Abstract: The sealing performance of the valves needs to be inspected during regular inspections of gate valves for containment isolation of nuclear power plants. The traditional test method is to pressurize on one side of the gate valve and measure on the other side. The advantage of this method is that the method of pressurization is the same as the pressure state in the valve working condition. The disadvantage is that its operating time is long and the detection process is exposed to radiation. According to the characteristics of the containment isolation valve, the feasibility of using the mid-cavity pressurization method in the sealing test of the gate valve for nuclear power plants is studied, and the current problems of the mid-cavity pressurization method in the Chinese valve industry are analyzed and solutions are given.
According to the setting requirements and guidelines for the isolation valve in the containment isolation device of the PWR nuclear power plant, at least one locked isolation valve is usually used in the system. According to the characteristics of valves, gate valves are generally used in nuclear power plants as containment isolation valves in large-diameter pipelines. Before the operation of the fluid system and during the service life of the valve, a sealing test should be carried out to verify that the containment can perform or continue its sealing function. The containment isolation valve of each system is an important part to ensure the integrity of the third barrier, so partial tests are required to verify functions under the design pressure.
Conventional test methods for gate valves
Test conditions
(1) The test is carried out at room temperature.
(2) Use water or air to pressurize the valve to the containment design pressure.
(3) When the valve is equipped with a remote operation device, it should not be closed with any other special operations. The electric valve should be closed by its remote operating device, and no manual operation should be carried out.
(4) The partial pressure method is used in the test. The pressure direction should be the same as the pressure bearing direction of the valve when performing its safety function, unless it can be proved that the opposite pressure direction can have an equivalent or more reliable result.
Test systems
During the test, the pipeline of the valve test system is pressurized. See figure 1. The pressure rise of the downstream pipeline of the valve is measured to judge the sealing performance of the valve. The criterion for judging the sealing performance of the valve is to measure its maximum allowable leakage rate.
(1) For every 1mm nominal diameter of the valve in the air pressure test, the leakage Q should be smaller or equal to 15.7cm³/h.
According to the setting requirements and guidelines for the isolation valve in the containment isolation device of the PWR nuclear power plant, at least one locked isolation valve is usually used in the system. According to the characteristics of valves, gate valves are generally used in nuclear power plants as containment isolation valves in large-diameter pipelines. Before the operation of the fluid system and during the service life of the valve, a sealing test should be carried out to verify that the containment can perform or continue its sealing function. The containment isolation valve of each system is an important part to ensure the integrity of the third barrier, so partial tests are required to verify functions under the design pressure.
Conventional test methods for gate valves
Test conditions
(1) The test is carried out at room temperature.
(2) Use water or air to pressurize the valve to the containment design pressure.
(3) When the valve is equipped with a remote operation device, it should not be closed with any other special operations. The electric valve should be closed by its remote operating device, and no manual operation should be carried out.
(4) The partial pressure method is used in the test. The pressure direction should be the same as the pressure bearing direction of the valve when performing its safety function, unless it can be proved that the opposite pressure direction can have an equivalent or more reliable result.
Test systems
During the test, the pipeline of the valve test system is pressurized. See figure 1. The pressure rise of the downstream pipeline of the valve is measured to judge the sealing performance of the valve. The criterion for judging the sealing performance of the valve is to measure its maximum allowable leakage rate.
(1) For every 1mm nominal diameter of the valve in the air pressure test, the leakage Q should be smaller or equal to 15.7cm³/h.
(2) For every 1mm nominal diameter of the valve in the hydraulic test, the leakage Q should be smaller or equal to 0.1cm³/h.
Figure 1 Pipeline systems for pressurization tests of pit valves EAS014VB
Performance analysis
The pit valve EAS014VB pipeline systems of pressurization tests meet the requirements of the tightness guideline of the containment isolation valve. The pit valve EAS014VB can prove that the sealing performance of the valve is consistent with the environmental conditions under accident conditions, and can verify the sealing performance and leakage under accident conditions. The shortcomings of the test system are the need to isolate the pipeline for a long time; the operation is cumbersome; the water filling and test process are longer, and the radiation or potential radiation during operation is relatively much. Due to the disadvantages of conventional methods, the pressurization test plan should be optimized according to the structural characteristics of the gate valve, and the operation should be simplified to shorten the test time. Design a pressurization method that can not only achieve the test purpose, but also is easy to implement.
Structural analysis of gate valves
According to the type of valve, gate valves can be divided into wedge gate valves, type C, figure 2, parallel gate valves with wedges, type W, figure 3 and parallel gate valves with springs, type V, figure 4.
