Because of the reciprocating action of the piston, care must be exercised to size the piping to minimize acoustical pulsations and mechanical vibrations. As a rule of thumb, suction and discharge lines should be sized for a maximum actual velocity of 30 ft/sec (1,800 ft/min) to 42 ft/sec (2,500 ft/min). Volume 1 contains the necessary formulas for determining pressure drop and velocity in gas piping.
Analog or digital simulators can be used to establish the pulsation performance of any compressor piping system in detail. API 618 Section 3.9.2 provides guidelines for piping pulsation and vibration control based on compressor discharge pressure and horsepower. In practice, many operators do not “analog” compressors of 1,000 horsepower or lower, but rather rely on extrapolations from proven designs. For larger horsepower sizes or where unusual conditions (e.g., unloading and loading cylinders) exist, an analog is recommended.
For smaller, high-speed compressors the piping sizing rules of thumb discussed above, in conjunction with pulsation bottles sized from Figure 11 -24, should be sufficient for individual field compressors. These rules of thumb can also be used for preliminary sizing of piping and bottles in preparation for an analog study.
To minimize pipe vibrations it is necessary to design pipe runs so that the “acoustic length” of the pipe run does not create a standing wave that amplifies the pressure pulsations in the system. The acoustic length is the total overall length from end point to end point including all elbows, bends, and straight pipe runs.
Typical pipe runs with respect to acoustic length are considered to be:
* Pipe length from suction pipeline to suction scrubber
* Pipe length from scrubber to suction pulsation dampeners
* Pipe length from discharge pulsation dampeners to cooler
* Pipe length from cooler to scrubber
* Pipe length from discharge scrubber to pipeline
The end of a pipe ran can be classified as either “open” or “closed.” Typically, closed ends are where the pipe size is dramatically reduced, as at orifice plates and at short length flow nozzles. A typical open end is where the pipe size is dramatically increased. Where the pipe run contains similar ends (closed-closed or openopen), prohibited pipe lengths are:
Where the pipe ran contains dissimilar ends (closed-open or open-closed), prohibited pipe lengths are:
The wave length may be calculated from:
Mechanical vibration of pipe is handled in the same manner as for reciprocating pumps. Normally, if the pipe support spacing is kept short, the pipe is securely tied down, the support spans are not uniform in length, and fluid pulsations have been adequately dampened, mechanical pipe vibrations will not be a problem. It is good practice to ensure that the natural frequency of all pipe spans is higher than the calculated pulsation frequency. The pulsation frequency is given by: