How electric control valve actuators can eliminate the problems of compressed air as a power medium


Courtesy of Rotork plc

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

Today, a new major technological advance is available that can help control-valve users avoid many of the problems and inefficiencies associated with using compressed air as a power medium.

The new solution uses electric power and eliminates dependence on compressed air. This totally electric solution is appropriate and cost-effective for a wide variety of control-valve applications, including those found in such sectors as power generation, chemical, petrochemical, and most other process industries. While the new generation of electric control valve actuators may not be suitable for all process applications, it is ideal for many situations, especially where users have experienced problems with frozen air hoses, lack of process precision, stick slip, and so on. Therefore, it is prudent for today's process control engineers to take a serious look at how the design features of the new generation of totally electric control-valve actuators can benefit them. In many situations, this technologically advanced equipment can substantially increase the output and efficiency of their process as well as help reduce maintenance and operating costs.
Control-valve actuation: a brief history

Before discussing today's latest technology, it's beneficial to understand how control-valve actuation has evolved. Decades ago, the main medium for controlling process control valves was by varying the pressure of the air supply to the valve's actuator. Typically, this air pressure varied between 3 and 15 PSI. A closed valve position would relate to 3 PSI and the open valve position to 15 PSI. This was an international standard for positioning linear control valves (and later rotary valves also) by balancing this air pressure against an opposing spring. The higher the pressure, the more compression was exerted on the spring, and the greater the movement of the control valve. As the pressure backed off down to 3 PSI, the spring pushed the valve stem back to the original position. This simple means of position control was used in a wide variety of process control plants. It was the mainstream solution offered by control valve actuator manufacturers as well as control system suppliers.

In the simplest form, compressed air was both the power medium and the control medium. Desired positions were achieved by varying this applied pressure and entire plants were controlled by compressed air channeled through small-bore copper tubing. The backs of control panels were a mass of tubes skillfully arranged by control systems craftsmen into symmetrical layers of carefully arefully laid pipe. However, with the advent of computers and programmable logic controllers (PLCs), the days of the 3-15 PSI control signals were numbered.

Soon, they were replaced by electronic signals carried on much lighter duty copper wire with control at the speed of the electrons rather than pressure waves. This was a revolution in control technology, bringing with it tremendous cost savings in installations as well as vastly improved control capabilities.

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