Rotor Dynamic Analysis Requirements in API Standards With Case Studies

This paper briefly describes the rotor dynamic analysis requirements in the API standards for process centrifugal compressors, induction motors and gearboxes. The paper also presents an example for each equipment type where the dynamic analysis performed by the manufacturer during the design stage in accordance with these standards identified critical aspects that were concerning to the purchase. The paper also highlights the measures taken to alleviate the purchaser’s concerns, the vibration behavior of these machines after commissioning and some lessons learned from these cases. The first case is for a 4,744 kW (6,400 HP) centrifugal gas compressor running at 11,889 rpm. The dynamic analysis shows that the compressor will run above its fist critical speed estimated at 5,400 rpm and below its second critical speed estimated at 20,600 rpm that is 16% less than the second multiple of the running speed. The analysis also shows that the second mode is lightly damped with an amplification factor equal to 116. The second case is for a 2,984 kW (4000 HP) induction motor running at 3,600 rpm. The dynamic analysis shows that the motor will run at almost twice its first critical speed. The first critical speed is estimated at 1,750 rpm with high amplification factor equal to 99. Also, the second critical speed is estimated at 7,400 rpm which is less than 5% from twice the line frequency and the second multiple of the running speed. The third case is for a 4,744 kW (6,400 HP) helical gearbox. The dynamic analysis shows the existence of a critical speed for the gear wheel rotor that coincides with the pinion rotor running speed at 50%–100% load range.