Craig Goch reservoir – butterfly valve failures

Craig Goch reservoir is the most upstream reservoir in the Elan Valley cascade and forms part of the water supply system serving Birmingham via Severn Trent Water and areas of mid-Wales via Dŵr Cymru Welsh Water. During a reservoir drawdown to facilitate diving in the reservoir as part of works for a ‘Measure to be Taken in the Interest of Safety’ under Section 10 of the Reservoirs Act 1975 the butterfly valve on the turbine bypass failed. The turbine bypass at Craig Goch is the most effective means of drawdown at the reservoir in normal operation and emergencies as it is considerably larger in diameter than the scour. Following the closing of the upstream guard valve it was found that the butterfly plate from the valve had sheared off and been ejected from the end of the pipe. The paper shares lessons learnt from a valve failure and covers the likely transient pressures experienced, the poor design of the valve that had been installed, the risk of using a valve for purposes for which it was not intended and the importance of an operable secondary means of isolation, especially where the asset is a critical water resource.