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All Electric Fuel System - Redundant Check Valves?

K

krw5927

Well Known Member
I'm in the middle of plumbing my all-electric fuel system for my 9A. Without turning this into a debate about the merits or dangers of an all-electric system, I'm seeking advice from the collective VAF brain on whether check valves need redundancy in a carb setup.

My fuel system will look like the following diagram:
mskpsp.jpg


The fuel pumps are Facet 40109 as mentioned elsewhere in the forums, and have built-in check valves. They will be fed by redundant power supplies and will be on independent switches, circuits, etc.

I've been thinking, if one pump fails AND its built-in check valve simultaneously fails open, the other pump will pump to the path of least resistance. That would probably be in a closed loop between the pumps, and not to the engine, which could lead to a fuel starvation event. To mitigate this risk, I could put an additional Andair check valve at the output of each pump, further reducing the possibility of fuel starvation.

Is this failure scenario realistic? Sure, one pump could fail during a given flight. One check valve could fail during a given flight. But both? Is this akin to losing both the vacuum system and the electrical system simultaneously in a spam can?

The pumps will be tested independently during the ground run-up, and as each pump is tested a failure of the opposite check valve would be immediately apparent as a loss of pressure. So the more I think about it, the more I think there would have to be two independent, simultaneous failures: one pump failure and one check valve failure on the FAILED pump.

Thoughts? Opinions? Thanks!
 
Put the pumps between the filter, and the selector valve.

Push the fuel, dont suck it.
 
I'd certainly considered that, but then if one pump fails you are stuck with the fuel in only one tank. Not that I have plans to go blazing through the sky with less than appropriate reserves in each tank, but the single pump for each tank doesn't meet my personal design goals.

I realize it's not the hydraulically ideal placement, but it's really no different from the stock design. And I still have the benefit of not heat-soaking the fuel with a gascolator or hot enging-driven pump.
 
Two in each wingroot in series eliminates the possibility of a check valve failure and also elminates the fuel selector.
 
2 in series...

doubles the pressure.

The selector is relatively immune to dirt, & so is the Facet; I'd put the filters after the pumps to avoid the pressure drop in the filters (unless they are the relatively open style of filter element).
 
Lots of different opinions here, very interesting debate. I debated putting two pumps in each wing initially, but decided the plumbing would be prohibitively complex in such a tight space.
I think I'm committed to at least TRYING this design out. Mainly wondering whether check valves need backups.
This is the stuff that makes experimental aviation so cool.
 
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Kurt,

I like the fuel system layout that you have above. I think it addresses most of the vapor lock "hot spots" while avoiding excessive complexity (four pumps).

To answer your question, I think that additional check valves would be a good idea. It seems that a common failure mode for facet pumps occurs when debris interferes with the operation of the pump plunger and it's associated valves. If one of your pumps' plungers were to fail in this manner, there seems an elevated probability that the same debris would also cause the small internal check valve in that pump to malfunction. In this scenario, fuel would be able to recirculate even though all failures were confined to one pump.

There is one potential mitigating factor that I see. Even a failed facet fuel pump may present a fairly significant flow resistance, and thus your engine side fuel pressure while reduced may still be tolerable for a carbureted Lycoming.

David
 
2 check valves

Disregard the pressures and return line in my diagram and omit(replace) the engine driven fuel pump.
If you install your set up just the way you mapped it out up to the selector
you can install 2 electric pumps with the additional check valves as per my diagram. This will solve your concern of creating a fuel loop and not sending fuel to your engine.
Maybe I am wrong but it seems that you are trying to install a fuel system that is imune to vapor lock and possible use of auto gas?
In that case you can just copy this set up.

IMG_2462.JPG
 
Very nice, Ernst. That is exactly my intent. Well, that and my O-320 E2D doesn't have a mechanical pump.

It's clear you have an injected system, and I assume your fuel servo is happy with up to 50 PSI fuel. If I run the output of one pump to the input of the second, I'll easily flood my carb or sink the float. And I'd definitely want to run both pumps on takeoff and landing. Pressures in series pumps are additive.

