“Introduction:

The subject 200 MW steam turbine has three bearings with high, intermediate, and low pressure stages (Figure 1). Considerable vibration trouble was encountered on hot starts of the 200 MW steam turbine after an accident in 1982 which left a bow in the rotor. This paper deals with the balancing activity after an outage where mechanical work was performed on the blading and low speed balancing was performed. The once-per-rev vibration levels on the no. 1 and 3 bearings were higher then normal during startup (when it did not trip) and operation. During the balancing process, it was found that, at times, the unit could not be brought to speed due to a thermal bow induced by a thermal mismatch in the high pressure rotor. Later it was determined that the first interstage metal temperature governed the ability to start up with minimal vibration.

“Balancing Effort:

After the outage, there was trouble getting the unit on-line due to a thermal bow in the high pressure rotor. However, after allowing the unit to stand for 10 hours to cool below 400°F, a smooth start was realized with a low pressure couple of 7 oz @ 330° installed at planes 3 and 4 (Figure 1). It was found in the 1982 balancing effort on this unit that the vibration on no. 1 bearing was sensitive to a low pressure couple. The peak vibration on startup (Figure 2) was 4 mils-pk to pk at the HP critical speed (2,200 RPM). Note the vibration drop out at 2,700 RPM due to residual bow. What the bow does to the associated polar plot is shown in Figure 3. This shot helped the no. 3 bearing and generator but not the no. 1 bearing (Table). The readings on the Y probe were always lower than the X probe readings provided in the Table. The probe locations are shown in Figure 4.