I thought I may as well add my 2 cents worth. I am a bit confused as to whether the original post was referring to a bleed valve in the pneumatic supply system or a bleed valve in the engine compressor casing. I am assuming it is the HP
valve in the pneumatic supply system, and with respect to this, I found an interesting photo in the database.
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Photo © Pier Picone
This photo is of the Rolls Royce Trent 700 (RR T700) bleed system. The heart of the system is the big "Y" shaped ducting arrangement. There are three main valves in this arrangement, these valves being situated on each of the three branches from the central "Y" shaped ducting piece.
The motorised valve on the top left branch of the central "Y" piece is the bleed valve. This opens and shuts off the bleed air supply (and regulates pressure?) from the engine to the aircrafts pneumatic system.
The motorised valve on the top right hand branch of the central "Y" piece is the High Pressure Bleed Valve (HPBV). This valve opens and shuts to admit and shut off HP
bleed air respectively. On the RR
bleed air is taken from the 14th stage of compression (not including the fan), which is also the 6th and last stage of the HP
Located on the bottom branch of the central "Y" piece is a non motorised valve known as the Intermediate Pressure Check Valve (IPCV). The IPCV controls the admission and shutting off of IP bleed air in response to movements of the motorised HP
bleed valve. IP bleed air is taken from the 8th stage of compression (not including the fan) which is also the 8th and last stage of the IP compressor.
During low engine power conditions, HP
air will be bled from the engine so that sufficient pressure exists in the pneumatic system. The motorised HPBV will be commanded to open. HP
air will them come down to the central "Y" piece from the right hand upper duct. HP
air now acts on the upper side of the IPCV, with IP air acting on the lower side of the IPCV.
Due to the pressure difference, the IPCV is forced to close. This simultaneously shuts off the IP bleed air supply, and prevents HP
air entering the IP compressor. The HP
air them passes through the upper left hand branch of the central "Y" piece, past the bleed valve and along the upper left hand duct to the aircrafts pneumatic system.
As the engine power increases, the pressure delivered by the HP
bleed will increase. At a certain point, the pressure delivered by the IP system will be adequate for the pneumatic system, whilst the pressure from the HP
system will become excessive. The HPBV is then commanded to close.
As the HPBV closes, the pressure on the upper side of the IPCV is reduced. Once below a certain value, IP compressor air can force the IPBV open and allow IP air into the lower branch of the central "Y" piece. IP air now flows past the bleed valve on the upper left hand branch of the central "Y" piece, and proceeds via the upper left hand duct to the aircrafts pneumatic system.
Beyond the upper left hand duct, the bleed air, whether HP
or IP flows through a large pre-cooler before entering the supply ducts to the pneumatic system. The pre-cooler is an air to air heat exchanger. It uses fan discharge air to cool the bleed air before it is transported onto the aircrafts pneumatic system. I believe that the large open ended duct to the right and above the HPBV may be part of the fan air supply or discharge arrangement from the pre-cooler
Anyway, does anyone know if the bleed valve also regulates bleed air pressure as well as providing shut-off function?