On Steam Solution Of A Prototype 8000 HP Motor Instability Problem.

Abstract

This paper presents a typical field problem that most maintenance engineers in any petrochemical plant could encounter. Two 4000 hp through-drive electric motors were totally destroyed when the inboard coupling failed. Market conditions dictated that the compressor be put back into service as expeditiously as possible. A 3600 rpm prototype 8000 hp motor which could be fitted on the existing foundation was acquired from a power company. The existing half shell journal bearings had to be converted to a full shell design in order to solve a vibration problem. After start-up, the motor was plagued with a random vibration problem which occurred during a load change. Vibration data taken during one of these excursions indicated sub-synchronous frequencies at halfspeed, which were equal in amplitude to the horizontal readings at synchronous speed. A computer model simulating the rotor and bearing systems indicated a whirl instability problem at 1800 rpm. The computer was used to design and optimize a "between the pad," four shoe tilting pad bearing which was installed during the December 1982 turnaround. The paper goes into details on the new bearing design and installation, motor mechanical and hot optical alignment, and start-up data. All of the design and fabrication of the bearings was done while the unit was running, and the bearings were installed during a normal scheduled downtime. The result was a "first try fix." This example illustrates that the "trial and error" method of problem solving should be done on paper and not with hardware, particularly when the stakes are high and the time short. Equipment and techniques used to solve this problem are well established and used daily in most petrochemical plants.