Non-return valves

by Rob Seaton » 7 years ago

3 1.32k 251

Rotax did a lot of testing back in the early 2000's, and found the series and parallel systems can cause a power loss.
(one test is worth a thousand expert opinions. Wernher Von Braun)

Re: Non-return valves

by Bill Hertzel » 7 years ago

Bill Hertzel

13 1.95k 7.98k

Genival,

You have Pictured it correctly! (Pun Intended!) ;)

You have the correct Check valve. (<1psi)

I would have chosen a higher quality Pump.
Something more like the Carter P60504.
It likely costs 2-3 time as much as the Facet Pump.
Still, the Piston/Vibrator type pump will probably serve you for many years of intermittent duty.

A warning to the backyard engineer.
There are other valves of the same appearance with stronger internal springs that do not open until more than10 psi is applied.
These are used to bypass clogged filters in high-pressure fuel injection applications.
They would not be appropriate for this application.
If you can't blow through the valve with typical lung pressure, you have the wrong valve.

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

Re: Non-return valves

by Bill Hertzel » 7 years ago

Bill Hertzel

13 1.95k 7.98k

The typical failure mode of the Rotary Vane pump is either the Motor Fails and it just stops,
Or the Vanes slowly wear down from rubbing the pump walls.
The microscopic vane particles are either collected in the following fuel filter or if smaller than 10u, they just pass harmlessly through the engine.
Eventually one of the vanes gets so short that it is thrown out of the rotor and instantly jambs the pump.
The Vane is way too big to exit the pump.

Whether series or parallel, both systems perform reasonably well.
If a pumps were "Spew its Guts" a catastrophic blockage is possible in either system.

These are the same pumps as used in the automotive world.
There is only one pump in your car. Do you ever worry about if it is going to fail on your next trip?
How often do you ever hear of a fuel pump that does not outlast the engine and need replacing?
I buy new cars and drive them until they no longer run, I typically get a $25 scrap value when trading it in.
I have never had an issue with any fuel pumps in my 6 cars. (57 Years) (Knock, Knock on Wood!)

2000 engine hours is TBO in an aircraft, It is about 70,000 miles in your automobile.
If you are going to have an engine out in flight, the odd of the fuel pump being the cause is extremely low.

The realistic differences....
In the Parallel System, with both pumps ON, the flow rate doubles and risks overwhelming the Fuel Regulator and/or Tank Return line.
The fuel pressure is limited to the better pump of the two.

In the Series system, With Both Pumps ON the Fuel Pressure has the capability of ADDing together.
In a Regulated system, The Regulator dumps the excess fuel back to the tank so the pressure never actually increases.
The flow rate stays the same and each pump shares the work-load at reduced heat and current draw.
If the fuel filter following the pumps were to become partially blocked, the increased fuel pressure has the capability of maintaining a nominal flow to the engine.

This Rotax Series/Parallel argument.
The Parallel pumps are obviously in Parallel.
The Series Pumps are in Series with each other and in parallel with the Check valves.
You can call it whatever you want, it still operates the same way.

Note:
The 914, 912is and the 915 engine have dedicated fuel pressure regulators.

The 912Uls and other engines with a 4 psi Mechanical Pump assisted by an Electric Pump are also pressure regulated.
If the Electric pump has slightly more pressure (4.2psi) than the Mechanical pump, you get the pressure of the Electric pump (4.2psi)
If the Electric Pumps pressure is slightly lower (3.8psi), The Mechanical pump will make up the difference (0.2) and deliver its the nominal pressure (4psi.)
The diaphragm spring in the Mechanical pump is a virtual regulator in that it will add only the pressure need to maintain 4psi and no more.
With both pumps operating, the pressures do not add up, and you do not end up with 8psi.

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

Thank you said by: James Redmon

Re: Non-return valves

by Rick Elliott » 7 years ago

Rick Elliott

12 8 33

I was originally looking for a definitive answer on the correct fuel line system to be used particularly on the 914 engine. It appears that both the parallel and in Series designs are both widely used without any apparent problems.
From the limited research that I have done the "parallel " design appears to be the most popular of the two systems used both with and without non-return valves.
I appreciate everyone's input on the subject and find it very comforting to know there is a wealth of knowledge out there to call on should the need arise.

Re: Non-return valves

by ivor phillips » 7 years ago

ivor phillips

1 4 2

I Have a 914 and it's plumb in as parallel with two non return valves, It's worked flawlessly for the last ten years and i am happy
with the lay out, it's a much neater installation, But i do have an 3/8 return pipe back to the fuel tank after the pressure regulator so perhaps that's the answer if you don't wish to use the serial setup,

Non-return valves

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