Control Valve Actuators Impact on Control and Variability

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.

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.

CVAs: Control Valve Actuators

A control valve actuator (CVA) 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.

Carefully consider the specific demands of your process

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.

Once motion is initiated, an electric actuator is restricted by the maximum speed of its motor, whereas a pneumatic actuator can move as quickly as the air can drive the piston or diaphragm. For smaller incremental changes in demand, the electric actuator's reaction time is significantly faster than an equivalent pneumatic actuator with a nominal dead time. Conversely, for large swings in demand signal, pneumatic actuators have the advantage of faster stroking speeds over longer distances.

Figure 1. Ideal control would have the set point very close to the limit of quality or capacity. The graph at the left showing large process variability, such as that associated with traditional pneumatic control-valve actuators, makes this difficult. On the other hand, the new generation of electric control-valve actuators reduces the variability, so that the highest quality and capacity can be achieved.

Other important considerations in actuator selection are resolution, repeatability, and precision.

Resolution is defined as the minimum change in demand signal that results in a change in output when moving in the same direction. This is an important measurement as it determines how finely the control valve can be positioned to affect the process.

Repeatability is the closeness of agreement of a number of consecutive measurements of the output for the same value of inputs when approaching from the same direction. The combination of resolution and repeatability impacts the precision in which the control valve can be positioned.

The benefit of precise control on process variability is well documented: the greater the precision, the greater the control that can be exerted over the process. That is, a greater precision can significantly reduce process variability, which can have a positive impact on the quality of the product produced as well as the production capacity the plant can achieve. These benefits accrue from using a more precisely controlled valve.

Actuators that are able to deliver high repeatability and high resolution are therefore more valuable to the process than actuators that do not have this capability.

New electric actuators can significantly improve traditional performance

While some pneumatic valve positioners catalog resolutions on the order of 0.1% that claim can be misleading. That is, once those positioners are coupled to pneumatic valve actuators, feedback linkage connections and other external factors diminish the resolution. Certain new electric actuators, however, combine the position feedback as an integral component of the actuator and are thus able to achieve genuine performance figures in the region of 0.1% repeatability and resolution.

It should also be noted that the nature and application of control valves often conspire to diminish the dynamic performance of the valve. Valve packing friction in globe valves or seat friction in ball valves can cause problems when trying to dynamically position a valve to a new set point in a minimum time.

Because air is a compressible medium, it has difficulty in providing precise control, especially when valves are "sticky." The static friction in the valve requires excess air to be introduced to the actuator in order to break the valve from the seat or the packing friction. Once the valve has broken free, the dynamic friction being less causes the excess air to overshoot the desired set point causing an oscillation.

The oscillation adversely affects process variability. Similarly, pneumatic actuators when mounted on globe valves tend to exhibit resilience under the action of a pressure spike or surge in the pipeline media.

Electric actuators, on the other hand, with their mechanical drive train are inherently stiffer and are able to hold the set point better. That means that under surge or cavitation conditions, the valve will hold its position and maintain the process set point.

Figure 2. Block diagram of control circuit.

Technological advances: Times Have Changed

Rotork CVA all-electric control valve
actuators (available in rotary and
linear actions) are pictured above.

Currently, the industry standard for control valve actuators (CVA) is the spring diaphragm unit with a digital positioner. Because of its simplicity, the spring diaphragm actuator is found in virtually every type of application and is simple, robust, and easily provides a fail open/close capability. Digital positioners have become sophisticated enough to overcome, to a certain degree, the problems of a stick slip and overshoot, but can be extremely demanding in terms of calibration, set up, and maintenance.

Until recently, the control of electric actuators was inferior to spring-diaphragm control valve actuators (CVA), either the electric drive was too slow to provide the response required, or the motor and drive train inertia of high-speed actuators precluded precise positioning.

New control technology has overcome these problems by sensing not just the output position of the actuator but also the motor position and speed.

