Need for a check valve?

[catmandu]

Ever mindful of making improper system modifications, I am looking to the braintrust here as GoogleFu is not coming up with anything.

RV-6A Carbed O320

I am rebuilding the cockpit, part of that is removing the center tower which means relocating the fuel selector valve. Upon disassembly, I found a check valve I did not know I had: Circle Seal Valve, Fuel Check 600psi Pat. 2538364 869A-6TT-8

It is installed after the fuel selector valve, prior to the electric fuel pump, which in turn is prior to the gascolator.

Turns out it was not operational due to some soft tank debris jamming it open. It was suggested locally that it may be there to keep a prime on the engine driven pump, but SOP for me is to turn on the electric pump prior to start, and I never noticed any sort of issues with my engine.

Given that is has collected debris that may have been trapped and discovered in the gascolator, is there a reason to keep this valve installed?

[Sam Buchanan]

Originally Posted by catmandu

Ever mindful of making improper system modifications, I am looking to the braintrust here as GoogleFu is not coming up with anything.

RV-6A Carbed O320

I am rebuilding the cockpit, part of that is removing the center tower which means relocating the fuel selector valve. Upon disassembly, I found a check valve I did not know I had: Circle Seal Valve, Fuel Check 600psi Pat. 2538364 869A-6TT-8

It is installed after the fuel selector valve, prior to the electric fuel pump, which in turn is prior to the gascolator.

Turns out it was not operational due to some soft tank debris jamming it open. It was suggested locally that it may be there to keep a prime on the engine driven pump, but SOP for me is to turn on the electric pump prior to start, and I never noticed any sort of issues with my engine.

Given that is has collected debris that may have been trapped and discovered in the gascolator, is there a reason to keep this valve installed?

A check valve is not part of the "standard" carbed RV fuel system. I would not want it in my plane.

[vic syracuse]

Most fuel problems are caused by modifications to a known good working fuel system, of which the RV's have a proven record.

Vic

[David Paule]

I concur with Sam.

Check valves have two ways they can fail, of course: they can fail open, as yours luckily did, or closed, blocking flow completely. And since they have an inlet and an outlet, there are two additional points that could leak. Sometimes they are built in a way that provides additional potential leak points.

If you want redundancy in a check valve, you need four of them, two in parallel which are themselves in series with another parallel pair, to ensure that a single failure can't block the flow of fluid or, like yours did, allow fluid to flow in both directions. Since the set of four could have a single failure without symptoms and without affecting performance, the set of them would need to be fitted with some sort of sensor arrangement that reports their status. This is because once a single check valve fails, the system is no longer redundant and is vulnerable to another single failure.

Since they aren't part of the design, it's best just to leave that thing out.

Dave

[catmandu]

Well, thanks all.

Out it goes, which will simplify shortening the tubing runs!

[JonJay]

Debris? Make sure it isn't slosh flaking off the tanks.

[catmandu]

Nope, slosh check on the tanks was good, but thanks for mentioning it. This stuff had some bulk, was mostly round on the edges, about 1/8" long. Looked like what I would expect to see breaking away from the edge of a seam.

[snopercod]

After the system is back together, I recommend that you thoroughly flush the lines all the way to the carburetor inlet before flying again. You should also remove and inspect the carburetor inlet screen and the gascolator screen. Call me anal, but I remove and inspect those screens during every Condition Inspection.

[Ralph Inkster]

In place of the check valve, an inline fuel filter (protecting the boost pump) would be acceptable. Be sure not to use a paper filament type automotive filter!

[Canadian_JOY]

Originally Posted by Ralph Inkster

In place of the check valve, an inline fuel filter (protecting the boost pump) would be acceptable. Be sure not to use a paper filament type automotive filter!

In this case it's a carb'ed engine so no boost pump to need protection, or so it would seem...

Fuel systems seem to work best when the three S's are employed... Seamless, Straight and Simple!