Effect of vent opening at the suction of a centrifugal compressor
Effect of vent opening at the suction of a centrifugal compressor
(OP)
Dear all,
Maybe an easy question for you:
I have a steam-turbine driven centrifugal compressor with the suction drum pressure-controlled. There is a vent valve (to flare) on the line between the drum and the compressor.
What happens of I open the vent?
- will this lower the suction pressure thus increasing the gas volumetric flowrate?
- will this decrease the volumetric flowrate as gas will escape through the vent?
- what about the mass flowrate?
Thanks.
Maybe an easy question for you:
I have a steam-turbine driven centrifugal compressor with the suction drum pressure-controlled. There is a vent valve (to flare) on the line between the drum and the compressor.
What happens of I open the vent?
- will this lower the suction pressure thus increasing the gas volumetric flowrate?
- will this decrease the volumetric flowrate as gas will escape through the vent?
- what about the mass flowrate?
Thanks.
"We don't believe things because they are true, things are true because we believe them."





RE: Effect of vent opening at the suction of a centrifugal compressor
For example, assuming that the discharge mass flow rate remains constant (unlikely, but easiest to describe), the suction pressure at the eye of the first stage impeller must go down (lowering the density and since volume flow rate is density times mass flow rate volume flow rate also goes down with a constant mass flow rate). The pressure upstream of the vent is most likely to go down (this is certainly not a sure thing, depends on the size of your suction reservoir and any losses), but the mass flow rate has to go up to add the vent mass flow rate to the (constant) discharge mass flow rate, so the most likely outcome is increased volume flow rate upstream of the vent.
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
RE: Effect of vent opening at the suction of a centrifugal compressor
David, volume is mass DIVIDED by density I believe...
"We don't believe things because they are true, things are true because we believe them."
RE: Effect of vent opening at the suction of a centrifugal compressor
The discussion remains the same--mass flow rate is constant after the vent, the volume flow rate at actual conditions changes from position to position through the system.
If you are trying to increase throughput of a saleable product by opening the vent then you are betting on a dog that can't hunt. Sales volumes are always settled in a set of units that are not pressure/temperature dependent (i.e., mass flow rate or volume flow rate at standard conditions). Volume flow rate at actual conditions is basically an intermediate step in determining velocity. I don't know of any other useful application of that number.
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
RE: Effect of vent opening at the suction of a centrifugal compressor
Assuming:
- speed of the compressor is fixed
- molecular weight of the gas is constant
Is it possible that the lower suction pressure causes the compression ratio to go up thus decreasing the mass flowrate at the inlet of the compressor?
"We don't believe things because they are true, things are true because we believe them."
RE: Effect of vent opening at the suction of a centrifugal compressor
"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek
RE: Effect of vent opening at the suction of a centrifugal compressor
"We don't believe things because they are true, things are true because we believe them."
RE: Effect of vent opening at the suction of a centrifugal compressor
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
RE: Effect of vent opening at the suction of a centrifugal compressor
I have at hand a centrifugal compressor curve showing on the y-axis: compression ratio and on the x-axis mass flowrate (not polytropic head vs volume flowrate by the way...)
So if compressors operate on their curves, then I believe a "significative" increase in compression ratio should lead to a decrease of mass flowrate, no?
"We don't believe things because they are true, things are true because we believe them."
RE: Effect of vent opening at the suction of a centrifugal compressor
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.