|The Rotork CVA offers an accurate and responsive method of automating control valves without the complexity and cost of a pneumatic supply. Photo courtesy Rotork|
By Russell Ray, Chief Editor
They work in harsh environments, and they get little or no recognition. But their impact on power plant efficiency can be significant.
Valves and actuators are critical in almost every aspect of power plant operations. They are used in a wide range of applications, including pollution control, feed water, cooling water, chemical treatment, bottom ash and steam turbine control systems. They are exposed to a variety of chemicals, abrasive materials and very high temperatures. They are critical in optimizing efficiency, and they are often the final control element in the operation of a power plant.
Although the basic technology for most valves and actuators has remained unchanged, innovative applications and design modifications for problem solving have led to notable improvements in actuator technology. These improvements can reduce costs by supporting the control valve’s ability to throttle accurately, thereby providing better performance for high-pressure steam bypass, turbine bypass and other critical power plant operations.
Actuators regulate mass and energy flows by adjusting valves, flaps and cocks. The actuator and valve create a single unit — the control valve. Actuators perform different motion sequences, including linear, pivoting and rotating motions, and they are powered by pneumatic, hydraulic or electrical energy.
Actuators receive a control signal from automation systems. The signal is converted into a motion so that the control element of the actuating element assumes a corresponding position. With control valves, this is a stroke motion. With flaps, ball cocks or rotary plug valves, this is a pivoting motion.
Power plants have traditionally used pneumatic actuators to drive the many control valves throughout their facilities. However, major improvements in electric control-valve actuator technology are helping power providers achieve their most important objectives at a lower cost.
The new electric actuators can hold up to the demands of continuous movement. In addition, they work effectively in harsh environments, and provide superior performance in a wide range of applications. The benefits include better efficiency, less maintenance and enhanced performance of the control valves.
The electric actuators include a new technology to meet the specific demands of a constantly modulating control valve using electricity as the mode of power. Like traditional pneumatic actuators, the new electric actuators are capable of constant modulation for long periods of time throughout the life of the valve. The electric actuator has an advantage in that it does not require recalibration over time. Temperature, contaminates and other factors have no impact on the unit’s calibration. Once calibrated, the electric control valve actuator can operate for months, even years, without adjustment.
Rotork has a line of all-electric, compact modulating actuators known as the Continuous Duty Modulating Failsafe Electric Actuator. The Rotork CVA is suited for almost all linear, quarter-turn control valve applications requiring precise position control and continuous modulation.
The electric actuator features a failsafe function, allowing the operator to program the actuator to lock in one of four positions if there is a loss of power.
The CVA does not require the infrastructure (piping/tubing to distribute compressed air) needed to operate a pneumatic actuator. What’s more, it is significantly more accurate, Kundin said.
The move toward electric actuators has led to the creation of more digital networks for controlling these types of actuators. But the transition has been slow
|Rotork IQ electric actuator being installed at a power plant. Photo courtesy Rotork|
While many pneumatic actuators have remained unchanged, except for the addition of smart positioners, there have been some new innovations in pneumatic actuation. A number of piston and rotary actuators have been creeping into power plants, which primarily have used diaphragm actuators on control valves.
Pneumatic actuators equipped with smart positioners now functionally compete with electric actuators in terms of fail-in-position operation on loss of signal at significantly less cost.
Hydraulic actuators are increasingly popular because of their ability to achieve high torque. Some companies offer a linear actuator that can be modified for rotary action through a gearbox. The device has been around for more than a decade and offers a digitally stepped servomotor pump to provide higher positioning accuracy than pneumatic actuators.
Hydraulic actuators have even been used to position small turbine control valves. The actuator is connected to a nearby smart programmable electronic box with an umbilical cable. Configuration and calibration is made easy through this box, which can be mounted away from the process for convenient access.
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