Addressing Pressure Locking Issues in Parallel Gate Valves

Parallel gate valves are widely used in various fluid control systems. However, under certain operating conditions, they may experience a phenomenon known as pressure locking (sometimes referred to as partition pressurization). This issue is primarily caused by thermal expansion of the fluid, especially when the gate valve is closed and the fluid is trapped in the valve body's chamber. As the temperature rises, the fluid expands, which may increase the pressure inside the chamber, potentially exceeding the valve's rated pressure limit and affecting its normal operation. Understanding the causes of pressure locking and implementing effective solutions is critical to ensuring the safety and reliability of the equipment.

Causes of Pressure Locking

Pressure locking occurs when fluid is trapped in the gate valve body's chamber, particularly when the valve is closed. As the surrounding temperature increases, the thermal expansion of the fluid raises the pressure inside the chamber. If the valve remains closed during this heating process, the pressure can exceed the valve's design pressure, causing pressure locking. However, if the gate valve is partially or fully open during heating, fluid can flow freely, preventing pressure buildup.

Pressure locking is particularly problematic in high-temperature or high-pressure applications. For example, in steam, hot water, or other fluid systems where temperature changes are significant, pressure locking can prevent the gate valve from opening or closing correctly, and may even cause damage to the valve body. Therefore, preventing and addressing pressure locking is an important consideration in gate valve design and operation.

Common Solutions to Prevent Pressure Locking

Several methods can be used to prevent pressure locking in parallel gate valves. Below are some common and effective solutions.

1. Small Hole Drilling Method

Drilling a small hole upstream of the gate valve seat is a simple and cost-effective method to prevent pressure locking. The small hole allows the trapped fluid to gradually escape, preventing pressure buildup. Since the valve operates with unidirectional flow, it is essential to clearly mark the inlet and outlet and add flow direction arrows on the valve body. This method is suitable for applications with low fluid volumes or lower pressure requirements, but for higher flow rates or pressures, other solutions may be necessary.

2. Installing a Pressure Balancing Pipe

Another common solution is to install a pressure balancing pipe, which connects the valve body's chamber to the high-pressure side of the system. This allows the pressure in the chamber to balance with the pipeline's high-pressure side, preventing pressure buildup. The advantage of this solution is its simple structure and easy implementation, providing a clear way to address pressure locking. However, this method limits the flexibility of the gate valve in some applications, as it results in unidirectional flow.

3. Adding an Isolation Ball Valve to the Pressure Balancing Pipe

To ensure bidirectional flow when necessary, an isolation ball valve can be added to the pressure balancing pipe. This design allows the ball valve to open and release pressure from the chamber in case of a pressure locking risk, while the valve remains closed during other operations. This method prevents pressure locking while maintaining bidirectional flow capability, making it suitable for a wider range of applications. During pipeline preheating or situations where pressure locking is likely, the ball valve ensures the pressure inside the valve body is released, preventing valve damage.

4. Balancing Bypass Valve

A balancing bypass valve serves as both a bypass device and a means to prevent pressure buildup in the chamber. During normal plant operations, the bypass valve remains closed. However, when the valve is closed, the pipeline is being preheated, or other conditions that might lead to pressure locking arise, the bypass valve should open. This design ensures that pressure locking is avoided while keeping the valve in normal working condition.

Hydraulic Locking and Stem Pressure Increase

Hydraulic locking is another phenomenon that may occur with parallel gate valves, especially in high-pressure feedwater applications. When the valve closes, the pressure in the valve's chamber may exceed the pipeline pressure, causing hydraulic locking. Specifically, when the valve stem opens or closes, water flow in the valve body's chamber is released, but when the valve disc contacts the seat, the flow stops, leading to pressure buildup. In hydraulic systems, if fluid cannot escape after the valve is closed, the pressure can become excessive.
To resolve this issue, the solution is similar to preventing pressure locking, namely using a bypass valve to release pressure from the chamber. Installing a bypass valve ensures that the pressure inside the valve body is released during any conditions that could lead to hydraulic locking, preventing failures caused by excessive pressure.

Electric Actuated Bypass Valves

For situations requiring electric operation of the bypass valve, the bypass valve can be equipped with an independent electric actuator. The actuator can be controlled locally or remotely, but it is important that the bypass valve is not opened solely for heating purposes. Additionally, the bypass valve should be interlocked with the main valve to ensure that the bypass valve only closes when the main valve is fully open. Conversely, the bypass valve should automatically close via interlocking when the main valve is fully open, ensuring proper operation sequencing.
For manual operation of the bypass valve, electrical interlocks can be used to link the operation of the main valve and the bypass valve, ensuring strict adherence to the operating sequence. This interlock system helps prevent the bypass valve from being mistakenly opened when the main valve is not fully open, thereby enhancing operational safety and accuracy.

Conclusion

Pressure locking in parallel gate valves is a common operational challenge, particularly in high-temperature and high-pressure applications. By incorporating appropriate design features and preventive measures, this issue can be effectively avoided. Common solutions such as small hole drilling, pressure balancing pipes, isolation ball valves, and balancing bypass valves can prevent pressure locking in various operational settings. Additionally, hydraulic locking can be mitigated with a well-designed bypass valve. By using these methods, parallel gate valves can maintain their safety, reliability, and flexibility under different working conditions, improving the overall performance of the system.