APU Bleed Valve Power

[Guest Ian_Riddell]

Just wondering what powers the APU Bleed Valve and the Duct Pressure Indications in PIC...From a dead ship, I started up the APU, but deselected the APU generator before the APU came up to speed, leaving only Standby Power, Battery power and the Ground Handling Bus on-line. Strangely, I thought, I was getting pneumatic duct pressure indications... indicating that the APU bleed valve was open. Is this normal on a 767?Thanks.Cheers.Ian.

[Guest HPSOV]

I think we need Ian to answer this one!Oh........

[Guest anthonyj]

The APU bleed is a seperate switch on the pnuematics panel between the two engine bleed switches. This must be selected off to prevent APU bleed air entering the centre duct and on to the cross duct. The APU generator switch controls gen power only, not pnuematics.

[Guest Ian_Riddell]

"I think we need Ian to answer this one!Oh........"LOL....I'll see what he has to say tomorrow when he goes back to work (I hear he has access to the Boeing maintenance manual) ;-)Actually, I've just found some diagrams for the APU bleed valve at home. They show that the APU Air Supply Shutoff Valve is powered by the Right 115Vac bus (with control relays powered by the Right DC Bus). I'm pretty sure that the APU valve would have autoclosed on normal shutdown after the previous flight... and in my scenario, without the main R (or the L) bus powered, there would be no way of re-opening it to let APU bleed air into the ducts. I seem to remember that PS1 also suffers from this anomaly. Crossfeed valves on the other hand, I hear, operate differently, and would stay in the last commanded position if there was no power to open/close them.Anyway, I should be able to find out what powers the duct indication tomorrow.Cheers.Ian.

[Guest Ian_Riddell]

"This must be selected off to prevent APU bleed air entering the centre duct and on to the cross duct."I'm not 100% sure of the operation of the valve, Anthony. The APU valve is designed to close automatically under the following circumstances even if the valve switch in the cockpit is selected to open/ON:If either engine bleed air valve (PRSOV) is open (except during engine start)APU running at less than 95%rpmFire handle pulled......but this may depend on the availability of elec power... Is the valve spring-loaded closed or does it need electrics to drive it completely closed?If the valve closed completely on shutdown (such as would likely happen if Ext Pwr was available in the transition from APU shutdown to dead ship), it would require electrics to re-open it. Without the Right 115V AC and Right 28VDC busses powered (as in my scenario), it would remain closed."The APU generator switch controls gen power only, not pnuematics."The APU provides power to the main busses which provides electrical power to the pneumatic valve control circuits, so you may or may not be correct :-)Cheers.Ian.

[Guest HPSOV]

This is a question not an answer....Sometimes after engine start the engine bleed valves do not open, and to open them you must advance thrust, to increase bleed supply, to push the valve open. So I was of the opinion that the valve could be closed electrically, but anytime there was bleed air pressure from "upwind" of the valve, it would the be pushed open.Could the APU valve not work somehow in the same way?

[Guest anthonyj]

Why would one want to do this anyway exactly? Normally electical power would be established prior to APU start, then transferred to the APU generator when the run light illuminates. Sounds like an odd way to do it (can't try it on the real aircraft as it is against SOPs).The only other reference in our manuals to the APU bleed valve's operation is the standard position disagree message and to close it or shut down the APU for an engine crossbleed start. Otherwise it is a bit of a mystery. HPSOV I agree with your statement but it will be interesting to hear a definitive answer!

[Guest Ian_Riddell]

After further reading, guys, I came to the conclusion that the APU valve is a kind of hybrid ... Not quite like an engine valve and not quite like an isolation valve. Unlike an engine valve, no air "muscle" is required to open it. It is driven open and closed with an electric motor like the isolation valves, but there is also quite a bit of automation involved (Isolation valves are only simple "door bell" circuits... Switch on, valve opens, switch off, valve closes).If the APU was shutdown properly (with a cooldown cycle), the APU bleed valve would be closed automatically with electrical power before the generator is tripped offline. If, for some strange reason the APU was restarted and the APU generator switch was selected off, the APU bleed valve would not re-open automatically at 95% rpm because there is no electrical power on the main busses. If power was removed from the main busses prior to APU shutdown, the valve would remain open. When the APU was restarted, say without main bus power, the valve would remain open, allowing air into the ducts (so it could be quite dangerous if other switches were out of place).I discovered today, that, without main bus power on APU startup, you wouldn't know that there was air in the ducts. Unlike PIC, the bleed pressure indications on the overhead panel will not operate without main AC power. The left side bleed duct pressure pointer requires the Left Elec Bus... and the right side pointer requires both Left and Right Elec busses to operate.Anyway, getting back to your question, HP... I'm not sure what causes this problem, but perhaps if we go through the start sequence, there will be a clue in there somewhere... It's a lot easier to understand this with a diagram, but...During start, the engine bleed valve known as the "PRSOV" is electrically triggered to open (automatically) 10 seconds after the engine has reached 50%N2. However, because another engine valve known as the "PRV" (upstream of the PRSOV...i.e. closer to the engine) is still closed, there is greater pressure on the APU side of the PRSOV... which keeps the PRSOV closed (even though it has been commanded open). 15 seconds after the engine has reached 50% N2, the PRV is also opened, putting a certain amount of air muscle behind the PRSOV, cracking it open. When the valve is cracked open, the "valve open" position microswitch on the PRSOV sends an electrical signal to the APU bleed valve control circuits to tell the APU bleed valve to close automatically (electrically). With the APU valve closed, the PRSOV can open completely.I'm only guessing here, but perhaps in your situation, the APU is still putting out too much air to allow the PRSOV to crack open. The APU goes into "overdrive" when, I believe, either of the engine start switches is in the start position, putting out masses of air. When an engine reaches 50%N2, the start switch flips back off as it normally does at around this speed, telling the APU to take it easy. Perhaps in your situation, one of the start system electrical circuits is not telling the APU to take it easy, and the APU bleed air is stopping the PRSOV from cracking open. In this case, it's up to you to advance the thrust lever, allowing the engine to pump out sufficient bleed air to override the APU bleed air, allowing the PRSOV to crack open, sending an electrical signal to the APU valve to close it electrically (irrespective of how much air the APU is pumping out). With the APU valve closed, the PRSOV should be able to operate normally.There may be a simpler explanation for what you are experiencing, but I can't think of any at the moment (I do apologize for the profusion of text). :-)Cheers.Ian.