Abstract: The design methods of main parts of gate valves, globe valves, check valves and ball valves for nuclear power plants are introduced and the general requirements for nuclear power valves are discussed in this article.
1. Overview
Nuclear power valves refer to valves used in nuclear islands (N1), conventional islands (CI) and auxiliary facilities (BOP) systems of power plants in nuclear power plants. According to the safety level, there are nuclear safety levels such as I, II, III and non-nuclear levels, among which nuclear safety level I has the highest requirements. They can be divided into level C1, C2 and C3 based on the design specification level. Nuclear power valves can be divided into Pressure I, II and III. The nuclear power valve is medium transmission control equipment which is used most in nuclear power plants, and it is an indispensable and important part of the safe operation of nuclear power plants.
1. Overview
Nuclear power valves refer to valves used in nuclear islands (N1), conventional islands (CI) and auxiliary facilities (BOP) systems of power plants in nuclear power plants. According to the safety level, there are nuclear safety levels such as I, II, III and non-nuclear levels, among which nuclear safety level I has the highest requirements. They can be divided into level C1, C2 and C3 based on the design specification level. Nuclear power valves can be divided into Pressure I, II and III. The nuclear power valve is medium transmission control equipment which is used most in nuclear power plants, and it is an indispensable and important part of the safe operation of nuclear power plants.
2. The design of the valve
2.1 Globe valves with small diameters
(1) C1 globe valves with small diameters
The valve body and bonnet of the C1 globe valves with small diameters should be designed as a whole. The valve body and valve bonnet can be sealed. If there are no other special requirements, welding should be done in the factory. Forging, die forging or rolling welding structure should be adopted for the valve body and bonnet. The use of cast steel is prohibited for the globe valve. Adopting cast steel for the regulating valve shall be approved by the user.
The upper sealing surface between the bonnet and disc of the Pressure I and II C1 small-diameter globe valve should be welded with hard alloys. The sealing surface of the Pressure I non-alloy steel valve bonnet should be welded with stainless steel. If the valve disc does not have integral design, its components should be assembled by welding to eliminate the risk of leakages through the sealing surface. The sealing surface of the valve seat and disc of pressure I and II valves is build up welded by hard alloys. For Pressure II pneumatic globe valves, their sealing surfaces must be build up welded by hard alloys and stainless steel can be used for the rest of the valves. The valve seat can be welded or a seat ring can be welded. The valve seat of the regulating valve can be integrated with the valve sleeve.
Sliding stems are only allowed to be used for C1 globe valves with small diameters, except for metal diaphragm valves that use rotating stems. The stem should be ground, and the diameter tolerance of the stem in contact with the packing should be less than 0.05 mm. Ensure that there is no corrosion or erosion for the finished surface of the valve stem due to the leakage of the medium between the valve stem and packing. The disc and valve stem can be moved slightly for centering, but they cannot be rotated freely on the valve stem. The disc of the regulating valve should be fixed on the stem.
(2) C2 globe valves with small diameters
If the C2 globe valve with small diameters is an isolation valve or a manual regulating valve, integral design should be adopted. The welded or threaded connection can be used between the valve body and bonnet. If there are no other special requirements, welding should be done in the factory. However, valves with pressure below 10MPa (600 LB) allow the bonnet to be bolted to the valve body with a gasket seal structure to prevent excessive compression. The valve body and bonnet adopt forging, die forging or rolling welding structure. Cast steel can be used for pressure I valves. Cast steel used for Pressure I and II regulating valves must be approved by the user.
Hard alloy surfacing is not required for the upper sealing surface between the valve bonnet and disc. Build up welding with stainless steel is required for the sealing surface of the Pressure I non-alloy steel valve bonnet. The upper sealing surface of pressure II and III valves can be directly arranged on the valve stem. When the disc is composed of multiple parts, spot welding or mechanical assembly should be used. The sealing surface of the valve seat and disc of pressure I and II valves are build up welded by hard alloys. For Pressure II pneumatic globe valves, their sealing surfaces must be build up welded by hard alloys and stainless steel can be used for the rest of the valves. The valve seat or seat ring can be welded. The valve seat of the regulating valve can be integrated with the valve sleeve.
Only non-rotating stems are allowed, except for metal diaphragm valves. There should be no deflection on the surface of the stem, and the diameter tolerance of the stem in contact with the packing should be less than 0.05 mm. The disc and valve stem can be moved slightly to adjust the centering, but they cannot be rotated freely on the valve stem.
(3) C3 globe valves with small diameters
The body and bonnet of the C3 globe valves with small diameters are connected by concave-convex flanges, and the flanges are equipped with sealing gaskets and fastened by bolts. If there are no special requirements, non-welded bolt connections can also be used. The valve body and bonnet are allowed to be made of cast steel. The Pressure II valve can adopt the sealing structure on the stem. If the upper sealing surface is on a valve with non-integral design, the combined parts of the valve should be welded as a whole. An upper seal is not necessarily required for Pressure I valves.
2.2 Gate valves and globe valves with large diameters
(1) C1 valves
Forged steel and cast steel can be used for the body of the C1 valve. When the welding seam on the pressure-bearing boundary can be inspected by radiography, the forged welded structure can also be used. The valve body and bonnet of the gate valve, large-diameter globe valve and regulating valve with different pressure are connected by bolts and pressure self sealing. If there are special requirements, the valve body and bonnet can be connected with a lip type bolt.
