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RR 250-C20W Bleed Valve

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I am due to swap out the bleed valve on my Enstrom 480B in the next 25 hours.  In looking at it, it seems to be what I would be termed in the car world as a "waste gate".  I had assumed that it was a control valve but it appears to be simply a preset pressure relief valve. 

Can anyone share some light on what it does?

Thanks

Bill

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On 4/8/2020 at 9:39 AM, Discap said:

I am due to swap out the bleed valve on my Enstrom 480B in the next 25 hours.  In looking at it, it seems to be what I would be termed in the car world as a "waste gate".  I had assumed that it was a control valve but it appears to be simply a preset pressure relief valve.  

Can anyone share some light on what it does?

Thanks

Bill

The ability of the compressor to pump air is a function of RPM. At low RPMs, the compressor does not have the same ability to pump air as it does at higher RPMs. To keep the blade angle of attack and air velocity within desired limits and prevent compressor stall, it is necessary to "unload" the compressor in some manner. In other words, the compressor needs to see less restriction to the flow of air through the use of a compressor  bleed air system.

CLICK PHOTO TO ENLARGE

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When the engine is not in operation, the bleed valve is positioned fully open by a spring located inside of the vented piston chamber. The spring along with Pi pressure, directed onto the bleed valve end, are used during engine starting and acceleration to position the bleed valve fully open. 

 During engine operation, Pc pressure is directed through an inlet filter and a restrictor (jet) into the Px chamber. Px air is then directed to Pa through a nozzle (venturi). The rate of air flow from Pc to Px to Pa determines the value of Px pressure for any given N1 RPM. Px pressure is used to provide the closing force on the bleed valve. Px pressure is separated from the Pa pressure and spring chamber by means of the rolling diaphragm. 

 Operation of the valve is a function of preselected ratios of Pi to Pa and Pc to Px to Pa pressures. When Px is less than Pi plus the spring force, the bleed valve is positioned open. When Px is greater than Pi plus the spring force, the bleed valve is positioned closed. The bleed valve is positioned open during engine starting and acceleration until the Px pressure increases sufficiently to overcome the combined value of the spring and Pa pressure. The bleed valve then closes and remains closed at all N1 speeds above the closing RPM.

Allison Gas Turbine 1981

 

Edited by iChris

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Chris:

Thanks for the help.  I don't know where you find all this documentation.  I looked and found nothing.  I am always impressed

 

Bill

 

 

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The one and only time I had an "engine problem?" was when the bleed valve failed and it built pressure then rolled the engine back to idle by itself. Luckily it was in a Bell 212 at low weight and elevation so we got it figured out while flying on the 2nd engine. 

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