Water treatment benefits from multidrop system
15 Jan 2000
Filter beds at Sluvad water treatment works in South Wales are the site of what Meggitt Mobrey believes is the largest multidrop installation of its MSP100 level transmitters. A total of 48 of the Hart-based devices have been put in by contractors Purac to monitor level, flow and pressure.
The requirement at Sluvad, which serves the Cardiff and Newport areas with 180 mega- litres of potable water a day, was to replace hydraulically-operated valves with electrically-actuated ones and to fit new instrumentation to interface with a new DCS.
The MSP100s are used to measure the levels in each of 28 rapid gravity filter beds, and to monitor flows from half of them. There are also Hart pressure sensors monitoring the differential pressures across each bed and other MSP100s measuring levels in other tanks.
The all-digital Hart installation offered considerable hardware and installation costs compared with traditional 4-20mA and other available fieldbus options.
All the Hart devices are connected in a multidrop, all-digital configuration to two DCSs each with 16-channel Hart field modules (HFM) to multiplex the signals. One HFM channel was allocated to each filter bed to safeguard the robustness of the control system. As Purac's Mark Cargill explains, ' it is a potential problem with multiplexing that if one channel goes down, then all the measurement on that loop is lost. Our arrangement puts all the measurements for each filter level, flow and pressure on to one channel, so that if there is a problem in any one loop, only one filter is affected, leaving the other 27 filter beds on line.
