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%