Capacity control in reciprocating compressors
Capacity control in reciprocating compressors
(OP)
Which system of capacity control in reciprocating compressors is more reliable?
1.- Hydraulically controlled variable-volume clearance pocket
2.-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 lot
Marc
1.- Hydraulically controlled variable-volume clearance pocket
2.-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 lot
Marc





RE: Capacity control in reciprocating compressors
I've never used finger-unloaders for activating valves for part of the stroke. My understanding is they're just a matter of activating or deactivating valves during operation for prolonged periods of time. Virtually none of the machines I see here in this particular field use them, if we need to go to single acting operating, we purge the machine and either pull the valve or install spacer rings under them for added clearance. I'm not sure I'd trust a pneumatic system to load and unload valves for part of their stroke - besides the fact I'd be surprised if you could get any kind of reliable timing, that has to be hard on the valve opening & closing time, I would think you'd see a lot of cracked valves. You also have to consider the same rod loading, reversal and interstage problems as with the VVCP.
RE: Capacity control in reciprocating compressors
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
RE: Capacity control in reciprocating compressors
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 it
Regards
Marc
RE: Capacity control in reciprocating compressors
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.
RE: Capacity control in reciprocating compressors
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