Airflow performance purge valve return line

andrew phillips · #1 06-22-2007, 07:24 PM

I would like to run the return line from the purge valve to the tank vent line at a point where the vent line is running vertical inside the cabin. i was thinking of putting a "T" in the vent line just above the rudder cable. This way the fuel shouid run down and into the tank and not up and over the top to the vent. Any comments? I know a direct line to the tank is the best but I am not sure I want to add more complication to this area. I already had to mount a gascolator in the wing root (dumb Canadian regs!).

Mark Burns · #2 06-22-2007, 07:39 PM

Andrew,
The fuel will go in whatever direction has the least resistance to flow.

I believe you'll wind up dumping a lot of the fuel out the vent. It will depend on the "pressure" in the tank at that time.

By that I mean is the fuel on a warming trend or cooling trend...It's almost always doing one or the other. If the fuel is warming up you'll be hard pressed to get fuel to run into the tank without going out the vent. If you are feeding from that tank of course that would help.

By leaving your "vent" alone and running a complete new return line the vent line can do it's job, which is vent. (mostly air).

This is just my opinion.

Mark

Rick S. · #3 06-22-2007, 08:14 PM

I ran mine to a Tee at the tank fuel supply on the right tank. I also installed a check valve on the purge line so fuel can only run into the tank. The procedure is to select the left tank, open the purge valve and run the boost pump. The fuel used to cool the distribution block is sent to the right tank.

This is one of the routing options given by Airflow for plumbing the line with the exception of the check valve. If I had the foresight while building the tanks I would have installed it's own return fitting the same as the vent line with an inner tube out to the outer portion of the tank but I didn't consider a purge valve and my BPE IO_540 came with it so since I paid for it, found by research that it may be helpful on hot starts I elected to keep it in the system. Also understand that the shut down sequence is different than a engine that doesn't have a purge valve. I highly recommend you use the same quality control cables as the mixture/throttle/prop and install a return spring to close the valve in the event the cable fails for some reason. The valve is a critical part of the engine controls.

But I don't have a root mounted colator to deal with either.

bruceh · #4 09-27-2012, 01:36 PM

I'm also wondering if it is OK to run the purge line back to the vent line in the cabin. Assuming you are purging on the same tank that is tee'ed into the vent line, this is a closed loop system. The fuel pump pushes the fuel into the distribution spider, then the fuel leave there via the purge line and into the vent line and back to the far end of the tank.

How would the fuel be able to go uphill and oppose the suction of the pump and gravity to flow out the vent opening?

Splicing into the vent line looks a lot more easy than having to change the size of the purge line to tee into the fuel tank line prior to the tank selector.

rleffler · #5 09-27-2012, 02:06 PM

Quote:

Originally Posted by bruceh

I'm also wondering if it is OK to run the purge line back to the vent line in the cabin. Assuming you are purging on the same tank that is tee'ed into the vent line, this is a closed loop system. The fuel pump pushes the fuel into the distribution spider, then the fuel leave there via the purge line and into the vent line and back to the far end of the tank.

How would the fuel be able to go uphill and oppose the suction of the pump and gravity to flow out the vent opening?

Splicing into the vent line looks a lot more easy than having to change the size of the purge line to tee into the fuel tank line prior to the tank selector.

What Rick was describing was to splice into the fuel feed line from the right tank prior to the fuel valve. In the RV-10, we can do this under the right seat. I suspect it would be similiar on the other models.

If you splice into the vent line, how do you control if it goes into the tank or out the vent?

bruceh · #6 09-27-2012, 04:02 PM

Quote:

Originally Posted by rleffler

If you splice into the vent line, how do you control if it goes into the tank or out the vent?

Gravity and suction. To have the purged fuel go up and over the loop in the vent line would require it to go against gravity. The fuel pump would be on when doing the purge operation. The pump is sucking fuel from the same tank as the vent line, so it should act as a closed loop system.

Has anyone plumbed it this way, and if so, does it work?

JoeBlank · #7 09-27-2012, 05:08 PM

Andrew,
I did it exactly that way on my RV-6. Over 900 hours and it still works fine. One placard you would need to add is to 'select' the tank that the purge line is plumbed to when performing a 'purge operation'. It forms a closed loop that way. Otherwise, if the other tank is selected, it will puke fuel out the vent since it is deadheaded...

Make sure your Tee is plumbed on the downhill side of the loop closest to the tank and NOT the vent side...

You will also notice a little occasional dribble from the vent when using the purge valve to shutdown the engine, which is standard procedure per Airflow. Not a big deal though...

longranger · #8 10-01-2012, 09:48 AM

My plan is to install a dedicated return fitting on the inboard rib of the right tank.

WingsOnWheels · #9 10-01-2012, 10:12 AM

I ended up with a carbed engine, but back when I built my tanks I added a second feed 'just in case'. I just used the same flange fitting as the sump drain.

I now have 6 of those flange fittings on my RV-6. 2 for the sump, 2 for the additional feed, and 2 to adapt the pop-rivet static port to the safeair fittings.