De-centralised control for Colchester WTW
As part of an extensive upgrade at Layer de la Haye water treatment works near Colchester, Essex & Suffolk Water is installing a new control system.
During the upgrade of the filters the actuated valves are being replaced with Tyco pneumatic valves, which are, according to Mr Gould, cheaper than electronically operated versions. Mr Gould: "For 96 readings from the valves each day, i.e. every 15 minutes, we would have needed 288 I/O memory cards. But these functions have now been networked by Tyco using the profibus-based AS Interface system." According to Tony Stark of Tyco: "AS Interface is the first and only bus system optimised for the first field level of automation." Where memory cards are still necessary, ABB Kent DP/PA units have been used to minimise the number of memory cards. Each DP/PA unit can deal with information from several memory cards.
AS Interface (ASI) modules are now being fitted to all the valves controlling the rapid sand filters. Unlike traditional multi-core systems, where a minimum of a three-core cable would be required for the remote indication and a further three-core cable for the solenoid valve, the ASI modules connect all actuators with a simple two-wire communications cable.
By getting rid of the PLC-based system, Tyco has reversed the trend for increasingly centralised plant control. Mr Stark explained the advantages of such an approach: "Here only the most important information is passed back to the central computer. This allows the use of shorter programs and simplifies commissioning. It also means the rest of the plant is more likely to continue working if a station fails. In many cases decentralised control will allow for faster system expansion."
Layer de la Haye is one of Essex & Suffolk's largest WTWs. Water is pumped to the works from the nearby Abberton Reservoir, which is filled primarily with raw water from the River Stour. Water is treated at the works using rapid and then slow sand filters, to produce a supply of 120Ml/d.