Recently we overhauled & commissioned 55 MW LMZ Turbine. Designed limit of axial shift is (-1.7 mm turbine end to +1.2mm Generator end) and designed differential expansion is +3.0 mm to -2.0 mm. On barring gear, axial shift was -0.55 mm (we had set total axial float as 0.55 mm during overhaul). On commissioning & full load of 55 MW, axial shift is -0.47 mm but differentail expansion is showing as -2.5 mm. This time new turbine & generator rotors have been used. With old turbine & generator rotors, Diff Exp used to be -1.50 mm, this time customer has changed the turbovisory Instruments also. We had set 13.0 mm as Generator rotor magnetic off-set value w.r.to stator . What could be cause behind high -ve axial shift and high -ve differential exapnsion value ?? what care to be taken ? what is remedial action ?? We are apprehending that if there is a full load throw off, since Axial shift is running at high negative value, rotor will go further to Non Working (surge) side and may damage the NW pads.
Anyone to suggest probable causes behind this problems & corrective measures to be taken ?? Thanks for valuable suggestion in advance.