Addressing Issues with Control Valve Small Opening Operation

Control valves play a crucial role in many industrial systems. They help regulate things like flow, pressure, and temperature. However, their performance can be heavily influenced by operating conditions, especially when they are working at small openings. When valves are set to small openings, they experience rapid changes in flow, which can lead to several issues. These problems not only affect how well the valve controls the system, but they can also shorten the valve's lifespan. Understanding the risks of running control valves at small openings and knowing how to optimize their working conditions can really improve both system stability and valve performance.

Common Issues with Small Opening Operation

When control valves work at small openings, they can still adjust flow to some extent, but several problems often arise. These issues can affect the valve's ability to control properly, and in some cases, they can even cause the valve to wear out faster. Here are the main problems that occur when control valves operate at small openings.

1. Severe Flow Resistance, Velocity, and Pressure Fluctuations

At small openings, the throttling gap becomes very small, which causes flow rates and velocity to fluctuate a lot. This results in dramatic pressure loss and velocity changes as the fluid moves through the valve, which leads to:

Maximum Throttling Resistance: When the valve is open just a little, the flow is restricted, and the resistance at the throttling orifice becomes the highest, which makes the flow velocity increase.

High Velocity and Erosion: At high flow velocities, the fluid can cause significant wear on the valve plug and sealing surfaces. This leads to erosion, which affects the valve's ability to seal properly and reduces control accuracy.

These changes in flow and pressure cause more wear on the valve and significantly shorten its lifespan.

2. Instability and Potential for Oscillation

When control valves are working at small openings, the pressure fluctuations (both dynamic and static) can cause the valve to exceed its design limits, leading to unstable operation. If the pressure fluctuations go beyond what the valve can handle, it may start to oscillate.

Oscillations: Oscillations can affect the overall stability of the system and make it difficult for the valve to maintain precise flow control.

Lower Control Accuracy: When the valve is unstable, the system may fail to maintain the desired operating point. This can lower control accuracy and even disrupt the normal operation of the system.

3. Valve Jumping or Spontaneous Opening/Closing

At small openings, control valves can experience "jumping" when they either shut or open unexpectedly. This happens because the valve plug doesn't make good contact with the seat at small openings.

Imprecise Regulation: The valve plug may not close or open smoothly, causing unstable fluid flow and making it difficult to adjust the flow precisely.

Damage to Sealing Surfaces: The jumping motion can cause the valve plug to hit the sealing surface, leading to wear or damage. This damages the sealing surface, which lowers control accuracy and reduces the valve's lifespan.

4. Damage to the Valve Plug Sealing Surface

When control valves are operating at small openings, the valve plug sealing surface is close to the throttling orifice, which means it gets eroded by the fluid flow. This can result in:

Damage to Sealing Surfaces: Since the flow rate and velocity are higher at small openings, the sealing surface is under greater pressure, making it more prone to wear, corrosion, or fouling.

Decreased Sealing Performance: If the sealing surface gets damaged, the valve's ability to seal properly is reduced, which negatively affects the flow regulation and the system's performance.

5. Valve Types Not Suited for Small Openings

Some types of valves are not designed for small opening operation, like butterfly valves and two-way control valves. These valves tend to have issues like:

Butterfly Valves: At small openings, the unbalanced torque can cause the valve to “jump open” or “jump closed”, which makes it hard to maintain stable control.

Two-Way Control Valves: At small openings, one valve plug may be open while the other is closed, which leads to uneven fluid forces on both plugs. This poor stability can cause oscillations, making smooth regulation difficult.

The Best Operating Conditions for Control Valves

To improve the valve's service life and stability, it's generally best to avoid running control valves at small openings for long periods. Based on experience, the optimal opening for most control valves should be at least 10% to 15%. However, this can vary depending on the type of valve and the specific application.

For special valve types, like high-pressure control valves, two-way control valves, and butterfly valves, the recommended opening should be at least 20% (for linear valves) or 30% (for equal percentage valves). This ensures the valve operates more stably and helps minimize the damage caused by excessive pressure fluctuations.

Ways to Optimize and Increase Valve Opening

To make sure control valves run under optimal conditions and avoid prolonged operation at small openings, it's important to implement effective optimization measures. Here are some common and effective ways to increase the working opening of control valves, improving their lifespan, stability, and control accuracy.

1. Start with a Larger Opening

One way to optimize the operation is to start the valve at a larger opening, like 90%. In this case, cavitation and erosion usually happen at the valve plug's top. As the plug wears, the flow rate gradually increases, and the valve begins to close. This helps extend the life of the valve plug by using it fully, until the sealing surface and the root of the plug are damaged.

Reduce Erosion: With a larger opening, the throttling gap is wider, which helps reduce erosion and significantly improves the valve's lifespan—sometimes by 1 to 5 times.

2. Reduce the Pressure Drop Ratio

The pressure drop ratio in control valves greatly affects their performance. A high pressure drop ratio can cause cavitation, erosion, and energy loss, which reduce the valve's stability. To reduce the pressure drop ratio, you can increase losses in other parts of the system. Here's how:

Increase Losses in Other Parts of the System: By adjusting pipe configurations or adding resistance elements, you can reduce the pressure drop ratio, which lowers the pressure on the valve and minimizes cavitation and erosion.

Add Throttling Devices: You can install orifice plates or other throttling devices downstream of the valve to absorb some of the pressure drop, which lightens the load on the valve.

3. Adjust Valve Size

If the control valve is oversized and tends to run at small openings, you can adjust the valve size. This can be done by:

Switching to a Smaller Valve: If the original valve is too large, you can choose a smaller one. For example, switch from DN32 to DN25.

Replacing the Plug and Seat: If you don't want to replace the whole valve, you can replace the valve plug and seat with smaller diameter components. This can help increase the opening range without needing a full valve replacement.

Conclusion

Using and maintaining control valves properly is key to ensuring system stability and extending equipment lifespan. By avoiding prolonged small opening operation and using optimization methods like increasing valve opening and reducing the pressure drop ratio, you can significantly improve both the valve's service life and its stability. Proper valve selection, precise control of the opening, and optimized system design will improve the accuracy and reliability of control valves, ensuring the efficient operation of industrial systems.