TE MIHI GEOTHERMAL POWER STATION- STEAM TURBINE GENERATOR PEDESTAL DEFLECTION

Abstract

The Te Mihi geothermal power station steam turbine generator (STG) is supported on a concrete pedestal structure that supports the STG over an underslung direct contact condenser. Following commissioning, both Units had experienced significant ongoing deflection of the pedestal table due to both permanent plastic deflection from concrete creep shrinkage and elastic deflection from vacuum loading. These ongoing deflections have exceeded the turbine manufacturer’s design criteria and required remedial works for STG rubs with multiple STG alignments and shimming of the STG casing.
Following an extensive investigation, a mitigation program was developed to address risk of STG rubs, potential coupling failure and future unplanned additional maintenance overhauls and alignments. A two stage implementation was used to initially relocate existing services, with a subsequent in-service installation of wide trussed columns. These works have been successfully implemented and pedestal deflection arrested.
Adequacy of support of the STG and the need for post commissioning monitoring of pedestal deflection is often not well understood by either the designer of the pedestal structure nor the operator.
This case study reviews the engineering investigations and mitigations developed for the excessive elastic deflection of the STG pedestal structure.