Non-return valves

Re: Non-return valves

by Bill Hertzel » 8 years ago

Bill Hertzel

13 1.95k 8k

It is a matter of function.

A Check Valve or One-Way Valve can be referred to as a Non-Return Valve or a By-Pass Valve depending on its use.
It is still the same One-Way valve no matter what you want to call it.

Consider the 912UL(S) type fuel system.
It is a low pressure(4psi) Inertial/Centrifugal Pump.
When it is not turning, fuel is free to flow with only minor restriction in either direction through the pump.
While the mechanical pump could theoretically pull fuel through this pump a By-Pass Valve is installed in parallel with the Aux pump to allow an additional flow path around the Aux Pump.
It is not there to prevent Backflow, it is there to allow forward flow.

In the 914 engine, The fuel pumps are medium Pressure(25psi) Rotary Vane type pumps.
When the pumps are not turning, the pumps present a major obstruction to the fuel flow.
There are By-Pass Valves in parallel with Both Fuel pumps.
Again they are not there to prevent Backflow, They are there to allow forward flow around the non-powered pump.
Without them, the 914 engine would suffer fuel starvation if either pump was de-energized.

So I guess it is a matter of semantics or just the quirks of the English language.
There are no Non-Return Valves installed in a typical Rotax installation. Just One-Way, Bypass Valves. :)

If you installed an engine in a low wing aircraft where the possibility of the fuel lines draining back into the fuel tanks while hangared was a concern,
Then the installation of a Non-Return Valve BEFORE the Aux Pump would be warranted.

In your case, you appear to have the pumps plumbed in parallel instead of the normal and highly recommended Serial configuration.
Apparently, your aircraft manufacturer has chosen to ignore the Rotax installation manual and installed the pumps in parallel and has found that Non-Return Valves were not needed.
Your concern is that the operational pump will backflow through the non-operational Pump causing a loss of fuel pressure.
If you are getting proper fuel pressure when operating on only one pump, this is the proof you are looking for that Non_Return Valves are not required.

If you were to Reconfigure your pump in Series, you would definitely need to add By-Pass Valves to allow the pumps to deliver fuel.

So, if you follow the Rotax installation Manual, The By-Pass Valves are absolutely Mandantory.

If you want to know if Non-Return Valves are needed to prevent Backflow through the 914 type pumps?
No, they are Not!

---

Bill Hertzel
Rotax 912is
North Ridgeville, OH, USA
Clicking the "Thank You" is Always Appreciated by Everyone.

Re: Non-return valves

by Genival Pacheco » 8 years ago

Genival Pacheco

4 87 135

Hi Bill,

You wrote

Consider the 912UL(S) type fuel system.
It is a low pressure(4psi) Inertial/Centrifugal Pump.
When it is not turning, fuel is free to flow with only minor restriction in either direction through the pump.
While the mechanical pump could theoretically pull fuel through this pump a By-Pass Valve is installed in parallel with the Aux pump to allow an additional flow path around the Aux Pump.
It is not there to prevent Backflow, it is there to allow forward flow.

I´m about to correct the fuel line system of my low wing plane and need to know if my assumption regarding the check valves and other are correct.

Today I´ve installed a FACET 40.105 installed in line whit the mec. pump withought any by-pass and check valves.
I bought two ANDAIR CHECK VALVE CK250-H that I´m planning to istall like the sketch attached.

Is that right?

Regards
Regards

Fuel_System.PNG (You do not have access to download this file.)

Re: Non-return valves

by Ken Ryan » 8 years ago

Ken Ryan

28 372 993

Bill,

This page http://contrails.free.fr/engine_pierburg_en.php indicates that Rotax allows either serial or parallel fuel pump installation on the 914 AND indicates that Pierberg recommends parallel installation.

edited to add: Also interesting is the observation that the Rotax "series" setup is not really series.

Could you take a look and comment?

Thanks,

Ken

Re: Non-return valves

by Rob Seaton » 8 years ago

Rob Seaton

3 1.32k 251

That is quoting 1996 Installation Manual....
The current 914 Installation Manual makes the check valves mandatory. For good reasons, and not to make anyone's life miserable :).
The setup can be described as both series and parallel.

Re: Non-return valves

by Ken Ryan » 8 years ago

Ken Ryan

28 372 993

I'm having trouble seeing the advantage of the Rotax "series/parallel" over a more conventional "parallel" setup. As pointed out in the article I previously mentioned [Link] it seems that the Rotax way introduces a greater probability of fouling the remaining pump with internal components of a failed pump, while offering no other offsetting advantage. Maybe the chances of a failed fuel pump blocking the remaining one is so small that it's not worth considering. I could buy that. But what is the downside to the conventional parallel setup (with check valves)? Does anyone know why Rotax uses the series/parallel rather than parallel?