The Effect Of Starvation On The Dynamic Properties Of Tilting Pad Journal Bearings

Abstract

A new 46 MW steam turbine became unstable as the load on the turbine was increased. The turbine had a relatively long slender shaft and the subsynchronous frequency was near the first critical speed of 1800 rpm. In an effort to allow the machine to reach full load a bearing redesign was undertaken. The bearing design was changed from the original 5 pad tilting pad journal (TPJ) bearings, orientated with the load on the bottom pad (LOP), to three pad TPJ’s also in the LOP configuration. Analysis indicated that the additional asymmetry in the dynamic coefficients would significantly increase the system logarithmic decrement (log dec) and allow the machine to run at full load without going unstable. With the three pad bearings installed the machine was brought up in speed and experienced subsynchronous vibration before reaching full speed. A design review of the bearings indicated that the top two pads of the three pad bearing were operating in the fully starved condition; that is they were not receiving sufficient oil to develop a hydrodynamic oil film. As such the horizontal stiffness went to near zero. This paper will present the results of the analysis work performed, including the starved bearing analysis, and the resulting redesign of the three pad bearing to run flooded. Operation as a flooded bearing allowed the machine to run fully loaded with no indications of unstable operation.