Reciprocating Compressors

The reciprocating compressor has several strikes against it when it comes to reliability, if the main consideration of reliability is long run times as would normally be expected from the dynamic type machines. It has a lot of parts and the parts are subject to wear. Before one jumps to the conclusion and totally excludes this compressor from consideration because of these factors, the positive aspects of this machine should be reviewed. For low-volume-high-pressure ratio service and particularly for low molecular weight gases it is difficult to match. When compared to the centrifugal for this service, the centrifugal will have to operate at a high speed and require many impellers. This end of the spectrum is not the best from a reliability point of view for the centrifugal.

If turndown is a consideration, again the reciprocating compressor, with its many unloading options, does meet the challenge. This flexibility is difficult to achieve reliably in the dynamic machine and may require the compressor to operate at an unfavorable point on its characteristic curve,

Several steps can be taken to maximize the run time for the reciprocating compressor. Since wear is a function of rubbing speed, the piston speed can be kept to a minimum. Chapter 3 made recommendations lor piston speed. Reliability problems due to valves are reputed to account tor 40% of the maintenance cost of the compressor. Valves are the single largest cause for unplanned shutdowns. Basically, valve life can be increased by keeping the speed of the compressor as low as practical. At 360 rpm, the valves are operated six times a second. At 1,200 rpm, the valves operate 20 times a second or 1,728,000 times in a day. It is not difficult to understand why the valves are considered critical. To keep the reliability in mind, valve type, material selection and application considerations such as volume ratio, gas corrosiveness, and gas cleanliness need attention by the experts. One final note is that while lubrication is an asset to the rubbing parts, it is not necessarily good for valve reliability.

While not necessarily improving the reciprocating compressor's individual reliability, using spared units does improve the overall plant reliability. With proper monitoring, the compressors may be removed from service in a timely manner for maintenance. If the program is properly administered, unplanned shutdowns can be avoided and a higher plant reliability achieved.

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