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Dual fuel filters, pumps, check valves stack - vibration question

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Flow

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Hello folks,

This is what ULPower, the engine manufacture recommend.
1595883927914.png

I am looking to move away from slip on fittings to AN (and braided teflon for the flex hoses).
This is one side of the 100 micron filter > Bosch 044 pump with AN both ends > Check valve stack.
Note the T fittings to separate the flow to the two pumps and then rejoin again to the post filter.
20200706_102231.jpg
I like how everything just bolts together with minimal moving parts - however...

The question is how should I mount this to the firewall to protect the filters pump and check valves from vibration as they will all have different resonant frequencies (and mounting radii).
I don't want the filters or check valves to snap off the pump at the threads.
 

rv7charlie

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And the answer is,
Don't. :) FWIW, here are my thoughts on the subject.

Firewalls are typically hot. I won't mount fuel pumps (or gascolators, for that matter) on a firewall because I don't want big massive chunks of metal collecting heat and inserting it into the fuel. Some people complain about high pressure fuel in the cockpit, but I much prefer that to high pressure fuel spraying into the engine compartment where I won't know about it (for a while). Keeping the pumps in the cockpit keeps all that mass and fuel cool longer, and all those required fittings where you can quickly detect a leak if it happens.

Inline gerotor and roller-vane pumps are typically positive displacement pumps. If they aren't running, nothing flows in either direction, so there's your checkvalve. Easy to check yours by blowing into each end. A checkvalve, on the other hand, *can* stick in 'check' mode for either direction of flow. So.....

Someone on the VAF forum just had an emergency landing, while still over the runway on takeoff, because his single inlet filter *and* his single output filter on his electronic injected engine both clogged from trash in the fuel. My choice for my dual pumps was a filter for each pump inlet, and a filter for each outlet, before T-ing the outputs together. There are some cleanable filters with AN fittings on ebay now for a pretty reasonable price. Do check carefully to be sure that the filter element is a decent sized cylindrical element; some of them, even from 'reputable' suppliers, have only a little disc about a half inch in diameter.

Hope that's useful,

Charlie
 

TFF

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I would be careful with those filters. Most cars with those pumps have filters about the size of the pump if not double.
 

Flow

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Thanks Charlie, good reading.

I would be happy to ditch the check valves on the Bosch 044s if the really don't fail open, they do come with check valves on the output with a slip on fitting though... are we sure they hardly ever fail open?

The question is really about solidly connecting the filters (and check valves) to the pump or using flex like the manufactures pictures.

For instance is the filter to pump thread eventually going to crack off from vibration if they is solidly screwed together? What sort of cradle for a riged stack design should I use?
 

Flow

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I would be careful with those filters. Most cars with those pumps have filters about the size of the pump if not double.
Those are the pre-filters, the post filter is the same size as the pump. There is a long version of the pre-filters if required. They are certainly a step up from the factory supplied pre-filters.

Back to the vibration/fatigue question when solidly screwing the filters etc into the pumps?
 

TFF

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I’m talking pre filters too. My Alfas and XJS have the filter after the tank and before the pump. They are bigger than the pump. Same style pumps.
 

rv7charlie

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Sorry; I should have tried to answer your specific question. The simplest way to compensate for the different diameters is to use something like an Adel clamps on each, and use different height standoffs to tie the Adel-like clamps to structure. To stabilize, you *could* roll your own clamp out of sheet aluminum, with it being a significant percentage of the length of the pump or filter, and use 2 fasteners to tie it down.

I can't offer any solid engineering data on whether you're safe just screwing them together. But I might be able to give some 'perspective'. Pretty much everyone says to keep aluminum -4 AN fittings away from the engine because of the risk of fatigue cracking, or even breaking from the stress of the hose/tube pulling on it. But my understanding is that Van's A/C ships some aluminum -6 fittings to mount on the engine, with their FWF kits. There's a lot more mass in the wall of a -6 & up, than in a -4. If you think about it, -6 'hard line' is a lot thinner than the adapters, and relatively highly stressed at the flare, but it survives being screwed to pumps. I used -6 adapters to assemble my pre-filters to pumps and I feel comfortable with it, but full disclosure; it hasn't flown. (BTW, I mis-spoke earlier about dual output filters; I only have one configured at this time, but I'm seriously considering two after hearing about the guy on VAF.)

I would tend to agree with TFF about the prefilters. The issues with prefilters are that they will catch the vast majority of trash (and their filter area will therefore begin to get blocked quicker), and they are also operating 'in a vacuum'. The pumps must suck fuel through them, and the more restriction there is in the supply side, the more likely we are to get fuel flashing into vapor(lock). That's one of the reasons that the prefilter is typically 100 micron and the post filter is something under 30 microns; to minimize pressure drop.

