Gate valves are distributed in the control system of each loop of the nuclear power plant and play an extremely important role in the safety of the nuclear power plant. The gate valve has a larger caliber and is mainly used in the reactor main circuit system (RCP), chemical
Most of the working media are radioactive liquids, which are considered to be the working temperature
The high temperature, working pressure and safety level play an indispensable role in nuclear power plants.
Analysis of the Causes of Leakage in the Leakage of the Gate Valve
The gate valve is a kind of cut-off valve. The movement direction of the gate of the opening and closing part is perpendicular to the direction of the fluid. The gate valve can only be fully opened and fully closed, and cannot be adjusted or throttled. The structure of the gate valve is relatively complex, generally by
The valve body, bonnet, gate, valve seat, valve stem, packing, studs, nuts, stop gaskets and corresponding actuators constitute the outer sealing part of the valve. The main parts of the valve are the fit between the packing and the valve stem and stuffing box. The connection between the valve body and the middle flange of the valve cover mainly involves the valve body, the valve cover, the connection position of the middle flange and the valve stem seal. The valve has an external leakage, that is, the medium leaks from the inside of the valve to the outside of the valve. The leakage of nuclear valves means that radioactive media will be released into the environment, which is not allowed by the design of nuclear power plants. Therefore, in the safety precautions of nuclear power plants, the possibility of external leakage of radioactive media should be avoided as much as possible in the equipment design.
Technical measures to prevent external leakage of gate valve
The main reason for the external leakage of the gate valve is caused by casting or forging defects in the production process of the gate valve, such as blisters, pores and cracks. In the design and production process, mainly through the selection of materials and strengthening of material inspection to prevent valve leakage.
(1) Selection of materials
As castings have many defects in the manufacturing process, some small cracks may also undergo creep deformation during the operation of the nuclear power plant. The forged valve body eliminates internal defects and cracks and has better stress resistance and intergranular corrosion resistance. The intergranular structure of the material is uniform and the reliability is high. In the design of nuclear power plants, forged valve bodies should be used for high-temperature and high-pressure gate valves.
(2) Inspection of valve body material
The gate valve materials used in nuclear power plants need to be tested through advanced equipment and scientific methods, and subtle defects on pressure-bearing components such as valve bodies and bonnets are found. Currently, the inspection of materials
The inspection methods are generally radiographic inspection, ultrasonic inspection and liquid penetration inspection, etc., and the operation inspection must be carried out by personnel who have these inspection certificates. During the inspection process, the gate valve
The materials are inspected one by one instead of random inspection.
Technical measures to prevent leakage at the flange of the gate valve
The middle flange bolt connection is the main form of connection between the valve body and the bonnet of the gate valve in a nuclear power plant. The gate valve is used in a high-temperature and high-pressure environment. The valve will be cooled during nuclear power plant shutdown and refueling.
Under the condition of constant temperature change, leakage may occur. The reason for the leakage is related to the failure of the middle flange gasket and the loosening of bolts and nuts. Therefore, in the design process of the valve, this
Consider these factors, select qualified and tested gaskets that meet the requirements of nuclear power, select bolts and nuts that meet the requirements of RCC-M, and add stop gaskets to ensure that the nuts will not loosen. The special remedy for the failure of the flange seal in the valve body and the bonnet is the lip welding, and the lip shall be able to be cut three times. Lip welding is only a backup method in the event of an external leakage accident, and is only used in emergency situations.
3. Technical Measures to Prevent Leakage at the Seal of Gate Valve Stem
(1) Packing and disc spring
The pressing force between the stem of the gate valve and the sealing packing of the bonnet needs to be calculated and determined. The pressing force is too large or too small to meet the requirements. When designing the valve stem sealing structure, the number of packing layers and packing should be reasonably determined.
Material compression force and packing size, and give a strict range of dimensional tolerances during processing, and there is evidence to check during processing and must be strictly implemented. When selecting fillers, not only must be considered
The working temperature should also consider the influence of the friction of the filler on the control process, the influence of the medium's radioactivity on the life of the filler, etc., and select qualified and tested that meet the requirements of nuclear power
Validated special packing material. Due to the wear and thermal burn of the packing, stress relaxation will occur. Spring loading is an effective way to compensate for the stress relaxation, such as loading a disc spring on the packing gland. Through the action of the disc spring, the compression degree of the packing can be adjusted to compensate for the deformation of the packing, thereby improving the sealing self-adjusting ability of the packing and improving the sealing performance.
(2) Leakage tube
In the design of nuclear power plant valves, especially for valves with radioactive media, in order to prevent leakage at the packing and collect possible leaks in a centralized manner, it is used in the middle of the packing
Plus the way of drain tube. This form of packing consists of 3 parts, the upper and lower parts are composed of several layers of non-metallic packings that play a sealing role, and a metal "lantern" ring is set in the middle. There is an annular space at the "lantern" ring to hold and collect the leaking medium from the lower packing. Holes are punched on the valve cover at the "lantern" ring and a leakage pipe is welded, which is used to lead the leaking medium from the leakage pipe to the collection and drainage system. The design of the leakage tube is equivalent to adding a protection method in the packing design. When the medium moves upward along the packing under pressure and reaches the position of the middle "lantern" ring, the pressure drops, and since the pressure at the leakage pipe is almost 0, the medium is forced to flow out of the leakage pipe instead of continuing to the upper packing. Flow, thereby avoiding the medium from continuing to leak upward along the valve stem. The medium flowing out of the leakage pipe is collected through the pipeline of the drainage system of the nuclear power plant, and processed by the three-waste treatment system.
(3) Upper seal
The upper seal is composed of the contact part of the valve cover hole and the valve stem head. The upper seal is also a measure to prevent the medium from leaking from the stem seal. When the upper seal is in full contact, the required leakage
Very small, no more than 0.04cm3/(td), where d is the diameter of the valve stem, mm; t is the time, h. The upper seal must not rely on the system pressure to achieve the specified sealing performance. The upper seal should have
The ability of the valve stem to withstand the pressure of the entire system. Under normal circumstances, the upper seal is not used, and it is only put into use when the gate valve packing leaks out, so as to ensure that the gate valve can run to the refueling period.
However, there is no leakage of a large amount of working medium from the valve stem packing position, or the packing can be replaced during the operation of the nuclear power plant to reduce the radiation dose.
Measures to prevent leakage of gate valve during use
During the commissioning phase of the nuclear power plant, a hydraulic test is carried out to ensure that the function of the equipment meets the requirements through the test. During this process, check to ensure that the gate valve has no leakage. In the operating phase of nuclear power plants, the root
According to the requirements of the in-service inspection program, the gate valve shall be inspected during the planned shutdown period and the gate valve packing shall be replaced regularly. Through regular inspection and replacement of packing, timely detection and elimination of hidden dangers of leakage, ensure
Ensure the safety of nuclear power plants.
