Actuator selection and sizing for valves

Abstract

An actuator is a machine or component installed on the top of an industrial valve for automatically moving and controlling the valve. The performance of a valve is largely dependent on its actuator. Three factors are important for engineers to consider when selecting an actuator: frequency of operation, ease of access, and critical functions. Valve actuators should perform several functions including moving the valve closure member to an appropriate position, holding the valve closure member in the desired position, providing enough force or torque for seating the closure member and meeting the required shut down leakage class, providing fully open or fully close or failure mode as is, or providing a certain amount of closure member rotation with the right speed. In general, actuator can be hydraulic, pneumatic or electrical. This paper discusses the mechanism plus advantages and disadvantages of these three types of actuators. Affected parameters for actuator selection include the availability of a power source, torque and size of the valve, failure mode, speed of operation, frequency and ease of operation, control accessories, hazardous area, and cost. This paper presents a case study of breakaway torque (break to open) calculation and actuator sizing for a full-bore ball valve in pressure Class 300 equal to 50 barg nominal pressure and 22Cr duplex body material. The valve is fail close with an emergency shut down function, and a pneumatic actuator was selected for the valve. Fail close means that the valve will close in case of losing the power used as a source of actuator operation. Other four torque values were provided from the valve supplier. The calculated breakaway valve force and torque were used as a basis for actuator air cylinder sizing assuming air pressure of 7 barg and system efficiency of 90%. Force and torque for closing the valve were used to calculate the spring movement as well as spring piston length through Hook’s law.