Gas Plant Recompressor Suction Pressures

[Bubba09]

I am looking at a cryogenic gas plant that is moving about half the amount of gas it was designed to.I would like to use the excess HP that I have in my recompressors to to lower my plants tower pressure thus cooling the plant and increasing my liquids recovery. The compressors are single stage recips.

How can I reconfigure the compressors? I will have the same discharge pressure, Same HP, less volume, and lower suction pressure than the original design.

Thanks in advance.

 

[JJPellin]

You need to contact the compressor manufacturer. Lowering the suction pressure while holding the discharge pressure constant will increase the rod load. If you hit a rod load limit, you could start breaking piston rods which is very dangerous. The manufacturer should be able to provide a rod load limit, and give you a correlation between differential pressure and rod load for your specific machine.

Johnny Pellin

 

[zdas04]

If you're looking for a way to asses it yourself go to the mfgr program. If your compressor is an Ariel, then you're in luck--their free "Perfomance" program is really excellent. Put in your parameters and hit "Performance" at the top and look for red or blue numbers in the output. If it is some other mfgr compressor then it can take some digging to find the program, but they all have them.

Single stage machines are pretty easy, but not trivial. The compression cylinder can do about 4.5 compression ratios Which means that the discharge pressure (in absolute terms) divided by the suction pressure (in absolute terms) is less than 4.5. If you go over 4.5 ratios, then you risk discharge temperature and rod-load problems. If you put your new conditions in and the program likes it, then you can move forward. If the program has any errors or warnings then you should resolve them prior to making the change.

The two parameters that you have to pay special attention to are discharge temperature and rod load. Rod load is kind of tough to measure, but most compressors have a way of seeing the sucction temp. You'll usually want this below 300F, and it is common for compressor-kills to be set slightly above this value.

If this all sounds too hard, then you can either go to your packager or the Mfgr and have them evauluate the change or you can hire one of us consultants to give you inconclusive answers.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

"It is always a poor idea to ask your Bridge Club for medical advice or a collection of geek engineers for legal advice"

 

[Bubba09]

Thanks. I got the answer from the DR sizing program.