Alignment of Turbine to a Generator

The main thing that is taken into account is the thermal growth of both the generator and the turbine. The turbine is going to grow more than the generator due to the steam driving the turbine. Then there are other factors that have to be accounted for. If it is a back-pressure turbine then it will rise more than the generator. If it is a condensing turbine then it will rise less and the generator will be set high to the turbine. This is due to the turbine growing more than the generator during startup but after the unit is on line the turbine will cool down and fall in line with the generator. There is a formula that can be used to calculate the thermal growth in mils but you have to take a hot reading and a cold reading to be able to do this. Most manufactors of these units will actually give the the growth of each unit by temperature. Alignment has nothing to do with gravity or weight. Only thermal growth and rotor sag. Hope this does not addd to your confusion. Dale

While your explanation has covered most of requirements of TG alignment - think we are ignoring the basic point mentioned in the query. "...... because of Jacking Pump...."

Appears that in this particular case, when the TG is on Barring Gear, normal method of lubrication bearing, which can be for reduction gear or for generator bearings, is not adequate at low barring speed. Hence to protect such bearings, these are jacked up by slightly lifted up by pressurised film of oil between concerned journal and bearing. And as a result of same Mustafamohmd suspects the initial difference of +0.14mm.

Besides, for very large TG Sets, Generator rotor length of order of 8 meters and diameter 1 meters, the sag in the generator rotor can not be ruled out. As a result of slight sag and long length of rotor the face of Generator Coupling Half is at angle with respect to turbine end rotor coupling half. In such case Co-Linearity of two axis is not possible. In such cases we have to make the face of two coupling halves parallel and axis interesting at face of coupling. To achieve same, we have to raise the non - drive end (exciter end) of generator pedestal bearing more than the drive end pedestal bearing to make the face of two coupling halves parallel. Similarly for Super Thermal Power Plants, using TG sets of 1000MW, possibility is of Synchronous alternator being coupled at both ends with turbines (I assume, I have not seen), the far ends pedestal of concerned turbines might have to be raised to take care of sag of Generator rotor.

As a general rule, as indicated by Tornado, hot equipment is intentionally kept lower than the relatively cooler equipment while doing alignment correction. The idea is that, on running condition both will grow (expand in vertical direction) and expected to be at equal level (both the shafts center lines are collinear). But bigger equipment suffer complex expansion patterns. While the frame expansion vertically (nevertheless) disturbs the alignment, the shaft expansion, linearly affects the DBSE (Distance Between the Shaft Ends). Both expansions strain the coupling, cause undesirable vibration and component failures. Despite careful design considerations, some amount of undesirable expansion are unavoidable. On a heavy machines the center line of shafts may not (mostly will not) grow parallel to the position fixed while alignment. For instance in the case of a big turbine, the vertical growth of the pedestal at inboard and outboard may be different because of the relative prevailing conditions like temperatures (nozzle end would be hotter than the exhaust end), the supports (bottom support at one end and center-line support at the other end), the insulations and many more. These factors lead to inclined placement of the shaft while running if not well predicted and accommodated at cold condition.

It is paramount important to keep the shafts (driver and driven) collinear in running condition rather than when not running. In other words, if only we keep the shafts in a (calculated) misaligned condition, they will assume a near perfect collinear alignment while running. OEMs do give 'cold alignment protocol' and preferred DBSE value in their manual or on demand.