Control valve actuators: Their Impact on control and variability

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Courtesy of Rotork

Sector: Oil & gas
Category: Electric actuators
Products: CVA

Electric control valve actuators provide excellent performance and are ideal for oil and gas wells in remote production fields. Instrument air supply systems are costly and require significant energy to run. If mains power isn't available, an instrument air supply isn't practical, especially when only a few control valves are in use at a location. Solar powered DC electric actuators are ideal for such an application.

In a process plant, the general function of a control valve is to restrict the opening of the valve so it affects the flow or pressure of the liquid or gas that is passing through it. In any given application, an installed valve, has one fundamental variable - the position of the moving element, which could be a profiled ball, plug, or sleeve in the valve. That single moving element determines the exposed orifice that allows greater or lesser flow through the valve, which in turn provides the control of the process.

The valve itself may be extremely sophisticated with exotic body and seat material, or it may have complex flow patterns that allow for a high pressure drop or some other complex feature. However, the fundamental requirement to move the valve stem to position the control element remains the same regardless of whether it's a simple or a sophisticated valve.

A control valve actuator is used to move the valve stem (which is attached to the internal control element) to the desired position and hold it in place. In addition to the act of moving and holding positions, there are many other parameters to that movement which determine the best type of actuator that should be used for every specific application. For example, other important considerations might include speed, repeatability, resolution, and stiffness.

The demands of the process significantly impact the demands placed on the valve, and, by association, the requirements of the actuator, so it performs adequately.

When selecting the proper actuator for an application, the first and most fundamental consideration is the actuator's ability to overcome the reactive force of the valve. That force is mainly a function of valve size and differential pressure across the valve as well as packing and/or seat friction.

Clearly, the force generated by the actuator must be sufficient to overcome valve forces. In many cases, control valves may have a seating force requirement in excess of the mid-travel force demand. Therefore, valve actuators are required to be sized to the maximum force generated by the valve.

Another important consideration is the dynamic performance requirement, or speed of the actuator, so that the valve can adequately meet process demands. There are two elements that should be considered when evaluating actuator speed. The first is the reaction time to initiate movement after a demand signal change, and the other is the speed of operation once motion is initiated.

It's important to note that electronically controlled electric actuators react almost instantaneously to a demand change when required. Pneumatic actuators, on the other hand, need to physically build up sufficient pressure in the piston or diaphragm to initiate movement. That generates a delay or dead time, which can negatively impact the process.

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