G4 Turbine Overhaul

After the successful refurbishment of the G3 horizontal turbine at North Power’s Wairua Falls site, McRaes Engineering asked for AIE to be involved in the redesign and refurbishment of the G4 vertical turbine. The main issue that had been making this turbine very difficult to obtain operational reliability was silt seizing the wicket gates in the bearing pots on the bottom ring. This in turn was causing the external regulating ring to distort during operation. There was no access to the bottom ring bearing pots without dismantling the bottom of the scroll casing.

North Power’s instructions for this project were:

  • Provide a sealing arrangement for the top and bottom rings to prevent silt entering the bearing pots
  • Provide a lubrication system to the bearing pots
  • Design and build a regulating ring that is internal to the wicket gate spindles
  • Design a linkage system that allows for easy adjustment of the guide vane position to ensure sealing strips contact one another
  • Incorporate into the linkage system a mechanism to prevent damage to the guide vanes if there is an obstruction when trying to close.
Together McRaes Engineering and Allied Industrial Engineering successfully undertook a complete redesign of the regulating ring, linkage and lubrication systems. Draft designs were continuously updated as we created fresh ideas to include reliability coupled with ease of maintenance.

The highlights of the final design were:

Externally removable bearing pots

  • There is no longer any need to dismantle the turbine to service the bearing pots. Each individual bearing pot can be extracted by the removal of three bolts.
  • The bearing pots were fitted with high pressure seals where previously these were not in existence.
  • The bearing itself has a drain, which ensures that any silt that bypasses the seal is expelled by grease during lubrication. The drain is a clear tube which allows the operator to see what is being expelled when lubricating the bearing pot.
A new regulating ring assembly was designed and subjected to Finite Element Analysis (FEA)
  • The static ring was spiggoted to the top cover to ensure repeatable location
  • The rotating ring had LG2 bushes cast and fitted to provide better reliability from any contamination
Linkages were designed to provide a same plane direct transfer of energy from the hydraulic ram that actuates the regulating ring through to the guide vane spindles. There was no chance of distortion from cantilevered fixtures.
  • Shear pins were designed and calculated to prevent any damage to obstructed wicket gates.
  • Eccentric pins were supplied to aid in seal strip alignment
AIE were also asked to inspect and subsequently machine the runner shaft to improve alignment of the runner within the machine. Throughout the redesign of the turbine clearances were re-calculated between the runner and top and bottom rings to provide a more efficient water path.

The turbine was installed and then commissioned in October by McRaes Engineering.