2018 17 Paper: Draft Tube Platform Supports to Eliminate Cavitation at Benmore

Benmore Power Station contains six 90 MW Frances Turbine Units, that have the first 3.1 metres of the turbine concrete draft tube below the turbine runner, lined with ¾ inch steel plate. Access into the draft tube for unit inspection and maintenance is through a small access hatch, giving access onto one of two temporary installed draft tube platform levels. Each draft tube is 4.4 metres diameter at the lower platform level, which is 7.6 metres from the draft tube bottom and 2.5 metres below the turbine runner.
The original Benmore draft tube maintenance platform supports are being replaced with a new platform support design to remove the cause of cavitation to the steel liner and original support. The draft tube platforms consist of an upper and lower platform, for draft tube and turbine runner access. The original platform supports were four support brackets per level, welded to the steel liner. The brackets extended out into the turbine water flow creating low pressure regions for cavitation to occur. Frequent repairs would be required to the steel liner and welds supporting the original platform brackets.
A new draft tube platform support design is being rolled out across all Benmore Units, which consists of recessed inserts welded into the steel liner in place of the original support brackets. The new upper platform supports enable the existing platform structure truss design to be unchanged. New removal brackets are used which bolt to the draft tube steel liner using the new wall inserts cut and welded into the liner profile. The upper platform is installed once the lower platform is in place. The new lower platform supports are larger wall inserts cut and welded into the liner profile to provide a slotted
ledge for the existing platform structure truss design to slot into for support. The following paper discusses the issues with the original support design, the new support concept for upper and lower platforms, and the installation project works. The site install works was completed by contractors for scaffolding the draft tube, cutting the steel liner and concrete, welding new supports and cavitation repairs, development of weld procedure, testing of welders and welding, and painting of draft tube.

• Author: Jason Fincher
• Exchange: 2018 - Albury AGL
• Company:
• Topic: Asset Condition, Hydrology, Refurbishment, Turbine, Welding
• Key Words: Cavitation, Draft Tube, Maintenance, Protection, Repair, Weld