Despite weighing-in at a massive 5000 kg, the 1600-mm diameter pneumaticallyactuated butterfly valve provides moreprecise and durable control than electric actuators.

Thames Water in Middlesex, England is justifiably proud of its Ashford Common water treatment plant. It processes about 690 million liters a day and satisfies a quarter of London’s entire potable water needs.

Five fixed-speed pumps supply the plant, and a 1600-mm butterfly valve – located halfway down a 30-m shaft – acts as a gravity bypass and controls flow into the works.

Recently, 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 cycling to adequately control flow. Although numerous attempts had been made over the years to optimize the valve’s performance, operators still had to actuate it about every 10 minutes. And constant oscillations under load, in turn, led to the need for frequent downtime and servicing. This proved to be inconvenient and costly, due to its relatively inaccessible location. Thames Water consequently tasked the process-valve manufacturer Erhard to come up with a better alternative.

Erhard based the new valve design on a pneumatic, rather than electric, actuator. This offered a much more robust control, with minimal maintenance requirements. The pneumatic actuator, custom-designed by Festo, couples directly to the valve’s butterfly vane without gears or spindles. A remote Siemens PLC controls the system, with local override capabilities for routine inspection.

Festo produces a wide range of pneumatic valve actuators and associated components for the process industry, including ATEX-compliant versions. This image shows a Copar quarter-turn actuator and butterfly valve.

Pneumatic valve actuators provide cost and performance benefits for process valve actuation. Pneumatic cylinders are inherently compact with high force capacities and overload capabilities. Compressed air provides a safe and reliable drive medium that copes with fluctuations in actuation forces, temperature, and humidity. It also delivers an operating life of more than 2 million cycles – up to 10 times more than a typical electrically operated valve.

Given the importance of Ashford Common’s contribution to London’s water supplies, the entire replacement was completed in just 16 hours.

According to Paul Davies, Thames Water’s project manager, “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, and we are now considering pneumatic actuation in other areas.”

For more information, visit www.thameswater.co.uk and www.festo.com.