Rotork valve actuators increase the Profibus benefits for Yorkshire Water

The introduction of Profibus-enabled Rotork IQT valve actuators on a filter bed automation upgrade for Yorkshire Water has facilitated the installation of a highly economical and efficient control network.

Stuart Goodwill, M & E Engineer at Mott McDonald Bentley - the partnership design and construction company for the project - explains: "Rotork's Profibus card enabled us to fully exploit the features of Profibus control technology when configuring the design of the network. Employing the analogue input fitted as standard on the Rotork card has saved the expense of either installing a separate Profibus interface on all thirty-five flowmeters or hard wiring them to the PLC. Instead, the analogue signal from each flowmeter is simply linked to the corresponding actuator for onward communication with the PLC via the Profibus network."

The automation programme is at Esholt, one of Yorkshire Water's largest waste water treatment works, serving more than 300,000 homes and industry in the Bradford area. The Rotork IQT actuators are fitted to previously hand operated plug valves on the secondary treatment percolating filter beds. Constructed over fifty years ago, the filter plant consists of seventy axial beds and extends over a distance of more than a mile. Effluent arrives from the distribution chamber, where it is mixed with re-introduced final effluent to improve consistency before being gravity fed into thirty-five troughs, each serving a unit of two filter beds. The automation upgrade is designed to improve the flow performance into the beds, optimise filtration efficiency and prevent damaging problems such as dry bedding and uneven flow distribution that were difficult to avoid when the plant was manually operated.

Electromagnetic flowmeters have been fitted in the pipes serving each trough, adjacent to the Rotork actuated plug valves. The flowmeters and actuators are connected to a new PLC in what is believed to be Yorkshire Water's first use of a Profibus network on a waste water treatment plant. Due to the sheer size of the filter bed site, the PLC is centrally housed and four separate control buses are used to prevent a reduction in the data transmission rate and eliminate the requirement for repeaters in the network.

The total rate of flow into the plant is divided by thirty-five and this figure (typically 80 litres/second) is compared by the PLC to the reading from each flowmeter. If the flowmeter reading is too low or too high the PLC instructs the appropriate Rotork actuator to step the position of the valve in either opening or closing direction until the flow reading is within the operating deadband.

The PLC software, written by Blackburn Starling, also enables the automatic operation of individual filter sets to be overridden if circumstances dictate. For example, the inverter drives that control the speed of the spray bars across the filter beds can be slowed down to enable the bed to be flushed, improving subsequent filtration performance.

The Rotork actuators also communicate valve position data to the PLC whilst integral data loggers store operating data including valve movements and torque profiles. All the operating and flow data from the PLC is transmitted via a radio modem to the Esholt central SCADA control room where the operation of the entire site is displayed. The filter bed upgrade was commissioned in 2005, since when it has successfully improved the treatment of up to 250,000m3 of effluent a day, helping the site to achieve stringent Environment Agency consent targets.