Profiting from Profibus
Mowlem is building a new filtration and treatment plant for Essex and Suffolk Water Company. The use of Rotork actuators is realising major savings for the water company. We find out how.
Mowlem Engineering and Mowlem Civil Engineering are building the new plant which will increase the supply of high-quality water to users in the Grays and Thurrock area by up to 4.5ml/d.
The well, pumping station and filter house were originally built in 1923 by the former South Essex Waterworks Company and the site is now being brought back into use after four years of redundancy.
An electric pump has been installed to bring the water to the surface where it is treated and filtered to remove a high iron and manganese content before entering the supply network.
The new plant comprises six pressure filters containing sand and manganese dioxide media, multiple chemical dosing stations and on-site filter backwash water processing, all under PLC automatic control using Profibus communication highways.
Most of the Rotork actuators installed are IQT units on butterfly valves controlling the flow through the filters and the sequence of backwashing operations, which is initiated either by time elapsed, operator intervention or the analogue input signal from a differential pressure sensor.
The backwash sequence uses the digital inputs and outputs and the analogue input available on the Rotork Profibus card to minimise the number of separate Profibus interfaces required and to simplify site wiring.
On receipt of the backwash signal, the IQT actuators are programmed to close the filter inlet and outlet valves and open the backwash valves that enable water stored on-site to be gravity fed through the filters.
At the same time, digital outputs from the Rotork Profibus cards operate smaller Rotork ROM2 electric actuators on adjacent air scour valves to aerate the filter media during backwashing. At the end of the backwash period the valve operating sequence is reversed to put the filter back into normal operating mode.
The same Profibus highway controls the actuators and monitors the instrumentation on the backwash WTW, which comprises a settling tank from where the separated water is drained into a wildlife lagoon in regenerated environmental land next to the site.
The Profibus highway is Scada supervised by a Siemens SL5 PLC at Essex and Suffolk's headquarters, with an Allen Bradley 50/4 PLC in the site's new control room.
Under normal unmanned operation, the PLC's data will be communicated by telemetry link to the Essex and Suffolk Water central control room, at Hanningfield Reservoir, where remote monitoring, interrogation and control can be performed.
Commenting on the Profibus solution's benefits, Mowlem project leader Paul Tompkins says: "The site PLC required the use of a Profibus card for the variable speed pump control, so electrical contractors were asked to submit tenders for conventional hard wiring and the Profibus alternative for all parts of site."
The adoption of Profibus facilitated substantial savings with electrical installation costs. Pre-commissioning and wiring checks were simplified and commissioning procedures were less arduous.
The cost of the Rotork Profibus actuators was comparable with the Folomatic current position transmitter alternative although the functionality of the Profibus version is greater.
Further installation time and material savings were experienced, says Tompkins, by using the spare l/O capability available within adjacent actuators to transmit flow, pressure, level and digital status signals back to the PLC for the control and supervision of the plant.
According to Tompkins, the Rotork actuators, reduced PLC l/O cards and smaller motor control centre ICA section have led to site cost reductions of up to 50%.
This is because of reduced cabling and cable management requirements, fewer cable glands, less design drawing, reduced installation time, less terminals and termination accessories and reduced Profibus interfaces.