Valves need actuators

Actuators are an essential element providing automatic control and monitoring of the operation of valves. John Pratt of UK company Rotork Controls Ltd discusses the evolving role of actuators and the functions available for the diverse applications in which they are used, and details the capabilities of Rotork's products and services.

In virtually all the relevant industries, the contemporary practice in plant design and operation is to automate. Automation comprises the control and monitoring systems, combined with a means of energy to perform the required activity. With a valve, all these elements come together and are contained inside the actuator.

Actuator basics

Valves and pumps are common components in many plants, but in many ways valve actuators are the antithesis of pumps. Generally, pumps are running virtually all of the time, whilst valve actuators can be inactive for up to 99% of their working life. The only exception is control valve actuation, but even here the activity is rarely continuous. On an isolating valve the actuator's function for most of the time is to communicate data about the valve's position and condition to the controller. However, when asked to change the valve position the actuator must respond with absolute reliability, especially as some valves only open or shut for safety reasons. It is also essential that the actuator communicates a signal to confirm that the requested action is being performed, or sends an alarm if it is not or cannot be completed.

Valves come in a range of designs, each with differing operating requirements for which various valve actuators are available, with a choice of power source encompassing electricity, air or hydraulics. Generally, electric actuation is in the ascendancy for reasons of economy, simplicity and ease of maintenance, although there are industries and applications where fluid power is preferred. For example, electric actuators are designed to stay put on power failure, leaving the valve in the position to which it was last moved, but there may be important reasons to open or close a valve under these conditions. Because it is easy to equip a pneumatic actuator with a spring that will operate the valve if the power fails, this type of actuator is traditionally specified for such failsafe applications.

Electric actuators can be used, but failsafe operation is usually only possible if a stored energy source such as batteries or an
uninterrupted power supply is provided, adding to the cost and complexity. To avoid this problem, the Rotork Skilmatic actuator has been developed to offer spring-operated failsafe benefits with the convenience and cost-effectiveness of electrical operation . The Rotork Skilmatic actuator design incorporates a totally sealed electro-hydraulic power unit with a spring-return actuator. When electric power is applied the hydraulic fluid is pumped to the actuator to operate the valve and compress the failsafe spring, which operates the valve in the reverse direction or in the event of power failure.

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