The control of internal leakages and external leakages
The internal and external leakages control of the valve is an important parameter that reflects the quality of the valve. The manufacturer has adopted various measures to minimize the internal leakage and external leakage of the valve. The internal and external leakages of oil and hydrogen will not only pollute the environment, but also cause a fire or an explosion. The internal leakage of the valve mainly occurs at the wedge, while the external leakage mainly occurs at the stem packing and bonnet's gasket. Therefore, we must take action for these three parts to obtain good sealing performance.
External leakages of the sealing of the packing
For gate valves and globe valves with DN greater than and equal to 50 mm, the use of flexible graphite rings and flexible graphite braided packing at the valve stem's sealing is a common practice in foreign manufactures. The top ring and the bottom ring of the packing are made of flexible graphite braided rings to prevent graphite from being squeezed into the valve cavity or outside of the valve cavity. They have advantages of good strength compared with mould pressing graphite rings. Mold pressing graphite rings are used in the middle. Sometimes, a braided ring is also added at an appropriate position in the middle. Because flexible graphite has good fluidity, anti-wear properties, good physical and chemical stability, it can be used as a filler to obtain a good seal and reduce the wear on the valve stem. For ball valves, plug valves, gate valves and globe valves with smaller stuffing boxes, using flexible graphite braided packing will be fine.
The pre-compression of the packing in the filling process is also one of the measures to ensure a good seal, especially a long-term good seal. The packing is only processed by pressing for most Chinese valves, so its sealing performance becomes worse with the increase of application time. Some foreign manufacturers use a pre-compression of 28MPa in the filling process and a special pre-compression tool is designed, so the packing can maintain a good seal for a long time. Of course, if the pre-compression of the packing is too great, it will increase the wear of the valve stem and opening force of the valve. Therefore, at this time, improvements should be made in the selection of the packing and surface treatment of the valve stem. The packing made of flexible graphite rings can not only reduce the wear of the valve stem but also the opening torque of the valve.
There are also some valves with a "the live load" structure at the packing gland, that is, a disc spring is provided to the bolts of the packing gland to make the packing gland always act on the packing with greater pressure, so as to prevent leakages due to the loosening of the gland plug or the packing.
External leakages of the sealing of gaskets
For gate valves, globe valves and check valves with DN greater than and equal to 50 mm, pressure-sealed bonnets can be used. For ball valves, plug valves (mostly lower bonnets) and gate valves, globe valves and check valves with DN smaller than and equal to 40 mm, flanged bonnets are generally used. There are two sealing parts for the pressure seal valve bonnet. One is the contact part between the valve bonnet and ring gasket, and the other is the contact part between the ring gasket and valve body. For some foreign valves, the contact surface between the ring gasket and valve bonnet is designed as a variable angle arc transition surface, forming a line seal with the valve bonnet, which significantly improves the sealing. However, the processing difficulty of the ring gasket is increased. In addition, the surface of the ring gasket and the corresponding valve body's sealing surface is plated with a layer of silver. Because silver has good plasticity, it is easy to fill the microscopic holes on the sealing surface, thus greatly improving the sealing performance of the valve bonnet's gasket.
For the flanged bonnet structure, most manufacturers use spiral wound gaskets, because they have good elasticity and high resilience rate, which makes them easier to ensure sealing while ensuring stability. The bonnet seal matched with the spiral wound gasket adopts a box structure, that is, the gasket is placed in a special groove where the bonnet and valve body are connected, thereby ensuring the stability of the spiral wound gasket. The stability of the gasket won't be affected by the great specific pressure. At the same time, the resistance of leakages of media is increased. Facts have proved that such a sealing structure is better.
