Southdown Turbine Diaphragm Repair and Rotor Machining

AIE were engaged to perform repairs to the steam turbine and associated diaphragms from the Southdown power station. The Southdown plant is a 175MW natural gas-fuelled, co-generation station capable of producing 1400GWh of electricity a year. At Southdown there are 3 gas turbines, 2 of which have an HRSG, the third is an open cycle unit. The gas turbines each drive a generator, which produces electricity. Heat recovery boilers capture the energy in the gas turbine exhaust gases to produce steam, which drives the steam turbine. The steam is then made available to the local industrial area near the station.

The scope of work included machining out the existing seals in the diaphragms, and then rolling, machining and fitting replacement seal strip carriers. This operation required AIE to develop specialised welding procedures in conjunction with an independent inspection organisation, and subsequent qualification of several members of our specialised welding team to these procedures.

The split line faces of the diaphragms were then machined in our CNC machining centre to ensure an acceptable fit of the mating surfaces was achieved. Following this, the diaphragms were assembled and set up in our vertical lathe to finish machine the new seal strip carriers.

After this process was completed, the new seal strips were caulked into the diaphragms, using a purpose made tool. The diaphragms were then reassembled on our vertical lathe to finish machine the new seal strips.

As our specialised welders had been qualified for the weld procedures, AIE was requested to perform on-site repairs to the turbine casing at the Southdown power station, south of Auckland.

During the refurbishment of the diaphragms, AIE received a call from our customer asking if we could assist with the repair of the steam turbine rotor as a crack had been discovered during the course of the non-destructive testing. Once the rotor had arrived at AIE’s workshops, it was set up in our Super-Lathe, and an inspection was undertaken by the OEM Works Engineer to verify the severity of the crack and to determine the remedial work requirements.

The repair procedure required progressive machining of the rotor followed by in-house NDT verification and technical analysis by the OEM engineers at each stage of the machining process. This measured approach to the repair was used to ensure the integrity and performance of the rotor was not compromised. It became apparent that the crack in the rotor was far more significant than originally thought, and the decision was made to completely remove the third stage disc from the turbine rotor.

The repairs to the diaphragms and rotor were successful and our customer has reported that after machining the disc off, the expected power loss was not seen and apart from a slight efficiency loss, output remained as before.