B757 RB211 Low duct pressure below switchover
 

I'm Stumped.
I have a low duct pressure in #1 eng at idle and LP while #2 is normal, split of about 5-15psi. This difference in duct pressures disappears above switchover when when engine bleed is from HP2, duct pressures are then equal throughout the HP range.
We have worked through the FIM and replaced PRSOV, pilot valves and checked all lines for leaks, IP check valve was also checked to see if HP 6 was recycling back through HP2.
This only occurs about 15mins after initial start before then it seems to operate as it should.
The characteristics of this fault indicate that below switchover #1 eng is operating in a low flow mode eg it is above 31,000 ft. Altitude switches were swapped over but fault remained in #1.

If anyone has experienced this or has any ideas on where to look next that would be great.
Cheers

Just thinking aloud... (and wild guessing)

Sticky HP valve?


Even though the engine may be producing enough (HP) pressure (upstream of the HP Valve), maybe the HP valve is sticking (closed) when cold, and IP is the only source of pressure(?) Is there any noticeable stickiness if you manually rotate the HP butterfly when cold? (Can you compare it with the other engine?).

On the other hand, it may be an HP Controller problem. Have you thought of purging the servo and sense lines. If the problem is related to operating temperature, maybe the HP Controller heater blanket is faulty (Do they have these on 757's?)

Regards
NSEU

Low Duct Pressure
 

You do not mention replacing the HPSOV & see previous poster comment.
The muscle air line from the compressor case can leak.
HPSOV lockout cam has been found out of detent restricting muscle air, be aware there are 2 distinct types pre & post -8 from memory. The diagrams of both are in the manual. good luck!

Thanks HPSOV has been replaced and original fault still exists. Fault isn't present on first start of the day but within 10-15 mins we get the split in duct pressure, so I believe HPSOV is opening.

Sorry,
Yes HPSOV has been replaced, all sense lines/pressure lines have been removed and checked with one found to be leaking this was replaced and fault still exists.

Sorry, I misread your original message. I thought the problem was occuring for the first 15 minutes of engine ops rather than after 15 minutes

But it seems to be closing, erroneously, after 15 minutes. If the lines and the HP valve have been checked/changed, then I guess you have to go further up the chain (HP Controller, ASCTU or even an ADC). I'm assuming the 757 is similar to the 767, in that one of the ADCs provides the altitude signal (only using the altitude switch as a backup).

Unfortunately, if the FIM is not helping, it may be a matter of replacing the system piece by piece :\ or calling RR or Boeing to see if they have this type of problem on record.

Cheers
NSEU