B737 Duct Residual Pressure

[LEM]

Was dispatched today with Wing Body Overheat.
Following the procedure (isolation valve closed, pack&bleed off)
I expected the press in the duct to fall to zero, but instead it remained at 40 psi in climb and 20 in cruise!

After several reset, also inverting the situation (right and left), problem confirmed.

I'd like to ask the excellent engineers we have on Pprune if this is usual for an old airplane, maybe, to have the bleed valves not closing properly.

Thanks

[Trash Hauler]

I have not worked on B737 however on other types I have worked on the residual pressure should be zero. I would right this up as it appears the isolation valve was not fully closed and therefore the area not isolated correctly.

Cheers

[LEM]

The isolation valve, when we checked it on the ground using APU bleed, seemed to isolate perfectly (40 psi on the left side, ZERO on the right one).

Just to complicate things further...

[Shaka Zulu]

Did you notice duct pressure changes when opening or retarding the appropiate thrust lever for the closed ducting?
And did it change when you opened up or closed the thrust lever on the other side?
When opening up the isolation valve, did the pressure between both sides equalize?

Trying to do some fault diagnosis here, cos having any indication is absolutely not right

[ukeng]

I'd say the PRSOV is leaking.
If the isolation valve is closed (as proved by yourselves using APU air on the ground) and you're getting air on the right system , the only place it can be coming from is the No 2 engine bleed air.
With Bleed off the PRSOV should be closed but it's possible it's not sealing properly.
Doesn't really help with the original snag as you'll still be getting hot bleed air in the ducting.

Edit - CFM powered 737's normally have PRSOV's which are spring loaded closed. What did your MEL say about locking out the PRSOV?

[LEM]

Did you notice duct pressure changes when opening or retarding the appropiate thrust lever for the closed ducting?

Affirm!

And did it change when you opened up or closed the thrust lever on the other side?

Didn't try asymmetrical thrust inflight. Should have done it!

When opening up the isolation valve, did the pressure between both sides equalize?

Affirm, isolation valve worked as advertised (tried on the ground only).


With Bleed off the PRSOV should be closed but it's possible it's not sealing properly.

That's what we thought, but amazingly this happened on BOTH sides, althought less on the left one (15 to 20 psi in cruise, 20 to 40 on the right one).
That's why I'm suspecting all old airplanes might present the same glitch

What did your MEL say about locking out the PRSOV?

There was no maintenance action, just operational (avoid icing bla bla bla...)

[LEM]

Affirm, ASFKAP, then it rises again when the pack is turned off again.


Actually, the proper closing of the bleed valves is almost never confirmed in normal ops, and I'm sure we would find out interesting statistics if we checked every airplane!

[LEM]

It also suggests to me that your original snag was an erroneous wing/body ovht indication problem rather than an actual duct leak.

Exactly, ASFKAP.


If they give me more details, I'll let you know.

Thanks.