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Problem: You have two reciprocating compressors in a NHT system, one service, the other on standby. When the duty trips the standby must be barred and then put into service.
To bar it, it must be depressurised, barred, pressured up and then started, this could take up to 15mins, so this makes it very tedious and labourious.
How long can a compressor be static until it needs to be barred? Some say it must be barred at least 1 hour before starting to ensure all liquid is displaced from the cylinders. Some say the compressor must be barred every 3-4 days.
Are there any systems that can eliminate the need for barring via reducing the chance of hydrocarbon condensation or otherwise.
 
Answers
11/08/2010 A: Balasubramanian Iyer, Nexen CNOOC Limited, balasubramanian.n@aol.in
1. You can schedule to drain your Suction Knock out Pot once in a week or once in two days to prevent Huge amount of Hydro carbon Condensing in the volume bottle making it time consuming to drain all at once during start up.
2. Try to Reduce the Level of the Separator Vessel and set it to a optimum level by trial and error checking for entrainment in the recycle gas by the suction volume bottle at various levels. This might work wonders if done correctly.
3.Try to incorporate De-mister Pads in the separator vessel section from where the gases goes to the compressor suction there by eliminating huge amount of entrainment in the gas leading to heavy condensation of Hydrocarbon in the compressor suction.
4. If you are not Passing your Gases to KOD prior to Compressor suction volume bottle, then after speaking to your process designer and licensor, you can incorporate in your next turnaround a line to Suction KOD from your Suction Voume Bottle.
This will also help you in starting of the Stand By Compressor in few minutes upon requirement.
18/05/2010 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
I agree completely with Mr. Bowers. It is a matter of design experience vs. on-the-job training. The deficiencies could also have been pointed out by the client's experienced operating representatives during P&ID or model reviews. Unfortunately, you must live with the layout you have. You must wait for all of the liquid to drain from the suction line before placing the spare in service. Some people skip the depressuring step before opening the drain valve, but that allows more vapor to the atmosphere and doesn't save enough time to avoid upsetting the unit . A flying switch is what you need, and that is provided by a proper equipment layout as Mr. Bowers has stated.
18/05/2010 A: keith bowers, B and B Consulting, kebowers47@gmail.com
Ridiculous--
Proper design of the suction piping is a key requirement as is the correct installation of the liquid knock out p ts ahead of the cylinders. All suction lines must slope to the knock-out pots. Compressor coolant should be kept warm and circulating.
Why is it kept connected to the system (suction and discharge valves should be closed when it is not rotating.) when it IS NOT an effective 'spare' in the present configuration?