Water dives into digital VALVE CONTROL

While the control world argues over which fieldbus will win out in the marketplace, many industries are simply making up their own minds and pragmatically picking what they see as the best for their own specific applications. The results of this `consumer power' can be clearly seen at Essex & Suffolk Water's Layer de la Haye water treatment works near Colchester, Essex.

As part of a refurbishment programme on its filter systems, ESW has opted for the Actuator Sensor interface (ASi) to provide digital control of the filters' valve systems.

The original system was conventional hard-wired 4-20mA analogue control based on Texas Instruments PLCs. But, says ESW senior project manager Norman Gould, the company wanted to replace this as part of its Y2K compliancy programme.

In total 144 valve actuators needed changing. After considering a switch to electric actuation, Gould selected pneumatic actuators from Tyco Valves and Controls, operating on the device-level ASi bus linked via a Profibus DP gateway into a local Allen-Bradley PLC. This in turn is connected via a Modbus link to the site-wide ABB Scada system.

The newly `digitised' valves control the first stage of the treatment process at the Layer works, which can produce 120 megalitres per day of drinking water drawn from the River Stour. After preliminary settling tanks, this stage is either upflow or downflow rapid sand filtration.

The Tyco package consists of Keystone double-acting pneumatic actuators (80psi), fitted with ASi interface modules, mounted on existing butterfly valves of varying sizes. Unlike traditional multicore systems where a minimum of a 3-core cable would have been needed for remote indication, with another 3-core cable for control of the actuator's solenoid valve, ASi connects all actuators with a simple, bright yellow (see picture), single two-wire cable.

There are a total of 12 filters, each with 12 valves. The majority require basic on/off actuation, provided by single coil solenoid valves, but there are 24 dual-coil solenoid control valves to enable the filters to be `slow-started' after each backwashing cycle. This slow build-up to full flow is now a regulatory requirement to minimise any breakthrough or carryover of cryptosporidia into the water supply.

To cope with this demand, Tyco's ASi system can supply either two outputs for control or one output for open/close. A traditional system would have needed an additional digital output card for the control valves, introducing extra costs at both the control cabinet and valve ends of the control loop. The ASi network, on the other hand, needs no extra cabling or cards in the control cabinet.

A look at the existing cable trunking to the filters shows just how this two-wire system has dramatically reduced cabling on the plant. Once packed with wiring between each valve and the control room, the 12in wide trunking now gives more than ample support to a couple of yellow cables - one for the valves, the other linking differential pressure cells on each filter to the control room.

In addition to the reduced cabling, the system has brought other cost savings to ESW. Although the filter valves need a variety of actuator sizes, the ASi modules and solenoid fittings remain the same for each valve, thus reducing the number of spares needed and making installation far easier.

Connection to the two-core cabling itself couldn't be simpler, with a flexible clip-on type connector replacing the conventional cable gland and terminal arrangements, while still maintaining IP67 integrity.

The £250 000 project took just four weeks to install by contractors that had not previously tackled an ASi system. Commented Gould: `The AS interface will not only reduce our capital costs, but also our revenue costs by enabling quicker and easier diagnosis at the plant level. It increases the flow of information, leading to control and process efficiency.' PE