THAMES WATER'S Ashford Common treatment plant processes around 690 million litres of water every day and meets about a quarter of London's potable water needs. The plant is supplied by five fixed-speed pumps, and uses a 1.6m butterfly valve as a gravity bypass and to control flows. The 5,000kg valve is located halfway down a 30m shaft. In late 2005, Thames Water commissioned a replacement for the electrically-actuated valve because, although it was only 12 years old, it needed to be operated frequently to provide adequate flow control. Although numerous attempts had been made to optimise its performance, the valve still required actuating every 10 minutes. Also, it oscillated constantly under load and required frequent servicing, which was inconvenient and costly because of its awkward location. Thames Water's main contractor, Costain, asked the process valve manufacturer Erhard to come up with an alternative design.
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