One thing I have never managed to understand, or managed to get a satisfactory explanation for:
The DUAL BLEED light illuminates whenever the APU bleed air valve is open and the position of the engine bleed air switch and isolation valve would permit possible backpressure of the APU. Therefore, thrust must be limited to idle with the DUAL BLEED light illuminated.
Basically, my doubt is: do we want to protect the engine or the APU?
According to the manual, we want to protect the engine: APU pressure could go into the 9th stage modulating and shutoff valve. So why on earth do we have to limit thrust to idle?
Increasing thrust would help preventing this problem!
Years ago I was told it's the APU we want to protect, actually....
The engines have a modulating/shut-off valve to protect it from reverse flow, but the APU bleed air valve is 'dumb', and will not be aware of any reversal of airflow. All the APU knows is that it's RPM is above 95%, and the bleed switch is commanding 'open' - so it does!
Going much above idle with the main engines has the same effect as turning off both packs, from HIGH, at the same instant, whilst on APU bleed, it creates a large back pressure, which can stall the APU, with a resulting overtemp, and auto shutdown!
"The engines have a modulating/shut-off valve to protect it from reverse flow, but the APU bleed air valve is 'dumb', and will not be aware of any reversal of airflow."
Don't know about the Classic, LEM, but the 737NG Maintenance Manual mentions an APU Check Valve (in the Aircon bay), and says that the valve is there to protect the APU. Perhaps the procedures are playing it safe? (The valve may not have the reaction time required for all situations perhaps?)
Yes as you pointed out the APU has got a check valve on all versions.
Quote from Operations Manual 2.20.2:
"With both the APU and engine bleed valve open, the engines operating at idle thrust, there is a possibility of APU bleed air backpressuring the 9th stage modulating and shutoff valve. This would cause the 9th stage valve to close."
From the above it is clear that what Boeing fears is APU pressure going into the engine, not the opposite. That's why it seems that increasing power would help preventing this problem.
The one way check valve is on the jurassics, classics and NGs. If memory serves me the check valve is calibrated to the maximum duct pressure so (in theory) will prevent backpressure from reaching the APU. So, LEM your question is valid why limit engine thrust to idle if the check valve is in place. In addition, the DB light illuminates if engine bleed switches and APU bleed value is configured in a manner so as to cause a dual bleed situation. So far so good....However, I have never understood why the checklist does not require the APU to be shut off in the event of a dual bleed light at any time other than engine starting. i.e. in flight. In other words, is placing the APU bleed to OFF sufficient to stop the problem? Why not place the APU switch to off? That would surely fix the problem or are Boeing engineers telling us that after shutting APU bleed if the Dual Bleed light remains illuminated the indication/warning is spurious.
"From the above it is clear that what Boeing fears is APU pressure going into the engine, not the opposite. That's why it seems that increasing power would help preventing this problem."
It sounds to me that whatever pressure is greatest, poses the threat. The APU on a Classic puts out fairly high psi with no load, posing a threat to the engines. When the engines throttle up, the pressure coming from the engines may be a threat to the APU.
Figuring out which poses the greatest threat to what on an NG may be a bit trickier to analyse as the APU puts out bleed pressure according to demand (Pack LO/HI flow, ECS demands, Main Engine Start, etc..).
It isn't so much the apu air back pressuring the engine as it is that the apu air will close the high pressure shut off valve and fool the high pressure regulator into thinking there's adequate pressure to the system. The NG PRSOV and HPSOV are commanded closed in a start sequence to prevent back pressure in the engines. Once the engines are running there's little danger from the apu air but there are other problems. If the engine(s) are powered up with the apu providing air to the same duct system there's the possibility of the HPSOV "popping" open with a resultant "water hammer" effect causing a duct rupture. Here's a link to a page and schematic that should help you make sense of the engineer's dream: http://www.b737.org.uk/pneumatics.htm
I'm still not 100% sure, but your post suggested me the solution to this dilemma.
Of course we have to thank Boeing for the very bad explanation given to us in the manuals...
What about this: the 9th stage air from the engine is there to help the 5th stage at low rpm, because the 5th stage alone is insufficient at low rpm. If I increase engine power in a Dual Bleed configuration, the system will think the pressure in the duct is sufficient and will prematurely close the 9th stage modulating valve.