I misunderstood your earlier post about checkvalves; I thought that they were integrated into the filter modules. I certainly wouldn't alter the fuel pump itself, if the check valve is integrated into (part of) the pump. I'm using Walbro GLS393 pumps, and have't had my hands on the Bosch pumps. They do have a great reputation, so if it ain't broke.... :)
Charlie
 

rv7charlie

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Followup on prefilters. I don't know where you are on the airframe, but my opinion about the ultimate solution in prefilters is to use an in-tank 'sock' filter like virtually all automotive applications, these days. They are large area, and barring the serious abuse of dumping pints of trash into the tank, effectively self-cleaning. That's why you never need to change a filter on modern cars, unless you're replacing an in-tank pump (also relatively rare). Rigging one can be a bit tricky, since they're made to press-fit on the inlet of the little turbine pumps, but it can be done.
IMG_2351.JPG
The sock and the tube sleeve (used as an adapter) are bonded to the tube with Flamemaster flowable fuel sealant, but regular proseal would work just as well. Hard to tell in the pic, but the safety wire continues behind the tube, to catch the mounting eyelet on the filter sock, to add 'suspender's to the 'belt' of sealant. I know it's pretty 'Rube' looking, but it was installed back in 2017 while I had the tank off to repair a leak, & was still flying when I sold the RV4 last fall.

Now, the ultimate would be a pair of turbine pumps in each tank. Zero risk of vapor lock, and lower power consumption than gerotor pumps. It would be somewhat more electrically complicated, though.

Charlie
 

Flow

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Thanks guys,

I have about 10 - 30" of head (depending on fuel level) to the pre-filters so I think I will be OK there.
I also have a long engine mount and quite a cool firewall so thinking I should be OK there as well.

I do like the reduction in the number of joints that could leak when connecting filters etc directly to the pump rather than having lengths of flex between them. Sound like this solid set up is not an absolute no no, and just needs secure mounting. Is this fair to say?
 

rv7charlie

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Under normal circumstances you'd be golden. But the vaporlock issue is between the prefilter and pump. If the filter starts to clog up and its effective area starts shrinking, it becomes more of an obstruction and the pump may suck the fuel into vapor at its inlet. The potential for both clogging and for vapor lock between filter & pump are the reasons for relatively large prefilters.

Charlie
 

Flow

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If the pressure drop across the pre-filters is less than 10" we are still even with a low tank, right?
I did a bit more digging on the Bosch 044 pumps, it looks like they do need their check valves to prevent reverse flow when used in parallel.
From what I can gather the main reason prefilters grew in size was to increase maintenance intervals. That said if I can fit the long versions of the SpeedFlows I think I'll use them.
 
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delta

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If you must mount your pumps on the firewall, make it as low as possible because they push fuel much better than pull. Put them in in a insulated, fireproof, vibration isolated box that has it's own fresh air supply, fuel dump overboard, and inspection port.
 

Flow

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That sounds good Delta, high pressure fuel in the cockpit sounds terrifying to me. I know drag racers that ended up covered in fuel and worse the cabin full of fuel vapor unable to shut anything (including the fuel pumps) down for fear of one little spark that would set it all off.
 
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wsimpso1

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We have had a number of discussions on hba.com about fuel systems, including mine. We did not get into forward or aft of the firewall, but it is pretty common to have valves and pumps aft of the firewall, even header tanks in the cabin.

Back to the original topic of securing the high pressure pumps and the associated plumbing... You are showing hoses, and they are generally to be minimized, as they are the source of all sorts of trouble. Hard lines and AN hardware are preferred for longevity and service. Filters should be big. The in tank bag filters are nice, but you had to have done that up front to implement them. There are lots of serviceable filters with AN fittings at the speed shop catalogs.

Then for the pumps themselves. Have you checked out www.sdsefi.com? Aircraft, then go down a ways and you will find SDS's dual EFI pump, with check valves, and fittings, structure for attaching to the airplane and shielding the wrench work at install and service from the pumps.

Your pictured setup fairly calls out that it will work the hose joints and damage the pumps and filters with those hose clamps. Ugh. While you may feel that you do not need to go full structure like SDS, at least go with a pair of padded clamps per pump and another per filter. Oh, and check valves are usually needed - both vane and gerotor type pumps can make fine hydraulic motors. No check valve, and fuel will get pumped in a circle. Sure, there will be some resistance, but it will make the pump set behave like it is short on both pressure and flow when only one is running.

Billski
 

rv7charlie

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Thank you for the correction on the gerotor pump/motor issue. I never even considered that they could be driven, as well as driving.

I hate it when I have to learn by saying something stupid...

Charlie
 

Flow

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OK so this is what I have got to so far.
2020-08-02 12.27.04.jpg
Any thoughts on how best to mount it?
I am thinking of a composite mold of half rounds with hose clamps glued into them that fastens the arrangement to the firewall.
The idea being to support the relationship between the filters and the pumps.
FYI, the 10micron is the post filter and the third check valve is for the return and the flex hoses are Aramid and Teflon.
 
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