Working Principle, Operation, and Maintenance of Globe Valves

Globe valves are widely used in fluid pipelines for regulating flow. Their primary function is to control the flow rate of fluids by adjusting the position of the valve plug. Typically, globe valves have a T-shaped body design, making them suitable for installation in straight pipelines. This design allows the fluid to enter through the center of the valve, make two 90-degree turns, and exit through the valve seat. The position of the valve plug determines the flow path, making the design and operation of the valve plug crucial for stable valve performance.

Flow Characteristics of Globe Valves

The flow characteristics of a globe valve are key to determining its flow control capabilities. These characteristics affect how the valve responds at various opening positions, making it essential to understand the different flow behaviors when designing and applying globe valves.

1. Flow Direction and Design Considerations

The flow direction in a globe valve is generally determined by the manufacturer or operating conditions. In most cases, the fluid flows from below the valve plug. As the fluid flows through the valve, it makes two 90-degree turns, causing a change in flow velocity and a drop in pressure at the valve seat. When the valve is closed, changes in fluid pressure and flow rate are minimal, making it easier to close the valve. Because the fluid pushes on the bottom of the plug, opening the valve is also relatively easy. Therefore, the design where the fluid flows from below the valve plug is often considered ideal.

2. Impact of Flow Characteristics on Valve Performance

Manual globe valves can be adjusted to different flow characteristics, such as percentage, linear, and quick-opening types. These characteristics affect the flow rate at various positions, usually represented by the flow coefficient (CV). For example, the percentage characteristic causes a nonlinear flow change, while the linear characteristic provides more uniform flow changes. By manually adjusting the valve wheel, users can fine-tune the flow rate to meet specific requirements under different operating conditions.
When the valve plug is in the throttling position, it slightly extends into the valve seat, limiting the flow rate. As the plug retracts from the seat, the flow increases; conversely, as the plug extends further into the seat, the flow decreases. During fluid passage through the valve seat, the flow rate increases, causing a pressure drop. When the fluid enters the lower part of the valve, the flow area increases, pressure recovers, and the flow slows down.

Design Principles of Globe Valves

Globe valves are designed to regulate fluid flow by adjusting the contact between the valve plug and the valve seat. The design focuses on controlling fluid flow, ensuring sealing performance, and ensuring ease of operation. The following are the key design principles for globe valves.

1. Importance of the Valve Body Workbench Area

The valve body workbench area is a critical design consideration. If the area is too narrow, especially behind the valve plug, the fluid velocity could become too high, leading to noise, wear, or even turbulence. Moreover, the force on the valve plug must be evenly distributed to prevent the plug from tilting or bending, which could affect the sealing performance and long-term operation. Therefore, a properly designed valve body workbench area is essential for enhancing the valve's stability and reliability.

2. Sealing System Design

Globe valves rely on end-face joint gaskets for static sealing to prevent fluid leakage through the valve body. Additionally, the packing box on the valve bonnet provides dynamic sealing to prevent leakage through the valve stem. During valve closure, the contact between the valve plug and valve seat must be tight to prevent any fluid leakage. When the valve is fully open, the entire valve seat area is available for fluid flow, maximizing flow capacity.

Operation and Maintenance of Globe Valves

Proper operation and maintenance of globe valves are essential for ensuring their long-term stable performance. Correct operation can extend the life of the valve and improve system efficiency, while regular maintenance helps prevent valve failures and ensures precise fluid control. The following are key points for operating and maintaining globe valves.

1. Proper Valve Operation

When operating a globe valve, it is essential to open or close the valve smoothly, avoiding excessive force. High-pressure globe valves are designed to withstand shock loads, but for standard globe valves, excessive force can damage the internal components. After fully opening the valve, the valve wheel should be turned slightly counterclockwise to ensure the valve wheel threads are tightened, preventing damage from loosening.

2. Handling of Pipelines During Initial Use

When a pipeline is first put into service, it may contain a lot of debris. In such cases, the globe valve should be opened slightly to allow high-speed fluid flow to flush out the debris. Rapid closing should be avoided to prevent damage to the sealing surfaces. This process should be repeated multiple times until the debris is completely cleared.

3. Maintenance of Normally Open Globe Valves

The sealing surfaces of normally open globe valves may accumulate dirt. Before closing the valve, the valve seat and plug should be cleaned using the same method. When closing the valve, smooth manual operation of the valve wheel is necessary to prevent damage to the sealing surfaces.

4. Replacing Damaged Parts

If the handwheel or handle is damaged, it should be replaced promptly. Do not use adjustable wrenches to replace the handwheel, as this could damage the valve stem, affecting its opening and closing function. If operation becomes difficult, it may be due to tight packing, in which case the packing should be loosened appropriately. If the valve stem is bent, maintenance personnel should be notified for repairs.

5. Effect of Temperature Changes on Valves

In some cases, after closing a globe valve, temperature changes can cause the valve components to shrink. In such cases, operators should reclose the valve to ensure that the sealing surfaces are completely tight, preventing any medium from leaking through gaps, which could lead to erosion.

6. Special Operating Conditions

If the valve is difficult to open due to thermal expansion after closing, the valve bonnet's threads can be slightly loosened to relieve stress on the valve stem. The valve can then be reopened to prevent damage to the stem or other components.

Conclusion

The design, operation, and maintenance of globe valves are closely related. Through proper design and adherence to operational standards, globe valves can perform stably under various conditions. Understanding the working principle, flow characteristics, and maintenance practices of globe valves helps extend their lifespan, reduce failure rates, and improve productivity. Regular inspections and timely repairs ensure the reliability and safety of globe valves in long-term use.