The sealing surface of the valve bonnet should be welded with hard alloys or stainless steel (minimum hardness of 25HRC). The upper sealing surface of the valve stem used for superheated steam should be welded with hard alloys, and 13% to 17% Cr stainless steel with minimum hardness of 20HRC can also be used. The sealing surface of the valve seat and disc should be welded with hard alloys.
The valve stem should have no deflection, and the surface should be ground with the maximum surface roughness of Ra being equal to 0.4μm. The diameter tolerance of the valve stem in contact with the packing should be less than 0.1 mm. The maximum surface roughness of the inner surface of the stuffing box Ra should be 1.6μm.
(1) C1 globe valves with small diameters
The valve body and bonnet of the C1 globe valves with small diameters should be designed as a whole. The valve body and valve bonnet can be sealed. If there are no other special requirements, welding should be done in the factory. Forging, die forging or rolling welding structure should be adopted for the valve body and bonnet. The use of cast steel is prohibited for the globe valve. Adopting cast steel for the regulating valve shall be approved by the user.
The upper sealing surface between the bonnet and disc of the Pressure I and II C1 small-diameter globe valve should be welded with hard alloys. The sealing surface of the Pressure I non-alloy steel valve bonnet should be welded with stainless steel. If the valve disc does not have integral design, its components should be assembled by welding to eliminate the risk of leakages through the sealing surface. The sealing surface of the valve seat and disc of pressure I and II valves is build up welded by hard alloys. For Pressure II pneumatic globe valves, their sealing surfaces must be build up welded by hard alloys and stainless steel can be used for the rest of the valves. The valve seat can be welded or a seat ring can be welded. The valve seat of the regulating valve can be integrated with the valve sleeve.
Sliding stems are only allowed to be used for C1 globe valves with small diameters, except for metal diaphragm valves that use rotating stems. The stem should be ground, and the diameter tolerance of the stem in contact with the packing should be less than 0.05 mm. Ensure that there is no corrosion or erosion for the finished surface of the valve stem due to the leakage of the medium between the valve stem and packing. The disc and valve stem can be moved slightly for centering, but they cannot be rotated freely on the valve stem. The disc of the regulating valve should be fixed on the stem.
(2) C2 globe valves with small diameters
If the C2 globe valve with small diameters is an isolation valve or a manual regulating valve, integral design should be adopted. The welded or threaded connection can be used between the valve body and bonnet. If there are no other special requirements, welding should be done in the factory. However, valves with pressure below 10MPa (600 LB) allow the bonnet to be bolted to the valve body with a gasket seal structure to prevent excessive compression. The valve body and bonnet adopt forging, die forging or rolling welding structure. Cast steel can be used for pressure I valves. Cast steel used for Pressure I and II regulating valves must be approved by the user.
Hard alloy surfacing is not required for the upper sealing surface between the valve bonnet and disc. Build up welding with stainless steel is required for the sealing surface of the Pressure I non-alloy steel valve bonnet. The upper sealing surface of pressure II and III valves can be directly arranged on the valve stem. When the disc is composed of multiple parts, spot welding or mechanical assembly should be used. The sealing surface of the valve seat and disc of pressure I and II valves are build up welded by hard alloys. For Pressure II pneumatic globe valves, their sealing surfaces must be build up welded by hard alloys and stainless steel can be used for the rest of the valves. The valve seat or seat ring can be welded. The valve seat of the regulating valve can be integrated with the valve sleeve.
Only non-rotating stems are allowed, except for metal diaphragm valves. There should be no deflection on the surface of the stem, and the diameter tolerance of the stem in contact with the packing should be less than 0.05 mm. The disc and valve stem can be moved slightly to adjust the centering, but they cannot be rotated freely on the valve stem.
(3) C3 globe valves with small diameters
The body and bonnet of the C3 globe valves with small diameters are connected by concave-convex flanges, and the flanges are equipped with sealing gaskets and fastened by bolts. If there are no special requirements, non-welded bolt connections can also be used. The valve body and bonnet are allowed to be made of cast steel. The Pressure II valve can adopt the sealing structure on the stem. If the upper sealing surface is on a valve with non-integral design, the combined parts of the valve should be welded as a whole. An upper seal is not necessarily required for Pressure I valves.
2.2 Gate valves and globe valves with large diameters
(1) C1 valves
Forged steel and cast steel can be used for the body of the C1 valve. When the welding seam on the pressure-bearing boundary can be inspected by radiography, the forged welded structure can also be used. The valve body and bonnet of the gate valve, large-diameter globe valve and regulating valve with different pressure are connected by bolts and pressure self sealing. If there are special requirements, the valve body and bonnet can be connected with a lip type bolt.
The sealing surface of the valve bonnet should be welded with hard alloys or stainless steel (minimum hardness of 25HRC). The upper sealing surface of the valve stem used for superheated steam should be welded with hard alloys, and 13% to 17% Cr stainless steel with minimum hardness of 20HRC can also be used. The sealing surface of the valve seat and disc should be welded with hard alloys.
The valve stem should have no deflection, and the surface should be ground with the maximum surface roughness of Ra being equal to 0.4μm. The diameter tolerance of the valve stem in contact with the packing should be less than 0.1 mm. The maximum surface roughness of the inner surface of the stuffing box Ra should be 1.6μm.
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