Purge valve return line

[hydroguy2]

I have an AFP injection unit and am looking for info and pics of the purge return line. I'm not wanting to go back to the tank, so where/how did you tie it back in?

[penguin]

Tie it in just after the fuel tap but before the electric fuel pump. You need to make a circuit so that the electric pump can circulate the fuel around. If you feed it in down stream of the electric pump that can't happen. The idea is to circulate cool fuel around the system, so dumping the returning fuel into a tank is best, but just downstream of the fuel tap is OK also.

Pete

[DanH]

Tie it into a tank line prior to the fuel valve. Select the other tank for start.

[Don at Airflow]

Originally Posted by penguin

Tie it in just after the fuel tap but before the electric fuel pump. You need to make a circuit so that the electric pump can circulate the fuel around. If you feed it in down stream of the electric pump that can't happen. The idea is to circulate cool fuel around the system, so dumping the returning fuel into a tank is best, but just downstream of the fuel tap is OK also.

Pete

Don't do this. It will only recirculate the vapor in the system when you purge, accomplishing nothing.

Correctly the purge return should be connected to one of the tank feed lines before the selector valve. Then you select the opposite tank when purging the system. You do not have to run the purge valve return line back to a tank. Look at the the schematic in Appendix I, page I-8 in your Installation and Service Manual.


Don

[RVG8tor]

Originally Posted by Don at Airflow

Don't do this. It will only recirculate the vapor in the system when you purge, accomplishing nothing.

Correctly the purge return should be connected to one of the tank feed lines before the selector valve. Then you select the opposite tank when purging the system. You do not have to run the purge valve return line back to a tank. Look at the the schematic in Appendix I, page I-8 in your Installation and Service Manual.


Don

Don,

Should a check valve be installed in the purge return line? I have read where some guys have added this but I don't think it is part of your diagram. Seems that the return line would just sit there with the fuel in it until the purge valve is opened, kind of like a closed branch in a system. I don't see the need for a check valve but maybe it would provide some protection from something I am missing.

[f1rocket]

A check valve would offer you the added protection if the return line developed a leak somewhere aft of the purge valve. Since most of that is higher than the fuel tank, I don't see where a check valve would offer up much for the complexity or cost.

Just FYI for archive purposes, on my last Rocket, I used a dual version of the Andair fuel valve, which is essentially two fuel valves in one. I plumbed the purge line to the second valve and routed the returned fuel to the opposite tank that was selected, That way, you didn't have to think about it when opening the purge line and running the fuel pump.

With that said, it was very complex and required a lot of expensive AN fittings. It worked, but probably was not worth the cost and complexity. It would be nice if Andair offered up a version of their fule valve that would do this automatically. I would think there's a market for this.

[DanH]

A check valve would offer you the added protection if the return line developed a leak somewhere aft of the purge valve. Since most of that is higher than the fuel tank, I don't see where a check valve would offer up much for the complexity or cost.

I installed a check valve at the tee in the tank line, but in retrospect I believe it unnecessary because of line routing. My original thought was to block free flow of fuel from the tank in the event of line failure/leakage/fire forward of the firewall. However, as the install progresed I realized that merely running the cabin side section of the line vertical and placing the bulkhead fitting high on the firewall would allow gravity to stop such flow. Gravity is very reliable.

A possible downside to a check valve is the pressure required to open it. With a check valve , shutdown by opening the purge isn't going be be as clean.

[Don at Airflow]

Actually because of the design of the purge valve the added restriction (within in reason) of a check valve would not change the operation or results of purging the system. The purge valve will shut off the port to the flow divider when in the purge position therefore the fuel would be forced through the check valve as long as the check valve had an opening pressure less than the boost pump operating pressure.

And like you said Dan, since the RV has low tanks unless you took the line loose and the connection was below the level of the fuel in the tanks the fuel would not run out so a check valve is not required on this installation. We typically install a check valve in the purge return line on high wing installations to keep from draining a tank if the purge line was removed for service or whatever.

Don

[airguy]

Originally Posted by Don at Airflow

Actually because of the design of the purge valve the added restriction (within in reason) of a check valve would not change the operation or results of purging the system. The purge valve will shut off the port to the flow divider when in the purge position therefore the fuel would be forced through the check valve as long as the check valve had an opening pressure less than the boost pump operating pressure.

Don

Gotta love it when the answer gets posted before the question!

[DanH]

Ahhh, got it, thanks Don. I was thinking the purge vented topside system pressure, indeed different from closing all flow to the divider.