Applications which require automated valve packages demand high quality performance with very little downtime. Chemical and petrochemical plants involved in the production of very dangerous chemicals require the optimal performance and efficiency of their control valve actuators. In these industries, the manufacturing of bulk chemicals is often a batch-controlled process using automated, frequently cycled valves. There are different types of actuators available to automate valves. Technologies include electric, hydraulic and pneumatic. Pneumatic designs include piston, vane, rack and pinion and scotch yoke. It is important to be familiar with fundamental procedures of sizing and selecting the particular actuator design being used.
Sizing and Selecting an Actuator
The first step in properly sizing and selecting an actuator is to understand the valve product that will be automated. The three most common rotary valve designs used are ball, plug and butterfly. Each design has a different torque profile which makes a significant difference in choosing accurate and safe actuators. When fluid flows through a rotary valve, static pressure does not act uniformly on the surface of the closure element. Dynamic torque results from non-uniform static pressure distribution on the closure element. Because dynamic torque acts on the valve stem, it can act either with or against the control valve actuator. If dynamic torque is of a magnitude that is greater than the frictional torque, it will cause rotary motion if unchecked by the actuator. In conventional ball and plug valves, friction torque is usually greater than dynamic torque and is not normally a concern when sizing actuators. Small dynamic torque acts to close conventional ball valves. In butterfly valves, friction torque is generally low except when the disc engages the seat at full closure, so dynamic torque has a much stronger influence. Dynamic torque in industrial butterfly valves with stem centered discs with symmetric faces acts to close the valve.
Valve manufacturers have the responsibility to properly test and publish torque data. They are also responsible of reporting the data accurately. This process is simplified and becomes more reliable when both valve and actuator are made by the same manufacturer. Once the torque requirement of the valve has been properly determined, the actuator can be properly sized.