Residual duct pressure (10psi) after isolating left duct

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Residual duct pressure (10psi) after isolating left duct

Postby blablubb » 11 Jan 2017, 00:12

Happy new year to all!

Hopefully i am allowed to state my question here, even if i am only the user of the system - but after doing a lot of research i still have no answer:

Condition:
737-800
In flight (ferry) isolation valve close, left bleed air sws close.
APU off.
APU bleed air sws off.

The checker asked what pressure i expect to be indicated on the left duct.
My answer was 0 psi.

But in fact it was somewhat about 10 psi.

He told me, that this was the same with all 737-800 in which he tried this out and he (nobody!?) has an explanation for that...

Sholud the prsov's be a 100% close? Or is there always (or often?) a rest of pressure leaking through the valve even if the switch is in position close?
Or does it has something to do with pressurisation of hydraulik fluid or water tank?

I thought, that you are the guys who may have an answer to this!

Best regards,
blubbi
blablubb
 
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Re: Residual duct pressure (10psi) after isolating left duct

Postby sjap » 15 Jan 2017, 15:45

yes there is always some leakage, if it is 0 to 10 psi this is normal, as soon as you switch on the pack on that side it will drop to 0 because you demand flow which is not present. The 10 psi buildup can be ignored and be handled as normal system behaviour. There are always some sense lines and internal valve leakages / clearances allowing a small amount of air passing through.
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Re: Residual duct pressure (10psi) after isolating left duct

Postby blablubb » 15 Jan 2017, 19:26

Thanks a lot! Your answer sounds good to me. But the instructor is still in doubt. I was not exact enough with my description above. The instructor told me, that the pressure is always right at 10psi. Not "in between" something. Still a little kind of magic to me.
Best regards,
Blubb
blablubb
 
Posts: 2
Joined: 10 Jan 2017, 23:46

Re: Residual duct pressure (10psi) after isolating left duct

Postby sjap » 15 Jan 2017, 20:19

No its not exactly 10 psi. Its in between 0 and 10 depends on the condition of many parts together. Plus the tolrance of the transmitter, the indicator etc. Even our testrun manual states that with bleed off, the duct press may be between 0 and 10 psi.
sjap
 
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Joined: 26 Jan 2007, 20:57
Location: Netherlands

Re: Residual duct pressure (10psi) after isolating left duct

Postby sibinz » 10 Feb 2017, 09:20

bav.JPG
bleed duct pressure after apu bleed switch off.
sibinz
 
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