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Capacity control in reciprocating compressors(2)

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 sinectica (Mechanical) (OP) 30 Jul 03 11:48
 Which system of capacity control in reciprocating compressors is more reliable?1.- Hydraulically controlled variable-volume clearance pocket2.-Stepless capacity control system that uses finger-type unloaders pneumatically actuated. The system unloads the valve for only a portion of its stroke.Does anybody have experience with any or both systems?It seems that the second system could have problems of leakage by the shaft of the unloader.What are the benefits and the problems of each system?Thanks a lotMarc
 Scipio (Mechanical) 30 Jul 03 13:40
 zdas04 (Mechanical) 31 Jul 03 6:57
 You really have to look at what you are trying to accomplish.  We have a bunch of 2 stage Ajax machines here in the southern Rockies with poppet VVCP to allow the compressors to react to changing conditons.  They say the same machine is good from 200-600 Hp with these devices.  I pay about $500k (US) for a 600 Hp recip and less than$175k for a 200 Hp recip -- why would I burden an operation that needs a $175k compressor with debt service, depreciation, and operating costs on a$500k machine?  Doen't sound like good economics or good engineering to me.My approach to fleet management has been to have 4 different size compressors (each one about half the Hp of the next larger machine), fabricate all the compressors in the fleet with the same size suction and discharge flanges located in the same place on the skid, and require all the skids have self-contained fuel gas.  I allow zero flexibility on these points, and I can do a two-compressor swap and have both machines running in the new location on the same day.  The fleet is all flooded-screw compressors under 600 Hp.  With this design scheme I maintain over 70% Hp utilization across the fleet and spend less than \$200k/year on compressor moves (68 compressors in the fleet rated at a total of 24,000 Hp).  The fleet operations and capital costs are far less this way than to try to build a rubber compressor.The VVCP idea seems sloppy to me.David
 sinectica (Mechanical) (OP) 1 Aug 03 14:34
 Thanks a lot for your kind answers, my question has been done because we are thinking to install a better capacity control than the one we have(which only allows steps of 25%)in three reciprocating compressors (Peter Brotherhood) that compress Hydrogen. The discharge pressure is aproximately 70 Kg/cm2 in 2 stages (4 cylinders).The system that unloads the valve for only a portion of its stroke seems to be the worse in order to guarantee valve life, but I think that it's the one which is going to be installed.I don't believe so much in actuate over the valves and that's why i posted this question.P.D. Sorry about my English, I'm not fluent using itRegardsMarc
 Scipio (Mechanical) 1 Aug 03 16:55
 Ah, never worked with Peter Brotherhood machines, just curious as I installed self-actuating hydraulic VVCP's on a Dresser Rand HOS frame a couple years ago.  I'm also curious though, if I understand right, your plan is to use finger unloaders to unload one or more suction valves of each cylinder for a fraction of each stroke, the idea being it will allow part of the hydrogen in the cylinder to be displaced back to suction for part of the stroke, and the rest to discharge, thus reducing overall capacity. I must be missing something though, are these really low speed machines?  I can't help but wonder how fast your machine runs and how you plan to synchronize the finger unloader with the compressor stroke so it only unloads the suction valve(s) for part of the stroke?  All the recip machines I see have speeds in the order of 600 to 1800 RPM, with time between valve opening and closing in the order of milliseconds.  That's a pretty small span of time to try and cut in half even further, especially with a system as spongy as one that's pneumatic driven.
 sinectica (Mechanical) (OP) 5 Aug 03 11:25
 You are completely right Scipio, because we work with compressors that run at 200/350 rpm and the system would be hydraulic not pneumatic.Although, I don't trust so much in this system, and I hope not to have so many problems in the future.Many thanks for your help!Marc

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