Required relief capacity for blower relief valve.

[duaneloh]

I'm evaluating a weight loaded pressure relief valves located at the outlet of a blower.

The protected blower is designed to have a capacity of 2,095 ICFM with inlet condition 70°F and 14.7 psia, discharge pressure of 3 psig and motor speed @ 2,637 RPM.

My PRV is set @ 4 psig. My PRV is fully open @ 25% overpressure, with that says, it will give me a relief pressure of 5 psig (1.25 x 4 = 5).

With these condition, DP = 5 psig and motor speed remains contant @ 2,637 RPM, the blower pressure performance curve shows that the inlet air flow will be increased.

My question is should the outlet of the blower be blocked, what would be the relief capacity? Should it be the design capacity of 2,095 ICFM or the increased capacity @ 25% overpressure?

i thought it would be the increased capacity of 2,180 ICFM however if the suction conditions and motor speed remains constant, the blower shouldnt be pulling in more air. Please help.

Thanks in advance,
Duane Loh

 

[quark]

I couldn't get your point. Actually the volume flowrate should get reduced(if at all) when you increase back pressure on blower. Lobe type roots blowers give constant volume output irrespective of the back pressure. Is there any reference on web for performance curve of your blower?

 

[Guidoo]

I agree with quark, you would expect the inlet flowrate to either decrease (centrifugal blower) or not change at all (positive displacement blower) when blower backpressure increases. Please explain what type of blower you have and why it behaves so different from what we would expect.

 

[CHD01]

You are approaching this problem incorrectly. To evaluate the relief device, first determine if the MAWP of the discharge piping system is really 4 psig. If the relief setpoint and the actual MAWP match then you will not have to change the setpoint of the relief valve. The next step is to determine the required relief capacity; this will be determined from the fan curve. The required capacity to be relieved by the relief valve is the amount of flow required to reduce the blocked deadhead pressure to the system MAWP and this required flow must be less than the flow capaity of the relief valve at the discharge systems maximum allowable overpressure. This max allowable overpressure is NOT necessarily the overpressure the relief valve is sized for. It could be 10%, 21% or 20% or if pipe standards are used can be greater than 21% for a fixed period of time.


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