Design Modifications And Active Balancing On An Integrally Forged Steam Turbine Rotor To Solve Serious Reliability Problems.

Abstract

This paper describes the design modifications and active balancing on a 19,000 hp integrally forged steam turbine rotor to improve its dynamic characteristics and reliability. The turbine was purchased as a replacement for an identical turbine used in a syn gas train. Ever since original installation, the turbine had been plagued with high vibration problems on the coupling end, excessive bearing temperatures, and reduced coupling life. A rotordynamic analysis proved that there was sufficient separation margin between the turbine's operating speed and any critical speed. However, at operating speed, the rotor had a high vibration response at the coupling, even with a very small unbalance. A design modification was thus incorporated on the rotor to make it less sensitive to unbalance. At operating speed, this modification lowered the predicted coupling end shaft deflection to approximately a third of its original value. An active balancer was then assembled on the rotor to provide the flexibility of online trim balancing. The replacement turbine has been installed and is operating with the lowest vibration and bearing temperatures in the compressor train's history. Presented in this paper is an overview of the rotordynamic analysis, the shaft welding technique used to accomplish the rotor design changes, and the balancing with the active balancer. The techniques presented in this paper can be used to improve the mechanical reliability of other machines with similar problems.