Effect of vent opening at the suction of a centrifugal compressor 1

[sheiko]

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.

"We don't believe things because they are true, things are true because we believe them."

 

[zdas04]

Need a little more precision in your problem statement. There are at least three possible points to consider: (1) piping upstream of the vent; (2) piping between the vent and the eye of the impeller, and (3) flow conduits after the eye of the impeller. There is absolutely no physical requirement for volume flow rate at actual conditions to remain constant and it doesn't.

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.

 

[sheiko]

"...and since volume flow rate is density times mass flow rate".
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."

 

[zdas04]

I knew that, but confused myself into thinking that density was volume/mass instead of mass/volume. I kind of overthought it.

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.

 

[sheiko]

David,

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."

 

[BigInch]

Isn't the vent connected to ... air?

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[sheiko]

No, to the Fuel Gas drum...

"We don't believe things because they are true, things are true because we believe them."

 

[zdas04]

A dynamic compressor is ... well, dynamic. A "small" change can often yield unpredictable results. With more ratios and a fixed speed you need more hp to compress the same amount of gas. Is that additional hp available? If so then the mass flow rate at a slightly lower suction might stay constant or even increase, it is really hard to predict. Each wheel can do somewhere around 2 compression ratios. If they are doing 1.6 CR/stage before you open the vent and the lower suction pressure gets you to 1.7 CR/stage then you are still within the operating envelope of the machine and (using more hp) you can probably move the gas that is there assuming that the reservoir you are pulling on is able to provide the gas for the vent and the 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.

 

[sheiko]

Dear David,
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."

 

[zdas04]

Every curve has some underlying assumptions. I'd bet that the one you are looking at is assuming constant power. If power input cannot change then increasing CR has to decrease mass flow rate in a dynamic machine. I've also seen those curves presented as hp vs. CR with mass flow rate constant. The usefulness of either really depends on what you are trying to do and a complete understanding of the assumptions.

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.