With so many options available, choosing the right control valve for a specific application can be a daunting task. My article in the March/April issue of Control Engineering, titled “Choosing the best control valve style for your application,” provides an overview of typical valve body styles to help end users make the right choice for their application.
Application considerations
Deep application understanding should guide all aspects of valve selection, from construction materials to body style. Pressure drops, dynamic range, valve flow characteristics, process fluid properties, and piping arrangement are all factors that deserve careful thought.
Types of control valves
Control valve body styles come in two major categories: sliding stem control valves where the stem rises and falls to throttle flow and rotary valves where the shaft turns (usually 90 degrees) to throttle flow. Each style has inherent strengths and limitations that play into valve selection.
Butterfly rotary valves control flow by rotating a flat disc. They are inexpensive, but they offer limited turndown and control range compared to other styles, and they are not suited for operation in high pressure drop, flashing, and/or cavitation conditions.
Ball control valves rotate a full ball with a hole or a segment of a ball to throttle flow.
Like butterfly valves, most ball valves have limited turndown, but some models have anti-cavitation and low noise options. V-ball control valves use a ball with a V-shaped notch to better modulate low flows and provide significantly higher turndown.
In a sliding stem control valve, the throttling element moves vertically to control the flow of the process fluid
Sliding stem valves often have higher throttling resolution than rotary valves, and they are designed to handle very high pressure drops, noise, flashing, and cavitation.
Selection criteria
Handling pressure drops is one of the key functions of control valves. High recovery valves, including rotary valves, experience very low internal pressures, which can lead to cavitation and noise, especially in applications with a large pressure drop.
This graph compares the vena contracta pressures of a high recovery (ball, butterfly) valve versus a low recovery (globe) valve for the same process conditions. Notice the vena contracta pressure in a high recovery valve is significantly lower, increasing the likelihood of cavitation
Adequate control valve gain is crucial for effective throttling and tight process control. Butterfly valves have an ideal gain between about 25 to 60 degrees. Ball valves have a broader range, especially v-ball valves, which can throttle effectively up to about 70 to 75 degrees. Finally, sliding stem valves provide a wide throttling range with optimal gain.
Choosing between sliding stem and rotary valves
Rotary valve designs generally have a limited range and don’t handle cavitation or high noise conditions well, but they are an inexpensive and effective option for basic flow control.
For precise control over a broad dynamic range or in cases of very high pressure drops, investing in a sliding stem valve is likely worthwhile.
Other considerations
Well-designed valves should incorporate a high-quality actuator, and a high-resolution positioner with tightly coupled or linkage-less feedback. Rotary valves should have an oversized shaft with spline connections to eliminate slippage and backlash, while sliding stem valves should have a stiff stem adequately supported along its length.
Make an informed valve selection
Selecting the right valve for a specific application requires a full understanding of application requirements and knowledge of the capabilities and limitations of each valve style and type. Valve vendors can help end users choose the right solution for their needs, leading to greater controllability, reliability, and performance.
