Spring-loaded parallel double-disc gate valves, also known as V-type gate valves, are commonly used in nuclear power plants. For example, in the Hualong One ACP1000 reactor type, they are widely used in critical systems such as auxiliary feedwater systems, turbine bypass systems, and main feedwater flow control systems, playing a vital role in ensuring the normal and stable operation of nuclear power plants. The sealing principle of spring-loaded parallel double-disc gate valves relies on automatic sealing, which has certain limitations under low pressure. The leakage rate of spring-loaded parallel double-disc gate valves in nuclear power plants has not met the requirements. Investigations revealed that the low pressure in the pipeline where the valve is located fails to provide sufficient medium force to enable the main sealing pair to form a seal, resulting in the valve's leakage rate exceeding the standard.
2. Characteristics of Main Seal Structure
The main sealing structure of the spring-loaded parallel
double-disc gate valve is shown in Figure 1. It is characterized by low opening and closing torque, and the valve is not prone to wedging. The gate is pushed against the valve seat by the medium force to achieve sealing, functioning as an automatic seal. The gate frame primarily serves a connecting function. The spring applies thrust to the valve seat, forcing the gate to close against it. When the gate assembly is raised, the limit frame prevents horizontal movement of the gate, acting as a constraint. The pin shaft connects the limit frame, gate frame, and valve stem. When the valve opens and closes, the gate assembly moves up and down with the valve stem.
1. Valve seat 2. Wedge 3. Spring 4. Wedge frame 5. Limit frame 6. Pin 7. Stem
Figure 1 Main seal structure
All spring-loaded parallel double-disc gate valves for nuclear power are metal seated. Many factors affect the leakage of their sealing surfaces, including but not limited to the following:
The sealing surface is generally welded with Stellite hard alloy, followed by heat treatment to relieve stress, ensuring sufficient hardness and service life.
The quality of the sealing surface includes the thickness, roughness, flatness, and hardness of the hard alloy cladding. The sealing surface of the spring-loaded parallel double-disc gate valve for nuclear power has strict requirements regarding these parameters. Additionally, ultrasonic and liquid penetration tests are conducted to detect defects on the sealing surface, ensuring the quality of the seal.
Sealing grease is not permitted on the sealing surface of the spring-loaded parallel double-disc gate valve for nuclear power.
The media in the system of the spring-loaded parallel double-disc gate valve for nuclear power typically include feed water, demineralized water, raw water, drinking water, and air. Different media have varying impacts on the leakage rate of the sealing surface. Since the medium in the ACP1000 reactor spring-loaded parallel double-disc gate valve is typically water under low-pressure conditions, this article uses ordinary water for specific analysis.
A spring is positioned between the two valve plates of the spring-loaded parallel double-disc gate valve. Given that the valve may be installed in a specific orientation, the spring force should be designed to offset the weight of a single valve plate when selecting the spring. If the spring-loaded parallel double-disc gate valve is installed horizontally, the spring force will also act on the sealing surface, providing additional support in sealing the valve. However, since the spring needs to be pre-loaded during valve assembly, excessive spring force can make assembly difficult. Therefore, when selecting the spring, its force should be slightly greater than the valve plate's weight.
The width of the sealing surface is a key factor influencing the sealing surface pressure ratio. It is typically determined by the valve designer based on the valve's purpose, the structure of the closing component, and the pressure ratio value on the sealing surface.
The spring-loaded parallel double-disc gate valve primarily relies on the medium's force to achieve sealing. The pressure difference between the front and rear of the valve is the primary factor influencing the sealing surface pressure ratio. For this analysis, the pressure behind the valve is assumed to be 0MPa.
The specific pressure on the sealing surface depends on its width, material, and the medium's pressure, and it is a critical factor in determining whether the valve can achieve proper sealing.
When selecting valves, the sealing principle should be considered. If the design pressure is low, it is advisable to avoid using the spring-type parallel double-disc gate valve. When the valve design pressure is low, and a spring-type parallel double-disc gate valve must be selected, it is recommended to make special design adjustments, such as modifying the sealing width to ensure the actual specific pressure on the sealing surface exceeds the required specific pressure, enabling the valve to achieve proper sealing.