Edit: as I look a bit harder at your diagram Ernst it appears yours differs from mine in one significant respect: The tee immediately after th electric pump with a check valve on only one leg - the other leg feeding the engine driven pump. Can you confirm what fuel pressure you see when both pumps are on? Is it 25, 50, or somewhere in between?
 
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Kurt, Talk with Curtis (RV-6 at Newton, KS), I believe he has an all electric fuel system as well. Best, -Jim
 
25psi

The AFP electric fuel pump has a built in pressure relieve valve set at 25 psi.
I never see pressures over 26 or maybe 27psi one or both pumps running.
I normally run the electric pump for take offs and landings just like you would for most low wing aircraft.
Not sure about carburetors and allowable pressures but the concept would be the same. Low pressure pumps, facet or mechanical ones, produce 4 to 5 psi and ordinarily both pumps are on for take off and landing and don't flood the carburetor. The additional check valves simply allow you to bypass the mechanical pump, in case it blocks fuel from passing through(vapor lock or other). My system allows you to run both, or one or the other, with the same results and the same pressures. And BTW I run 91 Octane ethanol mogas out of the right tank.
 
krw5927 said:
I'm in the middle of plumbing my all-electric fuel system for my 9A. Without turning this into a debate about the merits or dangers of an all-electric system, I'm seeking advice from the collective VAF brain on whether check valves need redundancy in a carb setup.

My fuel system will look like the following diagram:
mskpsp.jpg
Thoughts? Opinions? Thanks!
Click to expand...

Think about this....... Two 4104 pumps in series. Min. pressure 2#. Max pressure with both pumps on, 8#. Van's green arc for fuel pressure is .5# to 8#.

Install a 74 micron cleanable filter after selector valve (for ease of cleaning) and before Facet fuel pumps. No check valves. Each pump will pump through each other. Main pump should be wired direct to the battery and controlled with a quality oil pressure switch to act just like the manual fuel pump.
 
2 pumps in my -4

I am running 2 of the little Facet pumps in series in my -4. I have an O-320H2AD with no mechanical and an MA4SPA carb. I have one pump in the tunnel between my feet that is the "boost" and pressurizes the pushes pressure through gascolator (electric primer connected there), and the other is after the gascolator on the firewall, which I normally run continuous. Both can be switched on or off and have seperate breakers,and used as the primary pump. I have 120 hrs on this system, and no problems at all...very easy to verify either pumps operation via pressure gauge. I am not currently running any filters.
 
gasman said:
Think about this....... Two 4104 pumps in series. Min. pressure 2#. Max pressure with both pumps on, 8#. Van's green arc for fuel pressure is .5# to 8#.

Install a 74 micron cleanable filter after selector valve (for ease of cleaning) and before Facet fuel pumps. No check valves. Each pump will pump through each other. Main pump should be wired direct to the battery and controlled with a quality oil pressure switch to act just like the manual fuel pump.
Click to expand...

I really like this idea. I see the max flow for the 40104 pump is 25GPH. I assume that's more than enough for the O-320 at max power plus some reserve capability? I wouldn't think the O-320's fuel burn would be near that much.

Also, what is the reason for the filter after the selector instead of in each wing root as I'm planning? Is there a benefit that I haven't explored? I was mainly thinking about keeping the cleanable 74micron filters out of the cabin. That way I don't accidentally dump fuel all over inside when I clean the element.

Finally, I have no experience yet with oil pressure switches. Can a pressure switch control a 3A load or would I need to use the pressure switch to control a relay that controls the fuel pump? I like that this would simplify operations to be more "spam can-like" for others who might fly the plane, and remove a switch from my panel. But if oil pressure switches aren't as reliable as the fuel pumps I don't want the switch to fail and take down the pump unnecessarily often.

Thanks everyone for chiming in! These are some really cool ideas!
 
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Is there any reason you can't put the mechanical pump back in your O-320 E2D?

I have a friend with an O-290 powered Emeraude who just rigged up a dual-pump system... I'll see if I can find the email with the details.
 
Snowflake said:
Is there any reason you can't put the mechanical pump back in your O-320 E2D?
Click to expand...

