Bubbles in Return Fuel Stream

In the Rotax owners video section is a group of videos detailing the construction of an aircraft with a 912iS engine. The one about the fuel systems addresses the issue of return air bubbles. The way I understand it is that bubbles are normal due to the action of the fuel pressure regulator. Mine has bubbles when the engine is running. Perhaps Bill will enter the thread conversation with his knowledge of the engine.

I have a question about turning off the aux pump.  Wouldn't that make the main pump work harder to carry the entire load, thereby shortening its life?

Richard Boslaugh wrote:

In the Rotax owners video section is a group of videos detailing the construction of an aircraft with a 912iS engine. The one about the fuel systems addresses the issue of return air bubbles. The way I understand it is that bubbles are normal due to the action of the fuel pressure regulator. Mine has bubbles when the engine is running. Perhaps Bill will enter the thread conversation with his knowledge of the engine.

Check this 912iS Builder Series video, right about the 5 minute mark...

https://www.rotax-owner.com/en/videos-topmenu/builder/439-912is-ch750-8

An RVP of 7 is very conservitive. About the same as 100LL.
But this is California and they are known to be conservative.
At 7, the fuel will vaporize above 7000' Pressure Attitude if it were heated above 130°F.
You were not above 7000" and it was unlikely the fuel was above 130°F with 20+ GPM being circulated.
That would make the fuel tanks too hot to rest a hand on them.

The Gas Bubbles in the fuel lines are completely normal.
They give the appearance of a foam immediately after they leave the Pressure Regulator when the pressure suddenly drops 40psi but consolidate into propper bubbles within a foot or so downstream.
They are Not "Air" Bubbles, they are Fuel Bubbles, Mostly Butane, and will go back into solution if given the chance.

I would check the Coarse Fuel filter located Before the Fuel pumps.
If the filter is getting restricted, the suction of the pumps can lower the pressure in the line causing Vapor formation.
- - -

Turning OFF the Pump.
Yes turning OFF a pump will stress the operating pump.
It will also relieve the non-operating pump.
So, overall it is a wash.
The idea is for there to be a backup pump available if one of them should fail.
Not to have both of them die in unison.
If you leave Both pumps ON all the time, you will not know if one pump failed until the second one fails also.  Ouch!!!
The MAIN and AUX Pumps are identical and the labels are arbitrary.
They could just as well be labeled 1, 2 ... A, B ... or LEFT, Right.
It is not uncommon to alternate the pumps in flight just to confirm that both are operational and either one can handle the full load.
- - -

RVP (Reid Vapor Pressure)
This is a measure of the Vapor Pressure at 100°F Measured in PSI.
When the Vapor Pressure exceeds the Ambient pressure, The Fuel will Boil (Vaporize).
This can be accomplished by raising the temperature (Heating) or lowering the pressure (Altitude).
You can actually measure RVP pressure by sealing a quantity of gasoline in a container with a pressure gauge attached and heating it uniformly to exactly 100°F.
The pressure you measure in PSI will be the RVP.

This is why your plastic gas cans bulge under pressure if left out in the sun.
Do NOT Relieve the pressure on your bulging gas cans!!!  You will be wasting gas!!!
The Gas Cans are designed to handle the pressure. Leave them Pressure sealed!!!
Gas only goes "BAD" in Unsealed, Improperly Stored, Containers.

Bill,

I appreciate all of the information, it verifies what I was thinking.  The CA summer blend has an RVP of 7, but the winter blend is higher. It took a lot of looking to find it but I finally located a white paper on the Chevron site indicating the winter blend has an RVP of 13.  With further consideration I believe I was likely running the winter blend when this happened.  This makes a lot more sense mathematically.  Starting out with a fuel temp of 100, and allowing for the fuel cooling to 90 during climb out, vaporization at the pump inlet would be right around 4500'.  

I'm working on a spread sheet to predict vapor lock conditions for my particular aircraft, taking into account the affects of temperature and pressure on the RVP, to get TVP.   I measured the fuel pump inlet pressure at my airport, which is just 125' above sea level, and the result was -.3 PSI with both pumps running.  So, with a fuel TVP of 7, I start with a Net Positive Suction Head (NPSH) of 6.7 PSI.  I'm not 100% sure of the affect of altitude on the pump inlet pressure, but I believe that as atmospheric pressure falls, the pump inlet pressure will be reduced by the same.  Do you agree with this?