A newly installed production field natural gas compressor package was exhibiting excessive vibration throughout the inlet piping. Fearing a vibration related piping failure, the compressor could not be run at its rated RPM or gas capacity.
Equipped only with its SH2000 vibration meter that reads overall vibration, Metrix Instrument Company of Houston was challenged to determine the vibration source and to make a recommendation for permanently reducing the vibration. The compressor package components evaluated during the initial survey included the compressor frame, cylinders, engine, pulsation bottles, skid structural members, scrubbers, and inlet piping.
A sketch of the machine was quickly drawn, and a set of vibration readings and their locations recorded. This information promptly focused the attention on the suction side piping and scrubbers. The initial vibration readings were taken and analyzed. All vibration readings were made with the hand-held meter set to measure velocity (inches per second).
The Metrix workers determined that the scrubbers and inlet piping vibration was strongly influenced by engine speed. Between 1,050 and 1,100 RPM, the direct relationship between engine speed and inlet piping vibration was confirmed using a Fluke 87 meter set to read Hz, connected to a moving coil velocity transducer. When the engine speed was increased to 1120 RPM, the vibration at measurement Point 1 reached 6.1 IPS. But this speed was still far below the compressors 1,200 RPM operating speed rating.
Comparing the vibration readings to the compressor packages mechanical components, they decided to determine if increasing the horizontal stiffness of the inlet piping or scrubbers would raise their natural resonant frequency above the compressor sets operating speed range. They located a 15 to 16 foot length of pipe on the site and wedged it between a skid frame member and the primary inlet piping, (near measurement Point 1). At 1,100 RPM, the vibration at measurement Point 1 dropped from over 3 IPS to .7 IPS.
Additional vibration readings with the SH2000 determined that installing a brace between the two scrubber towers further reduced the scrubber and inlet piping horizontal vibration, making the diagonal brace unnecessary.
Today the gas compressor package is being run at rated speed and capacity with the vibration well within acceptable levels at all measurement points.
"This is not to imply that reciprocating compressor and piping problems can be identified and solved so easily," said John Polhemus of Metrix. "Clearly we were very lucky to find a relatively simple case where the scrubber towers were wagging the dogs tail, so to speak, and causing the dangerously high inlet piping vibration."
Whatever the degree of difficulty, this case shows that with reasonable care and thought, and by mapping out a machine and its related structural components, readings obtained with the SH2000 vibration meter are extremely useful. These readings can be used to profile a structures vibration and to test the effect of changing the structural stiffness or mass of different equipment and structural components.
For further information on Metrix Instrument Company in Houston, Texas, phone 713-932-6941.