The block diagram of the control circuit (see Figure 2) shows that the output of the actuator is fed back and compared to the demand position signal. The resulting error signal is fed into the motor speed profile. The actual speed of the motor is then compared to the demand speed and that error signal in turn is fed into the motor controller.

The accuracy of the sensors coupled with the control logic circuit can result in the elimination of overshoot normally experienced on "sticky valves." By eliminating overshoot, process variability is significantly reduced and many significant benefits result.

Electric Control Valve Actuators (CVA)

A fertilizer plant, which produces a
full line of nitrogen fertilizers and
industrial products, purchased five
CVA electric actuators so they didn’t
have to depend on instrument air. By
usingall-electric control-valve
actuators, it means that moisture and
particulate contamination from
instrument air is no longer a worry.
In the past, they had experienced
problems with check valves and
leakage with air receivers.
Since the installation of the CVA
actuators in 2009, the all-electric
actuators have performed extremely
well, and unlike pneumatic actuators,
there is no drift in the actuator
calibration, so accurate control is
ensured without continuous
maintenance intervention.

Also, new electric control valve actuators (CVA) put to rest a common perception that electric actuators are susceptible to mechanical wear when used for constant modulation. With careful gear design and material selection the drive train of the new generation of control valves actuators can achieve many millions of cycles, even under the full-rated load of the actuator. In fact, some tests have shown that over 200 million cycles can be achieved even at elevated loads.

Another obstacle when comparing functionality of conventional spring diaphragm actuators with electric actuators is the ability to fail to an open/close position. Recent developments in electric actuators have utilized stored energy in super capacitors. The electrical energy stored in the capacitor can generate a high power density enabling the electric motor and drive train to position the actuator not only to an open or close position but to any selected intermediate position. This versatility can deliver additional benefits to some processes where complete shut down of the process could be a problem.

Rotork CVA all-electric control-valve actuators are ideal for a wide range of applications that demand precise operation, reliability, and freedom from the encumbrances associated with instrument air.

Below are just a few examples of applications where CVA actuators are currently being used.

• Fuel terminals
• Power stations
• Chemical plants
• Offshore oil and gas production platforms
• Oil and gas wells
• Manufacturing facilities
• Water and wastewater treatment plants
• HVAC plant

There are a lot more

Summary

The technological developments incorporated in the new generation of electric actuators offer many significant functional and performance advantages over conventional control valve actuators (CVA).

Electric actuators can deliver improved process variability due to the precision of their performance. Reaction to changes in set point and maintenance of the set point, even when there are upsets in the process, can significantly reduce process variability.

In addition, the new generation electric actuators are easily installed and integrated into Bus systems such as Hart, Foundation Fieldbus, and Profibus.

Another significant benefit is that electric control valve actuators (CVA) eliminate the sometimes troublesome instrument air supply requirement.

Also, the new electric control valve actuators (CVA) have an integral thrust or torque sensor (depending on whether the actuator is a linear or quarter-turn output). The measurement of thrust and torque is invaluable when combined with simultaneous reading of position. These readings are retained in the control valve actuator's (CVA) built-in data logger and can be downloaded for detailed analysis.

Because thrust measurement or torque measurement is a direct reading - not one that is derived from the pressure over a piston or diaphragm area - the measurement is immediate and accurate. That means the changes in the characteristic of the valve can be monitored to predict when maintenance related to packing or seats is required.

Finally, the human machine interfaces (HMI) that are available for use with these new electric control valve actuators (CVA) provide easy access to important performance and analytical data. Simple and user-friendly HMI tools such as PDAs and laptop computers can communicate wirelessly via BlueToothTM to the actuator.

All of the settings, configurations, calibration, data logging, and analysis tools are available to facilitate easy installation of the control valve actuator (CVA) and to provide the information appropriate to the modern plant's asset management systems.

You can obtain a document that will help you calculate the cost savings of electric vs. air power by contacting by contacting Rotork Controls.