CASE STUDY: Tuning out a difficult torsional vibration problem in a reciprocating compressor installation.
Torsional vibrations are not always readily apparent, and are sometimes referred to as the silent killer in reciprocating compressor installations. EQT Corporation recently encountered a continuing series of failures at one of their compressor stations, which has two identical units with 1250 HP, 1190 RPM electric motors driving 4-throw reciprocating compressors. Over a period of just one year, there were five instances of either a cracked motor shaft, a cracked rotor, or both cracked rotor and motor shaft! Field measurements and torsional vibration analysis of the system with the existing flex coupling indicated torsional vibration levels at the auxiliary end of the compressor, vibration levels in the coupling, and motor shaft stress levels were all within allowable limits. A different approach was taken as a step toward solving the problem. The system was analyzed with a torsionally soft coupling in place of the existing coupling. Indications were that the soft coupling would tune the system, placing the lowest torsional natural frequency below the lowest operating speed. As a result, the largest portion of torsional vibration is taken across the coupling. A torsionally soft coupling was designed and installed to place the lowest torsional natural frequency at approximately 850 RPM. Torsional vibration levels were then measured over a range of loads and operating conditions. Results indicated that rotor shaft stresses were reduced by almost a factor of ten from those of the old coupling. The system has now operated satisfactorily since the soft coupling was installed more than one year ago.