As part of MBB’s O&M portfolio, MBB is contracted for the operation and maintenance of four hydropower stations in Mpumalanga. Two of which date back to the 1950s.
With high demand seasons a thing of the past, and very little water available for power generation, MBB undertook the challenge to refurbish the Densda turbine in September 2020. The original turbine was installed in 1955, and to our knowledge, no maintenance has been done on the guide vane system since.
An MBB modification to the original turbine design is the installation of a pressure gauge that measures the pressure behind the runner, which indicates the condition of the wear rings. Worn wear rings, guide vane bushes and links also resulted in difficulty synchronizing with Eskom. This further resulted in turbine overspeed, which occurs when the generator trips due to water leaking past the guide vanes. Excess pressure behind the runner resulted in significant axial forces, which in turn put a strain on the white-metal thrust bearing.
The overall focus was on the refurbishment of the turbine’s guide vane system and wear rings. The first step was to pull the bronze runner off the shaft – this required approximately 30 tons of axial tension before the runner started to move. The entire guide vane adjustment system and the guide vanes were then also removed.
Offsite machining consisted of the manufacturing of two new wear rings, a cut to the impellor faces, new bushes for the guide vane pins, and the polishing of the guide vanes. All cast iron elements were sandblasted and repainted.
During reassembly, all the guide vanes and the main shaft received new gland packings, and excess play on either side of the guide vanes was reduced by removing the gasket on the guide vane ring. Cavitation and mechanical damage to the bronze impellor were repaired with an epoxy composite reinforced with a silicon steel alloy. All components were repainted, bearing oil was replaced, and the cooling water piping was improved.
Once recommissioned, the pressure behind the turbine was reduced to zero, resulting in an effective axial thrust reduction of approximately 80%. Turbine rotational speed while tripped reduced from around 950rpm to less than 400rpm.
Hopefully, the turbine will now last another 65 years!