Pneumatics more reliable than electrics for valve actuation

Thames Water is justifiably proud of its Ashford Common water treatment plant. It processes about 690 million litres a day and satisfies a quarter of London's entire potable water needs. Ashford Common is one of the larger treatment plants clustered on the Thames above Teddington Weir – others include Hampton, Walton and Kempton.

The Ashford Common plant is supplied by five fixed-speed pumps and employs a 1600mm butterfly valve as a gravity bypass and to control flows onto the works. The valve is located halfway down a 30m shaft.

Towards the end of 2005, Thames Water commissioned a replacement for this main gravity valve because, although the existing electrically-actuated valve was only 12 years old, it required frequent movement to provide adequate flow control. Although numerous attempts had been made over the years to optimise the valve's performance, it still required actuating every 10 minutes. Also, the valve constantly oscillated under load conditions and required frequent servicing, which was proving inconvenient and costly due to its relatively inaccessible location. Thames Water's main contractor – Costain – consequently asked the process valve manufacturer Erhard to come up with a suitable alternative.

Erhard chose to base the new valve design on a pneumatic - rather than electrical - actuator. This offered much more robust control, with minimal maintenance requirements. The valve's pneumatic actuator is a custom-designed unit produced by Festo, which is coupled directly to the butterfly vane without gearing or spindles. The system is controlled by a remote Siemens PLC, with local operator override facilities for routine inspection.

Cost and performance benefits

Pneumatic valve actuators provide interesting cost and performance benefits for process valve actuation. Pneumatic cylinders are inherently compact actuators with high load capacities and overload capabilities. Using compressed air as the drive medium is safe and reliable, plus it copes with fluctuations in actuation forces, temperature and humidity, and delivers an operating life of more than 2million cycles – which is up to ten times more than a typical electrically-operated valve.

Given the importance of Ashford Common's contribution to London's water supplies, it was essential that the valve replacement was handled as quickly and efficiently as possible. Festo's service partner, Aspects Engineering, successfully carried out all of the system installation tasks. The entire procedure was rehearsed repeatedly prior to implementation to ensure that no last-minute problems would be encountered by the engineers while working in the confined space of the shaft. It says much for the professionalism of all parties that the entire task was completed in just 16 hours – some eight hours less than the allocated time. The valve was replaced on 7 December 2005 and went completely unnoticed by the company's customers - though it did win an accolade from Phil Stride, Thames Water's Director of Operational Control, who called it "an exemplary piece of work, well-planned and executed, which enabled Ashford Common to return to full production ahead of schedule."

Paul Davies, Thames Water's Project Manager, says: "We are exceptionally pleased with the performance of this valve. It is much more responsive to control signals and oscillates significantly less than its electrically-actuated predecessor. As a result, we expect less premature valve wear. It seems apparent that pneumatically-actuated valves offer numerous operational advantages over electrically-actuated valves in this environment. I have also been very pleased with the technical support provided by Festo during this project and we are now considering pneumatic actuation in other areas."