Internal leakages
① Inlets of gate valves
There is not much difference in the design of the structure of wedge for the domestic and foreign manufacturers. Generally, the gate valve with DN less than and equal to 40 mm adopts the wedge-shaped integral wedge, while the gate valve with DN greater than and equal to 50 mm adopts the flexible wedge-shaped wedge. As long as the processing accuracy of the sealing surface meets the requirements, this structure can meet the requirement for the initial sealing. However, users hope that the valve can maintain a good seal for a long time. Under normal circumstances, the initial sealing is easy to achieve, but it is not easy to achieve good long-term sealing. Here are two improvements for reference. One is to strive to improve the accuracy of the guide rail of the wedge, so that it and the wedge maintain a small matching gap to reduce the uneven wear of the wedge and valve seat and ensure a longer sealing life. The guide rail is generally casted integrally with the valve body, and it is not easy to be machined, so the matching clearance with the wedge is relatively large. It is recommended that the manufacturer should pay attention to this aspect. The other is to consider the corrosion of the wedge and lower part of the valve seat. When the valve is in the half-open and half-closed position, a high-speed medium flow will be formed, which will cause serious erosion to the area through which the medium flows. Because the adhesion of hydrogen sulfide and metal corrosion products is very low, it is very easy to be flushed by the high-speed medium, thereby accelerating the corrosion of the metal, that is, the formation of erosion corrosion.
The measures to prevent erosion corrosion can start with the selection of good materials, that is, surfacing suitable cemented carbide materials on the corresponding parts. In addition to hard alloys surfacing on the sealing surface, some manufacturers also surface hard alloys in a certain range of the lower part of the wedge and valve seat to prevent erosion corrosion of the non-sealing surface. This is not related to the sealing of the valve, but it is beneficial to the valve as a whole.
② Inlets of globe valves
First of all, it is necessary to ensure the processing accuracy of the valve disc and valve seat to obtain a good initial seal. In terms of anti erosion and corrosion, a foreign manufacturer adopts a stepped disc structure, that is, when the valve is opened or closed, the non sealed steps on the disc firstly form a smaller flow path to withstand the erosion corrosion; the sealing surface of the valve disc and valve seat is no longer directly subjected to erosion corrosion, or the degree of erosion corrosion is alleviated.
③ Inlets of ball valves
Eccentric or other frictionless spherical structure and metal-to-metal sealing should be adopted. This is because the structure has good sealing reliability, good fire safety, no friction, prolonging the effective life of the valve.
④ Plug valves
In foreign countries, an inverted cone balanced valve core is generally adopted. Generally, a mechanical balance structure is adopted for valves with pressure less than and equal to CL300, and a pressure balanced structure for valves with pressure greater than and equal to CL600, which can not only ensure a good seal, but also reduce the wear of the sealing surface metal. A foreign manufacturer sprayed the plug on this basis, thus saving the workload of maintenance of regular sealant injection.
The internal and external leakages control of the valve is an important parameter that reflects the quality of the valve. The manufacturer has adopted various measures to minimize the internal leakage and external leakage of the valve. The internal and external leakages of oil and hydrogen will not only pollute the environment, but also cause a fire or an explosion. The internal leakage of the valve mainly occurs at the wedge, while the external leakage mainly occurs at the stem packing and bonnet's gasket. Therefore, we must take action for these three parts to obtain good sealing performance.
External leakages of the sealing of the packing
For gate valves and globe valves with DN greater than and equal to 50 mm, the use of flexible graphite rings and flexible graphite braided packing at the valve stem's sealing is a common practice in foreign manufactures. The top ring and the bottom ring of the packing are made of flexible graphite braided rings to prevent graphite from being squeezed into the valve cavity or outside of the valve cavity. They have advantages of good strength compared with mould pressing graphite rings. Mold pressing graphite rings are used in the middle. Sometimes, a braided ring is also added at an appropriate position in the middle. Because flexible graphite has good fluidity, anti-wear properties, good physical and chemical stability, it can be used as a filler to obtain a good seal and reduce the wear on the valve stem. For ball valves, plug valves, gate valves and globe valves with smaller stuffing boxes, using flexible graphite braided packing will be fine.
