By Mike Dupuis, Hydro Tech Inc., Canada and
Ken Arthur, Volta River Authority Ghana
A 6 x 172 MVA generating station was currently experiencing thrust bearing failures. The bearing failures were due to a thrust bearing that was marginal in its design to withstand the new thrust loading due to an upgrade of the Francis turbine. The Babbitt on the thrust bearing pads were slowly deteriorating over several years to eventual failure. Several attempts were made to correct this problem by modifying the Babbitt pads, some with more success than others, but all bearing pad modifications eventually failed.
The thrust bearing failures did not become evident until after the turbines were upgraded by 30%. An increase in thrust load has been associated with each of the new turbine runners; the increase in thrust load produced higher pressure on the thrust bearings. Prior to the turbine upgrade, each thrust bearing operated without incident.
The Volta River Authority contracted Hydro Tech Inc. to develop a solution to the thrust bearings. Two issues were contributing to the thrust bearing failure, Babbitt pads that were operating above normal design capabilities, and a four segmented thrust bearing runner plate that was creating continuously pressure pulsations on the Babbitted bearing pad surfaces.
The solution to the failing bearings was to redesign the thrust bearing runner plate and to replace the Babbitt bearing pads with PTFE bearing pads. This solved both the pressure pulsations and the bearing pad loading.
The thrust bearing is an eighteen segmented tilting pad, spring-supported babbitt bearing. The runner plate is divided into four segments that are loosely supported. Each segment is supported by the thrust block and held in place by radial keys with two vertical bolts holding the each runner plate segment to the thrust block. The original runner plate segments were not...