What are Control Valves?
Control valves are used in many processes to control flow, pressure, temperature or other variables. The type of valve used will depend on the size of the pipe, the overall pressure that the system operates, the flowing media, process conditions, and other factors. There is also a balance between the cost of the valve and the cost benefits associated with tighter control.
There are many types of valves including:
Butterfly valves – These valves are typically lower cost and use less space than other valves. These valves are good for control but other valves can provide more precise control. They are available in large diameters.
Globe valves – These valves provide very precise control and are available with a wide variety of flow characteristics. The downside is that these valves tend to be more expensive that other types. These are also not well suited for anything but clean fluids
Ball valves – There are 2 main styles of ball valves, full and segmented. Typically the full ball is used for on/off service but these can also be used for modulating service. The segmented ball valve was originally designed for control of slurry flows. These offer high flow capacity and wide rangeability. The cost of these valves falls between that of the butterfly and globe valves.
Gate valves – These are typically used for shutoff and isolation. Occasionally they are used for throttling service but they are not well suited to this type of service.
Pinch valves – These are low cost valves usually used with abrasive or corrosive fluids. They are typically used for open/close service but can be used for modulation. These are typically operated using air pressure to collapse the sleeve.
Plug valves -These are similar to ball valves but are less expensive due to their design. They are used where there are high flow rates at low pressure drops.
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Valves can be manually operated or automated. Automation is typically done with rotary actuators for the butterfly, plug and ball valves. Linear actuators are typically used on the globe valves.
Valves are tested for flow rate versus valve opening using a constant pressure drop to determine the valve’s inherent flow characteristic. These are usually categorized as equal percentage, linear or quick opening. Most industrial processes use equal percentage type valves due to how the process varies as the flow varies. Butterfly and ball valves are most often equal percentage type while globe valves can be modified to produce any flow characteristic.
Selection of the optimal valve for your application is just part of the equation. Installing a valve actuator that takes advantage of the full capabilities of the valve is just as, if not more, important. Even if you are utilizing the highest quality valves poor valve actuator performance can eliminate the effectiveness of your process control. When selecting a valve actuator for your valve, you should keep some valve actuator perfomance characteristics in mind.
The key valve actuator performance characteristics are as follows:
- Precise, repeatable positioning typically better than 0.15% of span.
- The ability to start and stop instantaneously without dead time or position overshoot.
- Continuous duty rating without limitations on the number of starts per minute.
- Perform consistently and unaffected by load.
- Rugged industrial design capable of operating in difficult environments without an effect on performance.
- Minimal periodic maintenance required.
A valve actuator designed with these characteristics provides two extremely important advantages:
- An ability to follow the demand signal from the controller precisely and instantly. This ensures that the actuator responds exactly as directed by the controller. Thus, the actuator is not the limiting factor in the control loop and the controller can function to its optimal levels.
- A high degree of maintenance-free reliability. An actuator designed to function as outlined above by default is more rugged than typical actuators. By design, then, it is capable of a much higher degree of reliability.
When it is necessary to improve process control performance, the first step is to improve valve actuator performance. Beck electric actuators provide the necessary control and reliability that is required for many valve applications. The Group 11 quarter-turn valve actuator product line is often found on low to medium torque applications (20 lb-ft to 1,800 lb-ft). Group 14 linear valve actuators and Group 29 linear valve actuators are utilized for linear valve applications (up to 6,100 lbs thrust and 4-1/2″ stroke), and Group 31 compact rotary valve actuators are utilized on low torque applications (15 lb-ft to 30 lb-ft).