ArticleProcedures for Preventing Machinery Thrust Failures on Compressors and Turbines
by Charles Jackson
This is a well written paper regarding the practical application and use of thrust protection systems on turbo-machinery using a compressor/turbine machine train as a case study. This paper is especially good for beginners and anyone else with minimal experience around Babbitt type thrust bearings and thrust protection systems. The paper uses a number of diagrams and pictures to assist with the explanation of the material. This is a very practical paper but reinforces the fact that thrust protection is for machine protection and not bearing protection. The thrust bearing is a sacrificial component to protect the rotating machine and if it begins to wear, then indication and protection systems will shut the machine down before potential damage to the main components of the machine occur. This paper would be more valuable for an engineer or analyst that assists in overall machinery troubleshooting, condition evaluations, and setup of a protection system. This is a well done paper for anyone that is requested to assist in setting up thrust protection for rotating machinery with procedure that is easy to follow and technical reasons for the choices and recommendations made in the procedure.
“Introduction and Problem or Needs:
The thrust failure of a turbine, compressor, or related machine costs over a million dollars, and the production outage may well be 4:1 higher. A referenced paper was presented on this subject in 1985, after many improvements in API 670’s standards, but this needs to be re-visited due to many changes which have removed the Technical people from User and related companies.
The thrust bearing on most equipment separates the running components-rotor from the stationary components-stator. It was never intended to Save the “thrust bearing”; but many Saves of the machine, have been documented. In all cases; there was distress to the babbitt when the shutdown limits were reached. Many OEM shutdown limits were too short; e.g. 5-10 mils (0.127- 0.254 mm); therefore causing the owner’s to bypass the protection and fail a total casing/train. Setting proper limits for alarms and automatic shutdown causes arguments. The typical limits, in this procedure, is 15 mils (0.381 mm) for Alarm and 25 mils (0.635 mm) plus “automatic” Shutdown, within 3 seconds, when using a minimum of 30 mils (0.762 mm) of babbitt (white metal) on the thrust pads. The success has been repeated. Further, the difficult positioning of the rotor at center-of-float or using the instrumentation center-of-range calibration is performed in a “slightly” different manner. Often, the responsibility rest with the Owner’s I&E department with assistance from Machinists, Electricians and Technical Staffs.
Turbines often have the normal/active thrust towards the exhaust which is a “gap opening” direction with the dual voting proximity probes viewing the rotor shaft near the thrust bearings. Compressors may have the normal/active thrust towards the suction and may be driven through the discharge or the suction of the compressor. Further, the compressor could shutdown with a settle out pressure still existing within the case; e.g. 700 psig (~ 48 bars), per the most recent incident; causing a total thrust reversal (~ 22 tons force- 199 Kilo-newtons) on shutdown with some pre-planning to prevent the axial thrust probes from being damaged (travel range). Re-starting the compressor, under pressure, may become a design challenge to the thrust bearing and lubrication design.
An example steam turbine driving a centrifugal air compressor through a Lucas dry type coupling is used to illustrate. The same procedure applies to other centrifugal compressors; e.g., a vertical barrel has a thrust bearing on the suction end being driven through the discharge end; a preference.”
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