Figure 2 Wedge gate valves type C
Figure 3 Parallel gate valves with wedges type W
Figure 4 Parallel gate valves with springs, type V
The characteristic analysis of the three structural types of gate valves is shown in Table 1. According to the analysis of the characteristics of the three types of gate valves, the valves that can be used as containment isolation valves are Type C and Type W. C-type and W-type gate valves are two-way sealed valves. The center cavity of the valve is pressurized in the sealing test based on their characteristics.
Table 1 The characteristic analysis of the three structural types of gate valves
Figure 1 Pipeline systems for pressurization tests of pit valves EAS014VB
Performance analysis
The pit valve EAS014VB pipeline systems of pressurization tests meet the requirements of the tightness guideline of the containment isolation valve. The pit valve EAS014VB can prove that the sealing performance of the valve is consistent with the environmental conditions under accident conditions, and can verify the sealing performance and leakage under accident conditions. The shortcomings of the test system are the need to isolate the pipeline for a long time; the operation is cumbersome; the water filling and test process are longer, and the radiation or potential radiation during operation is relatively much. Due to the disadvantages of conventional methods, the pressurization test plan should be optimized according to the structural characteristics of the gate valve, and the operation should be simplified to shorten the test time. Design a pressurization method that can not only achieve the test purpose, but also is easy to implement.
Structural analysis of gate valves
According to the type of valve, gate valves can be divided into wedge gate valves, type C, figure 2, parallel gate valves with wedges, type W, figure 3 and parallel gate valves with springs, type V, figure 4.
Figure 2 Wedge gate valves type C
Figure 3 Parallel gate valves with wedges type W
Figure 4 Parallel gate valves with springs, type V
The characteristic analysis of the three structural types of gate valves is shown in Table 1. According to the analysis of the characteristics of the three types of gate valves, the valves that can be used as containment isolation valves are Type C and Type W. C-type and W-type gate valves are two-way sealed valves. The center cavity of the valve is pressurized in the sealing test based on their characteristics.
Table 1 The characteristic analysis of the three structural types of gate valves
| Wedge gate valves type C | Parallel gate valves with wedges type W | Parallel gate valves with springs type V | |
| Applications | C-type gate valves are used for high pressure, high pressure difference and high flow conditions. |
They are used in working conditions with low pressure and good sealing performance. |
They are used in working conditions such as low pressure and high pressure difference of temperature higher than 450℃, and low and medium pressure for sealing. |
| Advantages | The sealing function is separated from the guiding function, and there is only one part in the flow channel, which have excellent reliability. It can be wedged under any pressure conditions, which provides great sealing performance. The valve disc guiding is fine machined, which is suitable for working conditions of high pressure difference. In the case of high flow rate, the disc will not tilt. The flexible valve disc is adopted, which makes the C-type gate valve suitable for relatively high temperature working condition that is below 450℃. The closed part is not vibrated. The valve seat has less friction and less wear when the valve is opened or closed. The torque of the actuator is balanced by the torque rod, and the valve seat doesn’t beat extra loading. C-type gate valves hardly need to be repaired, and they can be used for the RCP system. | They have good sealing performance in low pressure, which can meet containment isolation requirements (RIN:J) of low pressure sealing. The type W synthesizes advantages of wedge gate valves and advantages of parallel gate valves used in low pressure working conditions. Because of the wedge parts between valve discs, the loading can be applied on the valve seat when the valve is fully closed. They have low friction and less wear. The valve plate can rotate when the valve is closed, so the valve has functions of self grinding and self cleaning. They don’t need to be repaired. |
They are easily to design and process, and also have low costs. The actuator of the valve is easily adjusted, because the accuracy of the closed position is not high. They are used for closing in the no medium state. Extra loading is not required to open the valve, which is used for matching pneumatic actuators. |
| Disadvantages | They requires fine machining. They can’t be used for working conditions which are higher than 450℃. They have relatively complicated actuators. Thermal expansion may make the wedge blocked, causing the valve being difficult to be opened. | They requires fine machining. The valves used for high pressure are difficult to design due to the big wedges inside. Thermal expansion may make the wedge blocked, causing the valve being difficult to be opened. |
Pressure tightening easily occurs. The sealing depends on the pressure. The sealing performance in the low pressure is poor. The valve seat is used as the guiding of the valve disc, making the valve easily worn. They need to be repaired frequently. The torque of the electric actuator can not be balanced. |
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