Among the numerous equipment in nuclear power plants, gate valves are widely used in high-temperature and high-pressure systems, especially in primary loop systems, due to their excellent cutting ability. Its stable operation is crucial for ensuring the safety of nuclear power plants. However, due to the extremely high requirements for sealing in nuclear power systems, the structural characteristics and potential leakage risks of gate valves have also become a focus of attention in the operation and maintenance management of nuclear grade equipment.

1、 Overview of Gate Valve Structure and Classification

The basic structure of a gate valve consists of a valve body, a gate plate, a valve seat, a valve stem, and a valve cover. It uses a gate to lift and lower the pipeline medium in the channel, and is only suitable for "fully open" or "fully closed" states, without flow regulation capability.

According to the gate structure, nuclear power gate valves are mainly divided into two categories:

Wedge gate valve: such as C-type (elastic gate) and K-type (double gate);

Parallel plate gate valve: such as V-shaped (spring loaded) and W-shaped (with wedge structure).

Due to the inability of rigid gate plates to effectively cope with fitting errors caused by valve chamber compression or high-temperature deformation, they have been basically eliminated in nuclear power applications and replaced by sealing structures with certain elasticity to enhance sealing reliability.

2、 Internal leakage problem: dual challenges of manufacturing and application

As a shut-off valve, once an internal leakage occurs, the safety of the system is directly threatened. Internal leakage of gate valves in nuclear power plants is often caused by the following factors:

Poor cleanliness of the medium: Impurities enter the sealing pair with the medium, causing scratches or jamming on the sealing surface;

Improper stroke control: The stroke or torque switch is adjusted incorrectly, resulting in the valve not being fully closed;

Insufficient sealing processing accuracy: The valve disc and seat do not fit tightly, resulting in leakage;

Material defects: such as pores, slag inclusions, etc. that affect sealing performance;

Mismatched structural selection: If a self sealing V-shaped valve is mistakenly used in a low-pressure differential system, the seal is not easily formed.

Prevention and control measures should start from both the manufacturing and application ends: strengthening the quality of sealing surface processing, optimizing the selection and debugging process, and improving the level of medium purification to reduce the risk of impurities during valve operation.

3、 Leakage hazard: High voltage alarm for safety and environmental protection

Once the nuclear power gate valve leaks, it not only endangers equipment safety, but also may cause the leakage of radioactive substances, posing a serious threat to the environment. Common leakage areas include:

Connection between valve body and valve cover: casting defects (such as sand holes, cracks), flange gasket failure, etc;

Packing box location: Sealing failure caused by aging of the packing or damage to the valve stem;

Upper sealing structure: Leakage occurs due to misoperation or structural deformation.

To address these risks, protection can be strengthened from both structural design and manufacturing control dimensions:

Select high-quality forgings: It is recommended to use forged valve bodies for important working conditions to reduce the risk of casting defects.

Reasonable design of flange connection: Nuclear power gate valves often use graphite wrapped gaskets and sealed welding design, with the ability to perform three lip cutting and welding repairs.

Optimization of packing structure: Adopting a spring-loaded compression mechanism to dynamically compensate for packing relaxation and improve sealing stability.

Set up a leakage pipe: A leakage pipe is installed at the filling point in the middle of the valve for radioactive media, which can be collected and discharged in case of leakage.

Configure sealing structure: In the event of packing failure, provide a second line of defense through upper sealing to ensure emergency sealing capability.

4、 Nuclear power gate valve, balancing precision and safety

Nuclear power plants have strict requirements for the sealing performance of valves. As a key equipment in large-diameter, high-temperature and high-pressure systems, gate valves have a direct impact on the safe and stable operation of nuclear power plants due to their internal and external leakage problems. Therefore, it is necessary to work together at multiple levels such as product design, material selection, manufacturing process, and operation and maintenance to improve the reliability and sealing performance of nuclear grade gate valves from the source. Only in this way can we truly build the "first line of defense" of the nuclear power system.