How to plot actual compressor performance on manufacturer' performance

[enginepuppet]

Hi everyone, this is my first post. I hope we can help each other out.

I have problem on plotting the actual compressor performance on manufacturer' performance chart. Can someone guide me?

Please see the attached file for detailed information.

Thanks,
EP

 

[pmover]

the first thing i noted are differences in the gas MW. with a different gas MW during actual operating conditions, the compressor predicted head will be different than originally designed. so, you can either redo the predicted performance curves yourself at the actual MW or request the compressor mfg to provide the modified chart for you (recommended).

basically, obtain the inlet/outlet process conditions and gas composition and then determine compressor inlet flows and the head developed. plot the data point and compare actual compressor rpm to that of predicted. they should be nearly the same provided the actual gas composition is relatively close to that of the predicted composition to develop the curves.

good luck!
-pmover

 

[enginepuppet]

Please give your opinion about my explanation below. Is it correct?

The situation is:

on the actual operating performance:
The compressor needs to run on 13485 rpm in order to achieve the expected flow of 9328CFM with the Hisen of 64678 ft-lbf/lbm.

ideally (according to the mfg performance map):
the compressor originally can run on 11650 rpm[/COLOR][/B] with these conditions (Flow=9328CFM and Hisen=64678 ft-lbf/lbm).

Summary:
The unit needs more power to deliver the expected flow. it means the unit is not so efficient as before, there is LOSS of ENERGY on the unit. As an indication, the unit has high discharge temperature. (Energy loss --> heat).

Another question: what are the possible causes to this "MORE POWER/SPEED"?

please see the attached file.

Gruss,
EP

 

[zdas04]

No it is not correct. Read Pmover's post. You can't just ignore a 12% difference in specific gravity. You are compressing different stuff than the manufacturer was expecting when he created the chart. The chart is meaningless for the gas you are compressing.

David

 

[enginepuppet]

Thanks David and -pmover it's very helpful.

gruss,
EP

 

[enginepuppet]

Hi, I want to ask another question that will boost my understanding up about this whole performance map. (hopefully)

I now understand that gas with different MW or SG (if it out of tolerance off-course), should have different performance map.

my question:
if the situation I explained above has had the same specific gravity, how about that?

to be clear:

SG actual = SG design
Head(isen) actual = Head(isen) design
Flow actual = Flow design

but

Speed actual > speed design
(let's say delta speed is 10~15%)

what does it mean? what indicates this situation?

gruss,
EP

 

[bcs5274]

if you have to spin it faster to acheive the same or close to the desired flow there might be something wrong with the compressor, but to be sure performance of unit with the actual specific gravity of the gas in question has to be done first to prove to the manufacturer that there is some thing wrong with said unit.

fyi

 

[enginepuppet]

thanks for all the helps. I'll go get the new map!

mfg,
EP

 

[pmover]

enginepuppet,

after my initial posting, i thought about my response and realized my error . . .

that is the compressor curve (isen head vs acfm) will not drastically change even with a different moleweight of gas as the compressor wheel will generate that amount of head at the specified inlet flow conditions and rpm. what will change are the compressor outlet conditions; thus the overall calculated head. do understand that due to the heavier MW gas, the compressor will operate at a different speed than originally designed. i believe this may be a little confusing, so i am attaching an article that may further assist you. take your time reading it and i hope this helps.

been having internet comm problems lately; hence this late follow-up posting.

hope this helps.
-pmover

 

[dcasto]

The only thing you know for sure is that you are the green line. If you take the differential pressure across the compressor, you can back into the head based on the density of the inlet gas. That will then tell you what is the inlet flowrate.

 

[enginepuppet]

i think i need some times to read that.

thanks to you pmover.