CUSTOMER: Alstom Power Group New Zealand  

PROBLEM: When international energy company Alstom Power performed a survey of the existing stator frame on the second of the generators, during Meridian Energy’s overhaul of the seven 105 MW generators at Manapouri Power Station on the South Island, New Zealand.  it was discovered that the support surface was not flat, and was more than 6 mm out of true. This would have made levelling the stator core difficult, with the need for shimming or placing packing under the core to correct any imbalance.

SOLUTION:  Furmanite, already known to Alstom Power, was called in on the basis of its extensive experience and high-tech equipment, to machine the surface flat.  The work was carried out using unique and highly sophisticated circular self-levelling milling (CSLM) technology, developed by Furmanite to machine circular structures up to 24 metres diameter, which is capable of achieving extremely high levels of accuracy to workshop tolerances and exceptional overall flatness, on-site.  Moreover, the work was completed by three Furmanite technicians in just three days – significantly faster than could be achieved by workshop methods.

The CSLM machine uses a scanning laser source or independent datum reference which is interfaced into the electronically-controlled hydraulic levelling system.  The 10,000 kg of equipment was transported to the underground site and the machine was assembled inside the generator frame to be concentric to the inside diameter of the core support ring, and levelled precisely to the surface to be machined.  Once set up, Furmanite's technicians then mounted the clocking arm on the machine which clocked the 230 mm wide surface to obtain the optimum machining plane.  The specifications were onerous and a high level of accuracy was required: concentricity within +/- 1mm tolerance, and a flatness tolerance of +/- 0.5 mm over the 6.38m diameter, with a surface finish of 3.2µm/Ra.  The optimum machining plane was transferred to the datum ring via the clocking arm and, with the milling cutter then fitted, the electronically-controlled hydraulic self-levelling arm travelled over the support ring surface supported by the central pivot (the unique rigid support eliminating any distortion of the equipment and surface), machining it flat.  All the specified requirements were met, including achieving a flatness tolerance of 0.3mm.

OUTCOME: Fumanite's specialist precision on-site machining equipment created major time savings by enabling a generator frame stator core support ring to be restored to flatness more accurately and quickly than other machining methods could achieve.