In the past few years, it has become commonplace to install centrifugal compressors in parallel with reciprocating compressors. Among other concerns related to this system layout, centrifugal compressors cannot efficiently operate with high-amplitude pressure fluctuations (pulsations) in the attached piping system.
As reciprocating compressors create higher-amplitude pulsations than centrifugal compressors, it is important to determine and reduce, if necessary, the pulsations created when the reciprocating compressors are operating in parallel with centrifugal compressors.
In areas where pulsations are a concern, system mechanics can also be an issue. Centrifugal compressor piping systems are typically installed with little thought of vibrations and piping dynamics in mind. Therefore, restraints are sometimes designed only for static loads, which can lead to high vibrations.
The Gwinville Compressor Station in Jefferson Davis, MS, for example, experienced severe 30-inch piping vibration problems in early 2009. At this location, parallel operation of a new centrifugal compressor and eight reciprocating compressors was planned for a 1,000 psia, 60°F suction piping system. Seven of the eight reciprocating compressors operate over a speed range of 250 to 300 rpm. Unit 8 operates over a speed range of 200 to 250 rpm. Pipe routing consisted of approximately 300-feet of piping between the new centrifugal compressor and the closest reciprocating compressor.
Severe piping vibrations were noticed near the centrifugal compressor shortly after startup. Even relatively small amplitude pulsations can cause concern regarding efficiency and operation closer to surge than would be predictable with no pulsations.
More in March/April 2013 issue of Turbomachinery International