Full load shutdowns of reciprocating compressors can generate higher torsional responses than a normal unloaded shutdown. A transient analysis can be performed to calculate the maximum torque in the coupling during a shutdown with the compressor at full load.
EDI calculates the full load compressor torque values for various compressor speeds. The torque due to the gas forces will be fairly constant during shutdown as long as the discharge pressure remains the same. However, the dynamic torque due to inertial forces decreases as a function of the compressor speed squared.
The transient analysis simulated several seconds at a constant compressor
speed and then the unit is tripped. The coast down time is predicted. During coast down, the dynamic torque at the compressor orders may excite the torsional natural frequencies.
The calculated peak torque must be compared to the allowable level for the coupling. This is particularly important for systems containing rubber couplings. A cumulative fatigue analysis can be performed on the predicted shaft stresses.
