Vital valves

A CHANNEL Island water-treatment company is trialling actuator-driven pinch valves to reduce maintenance on one of their plants.

Pneumatic and electrical automation specialists Festo supplied Guernsey Water with valves that have a claimed lifespan of more than eight years. The previous valves lasted only a year.

Guernsey Water is trialling the Festo valves with a section of the filtration system at the company’s treatment plant in St Saviours. If the trial is successful, Guernsey Water will install the valves across the entire filtration system.

Guernsey Water, which also operates plants at Longue Hougue and Kings Mill, processes 12 million litres of water a day. That figure rises significantly during the summer months when the island’s population is boosted by tourists.

In 2004, Guernsey Water upgraded the St Saviours plant with an ultra-fine membrane-filtration system. The plant originally used chemical coagulation and rapid-gravity filters.

The membrane-filter systems are designed to provide water-treatment plants with a more compact, higher-performance filtration solution than traditional systems that use chemical clarification and sand-bed filter combinations. These membrane-filter systems are also said to be more cost effective in use.

Most of these systems comprise a series of removable modules, each containing a set of fine-tube membranes that allow the passage of water but prevent the passage of suspended particulates and biological contaminants.

The impurities are left clinging to the outsides of the hollow-fibre membranes and are removed by a series of regular purging or scourging routines.

The membrane-filter system at St Saviours comprises four banks of filter modules, together with valves and a control unit. Operating automatically, it uses compressed-air back pulses to clean the filters every 15 seconds.

When the system was first installed, it was able to deliver a high flow rate of quality drinking water to Guernsey Water’s customers and load-balancing service reservoirs with minimal maintenance requirements.

However, after about 18 months of operation, Festo claims the system’s purging valves began to fail, resulting in higher maintenance overheads and high levels of system downtime.

The valve supplier recommended installing an air-lubrication system, but Festo believed that this merely compounded the problem.

Andrew Benstead, water production manager for Guernsey Water, said: ‘During a visit to a trade show last year, I happened to mention our valve issues to an automation specialist, who suggested that I should contact Festo.

‘By then, we were experiencing up to three faults a week — some of these were genuine, while others were being flagged up erroneously due to signalling problems,’ he added.

After conducting an on-site survey, Festo determined that the principal cause of the purging valve problem was the choice of technology.

The existing valves were butterfly-type units, which have an estimated operational life of between 500,000 and one million cycles.

Given that the membrane filters at the St Saviours plant are purged every 15 seconds and that each bank of filter modules is generally in use for 200 days a year, the valves could have failed about a year after installation, according to Festo.

The plant was also experiencing a high incidence of solenoid-valve failures, which Festo determined was caused by water ingress, exacerbated by incorrect valve orientation and inadequate environmental protection.

Another problem was that valve wear had damaged some of the actuator-drive shafts, leading to errors in their end-position signals, which culminated in false indication of operational status.

To overcome the premature failure of the air-purging valves, Festo recommended that Guernsey Water replace them with actuator-driven pinch valves.

It estimated that these valves have an operational life of 10 million cycles, which is equivalent to more than eight years of use with the duty cycle employed at the St Saviours plant.

Festo also recommended using non-contact end-position sensors on the pinch valves to eliminate the effect of drive-shaft wear. It was also recommended that Guernsey Water install a cabinet to help keep the solenoid valves clean and dry.

Further refinements included removing the unnecessary air-lubrication system and adding quarter-turn actuators and switch boxes, a 25cm butterfly valve for water-flow control and a silencer to reduce the sound of exhaust air.