How to Get the Most from Your Control Valves
There’s a lot to know about valves in order to select the right ones and use them effectively.
For example, actuators are often specified based on the forces required at room temperature for valves with loose packing. In the actual application, packing is likely to be tighter, producing higher friction. Higher operating temperatures will likely also increase the demand on the actuator. As a result, actuators are often under-sized.
Also, the valve’s test data may look great, McMillan said, but if the test shows performance only at positions 40% open or greater, it’s missing the often-problematic behavior nearer to the closed position. Likewise, response time tests done for large steps may mask problems that occur at smaller steps of 1% or less.
Important mechanical characteristics such as shaft windup and backlash are not usually measured, so suppliers do not provide them to end users.
VALVE/ACTUATOR CHARACTERISTICS TO KNOW
Resolution is the minimum change in the stem position of a control valve. This limits the control system’s ability to position the valve exactly where it needs to be. A lower resolution valve/actuator can continually oscillate around the desired setting if the adjustment increment is so large that the flow alternates between too high and too low. Some users do not thoroughly understand the concept of resolution, though it is an important factor in designing a stable control system.
Stiction is a combination of “stick” and “friction.” In a valve, especially at or near its closed position, a control signal for an increment of movement is followed by force from the actuator that needs to overcome stiction before the valve can respond.
Backlash, a characteristic of valves and other mechanical devices, happens because of clearances between mating parts. When a reversal of direction occurs, no movement happens until all the clearances are taken up.
“Whenever we reverse direction, we need to go through a deadband due to backlash,” said McMillan (see graph). It tends to be highest near the closed position, especially for rotary valves, he said. “Unless you specify otherwise, suppliers will not give you the deadband and resolution near the closed position.”
Piston actuators for rotary valves are converting a linear motion to a rotary motion to actuate the valve. Inherent in the design is a number of mechanical connections (pinned joints, rack and pinion or slots) that have clearances, resulting in more backlash than designs such as diaphragm actuators that have fewer connections.
Deadband is sometimes confused with hysteresis, which is the phenomenon of different valve positions resulting from the same signal depending on whether the valve is being opened or closed. In a control valve, this is a much smaller effect than deadband, McMillan said. Hysteresis is an important consideration in some places, but not in valves, he said, where it typically runs less than 0.1%.
SELECTING THE BEST VALVE AND ACTUATOR FOR THE JOB
Sliding stem (globe) valves generally have the least backlash and stiction and provide a wider throttling range than rotary valves. However, they may not be suitable for sticky fluids that can increase friction or for handling slurries. Also, they may be costly in larger sizes.
Diaphragm actuators offer a number of advantages. They typically have less backlash and generally have a resolution of 0.1%, in comparison to 1% for piston actuators. To achieve more thrust, high pressure diaphragm actuators are now available that can operate at up to 90 psig (621 kpa); for traditional diaphragm actuators the maximum air pressure is 30 psig (207 kpa).
A splined, short-shaft connection to rotary valves gives more precise response with considerably less windup and/or backlash than other options.
Segmented V-notch ball valves designed for control valve service have lower torque requirements (and friction) than other rotary valves and improved flow characteristics at low flow. Using a splined shaft connection reduces backlash.
Just any V-notch ball valve isn’t necessarily the right one for the job, however. McMillan recalled on one occasion recommending this type of valve for control applications. Later, he received an email from someone who said his V-notch ball valve didn’t work right. His valve had little flow until it was 15% open and then the flow jumped way up. Since then, McMillan said, he is careful to note that users must know the design purpose of the valve under consideration: throttling or on/off (isolation).
McMillan offered guidelines on valve and actuator selection.
- Use on/off and isolation valves for sequences and safety instrumented systems, and use low stiction and low backlash throttling valves with smart positioners for loops. Many loops require both types of valves.
- If size and process conditions permit, preferentially use sliding stem (globe) valves with diaphragm actuators and ultra-low friction (ULF) packing.
- Make sure the valve drop is at least 25% of maximum system drop
- Make sure the actuator is sized for 150% of maximum torque and thrust.
In order to have all the information necessary for effective control system design, McMillan recommended adding a number of requirements to control valve specifications, including
- Resolution and deadband less than 0.2% to 0.5% at minimum flow position
- Stem position feedback (readback error) less than the unit’s resolution
- Small step (resolution +0.1%) 86% response time: 1 to 5 seconds (The 86% response time is the time for a valve to reach 86% of its final response.)
- Large step (e.g., 20%) 86% response time: 1 to 20 seconds
- Minimum flow valve position greater than 5%
See the complete webinar for more detailed discussion of other aspects of valves and their use, including the effect on flow response of changing various control parameters.
Barbara Donohue is web editor of VALVE Magazine.
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