Actually, this is not true, pressure from the 5th stage is still insufficient alone, and APU air pressure will overcome engine air. This will cause the engine 5th stage no-return check valve to suddenly close, creating a shock in the duct and , as you suggested, possible rupture of the duct itself.
If I increase even further engine power, the 5th stage pressure will be strong enough to reopen 5th stage check valve, overcoming APU pressure: now it's the APU check valve which will suddenly close, creating the same kind of water hammering effect.
Accelerating all this sequence in real time in our mind, we can see how increasing engine power in a Dual Bleed config. will create shocks to both the engine and the APU check valves.
A lot of speculation above! This is the true answer to this,although it may sound a bit unlikely at times!
The Dual Bleed light is illuminated when the APU bleed valve is open and the engine bleeds are selected on,regardless if the APU is running or not. The reason for engine idle power only in this scenario is as follows. There was an instance years ago when the APU bleed valve was open [not sure if the APU was running or not] and the APU bleed air duct check valve failed in the open position. This resulted in engine air back-feeding through the [failed] open apu check valve,which is,in fact,a simple flapper valve,into the APU and causing it to drive the compressor blades. This resulted in a overspeed condition which the crew were unaware of and an uncontained failure of the APU was the result.Not sure of the exact consequences this caused but it obviously would have had the potential for severe problems. This is the reason for lower engine power.It is obviously irrelevant if the APU is running or not with these circumstances as the engines will produce higher duct pressure at cruise power than the APU.Also remember that normally the APU will not be running in flight.
By the way,before someone says the APU bleed valve cant be open unless the APU is above 95%,just remember that it can fail in the open position.
LEM I'd say you about have it. There's other things going on as well. In the NG there's a 110psi regulator in the HS regulator that acts as a "door slammer" in the event that the regulation system doesn't function. The High Stage (9th) valve is self regulated to 34psi by pressure downstream of the butterfly so once 5th stage gets to >34psi the HS valve is closing. If that doesn't work then when 9th stage pressure gets to 110psi the HS valve is commanded closed. The PRSOV regulator has a 180 or 220 psi overpressure switch (depending on aircraft and operator) that electrically trips the system if duct pressure upstream of the PRSOV gets that high. The NG APU now uses a load compressor so back pressuring the APU isn't the problem it once could have been. These things along with what's described above all pretty much guarantee the ground engineers will remain gainfully employed for the remainder of their natural lives.
I had a Dual Bleed light illuminate during taxi with the APU Bleed Air switch in the OFF position. I elected to shutdown the APU and the problem went away. So, in my case whether the APU was running or not was indeed important. I believe the catch here is not the engine bleed switches but rather the position of the APU Bleed Air Value. Are there any pprunes out there that could shed some light on the exact location of the APU bleed valve sensor? Is it on the value itself , located downstream of the duct, or on the one way flapper check valve??
I guess closely examining a rise/drop in APU EGT indication would be the only way in which one could determine bleed air valve status?? However, placing the APU off was a quick and effective solution at the time. I was just wondering why Boeing elected to leave this step out ???
According to the schematics in 36-11-11 and 49-52-31 the ground to turn on the light comes from the Engine bleed switches and the APU Bleed Control Valve. If the APU Bleed switch was off and you still had a Dual Bleed light you had a malfunction of the Bleed Valve in the APU compartment or a short somewhere between the APU and the cockpit. At least that's what the schematics for my company show.
You are missing my point.Even with the APU off,you can still get a back pressure causing the engine bleed air to drive the APU.Admittedly you would need a dual failure,but if the check valve was stuck in the open position AND the APU bleed valve was also stuck open,there is nothing to stop the engine bleed air from driving the APU via the compressor blades.That is what I meant by saying it's irrelevant if the APU is running or not.It could be argued that having the APU off is worse than having it running in this case.Remember,this is only a problem in the unlikely case of both check valve and APU bleed valve being stuck open. By the way,there is no indication as to the position of the check valve,it's a simple flapper valve and the dual bleed light receives an earth through the 'open' switch in the bleed valve itself and the engine bleed switches as is stated above.
The high stage valve should never "pop" open. However should it open abruptly at an inappropriate time a duct rupture would be the result. That's why the 110psi close control. It provides a positive force to prevent such an occurance.