5 replies to this topic

[shiniez]

Dear All,

I just the beginning to start about the compressor. I have to determine the effect of changing feed inlet composition from the heavy gas to light gas for example change from feed C1-C7 hydrocarbons to C1-C4 hydrocarbons instead. Could someone help me to explain if I change the feed like the example case, what will be the effect to compressor performance like the energy consumption, etc.

[ankur2061]

shinez,

To provide a quick answer if the gas composition changes from a heavier gas to a lighter gas with other inlet conditions (pressure, temperature) remaining same the power or energy consumption will increase.

For further details, use the search feature of the forum for centrifugal compressors and you will find several posts related to theory and thermodynamic equations of gas compression. Most of the posts related to centrifugal compressors where I have posted replies, I have tried to provide thermodynamic equations governing gas compression and the definitions of the variables in these equations.

Regards,
Ankur.

[shiniez]

Thank you so much for your advice

[sheiko]

Shiniez,

As the gas is becoming lighter, the molecular weight is decreasing, thus decreasing the gas density (assuming fixed suction temperature and pressure).

If you deal with a CENTRIFUGAL compressor, the result of the decrease in gas density is:
- the compressor head remains the same (not directly affected by the change in density)
- the system head decreases because of less friction head due to the lighter gas
- the volume flowrate increases because of the initial high head from the compressor
- then, the compressor drops its head because the volume flowrate has increased

So the net result is:
- a lower density
- a higher volume flowrate
- a lower compressor head

Electrical power being proportionnal to (density)*(volume flowrate)*(compressor head), I believe its evolution will depend on the relative change of the 3 variables.

But that's not all.
Now, depending on your compressor process control system actions (suction throttling pressure control, spillback suction pressure control, ....) and on which part of its curve your compressor is operating (flat part or steep part), the electrical power consumption will further change.

Edited by sheiko, 09 October 2012 - 11:20 PM.

[sheiko]

To reinforce what I have said and to enrich the discussion:
http://www.chemicalp...essors/show/232

Regards.

[benabed]

Dear freinds,
I have to jump on this interesting topic because years ago i was facing the same problem when i was working in an oil treatment plant.The centrifugal compressor for the recovery of the flash gas was experiencing a decline of the performance as a result of the operation in a recycle mode for a prolonged time.The gas then becomes leaner due to cyclic compression and cooling/expansion resulting in lower discharge pressure so compressor fail to feed forward and the incomming gas start to escape to flare due to the increase in suction pressure.As a solution we usualy open the flare valve in the discharge of the compressor in order to release the light gas trapped inside. This solution will not last for long time and the problem will be repeated again in a vicious cycle.I am looking for someone in our community who has experienced the same problem to help me solve this problem.
Hope that my discription was clear . Looking forward to hear from you.

Best reguards.

Benabed