Wenzhou Unique Valve Co., Ltd.
Improvement in Nuclear Grade Lift Check Valves (Part One)
Jul 12, 2021
uv*******01
On this page
Abstract: Taking the nuclear grade 2 lift check valve of the boric acid recirculation loop of the safety injection system of Qinshan Second Nuclear Power Plant as an example, the structural features of the check valve are analyzed from the back pressure spring, form of the sealing surface, pressure relief hole, and valve spool stroke, and the improvement direction is proposed, hoping to help the improvement in nuclear-grade check valves.
For a long time, the boric acid recirculation loop's flow rate of 1# unit RIS (the safety injection system) of Qinshan No. 2 Nuclear Power Plant (referred to as Qin No. 2 Plant hereinafter) has been at a low level. After checking and analyzing reports of installation, trial run and tests, it was found that the flow rate of 1RIS030MD has been maintained at 3.2 to 3.4 m3/h, and the designed rated flow rate was 4.1 m3/h; the alarm value of the low flow of the RIS boric acid recirculation loop was 3 m3/h, and slight flow fluctuations would cause the alarm to go off. Combined with the external experience feedback and structural analysis of valves, the root cause of the low loop flow is the insufficient flow capacity of the on-site check valve. Therefore, it is necessary to improve the structure of the valve to ensure the safe and stable operation of the subsequent units.
1. Principles and structure of check valves
A check valve is also known as a one-way valve or a reverse flow valve, which refers to a valve that relies on the flow of media in the pipeline to open and close the disc. A check valve belongs to an automatic valve, and its main function is to prevent the medium from flowing back, the pump and drive motor from reversing, and the leakage of the container's medium. According to the structure, Qin No. 2 Plant owns swing check valves and vertical lift check valves, accounting for 18% of the total number of valves. The outlet check valve of boric acid recirculation pump is a spring-loaded lift check valve with auxiliary force. The valve is mainly composed of a valve body, a bonnet, a valve clack, a spring, a valve seat and other parts. The valve seat and sealing surface of the valve clack are welded with Stellite alloy. Two axial relief holes and a pressure relief groove are set on the valve clack. The two balance holes are connected to the upper cavity of the valve by converging into a pressure relief hole (Figure 1).
The working process of the check valve: When the medium flows from the inlet, the valve disc is pushed and lifted by the medium; in the ascending process of the valve disc, the medium is discharged through the pressure relief groove to reduce the resistance when the valve disc is opened, so that the valve disc leaves the valve seat to allow the medium to pass; when the medium stops flowing or flows back, the valve disc is quickly closed by the upper spring and its weight.
2. Influencing factors of insufficient flow capacity of check valves
2.1 The status analysis of valves
A check valve disc tends to be closed under the force generated by the medium pressure difference and spring force. In addition to the medium pressure difference and spring pressure in the opening process, a check valve is also subjected to the limited gravity and friction between the valve bodies, that is, the sealing force being equal to medium pressure difference force plus elastic force plus gravity of the check valve disc minus the friction force between the valve bodies. Therefore, the check valve uses spring pressure to press the disc against the sealing surface of the seat to achieve the purpose of sealing. When the medium flows through the check valve in the specified direction, the medium overcomes the spring pressure, weight of the valve disc and friction force between the valve bodies to push the valve disc and allow the medium to flow.
Figure 1 The structure of the check valve at the recirculation loop outlet of the boric acid
Because the working medium of the check valve described in this article is medium pressure, the gravity of the valve disc and frictional force between the valve bodies are ignored. In this way, when the valve is about to open, the force of the medium acting on the valve disc is
. In the formula, F is the acting force of the valve disc's medium (MPa). Di means the inner diameter of the sealing surface (mm). Bm is the width of the sealing surface of the seat (mm). P stands for the pressure of the inlet medium (MPa). Pi is the pressure behind the valve disc, which is the back pressure (MPa). Δp means the pressure difference between the front and rear of the valve disc (MPa).
According to formula (1), the sealing specific pressure required for valves F0 equals to
. PN is the nominal pressure of the valve, which refers to the maximum allowable working pressure of the valve at a specific nominal diameter and working temperature. Therefore, the opening of the valve needs to meet F being greater than F0, and the force of the medium acting on the valve disc must be greater than the required force of the seal specific pressure.
2.1.1 Spring pressure
The sealing performance of the valve is determined by sufficient sealing specific pressure and good sealing surface processing quality. The processing quality depends on the processing engineering. When the check valve is in the closed position, the valve spool relies on the spring pressure to ensure that it has sufficient sealing force. The spring is the key to controlling the opening pressure in the opening or closing process. Excessive spring pressure is beneficial to the closing process, and makes the valve spool close quickly under the force of the spring to ensure a better sealing effect. However, it also increases the sealing pressure of the valve. Therefore, under the premise of ensuring sufficient sealing force between the sealing surfaces, the spring pressure should be reduced.
