Abstract
In industrial practice, the high-pressure sealing of swing check valves is generally guaranteed, but the low-pressure sealing is often overlooked, which may cause serious abnormal working conditions. The principle of low-pressure sealing of swing check valves, the reasons for low-pressure sealing being neglected, and the applicable standards of low-pressure sealing are discussed based on this to improve the reliability of low-pressure sealing of swing check valves.
Treatment of the reverse leakage of four swing check valves in a nuclear power unit
In October 2016, it was found that reverse leakage occurred for the four swing check valves of the chemical and volume control system on site in a thermal test of a nuclear power plant. The leakage caused the four check valves of the make-up pump of the chemical and volume control system to fail to return to their seats after the pump was stopped.
After the design management department intervened to deal with the problem, a letter was first sent to the original designer (a US company) to increase the leakage rate under low pressure conditions in the design documents, but the original designer stated that the swing check valve does not have a requirement for leakage rates under low pressure conditions. In order to further solve the problem, the design management department coordinated with another Chinese design institute to propose requirements for low-pressure sealing tests. After research, the design institute proposed parameters such as the test pressure of the low-pressure sealing test, the pressure holding time, and the leakage rate of the valve seat per mm of nominal diameters.
In December 2016, the design management department coordinated the equipment procurement department to repurchase the check valve in accordance with the new low-pressure sealing requirements. However, leakage continued in subsequent tests and operation due to the manufacturing process and machining accuracy of this batch of check valves. After 2018, combined with experience feedback, the check valves of this batch were gradually replaced with imported swing check valves produced by a foreign manufacturer, and the problem was successfully solved in the end.
2. Analyzing the root cause of the above problems based on the principle of the swing check valve
Compared with other types of check valves, the main movement mode of a swing check valve is that a valve disc rotates around a pin shaft. This kind of check valve has the advantage of lower flow resistance compared with lift check valves and pipeline check valves, so it is widely used in various process systems of water supply.
Specific to the aforementioned problems, the main standard that the original designer followed when designing the valve was ASME B16.34-1996. The valve's design specification stipulates that the sealing test of the valve should be in accordance with the requirements of ASME B16.34-1996 and conduct a sealing test not less than five minutes at a temperature of 37.8℃ and under 1.1 times of the rated pressure. However, the sealing performance under the low-pressure difference working condition is not covered in the design document. As a matter of fact, the original design party doesn't have a thorough understanding of ASME B16.34-1996, which is the fundamental reason that the original designer believes that there is no requirement for leakage rates under low pressure working conditions for the swing check valve.
In industrial practice, the high-pressure sealing of swing check valves is generally guaranteed, but the low-pressure sealing is often overlooked, which may cause serious abnormal working conditions. The principle of low-pressure sealing of swing check valves, the reasons for low-pressure sealing being neglected, and the applicable standards of low-pressure sealing are discussed based on this to improve the reliability of low-pressure sealing of swing check valves.
Treatment of the reverse leakage of four swing check valves in a nuclear power unit
In October 2016, it was found that reverse leakage occurred for the four swing check valves of the chemical and volume control system on site in a thermal test of a nuclear power plant. The leakage caused the four check valves of the make-up pump of the chemical and volume control system to fail to return to their seats after the pump was stopped.
After the design management department intervened to deal with the problem, a letter was first sent to the original designer (a US company) to increase the leakage rate under low pressure conditions in the design documents, but the original designer stated that the swing check valve does not have a requirement for leakage rates under low pressure conditions. In order to further solve the problem, the design management department coordinated with another Chinese design institute to propose requirements for low-pressure sealing tests. After research, the design institute proposed parameters such as the test pressure of the low-pressure sealing test, the pressure holding time, and the leakage rate of the valve seat per mm of nominal diameters.
In December 2016, the design management department coordinated the equipment procurement department to repurchase the check valve in accordance with the new low-pressure sealing requirements. However, leakage continued in subsequent tests and operation due to the manufacturing process and machining accuracy of this batch of check valves. After 2018, combined with experience feedback, the check valves of this batch were gradually replaced with imported swing check valves produced by a foreign manufacturer, and the problem was successfully solved in the end.
2. Analyzing the root cause of the above problems based on the principle of the swing check valve
Compared with other types of check valves, the main movement mode of a swing check valve is that a valve disc rotates around a pin shaft. This kind of check valve has the advantage of lower flow resistance compared with lift check valves and pipeline check valves, so it is widely used in various process systems of water supply.
Specific to the aforementioned problems, the main standard that the original designer followed when designing the valve was ASME B16.34-1996. The valve's design specification stipulates that the sealing test of the valve should be in accordance with the requirements of ASME B16.34-1996 and conduct a sealing test not less than five minutes at a temperature of 37.8℃ and under 1.1 times of the rated pressure. However, the sealing performance under the low-pressure difference working condition is not covered in the design document. As a matter of fact, the original design party doesn't have a thorough understanding of ASME B16.34-1996, which is the fundamental reason that the original designer believes that there is no requirement for leakage rates under low pressure working conditions for the swing check valve.
Next: The Analysis of Low-pressure Sealing of Swing Check Valves (Part Two)
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