Providing superior control performance

Electric control valve actuators eliminate the problems of compressed air as a power medium.

Today, a major technological advance is available to help control valve users avoid many of the problems and inefficiencies associated with compressed air. The new solution, using electric power, is appropriate and cost-effective for a wide variety of control valve applications in most process environments, especially where users experience problems with frozen air hoses, lack of process precision and stick-slip. In many instances, this technologically advanced equipment can substantially increase the output and efficiency of the process whilst helping to reduce maintenance and operating costs.

Before discussing today’s latest technology, it’s beneficial to understand how control valve actuation has evolved. Decades ago, process control valves were controlled by varying the pressure of the air supply to the actuator between 3 and 15PSI. By balancing this air pressure against an opposing spring, a closed valve position would relate to 3PSI and the open valve position to 15PSI. Entire plants were controlled by compressed air channeled through small-bore copper tubing. However, with the advent of computers and PLCs, the days of the 3-15PSI control signals were numbered. They were replaced by electronic signals carried on much lighter-duty copper wire. This was a revolution in control technology, bringing with it cost savings as well as improved control capabilities.

Another benefit of this change was the elimination of labor-intensive maintenance of the pneumatic control system. Filters, regulators, lubricators, and a multitude of small pilot control valves were replaced with PLCs and their final element controllers. In place of the 3-15PSI pressure signal, a 4-20milliamp current control signal was adopted. However, having used instrument air as the control medium in the past, it was perceived that there were benefits in retaining it as the power medium, so in time the instrument air was upgraded from 3-15PSI to 80PSI, allowing greater forces to be generated by smaller pistons or diaphragms. The resultant pneumatic spring diaphragm and piston actuators have become the default standard for positioning control valves.

A “Positioner” is used to translate the control signal from 4-20milliamps to high pressure acting on the diaphragm. The simple pneumatic positioner has evolved from the basic functionality of controlling high pressure air using a low pressure signal to today’s smart positioner, which gathers information to provide diagnostic information that can be transmitted back over the 4-20milliamp signal using a communications protocol such as Hart. This method is currently the standard for most process control valves.

Why electric actuation may be better
However, this may not be the best solution for every application. Just as electronics have usurped control signal technology, electric actuators now offer a viable alternative to pneumatic spring diaphragm and piston designs. There are many drawbacks to using compressed air as a power medium. Converting electrical energy into compressed air, transporting it via a filter regulator and lengths of tubing before directing it into a chamber for expansion is an inefficient method of moving power from one point to another.The inefficiencies of compression and friction losses in transmission can account for a 50% net loss of the applied energy. Compare this with the much more energy efficient method of transmitting the power via electricity and converting it into kinetic energy in a motor located directly at the valve. Basically, the electric motor drive has been transferred from the compressor to the actuator, eliminating the intermediate conversions and transmissions and their attendant losses.

When considering the constant movement of large numbers of process control valves in a plant, the elimination of compressed air can be significant and result in a much more productive and cost-effective operation. In addition, plant reliability and availability is a significant factor. Air supplies require proactive and expensive maintenance to ensure that moisture and contaminants to do not accumulate and clog the small orifices in smart controllers.

Although many processes are in buildings that protect the valves and instruments controlling the process, equally often valves are located in open areas and are vulnerable to temperature swings that can drop below the freezing point of water. Freezing temperatures can cause air lines to freeze and incapacitate pneumatic control valve actuators.

Case studies: electric control valve actuators
In Nova Scotia a refinery needed to replace frozen air lines every year because they had ruptured. After the hoses ruptured, control valves could only be manually operated, which defeats the object of investing in automatic process control. In another example, a power plant in New Hampshire recently replaced all of its spring diaphragm actuators on fuel control valves 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. The valves became very difficult to control due to the stick-slip effect, causing “over shoot” of the desired positional set points.

In Chilean and Peruvian mining applications, reduced temperatures and high altitudes combined to make air supply maintenance and running costs extremely costly. In such environments, freezing air lines were also an ongoing problem causing failures and loss of production.

There are some instances where an air supply is only required for a single control valve actuator, so a small air compressor system is needed, taking up space, weight and cost. Examples include small package boilers that require a steam control valve, often with a fail to position capability. The traditional method of doing this is to use a spring diaphragm actuator where a shutdown signal vents the air to allow the spring to close or open the valve.

New technology electric actuators are capable of storing electric energy so that a loss of electric power can trigger a pre-programmable fail to position. Furthermore, because of the greater degree of control available with an electric actuator, a preset failsafe position – fully open, fully closed or anywhere in between – can be easily programmed. A different failure position can be programmed for either loss of power or loss of control signal.

Finally, compressed air is by definition a resilient, or springy, medium. Pneumatic control valve actuators therefore do not always have the stiffness required for precise process control. For example, with a globe valve with high friction in its stem packing or a ball valve with high friction on its seat, the static friction requires an excessive amount of air pressure to initiate valve movement. 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 valve runs away with itself and often overshoots, causing a correction to be made resulting in oscillation around the set point. This problem is eliminated with electric actuators due to the higher stiffness and controllability of today’s electric drive trains and the advent of sophisticated dual sensor technology.

Summary of features and benefits
Electric control valve actuators can provide superior control performance, are easy to programme and eliminate the need for problematic air supplies.They are suitable for a wide range of linear and quarter-turn process industry applications including power generation, pipeline and gas installations, petrochemicals and mining. They are easily integrated into process control protocols, including Hart and Foundation Fieldbus.

Rotork’s CVA actuators, for example, can be specified for single-phase AC or DC electrical supplies and provide extremely precise control valve operation, with repeatability and resolution performance at <0.1% of full scale. In addition, they utilise wireless Bluetooth communication technology for quick and easy set-up and adjustment. Rotork CVA actuators incorporate a data logger, which provides an extensive record of operational and maintenance-related data including valve torque profiles, dwell times and relevant statistical information. They also utilise built-in super-capacitors, providing an advanced, programmable method for fail-to-position protection.

Article by Chris Warnett, Sales and Marketing Director, Rotork Process Controls


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