Electric control valve actuators: eliminating the problems of compressed air as a power medium

Electric actuation can help control valve users avoid many of the problems and inefficiencies associated with using compressed air as a power medium.

A totally electric solution for control valve actuation 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 totally electric control valve actuators can benefit them. In many situations, this 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 electric valve actuation technology, it’s beneficial to understand how the control valve actuator 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 also later rotary valves) 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 systems 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 channelled through small-bore copper tubing. The backs of control panels were a mass of tubes skilfully arranged by control systems craftsmen into symmetrical layers of carefully laid pipe. However, with the advent of computers and 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.

Figure 1: An example of an electric control valve actuator.

The other great benefit of this change in technology was the elimination of the labour-intensive maintenance of the pneumatic control system. Filters, regulators, lubricators and a multitude of small pilot control valves were eliminated and replaced with PLCs and their final element controllers. In place of the 3-15 psi pressure signal, a 4-20 mA current control signal was adopted as a global standard.

Although the control air signal has been superseded by a control current signal in most process plants, the power to move many of these process control valves has remained as compressed air.

Having used instrument air as the control medium in the past, it was perceived by many that there were some benefits in retaining instrument air as a power medium. The air can be used to transport energy from one place to another to operate remotely positioned control valves, dampers and other equipment. In the evolution of pneumatic process control, the instrument air was upgraded from the 3-15 psi supply to a nominal 80 psi. This allows greater forces to be generated by smaller pistons or diaphragms. The result is that pneumatic spring diaphragm and piston type actuators have been the default standard for positioning control valves for many years.

The simple pneumatic positioner has evolved, from the basic functionality of controlling applied high-pressure air using a low-pressure signal, to the smart positioner of today that not only directs high-pressure air to the valve actuator, but also is able to gather information at the various pressure positions within the actuator assembly to provide diagnostic information, which can be transmitted back over the 4-20 mA signal using a communications protocol such as HART.

This method is currently the de facto standard for the majority of linear and rotary process control valves in almost every industry, from oil and gas production through power generation to the chemical, petrochemical and many other process industries.

Why electric-powered actuation may be better

However, the de facto standard of coupling instrument air with a smart positioner may not be the best solution for every application. Just as electronics have taken over control signal technology, electrically powered actuators now offer a viable alternative to those using the pneumatic spring diaphragm and piston design.

Specifically, there are many drawbacks to using compressed air as a power medium. For years, process engineers have had to engage in complex workarounds to overcome them. The drawbacks vary in degrees depending on particular applications.

Generally speaking, taking electrical energy, converting it to compressed air then transporting it via a filter regulator and lengths of tubing with fittings, before directing it into a chamber for expansion is an incredibly inefficient method of moving power from one point to another. The inefficiencies of compression and the friction losses in transmission can easily account for a net loss of 50% of the applied energy. This can be directly compared to the much more energy-efficient method of transmitting the power via electricity and translating that electrical energy to kinetic energy in a motor located directly at the process control valve. In effect, the electric motor drive has been transferred from the compressor to the actuator, eliminating all the intermediate conversions and transmissions, together with their attendant losses.

When considering the large number of process control valves in a plant and compounding this with the constant movement of process control valves, the elimination of compressed air in a plant can be significant and can result in a much more productive and cost-effective operation.

In addition, plant reliability and the associated availability are significant factors to be considered. Air supplies require proactive maintenance to ensure that moisture, dirt and other contaminants to do not accumulate in the air lines and cause the small orifices in smart controllers to plug up. This proactive maintenance has a significant cost, which should be included in every objective analysis.

Although many processes are enclosed in buildings that protect the valves and instruments controlling the process, this is not always the case. There are several examples where valves are located in open areas and are vulnerable to temperature swings that can drop below the freezing point of water. This affects not just European and North American plants, but also Asian plants such as the many new process plants being built in China, Japan, Korea and other areas.

A drop in the temperature below freezing point can cause airlines to freeze and incapacitate the pneumatic control valve actuator or controller.

Examples of why electric control valve actuators are often the best solution

In Halifax, Nova Scotia, a refinery technician recounted that every year he needed to replace frozen airlines because they had ruptured. After the hoses ruptured, certain modulating control valves could only be operated by hand. This, of course, defeats the basic reason for investing in automatic process control.

Another example is from a power plant in New Hampshire which recently replaced all of its spring diaphragm control valve actuators on fuel control due to the effects of reduced temperatures. Low temperatures not only had an adverse effect on the actuators, but also on the viscosity of the medium being controlled and the friction effect on the valve seats. That is, the valve became very difficult to control due to the stick slip effect, causing ‘overshoot’ of the desired set point of the valve.

In high-altitude mining applications, such as those found in Chile and Peru, reduced temperatures and high altitude combined to make power air supplies extremely costly. Maintenance and running costs were problematically high. In such an environment, freezing airlines were also an ongoing problem which caused valve actuator failures with attendant loss of production capability.

Figure 2: Typical stick slip action in a control valve.

There are some instances where an air supply is not required for anything other than controlling a single process control valve actuator. In these circumstances a small air compressor complete with air set needs to be provided, taking up space, weight and cost. There are many small package boilers, for example, that require a steam control valve. Quite often this requirement also includes the need for a fail-to-position capability. The traditional method of doing this would be to use a spring diaphragm actuator where a loss of air or a shutdown signal would vent the air to allow the spring to close or open the valve, depending on the requirements of the process.

Electric actuators are capable of storing electric energy such that a loss of electric power can trigger a fail-to-position, which has been pre-programmed into the actuator. Furthermore, because of the greater degree of control available with an electric actuator, a preset position - either fully open or fully closed or anywhere in between - can be programmed easily into the actuator should the power or the control signal be lost. A different failure position could be programmed dependent on either loss of power or loss of control signal.

Finally, compressed air is by definition a resilient medium. In fact, some cars even use it as a method of suspension. Because compressed air acts like a spring, then pneumatic control valve actuators do not often have the stiffness required for precise process control.

For example, consider a globe valve with a high degree of friction in its stem packing or a ball valve with a high degree of friction on its seat. In either case, the static friction is quite high, requiring an excessive amount of air pressure in order to initiate movement in the valve. Once the valve moves, static friction is replaced by dynamic friction, which is invariably lower. This causes the resistance to the excessive air pressure to drop. The result is the valve runs away with itself and often overshoots the desired set point, causing a correction to be made resulting in oscillation around the set point.

Figure 3: Graphic description of stick slip action.

This problem is eliminated with an electric control valve actuator due to the higher stiffness and controllability of today’s electric drive trains and the advent of sophisticated dual sensor technology in the actuator.

Electric control valve actuators: a summary of features and benefits

Electric control valve actuators can provide superior control performance, are easy to set up and they eliminate the need for troublesome power air supplies and all their problems.

They are available in linear and quarter-turn actions and are suitable for a wide range of control valve applications throughout a wide range of process industry applications including power generation, pipeline and gas installations, petrochemical and refinery facilities, mining and many other process applications.

The new actuators eliminate the need for costly air supplies and are easily integrated into most process control environments, including those that use HART and FOUNDATION Fieldbus protocols.