure gives you a 19% chance when in parallel...81% chance of not passing stuff through.

 

[Olas]

If something gets through filter 1 why would you think that filter 2 would catch it ??
[/quote

I can't give you a reason based on filtration technology because in my head, if a particle is small to get through the first filter. it should also be small enough to get through the second filter (because they are both the same filter using the same medium)
HOWEVER,
Every time I pop the bonnet I have a look at the fuel filters just to see if they look plugged up yet...the rear filter discolours quite quickly as it catches debris but the front filter doesn't start to show any discolouration until near the end of the OCI

So it seems to me that there are some particles getting by the first one and being caught in the second - I only think it would happen because I've seen it happen...but I can't quantify it scientifically, 'it just does'..

I may try to cover all the bases by having...
fuel line from the tank,
a T in the fuel line,
two filters in parallel,
a T to join those two filters back into one line,
one more filter, in series with those two which are in parallel with each other,
line to the suck side of the pump,

This way the two in parallel double the holding capacity, running into a third (series) filter should remove more particulate matter, and if one should fail there is still at least one back up filter to provide some amount of cleaning.

Why do we overthink and over engineer solutions to problems that don't exist? It was only 2-3 years ago I replaced the filler neck, the tank, all the lines and the pump!!

I guess I feel over-protective of my car as a whole, but even more so of individual systems that have been updated and cost blood, sweat, tears and money in the process!

 

[hattaresguy]

With marine applications its pretty much standard. You have say a 20 micron first and than a 10 micron secondary. Lastly you have the manufacture supplied on engine filter which is probably 2-5 micron.

The best setups have restriction gauges on them, and have another set of filters next to them with a valve so you can swap while running if they start to clog up.

I don't think fuel is that much of an issue, unless you plan on doing expeditions into third world countries. In that case I'd recommend a Baja filter as well.