This E2D came off a Cessna and has no accessory case provisions for a mechanical pump. I think maybe all E2Ds are the same. I could have the accessory case machined, then buy the parts and the pump, but as I understand the mechanical pump is in a less-than-ideal location hydraulically and receives a lot of heat from the engine which contributes to the possibility of vapor lock. I also want to be able to use ethanol-laced auto fuel, and the mechanical fuel pump diaphragms are not compatible.
 
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rocketbob said:
Two in each wingroot in series eliminates the possibility of a check valve failure and also elminates the fuel selector.
Click to expand...

That only eliminates the failure mode of failing open. To also eliminate the failure mode of failing in the closed position, you need a second pair in series, which are located parallel to the original two. That gives you four on each side for redundancy to keep working in the event of a SINGLE check valve failure.

Dave
 
I wouldn't worry about a closed check valve condition as its not likely to occur, and the backup in that case is to switch to the other tank. Assuming you have enough fuel in it to make a precautionary landing.

I have my pumps in the wing roots in my rocket. Inline filter,pump,pump, all in series. 2nd pumps normally stay off. No fuel selector.
 
krw5927 said:
Also, what is the reason for the filter after the selector instead of in each wing root as I'm planning? Is there a benefit that I haven't explored?

Finally, I have no experience yet with oil pressure switches. Can a pressure switch control a 3A load?
Click to expand...

1. You need to be able to shut off the fuel to remove the filter.

2. The unit (1) draws 1.4 amps. The instructions say to install an oil pressure switch so if you have an accident and the motor dies, the fuel pump will stop flowing. No relay needed.

With one fuel pump installed like this, it will act just like the mechanical fuel pump. Remember, It must be wired direct to the battery so if you must shut off the master during flight, you still have fuel going to the motor.
 
krw5927 said:
I'm in the middle of plumbing my all-electric fuel system for my 9A. Without turning this into a debate about the merits or dangers of an all-electric system, I'm seeking advice from the collective VAF brain on whether check valves need redundancy in a carb setup.

My fuel system will look like the following diagram:
mskpsp.jpg


The fuel pumps are Facet 40109 as mentioned elsewhere in the forums, and have built-in check valves. They will be fed by redundant power supplies and will be on independent switches, circuits, etc.

I've been thinking, if one pump fails AND its built-in check valve simultaneously fails open, the other pump will pump to the path of least resistance. That would probably be in a closed loop between the pumps, and not to the engine, which could lead to a fuel starvation event. To mitigate this risk, I could put an additional Andair check valve at the output of each pump, further reducing the possibility of fuel starvation.

Is this failure scenario realistic? Sure, one pump could fail during a given flight. One check valve could fail during a given flight. But both? Is this akin to losing both the vacuum system and the electrical system simultaneously in a spam can?

The pumps will be tested independently during the ground run-up, and as each pump is tested a failure of the opposite check valve would be immediately apparent as a loss of pressure. So the more I think about it, the more I think there would have to be two independent, simultaneous failures: one pump failure and one check valve failure on the FAILED pump.

Thoughts? Opinions? Thanks!
Click to expand...

I am very interested to know what you ended up with, as I have to build a similar system myself.

I am running 2 of the little Facet pumps in series in my -4. I have an O-320H2AD with no mechanical and an MA4SPA carb. I have one pump in the tunnel between my feet that is the "boost" and pressurizes the pushes pressure through gascolator (electric primer connected there), and the other is after the gascolator on the firewall, which I normally run continuous.
Click to expand...

What is the reason for mounting one pump after the gascolator on the firewall?


Since the failure mode is "fail open" on these Facet pumps, then mounting two in series (and no return valves) must be the simplest and best option in my opinion. But I am thinking of mounting one directly after the other, unless there is a particular reason for not doing so that I don't know about.


Edit: Has anyone though about using a separate battery for the "aux" pump? for added redundancy in case the main battery goes down. How is this done on EFI systems? I would think batteries are not the weak link, and is easily monitored, thus an extra battery does not add anything except complexity?
 