For a long time, the boric acid recirculation loop's flow rate of 1# unit RIS (the safety injection system) of Qinshan No. 2 Nuclear Power Plant (referred to as Qin No. 2 Plant hereinafter) has been at a low level. After checking and analyzing reports of installation, trial run and tests, it was found that the flow rate of 1RIS030MD has been maintained at 3.2 to 3.4 m3/h, and the designed rated flow rate was 4.1 m3/h; the alarm value of the low flow of the RIS boric acid recirculation loop was 3 m3/h, and slight flow fluctuations would cause the alarm to go off. Combined with the external experience feedback and structural analysis of valves, the root cause of the low loop flow is the insufficient flow capacity of the on-site check valve. Therefore, it is necessary to improve the structure of the valve to ensure the safe and stable operation of the subsequent units.
1. Principles and structure of check valves
A check valve is also known as a one-way valve or a reverse flow valve, which refers to a valve that relies on the flow of media in the pipeline to open and close the disc. A check valve belongs to an automatic valve, and its main function is to prevent the medium from flowing back, the pump and drive motor from reversing, and the leakage of the container's medium. According to the structure, Qin No. 2 Plant owns swing check valves and vertical lift check valves, accounting for 18% of the total number of valves. The outlet check valve of boric acid recirculation pump is a spring-loaded lift check valve with auxiliary force. The valve is mainly composed of a valve body, a bonnet, a valve clack, a spring, a valve seat and other parts. The valve seat and sealing surface of the valve clack are welded with Stellite alloy. Two axial relief holes and a pressure relief groove are set on the valve clack. The two balance holes are connected to the upper cavity of the valve by converging into a pressure relief hole (Figure 1).
The working process of the check valve: When the medium flows from the inlet, the valve disc is pushed and lifted by the medium; in the ascending process of the valve disc, the medium is discharged through the pressure relief groove to reduce the resistance when the valve disc is opened, so that the valve disc leaves the valve seat to allow the medium to pass; when the medium stops flowing or flows back, the valve disc is quickly closed by the upper spring and its weight.
2. Influencing factors of insufficient flow capacity of check valves
2.1 The status analysis of valves
A check valve disc tends to be closed under the force generated by the medium pressure difference and spring force. In addition to the medium pressure difference and spring pressure in the opening process, a check valve is also subjected to the limited gravity and friction between the valve bodies, that is, the sealing force being equal to medium pressure difference force plus elastic force plus gravity of the check valve disc minus the friction force between the valve bodies. Therefore, the check valve uses spring pressure to press the disc against the sealing surface of the seat to achieve the purpose of sealing. When the medium flows through the check valve in the specified direction, the medium overcomes the spring pressure, weight of the valve disc and friction force between the valve bodies to push the valve disc and allow the medium to flow.
Figure 1 The structure of the check valve at the recirculation loop outlet of the boric acid
Because the working medium of the check valve described in this article is medium pressure, the gravity of the valve disc and frictional force between the valve bodies are ignored. In this way, when the valve is about to open, the force of the medium acting on the valve disc is
. In the formula, F is the acting force of the valve disc's medium (MPa). Di means the inner diameter of the sealing surface (mm). Bm is the width of the sealing surface of the seat (mm). P stands for the pressure of the inlet medium (MPa). Pi is the pressure behind the valve disc, which is the back pressure (MPa). Δp means the pressure difference between the front and rear of the valve disc (MPa).According to formula (1), the sealing specific pressure required for valves F0 equals to
. PN is the nominal pressure of the valve, which refers to the maximum allowable working pressure of the valve at a specific nominal diameter and working temperature. Therefore, the opening of the valve needs to meet F being greater than F0, and the force of the medium acting on the valve disc must be greater than the required force of the seal specific pressure.2.1.1 Spring pressure
The sealing performance of the valve is determined by sufficient sealing specific pressure and good sealing surface processing quality. The processing quality depends on the processing engineering. When the check valve is in the closed position, the valve spool relies on the spring pressure to ensure that it has sufficient sealing force. The spring is the key to controlling the opening pressure in the opening or closing process. Excessive spring pressure is beneficial to the closing process, and makes the valve spool close quickly under the force of the spring to ensure a better sealing effect. However, it also increases the sealing pressure of the valve. Therefore, under the premise of ensuring sufficient sealing force between the sealing surfaces, the spring pressure should be reduced.
