Zero of Axial Shift Indicator.

What I learnt in the year 1977 while boxing up bearings of India's first 200 MW unit at Obra (Unit 11) was as follows:

You move the rotor towards the Generator side and the thrust pads which get tightened are called working pads. If the Millwright Fitter had no brains and scrapped the parting plane of the bearing and made it non perpendicular to vertical plane against which the pads are pressed the two halves of the pads will be in different planes, therefore, you must ensure that bottom half as well as top half pads get tightened.How? Measure the axial float with bottom half pads, press the rotor against working pads, put a dial indicator to fix the axial position of the rotor to zero, assemble the top half pads, measure the axial float again, press the rotor against the working pads and see whether you get the zero in the dial indicator and nearly the same axial float.

The activities in the previous paragraph are supposed to be done before the box up of the turbine casings because the reference point of steam flow path is  when the rotor is pressed against working pads. Since I was not present before box up I re-checked to confirm and to give clearance for final checking and calibration of Axial Shift Indicator which should also indicate zero with rotor pressed against working pads.

My curiosity was what position the rotor takes while the machine is on load. The unit was synchronized on 31st December 1977 and it was fulfilled. The rotor gets pressed towards the working pads and the axial shift indicator indicates negative value corresponding to the thickness of the oil film which also varies. What happens when the unit trips? The rotor moves towards the non working pads (also called surge pads) and indicates higher negative value. While rotating on Turning Gear (also called Barring Gear) sometimes it almost touches the surge pads and indicates a negative value equal to the axial float in the bearing and this is the acid test of the erection work.

My next curiosity was whether the Turbine trips on axial shift. The answer was it may never trip in 30 years life. Then why the axial shift indicator? To indicate:

• wear of thrust pads if it indicates zero or positive value and
• axial float in thrust bearing sometimes when on Turning Gear.

After many years I am associated with 600 MW Steam Turbines at Jharsuguda and the same holds good but people who have come from various power stations have doubts.

One person confidently says that my knowledge is obsolete and the steam flow path is adjusted with the rotor kept in the middle of the axial float and the axial shift indicator is set to zero at this position of the rotor. When I ask that person which way the rotor goes when the unit is on load he has no answer. He thinks it is the job of operation to see that. When I tell him that the axial shift indicator is not an important parameter for operation because the Turbine is not going to trip on axial shift for its entire lifetime he says then how does it matter what way he sets the zero.

It matters because if you set the steam flow path and the axial shift indicator zero in the middle of the float the Turbine will come on working pads on load and not operate at the intended position corresponding to steam flow path. In addition the axial shift indicator will not serve the purpose of indicating the wear of working pads accurately.

My only advice to such people is to observe what the axial shift indicates while the machine is on load and on Turning Gear and speak accordingly.