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My Subaru fuel system is similar to your diagram with the following exceptions:
- I have filters between the selector and each pump (as mentioned earlier, so fuel can be shut off to replace the filters).
- I have another (finer - Andair) filter after the pumps and before the engine. The engine has an internal filter as well.
I also put automotive 'socks' on my fuel pickups in the tanks. Any FOD that wants to get to my engine has a multi-stage gauntlet to pass first.

My pumps also have built-in check valves. As you mention, the pre takeoff check tests the whole system. I obviously feel redundant check valves are not needed. These valves are very simple and if both a pump and it's check valve fail simultaneously, well it's just not your day :eek: You will win the lottery first :rolleyes:
 
Dual Parallel Electric Pumps

The layout:
I have fuel inlet screens in the fuel tanks, then the fuel selector valve, then 2 automotive fuel pumps in parallel (next to the left rudder pedal). Each pump discharges thru an Andair check valve then they Tee together and pass thru the firewall bulkhead fitting to a steel automotive fuel filter, then to the injection.
Excess fuel pressure is dumped by a regulator, back thru the fuel selector valve, which returns the unused fuel to the selected tank.
The fuel pumps receive electric power from independent switches & fuses from the Emergency Bus. Running both pumps at the same time for take off and landing increases the fuel pressure minimally.
Why I did it:
I'm switching from an Auto Engine to a Lycoming O-360 with Airflow Performance Fuel Injection, and I'm keeping the dual electric pump system.
Although it is unconventional for this application, the AFP manual indicates an advantage: reduced flooding if the engine is shut down by turning off the fuel pump. The traditional mechanical pump can discharge one last stroke of fuel thru the Bendix & AFP injection fuel control valve after shutdown. This fuel ends up in the intake manifold or air filter and can cause hot start flooding. Shutting down the engine by running the injection dry reduces the need for a purge valve.
 
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Another system...

Two things, first search FrankH threads. He posted his all electric system.

Second, here's what I'm planning, but first a little history... The reason I'm planning on this change was something that occurred during phase I testing. I was flying at about 5000 near the airport when the engine started stumbling. I did all the usual things, turn on boost pump, switch tanks, etc. the boost pump made the problem slightly better, briefly. Anyway, after a nice sweaty landing, I shut the engine down and immediately the fuel filter started leaking. I disassembled the filter and noted that the o ring wasn't seated in the groove in one spot. I'd never had the filter apart before, and never would have thought to look at that. Ever since then I'm paranoid about any possible source of leaks.

What I plan on doing is this. A fuel filter (or gascolator, yet to decide), then facet electric pump in each wing root. In the fuselage both lines to a tee then a shutoff valve. Eliminate the firewall mounted gascolator, and run the line directly to the engine driven fuel pump.

For takeoff/landing both pumps on. For cruise both pumps off and let the engine driven fuel pump empty both tanks at the same rate ( mostly). If I need to pump out of a single tank for whatever reason, I turn that pump on.

The problems this eliminates:
1. Fuel starvation from an air leak (since the pumps are operating in pressure mode vs suction).
2. I get the fuel pump and filter out of the cockpit.
3. It allows the option to use both tanks at the same time.
4. It gets rid of the firewall mounted heatsink/gascolator.

Ok, fire away!

Lance
 
What fuel pumps for a 540?

rocketbob said:
I wouldn't worry about a closed check valve condition as its not likely to occur, and the backup in that case is to switch to the other tank. Assuming you have enough fuel in it to make a precautionary landing.

I have my pumps in the wing roots in my rocket. Inline filter,pump,pump, all in series. 2nd pumps normally stay off. No fuel selector.
Click to expand...

What pumps are you using to feed a 540?
 
SvingenB said:
I am very interested to know what you ended up with, as I have to build a similar system myself.
Click to expand...

I ended up with just the check valves built into the Facet 40109 pumps. In my installation I found the pumps offered enough flow and natural resistance to keep the carburetor fed in the case of a failed-open valve valve and a simultaneous failure of the power supply to the same pump. So far it has worked flawlessly.

Make sure to test your installation until you are comfortable with its operation.
 
judoka5051 said:
Ok, fire away!
Lance
Click to expand...

