RECIP BASICS 1

[erjainesh]

A Recip compressor with below data

suc. press : 9.1 Barg and disch. press. : 41 barg ( 2 stage machine)
flow: 21700 nm3/Hr

now if suc press. is increased to 10.1 barg with no change in flow and discharge pressure
what wil happen in compressor output and other things?

may be a basic question but i m new to this field

 

[georgeverghese]

Compressor mass rate increases.

Though total press diff decreases, the net effect is a power demand increase since the mass throughput increase overshadows the power demand decrease due to decrease in press diff.

Am assuming there is no constraint on intercooler heat rejection duty.

 

[LittleInch]

erjainesh,

Your question is not physically possible.

If you're increasing the inlet pressure (mass) per stroke and the machine is the same speed and stroke length, then mass flow out of the machine will increase. Where else is it going??

So either discharge pressure will rise to accommodate more mass in the same actual volume (increased density) or the actual volume will increase at the same pressure.

George is correct - power will also increase.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[nickelkid]

With some type of capacity control, like volume pockets or speed control, erjainesh could maintain his capacity and discharge pressure.

 

[LittleInch]

True, but we have assumed that there is no change in speed.

If there is then the question is meaningless as he has specified that the output does not change. The impact would be a small reduction in power as the amount of work has reduced marginally, but the mass flow remains the same as output flow and pressure are the same.

As he or she said - a fairly basic question, but with no real inputs

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[zdas04]

I've attached a couple of pages from a class I taught yesterday that should be germane. This example is based on an oil-flooded screw compressor, but the analysis and conclusions are the same for a recip (i.e., each displaces the same amount of space in each revolution of the driver shaft). It doesn't address exactly your question, but I think it addresses the question that is possible to answer (i.e., your question of raising suction pressure with no change in flow rate is not possible without some other change like rpm or clearance).

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[erjainesh]

hii Georgeverghese

my calculation are showing what you said

increase in power however my rod load are acceptable range and also my motor hp are higher i can try this case

 

[georgeverghese]

Pls analyse this case further for details.

Interstage pressure will increase - what about relief valve settings in comparison to higher interstage pressure?

As you say, other mechanical loads ( of which I know little of for recips) such as road loads and crankshaft loads / bearing loads should be checked preferably with machine supplier.

And intercooler duty will go up also - checked ? What about higher intercooler dp - taken into account for second stage throughput? Also check recycle valve duties / other PSHH settings.

If your motor can operate at this new hp, what will be new overcurrent trip setting ?

Mechanical, instrument and electrical engineers, together with machine manufacturer should all study this new operating case carefully before you go to this higher duty. If anything happens to this machine later,and plant production is affected, and it is found to be related to this new operating case, you should be able to stand up to countless accusations that may go on for weeks...

 

[erjainesh]

this is one of the case of the compressor
Actually the compressor is designed for other gas composition with high suction pressure and discharge pressure
but thank you will check the recycle valve duties that was not thought by me

 

[gwalkerb]

It's fairly common for recips to have a suction pressure control valve (SPCV), which would regulate your suction pressure to a setpoint. Does this machine have a SPCV? If it does, you don't really care if your suction pressure is higher than design pressure (assuming you're not exceeding piping or valve MAWPs), as the valve control loop will set the valve to a point to let the right pressure through.