Feasibility of mid-cavity pressurization
Technical regulations
The gate valve technical specification stipulates that in addition to the containment isolation valve, test holes need to be drilled for valve bonnets of some valves for periodical inspection and plugged with a screw plug. See the figure 5. The hole drilling should meet the relevant requirements for the specific technical design.
Figure 5 Drilling inspection holes on the valve bonnet
For containment isolation gate valves of nuclear power plants, a small hole should be drilled on the valve body or valve bonnet and wedge according to the design requirements, unless the existing small hole in the structure of the filling purge device can be used. This part of the gate valve is also provided with these holes in the actual design according to the specifications. This structural design provides convenience for the pressure test by using the small holes under the filling purge device. The structure of the packing purge hole is shown in the Figure 6.
Figure 6 Valve bonnets and valve bodies
Therefore, it is feasible to pressurize from the middle cavity based on the analysis of the design structure of the containment isolation valve.
Installation process tests
In the on-site installation stage of the gate valve, a pressure test should be carried out to check the sealing of the valve. The pressure method is to pressurize the middle cavity to have the sealing test and verify the sealing performance of the valve based on experience. In the commissioning stage after installation, the pressurization test operation of the relevant conventional method is carried out according to the requirements for the commissioning outlines.
Current status of pressure tests abroad
A foreign valve company has a special sealing test-bed or TRESOR, which can be used to test the performance of the gate valves they manufacture. The test method is to measure the sealing performance of the gate valve by pressurizing in the middle valve cavity. During the overhaul of the nuclear power plant, the tightness of 20 to 30 valves will be tested each time, which reduces the time for system testing and maintenance and improves the work efficiency and safety of the power plant.
Analysis of problems in the middle cavity pressure test
At present, if Chinese nuclear power plants adopt pressurization method in the middle cavity to conduct periodic tests on gate valves, the following problems need to be solved:
(1) There are currently 158 containment isolation valves for nuclear power plants under construction. Valves are supplied by different manufacturers. Among them, 34 valves are supplied abroad and 124 valves are from China. The TRESOR can be used for foreign valve products, but it is not suitable for Chinese valves.
(2) The mid-cavity pressurization method and test-bed have been recognized by some foreign organizations and institutions. However, there is still a lack of experience in fully adopting this method for regular inspections in China and they have not been recognized by the regulatory authorities. Standard specification and technical research work need to be carried out further.
(3) Under the current scope of application, the equipment cost of the test-bed is relatively high, and the equipment needs to be calibrated and maintained during use.
(4) At present, the Chinese gate valve design and manufacturing process lacks the basic data of valve field test, and the accumulation of the relevant test data and equipment development are needed.
Conclusion
With the construction and operation of nuclear power plants in China, more and more valves need to be regularly inspected and repaired. The time and repair costs required for traditional periodic inspection methods are relatively long and high. If the middle cavity pressurization method is adopted, the test process will be simplified and reasonable, and the period of regular inspection, maintenance and radiation time can be shortened. Therefore, there is a good application prospect on the relevant middle cavity pressurization test research.
Technical regulations
The gate valve technical specification stipulates that in addition to the containment isolation valve, test holes need to be drilled for valve bonnets of some valves for periodical inspection and plugged with a screw plug. See the figure 5. The hole drilling should meet the relevant requirements for the specific technical design.
Figure 5 Drilling inspection holes on the valve bonnet
For containment isolation gate valves of nuclear power plants, a small hole should be drilled on the valve body or valve bonnet and wedge according to the design requirements, unless the existing small hole in the structure of the filling purge device can be used. This part of the gate valve is also provided with these holes in the actual design according to the specifications. This structural design provides convenience for the pressure test by using the small holes under the filling purge device. The structure of the packing purge hole is shown in the Figure 6.
Figure 6 Valve bonnets and valve bodies
Therefore, it is feasible to pressurize from the middle cavity based on the analysis of the design structure of the containment isolation valve.
Installation process tests
In the on-site installation stage of the gate valve, a pressure test should be carried out to check the sealing of the valve. The pressure method is to pressurize the middle cavity to have the sealing test and verify the sealing performance of the valve based on experience. In the commissioning stage after installation, the pressurization test operation of the relevant conventional method is carried out according to the requirements for the commissioning outlines.
Current status of pressure tests abroad
A foreign valve company has a special sealing test-bed or TRESOR, which can be used to test the performance of the gate valves they manufacture. The test method is to measure the sealing performance of the gate valve by pressurizing in the middle valve cavity. During the overhaul of the nuclear power plant, the tightness of 20 to 30 valves will be tested each time, which reduces the time for system testing and maintenance and improves the work efficiency and safety of the power plant.
Analysis of problems in the middle cavity pressure test
At present, if Chinese nuclear power plants adopt pressurization method in the middle cavity to conduct periodic tests on gate valves, the following problems need to be solved:
(1) There are currently 158 containment isolation valves for nuclear power plants under construction. Valves are supplied by different manufacturers. Among them, 34 valves are supplied abroad and 124 valves are from China. The TRESOR can be used for foreign valve products, but it is not suitable for Chinese valves.
(2) The mid-cavity pressurization method and test-bed have been recognized by some foreign organizations and institutions. However, there is still a lack of experience in fully adopting this method for regular inspections in China and they have not been recognized by the regulatory authorities. Standard specification and technical research work need to be carried out further.
(3) Under the current scope of application, the equipment cost of the test-bed is relatively high, and the equipment needs to be calibrated and maintained during use.
(4) At present, the Chinese gate valve design and manufacturing process lacks the basic data of valve field test, and the accumulation of the relevant test data and equipment development are needed.
Conclusion
With the construction and operation of nuclear power plants in China, more and more valves need to be regularly inspected and repaired. The time and repair costs required for traditional periodic inspection methods are relatively long and high. If the middle cavity pressurization method is adopted, the test process will be simplified and reasonable, and the period of regular inspection, maintenance and radiation time can be shortened. Therefore, there is a good application prospect on the relevant middle cavity pressurization test research.
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