Caution should be exercised when modifying reciprocating compressor suction systems because changes may alter the acoustic response. Unacceptable levels of vibration, high piping and nozzle stresses, and compressor valve problems may result from the addition of knockout vessels and coalescers, or from piping changes. Chances are often good that problems will not result, but there is a very real risk, and your project may be the statistic. In new equipment installations, an acoustic study is the tool used to mitigate this risk.
Installing a coalescer or other piece of equipment in a reciprocating compressor suction line changes the acoustic length of the line, or creates two new acoustic lengths where there was previously one. An acoustic study is a design review of these lengths to determine if any acoustic resonances will occur and if they will coincide with the mechanical natural frequencies in the piping system. In some situations it is prudent to revisit this work when making field changes.
API 618 gives guidance as to when an acoustic study is recommended for new machinery installations. An acoustic study should be considered when any of the following are true:
• Two or more compression stages
• Three or more cylinders per stage
• Final discharge pressure exceeds 1000 psig
• Driven equipment horsepower is 500 Bhp or greater
• Service alternates between gases of significantly different molecular weights
• Interaction is anticipated between compressors of 150 Bhp and greater
Acoustic studies are usually not performed for machines of less than 150 Bhp.
The API recommendations provide a good basis for deciding whether to perform an acoustic study when altering a system, but risk assessment should also play a part in decision making as these studies can be quite expensive. Generally, risk of having harmful pulsations increases as compressor running speed decreases and as the gas acoustic velocity increases (usually as molecular weight decreases). As these parameters change, acoustic lengths get very long and fall out of the normal range of field piping lengths. Also, the intended use of the compressor plays a part in risk assessment. The cost of production losses should be weighed against the cost of performing an acoustic study as part of an alteration.
All things considered, a packaged high-speed compressor in a producing field gas application would be considered low risk because it is high-speed, high molecular weight, with low to moderate production losses. Conversely, a large hydrogen booster compressor in a refinery would be considered high risk for acoustic problems because it is low speed, low molecular weight, with high production losses.