Imagine you're descending at the end of a flight with only reserve fuel left (a few gallons in each tank). The electric pump in the right tank fails - but how do you know a pump has failed, and how do you know which one it was? Better yet, what do you do about it?

In this installation both pumps will be turned "on" but only one will be pumping. And you may not know anything is wrong until the engine quits. Fuel can free flow from one tank to the other because the tee is before the selector valve. So the left pump simply (and quickly with no restrictions) pumps that last couple gallons of usable fuel out of the left tank and into the right. The left tank fuel pickup un-ports, so your engine driven pump sucks air. You cant get fuel out of the right tank because its pump has failed. A single point failure can cause fuel starvation in such an installation.

A check valve on the output of each electric pump would prevent this problem. But I found the check valves on Facet 40109 pumps are pretty restrictive even in the forward direction. In my installation that is a good thing. In yours, it may cause the engine driven pump to vaporize fuel. You may opt for a less restrictive Andair check valve on the output of a standard, non-checked Facet pump.

Also be aware that pumps operating in series are additive in pressure. So if your Facet pump is rated to 5 psi, and your engine driven pump is rated to 5 psi, then when both are operating your carburetor (assuming a carb) will receive roughly 10 psi. That might be enough to sink a carb float.

Please be very careful and consider every possible failure mode when designing alternative fuel systems. This is some of the most dangerous stuff we play with in experimental aircraft building. A very wise idea is to stick with the Vans setup. The parts and machining on an accessory case is not all that expensive or troublesome.
 
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Good points,,, but I've got answers!

Hey Kurt,

I'll address your concerns in order of posting. I won't be using the electric pumps while descending, only the mechanical pump until I'm on final. I only turn the boost pump on for the unlikely need to 'go around' anyway. Plus, I'm pretty conservative about fuel and much less than 5 gallons a side makes me uncomfortable.

The pumps I'm going to use have the built in check valve (already using this pump for a boost pump). If 1 pump fails then I'll only be able to pump out of the other tank with the electric pump, or use just the mechanical pump only and continue to use 'both'. The built in check valves will stop the inadvertent fuel transfer problem.

As far as running the pumps in series, it will be no different than it is right now with the single boost pump, then mechanical pump installed as shown in the plans. The pumps will be operating in parallel, and producing the same pressure as one (electric) pump.

Thanks for the input and comments,

Lance
 
Sounds like you've given this a lot of thought, Lance. Let us know how it works out, and if I may suggest one more thing, test your fuel supply thoroughly on the ground before you fly it.
 
I sure did. I agonized over every eventuality. You can bet I'll be very thorough on the ground, and probably remain within gliding distance of the field until I burn through both tanks.

Lance
 
krw5927 said:
This E2D came off a Cessna and has no accessory case provisions for a mechanical pump. I think maybe all E2Ds are the same. I could have the accessory case machined, then buy the parts and the pump, but as I understand the mechanical pump is in a less-than-ideal location hydraulically and receives a lot of heat from the engine which contributes to the possibility of vapor lock. I also want to be able to use ethanol-laced auto fuel, and the mechanical fuel pump diaphragms are not compatible.
Click to expand...

This is for those who follow and find this thread:

OK, I'm old school on this one and had to chime in.

The O-290-D2 that my -9 started life with was not setup for a fuel pump. I thought long and hard about doing the dual electric setup like you have done. At the time, one of our chapter members had an RV-6A with an O-320H2AD up front and rather than putting blisters in the top front of his cowl, he went with two pumps, similar to what you did.

With a lot of thinking, research, and talking I elected to change the accessory case on the O-290. All told, I think it cost me around $350 plus the cost of the engine driven fuel pump. That included a used, yellow tagged, accessory case, cam gear, plunger, and gaskets.

Changing the accessory case was easy enough to do.

There is a good reason why our fuel systems are designed the way they are and thousands of RV's are flying with this setup.

Most Phase 1 engine stoppages are related to the fuel system, so be cautious when making changes.

The main reason for the "standard" set up is so that should the engine pump fail, you have an option of turning on an electric pump and should the electric pump fail, the engine pump will get you home.

