Valves and Actuators

21 August, 2015 by Russell Ray

http://www.power-eng.com/

A single power plant uses hundreds of valves to control almost every aspect of its operation.Valves, in conjunction with a controlling actuator, are used for pollution control, feed water, cooling water, chemical treatment, bottom ash and steam turbine control systems.They work in harsh environments and are exposed to a variety of chemicals, abrasive materials and high temperatures. They are critical in optimizing efficiency, and they are often the final control element in the operation of a power plant.

What's more, additional demands are being placed on valves and actuators as power plants are forced to be more flexible to accommodate the growth of intermittent sources of renewable power and mandates to curb carbon emissions. As a result, valves and actuators must operate at higher pressures, temperatures and frequency.

Although the basic technology for most valves and actuators has remained unchanged, innovative applications and design modifications are being developed to withstand these demanding environments. In addition, 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.

There are three common types of actuators: electric, pneumatic, and hydraulic.Pneumatic valve actuators are powered with air or gas. The air pressure acts as a piston to create linear force to close and open the valve. 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 producers lower costs and boost efficiency. Valve actuators powered by an electric motor can withstand 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. What's more, electric actuators do not require recalibration over time. Once calibrated, the electric control valve actuator can operate for months without adjustment.

Hydraulic actuators, which use pressurized hydraulic fluid to open and close valves, are increasingly popular because of their ability to achieve high torque. Hydraulic actuators are designed to carry out linear movement of all kinds. When a large amount of force is required to operate a valve, hydraulic actuators are normally used. The most common type of hydraulic actuator uses pistons that slide up and down within a cylinder containing hydraulic oil and a spring.

Young & Franklin offers electromechanically actuated gas control valves designed specifically for industrial gas turbines. These sonic flow valves are available in a range of sizes suitable for any industrial or power turbines. The valve body is coupled to an actuator assembly that contains a fail-safe spring to quickly close the valve, halting fuel flow in the event of a power failure or turbine trip condition. When the valve and EMA are coupled to the Y&F series 1100 Digital Motor Controller, the complete system provides precise fuel flow delivery with reliable operation.

Date: 2015-12-11; view: 1138