Valve actuation for solar plant

The Andasal solar plant in Spain has a 510,000 m² collector surface area which provides a generating capacity of 50 MW, enough to meet the annual electricity demand of 50,000 households or 200,000 people.

The parabolic trough is constructed as a long parabolic mirror with a Dewar tube running its length at the focal point. Sunlight is reflected by the mirror and concentrated on the tube, where it is absorbed by heat transfer oil flowing through it. The oil is used to heat steam in a conventional turbine generator.

Rotork IQ intelligent electric actuators with Pakscan 2-wire digital control have been specified for valve control in all areas of the generating process at the site.

Rotork Iberia worked closely with the plant’s engineering company, Sener, to integrate an economical and efficient actuation and control system into the overall plant design.The decision to use Rotork’s Pakscan 2-wire digital control was assisted by the system’s long-range bus capabilities. Designed specifically for the spacious environments associated with the majority of valve actuator installations, Pakscan can operate a loop of up to 20 km in length without any deterioration in communication performance or the need for repeaters. This has enabled over 100 actuators at the site to be controlled and monitored with a single bus loop. Each loop is supervised by a Pakscan P3 120-channel master station, which provides the communication interface with the plant’s control centre.

Control, monitoring, interrogation and configuration of each actuator is also available at the master station, offering increased flexibility to the operator. In addition, thanks to the web server installed as standard in the P3 master station, the operator in the main control centre will always have a clear picture of the condition of all the actuators on the loop at all times, even in the event of a failure of the main plant control system.

The main pipework circuits on a parabolic trough power plant comprise the HTF (heat transfer fluid) thermal oil pipes that carry the heat transfer oil around the mirrors, the steam plant and the power generation circuits. In addition, a liquid salt heat storage circuit is installed to enable electricity to be generated for up to 7½ hours after the sun has set. In all plant areas, on-off valve control has been achieved using IQ multi-turn and IQT quarter-turn electric actuators. For the 10 control valves on each of the networks, IQM modulating actuators were selected, equipped with Rotork Folomatic proportional controllers and CPT current position transmitters, operating from a 4–20 mA control signal.

The Pakscan digital control system offers economical installation costs allied to reliable and comprehensive real-time communication with the IQ intelligent actuators. Data loggers within each actuator facilitate diagnostics by recording historical operating data and valve torque trends. This data can be downloaded either through the actuator’s ’non-intrusive’ infrared link to the handheld setting tool or laptop, or via the Pakscan bus loop. Using Rotork’s IQ-Insight software, this data can be analysed in the office to predict any potential operating issues. Maintenance can therefore be planned in advance of any unplanned interruption to normal operations, optimising the overall performance of the plant.