With a dual electric fuel pump set up, you are depending on the electricity always being there. Yes, you can add batteries, diodes, check valves, etc. but any time you add complexity you add risk.

Good luck with your setup! I wish you many trouble free hours.

PS. When my friend sold his -6A the new owner removed the dual electric pumps, added a fuel pump to the front of the engine, single electric pump, and matching blisters on the top of his cowl.
 
F1R said:
What pumps are you using to feed a 540?
Click to expand...

Four 40185's with fuel return lines. Rotec, not bendix injection.

Two in each wingroot. Tank->filter->40185->40185->tee from other side->engine.

Return plumbed to left tank.

40185's have built in check valves.

Only one in each wingroot is ran normally, the other one is a backup.
 
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N941WR said:
The main reason for the "standard" set up is so that should the engine pump fail, you have an option of turning on an electric pump and should the electric pump fail, the engine pump will get you home.

With a dual electric fuel pump set up, you are depending on the electricity always being there. Yes, you can add batteries, diodes, check valves, etc. but any time you add complexity you add risk.
Click to expand...

This is understandable, but debatable. I won't argue with the merit of having two separate systems (mechanical and electrical), but the complexity argument is not correct in my opinion.

The simplest redundant system is two electric pumps (facet) in series. When replacing one facet with a mechanical pump, you add complexity (more fuel lines that runs inside the heated cowling, and the added complexity of the mechanical pump itself. But you have added "better" redundancy because a mechanical fuel pump runs with no electricity. You have in fact traded simplicity for redundancy.

No electricity is the failure mode for the simpler all electric system. With separate buses for each pump, this means a dead battery is the failure mode. How likely this is to occur so fast that you won't have time to land somewhere before the battery is dead is hard to tell, but it's certainly much more unlikely than likely. This means the failure mode is most likely not a sudden and "catastrophic" failure mode.
 
My all electric -4

I built my -4 with an O320H2AD (carb), and didn't want the big cowl bump, so I run the much debated "all electric" fuel system. I have two Facet pumps, one in the tunnel per plans(boost), and one on the firewall (always on). Either runs the plane fine alone, I use both for T/O and Landing. They are plumbed in series with a gascolator . I have a bit over 200 hrs. on it now, and it works perfectly. A month ago, I lost my alternator(crimped terminal broke) while flying formation with some friends, and was close to my home airport, as well as several others, so I did a little test. Flew 45 minutes with less than .5V drop in my voltage, still using radios, electric flaps ect. This gave me comfort in how little draw comes from all my "stuff", but as a backup, I carry a rechargeable battery pack that can be plugged into a cig lighter and will carry all the essentials. Winter upgrades will be a solid state B/C alternator on vacuum pad, and small back up battery .
 
Fuel system mod update

Hey all,

As explained earlier in this post I decided to alter my fuel system and explained the reasons why. Someone asked my to provide an update after using the system for a period of time. Well, it's been a year and 3 months and it has worked perfectly the entire time. Actually, I found one more benefit than I planned. Reduced autopilot 'bobble'. During early testing I noticed that as one tank burned off the autopilot was making more adjustment cycles per minute to counter the uneven weight of the fuel load. It resulted in the stick pulsing and a noticeable side to side wobble as a result. With my system, which can be left, right, or both, I can balance the airplane, then let the tanks empty at the same rate. I just flew the plane 1100 miles and never had to play with the fuel system other than running the boost pumps for takeoff.

Here's a brief description of the system:

Fuel filters and facet boost pumps (built in check valves) in wingroots to a tee in the cockpit. Then to the fuel valve (on/off only), gascolator, engine driven fuel pump, carb.

Boost pumps on for takeoff, then both off for cruise flight with engine pump only. If for whatever reason I want to use one side I can turn on that pump.

I was originally going to remove the gascolator, but was overcome by laziness.

In other news, I listed elsewhere that the plane is very light and what I did to get it that way. I also spent some time shamelessly using speed mods picked up on this site and had very good performance with this plane. I usually see 205mph at 2700rpm at 10.5k, not bad on 160hp! Plus I was still getting about 1700fpm solo with full fuel.

Hope this helps someone, Lance
 
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