Paper No. 105
Engine-driven compressor units at a gas transmission pipeline station experienced multiple coupling failures. These units had a torsionally soft coupling between the engine and compressor that utilized radial leaf springs. Pressurized oil was supplied to the coupling from an engine bearing through a central bore in the engine crankshaft extension. Torsional damping occurs when oil is forced through internal clearances as the coupling springs flex.
Field measurements taken on one of the units showed that at certain operating points, the design limits of the coupling were exceeded in terms of angular oscillation and vibratory torque. Physical evidence (cracks at a 45 degree angle) and oil analysis containing copper also supported this finding. The worst condition was identified when operating near the first torsional natural frequency (TNF) with some of the compressor cylinders single-acting.
As a short-term solution, restrictions were placed on the compressor speed and load steps to avoid exciting the first TNF. The long-term recommendation involved de-tuning the first TNF below minimum running speed by adding inertia to the system with a compressor flywheel.
The purpose of the case study discussed in this paper is to raise awareness of how a torsional vibration problem can occur in reciprocating compressor systems.