Rotork is a key player at Wessex Water's first Profibus valve actuation site

Rotork intelligent electric valve actuators with Profibus DP open control system connectivity are helping Wessex Water to improve the sewage treatment regime and protect the popular tourist beaches on the Somerset coast at Highbridge.

A new storm overflow tank installation at Highbridge is Wessex Water's first use of the Profibus DP protocol for the monitoring and control of valve actuators, which, says Wessex Water's commissioning engineer Iain Harris: "should provide us with the prototype for similar installations in the future. Wessex Water is always keen to look into using the latest developments in technology in order to continue to be recognised by Ofwat as being the most efficient water and sewerage operator."

At Highbridge, two new storm tanks have been built to double the storage capacity of the existing installation, which receives the flow from eight pumping stations in the surrounding area. The plant expansion is being designed, installed and commissioned by the principal contractor, M. J. Gleeson, in order to meet strict new consent targets introduced by the Environment Agency to minimise the risk of untreated discharges to sea.

At the same time new screens, filters, aeration tanks, settlement tanks and ultra-violet treatment plant have been installed at the West Huntspill STW, which receives the outflow from the Highbridge site, at a controlled rate of no more than 300 litres/second.

Rotork IQ actuators control the flow rate through the Highbridge plant as well as the sequence of filling and emptying the storm tanks when the full flow to site rate is exceeded. At this point the additional volume is automatically diverted into the storage tanks until the flow rate returns to normal.

All the processes at Highbridge are controlled on Profibus two-wire networks, linked to a new Mitsubishi Q2AS-S1 PLC utilising a software programme written by Wessex Water automation. Wessex Water senior automation engineer David Evans explains: "A total of four independent Profibus networks are installed at Highbridge, each network linking flow meters, high and low level sensors and valve operating equipment from different manufacturers. This type of flexibility is a particularly useful benefit of using an open system such as Profibus when designing and operating plant processes."

The software programme fills tanks 1 to 4 one after the other but empties them in an especially configured eight-part sequence to enable the process to react immediately to fluctuating ambient conditions. For the offsite monitoring and reporting of alarms from the fully automated site, the PLC is linked by a Seprol S500 telemetry output to Wessex Water's Regional Operations Centre at Bath and can be accessed from other communication centres.

Commenting on the decision to use Rotork IQ valve actuators on this significant Profibus application, Iain Harris says: "Rotork has a good track record of reliability at other Wessex Water sites. As well as integrating successfully with the other Profibus controlled equipment on site, the IQ actuator gives us greater flexibility with control and instrumentation, particularly with the maintenance and diagnostic abilities provided by inbuilt data loggers."

Wessex Water operations provide water supply and sewerage services across an area of south west England covering 10,000 square kilometres including Dorset, Somerset, Bristol, Bath, most of Wiltshire and parts of Gloucestershire and Hampshire.