ABSTRACT

Pressure pulsation from reciprocating compressors can cause excessive piping and structure vibration on offshore platforms, which may result in shutdowns or piping fatigue failures. This paper describes how to avoid such problems through proper use of pulsation dampeners, analog computer analyses, piping layout and supports. Field test results on actual installations are used to illustrate the effectiveness of these measures.

INTRODUCTION

Reciprocating compressors are sources of pressure pulsation. This pulsation propagates through the compressor piping, frequently causing appreciable vibration of both the piping and its supporting structure. The vibration can be strong enough to result in piping fatigue cracks and unscheduled shutdowns.

Particular attention must be given to pulsation induced compressor piping vibration on offshore platforms because of the greater potential for exciting other systems coupled through the structure. The common compressor pulsation frequency range of 5 to 100 Hz. often coincides with the natural vibratory frequencies of both the piping and structural members supporting the piping. When excitation and natural vibration response frequencies coincide, high vibration amplitudes and cyclic stresses can develop.

This paper discusses prevention of pressure pulsation induced vibration problems through the proper selection and use of:

  1. Pulsation dampeners.

  2. Analog computer analysis.

  3. Piping layout and supports.

PRESSURE PULSATION CHARACTERISTICS

By its inherent design, a reciprocating compressor does not provide a smooth steady flow but, instead, produces a series of periodic displacements produced by the intake, compression, and 0ischarge cycle basic to this type of compressor. This periodic displacement action superimposes a pressure pulsation wave on the intake and discharge system mean pressures. In addition, higher harmonic pulsation components are usually present because of the abrupt opening of the compressor valves which excites the acoustic response properties of the gas columns within the compressor passages and piping.

The origin of a pressure pulsation wave can be illustrated by the single reciprocating compressor cylinder shown in Fig. 1. The gas is compressed until the cylinder pressure exceeds that of the discharge system sufficiently to open the discharge valves. The discharge piping and passages are acted upon by a pressure pulsation having a fundamental frequency coinciding with the compressor displacement frequency and higher harmonic components. Similar effects are seen in the suction passages and piping.

Pressure pulsation waves propagate at sonic velocity (approximately 1400 fps for natural gas) through gas contained in both the suction and discharge piping. The pulsation travels upstream and downstream through the gas media which is flowing at a slower velocity of approximately 3D to 60 ft/sec. This can be visualized as being similar to the ripple action produced from a rock thrown into a flowing stream.

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