The pre-compression of the packing in the filling process is also one of the measures to ensure a good seal, especially a long-term good seal. The packing is only processed by pressing for most Chinese valves, so its sealing performance becomes worse with the increase of application time. Some foreign manufacturers use a pre-compression of 28MPa in the filling process and a special pre-compression tool is designed, so the packing can maintain a good seal for a long time. Of course, if the pre-compression of the packing is too great, it will increase the wear of the valve stem and opening force of the valve. Therefore, at this time, improvements should be made in the selection of the packing and surface treatment of the valve stem. The packing made of flexible graphite rings can not only reduce the wear of the valve stem but also the opening torque of the valve.
There are also some valves with a "the live load" structure at the packing gland, that is, a disc spring is provided to the bolts of the packing gland to make the packing gland always act on the packing with greater pressure, so as to prevent leakages due to the loosening of the gland plug or the packing.
External leakages of the sealing of gaskets
For gate valves, globe valves and check valves with DN greater than and equal to 50 mm, pressure-sealed bonnets can be used. For ball valves, plug valves (mostly lower bonnets) and gate valves, globe valves and check valves with DN smaller than and equal to 40 mm, flanged bonnets are generally used. There are two sealing parts for the pressure seal valve bonnet. One is the contact part between the valve bonnet and ring gasket, and the other is the contact part between the ring gasket and valve body. For some foreign valves, the contact surface between the ring gasket and valve bonnet is designed as a variable angle arc transition surface, forming a line seal with the valve bonnet, which significantly improves the sealing. However, the processing difficulty of the ring gasket is increased. In addition, the surface of the ring gasket and the corresponding valve body's sealing surface is plated with a layer of silver. Because silver has good plasticity, it is easy to fill the microscopic holes on the sealing surface, thus greatly improving the sealing performance of the valve bonnet's gasket.
For the flanged bonnet structure, most manufacturers use spiral wound gaskets, because they have good elasticity and high resilience rate, which makes them easier to ensure sealing while ensuring stability. The bonnet seal matched with the spiral wound gasket adopts a box structure, that is, the gasket is placed in a special groove where the bonnet and valve body are connected, thereby ensuring the stability of the spiral wound gasket. The stability of the gasket won't be affected by the great specific pressure. At the same time, the resistance of leakages of media is increased. Facts have proved that such a sealing structure is better.
Internal leakages
① Inlets of gate valves
There is not much difference in the design of the structure of wedge for the domestic and foreign manufacturers. Generally, the gate valve with DN less than and equal to 40 mm adopts the wedge-shaped integral wedge, while the gate valve with DN greater than and equal to 50 mm adopts the flexible wedge-shaped wedge. As long as the processing accuracy of the sealing surface meets the requirements, this structure can meet the requirement for the initial sealing. However, users hope that the valve can maintain a good seal for a long time. Under normal circumstances, the initial sealing is easy to achieve, but it is not easy to achieve good long-term sealing. Here are two improvements for reference. One is to strive to improve the accuracy of the guide rail of the wedge, so that it and the wedge maintain a small matching gap to reduce the uneven wear of the wedge and valve seat and ensure a longer sealing life. The guide rail is generally casted integrally with the valve body, and it is not easy to be machined, so the matching clearance with the wedge is relatively large. It is recommended that the manufacturer should pay attention to this aspect. The other is to consider the corrosion of the wedge and lower part of the valve seat. When the valve is in the half-open and half-closed position, a high-speed medium flow will be formed, which will cause serious erosion to the area through which the medium flows. Because the adhesion of hydrogen sulfide and metal corrosion products is very low, it is very easy to be flushed by the high-speed medium, thereby accelerating the corrosion of the metal, that is, the formation of erosion corrosion.
The measures to prevent erosion corrosion can start with the selection of good materials, that is, surfacing suitable cemented carbide materials on the corresponding parts. In addition to hard alloys surfacing on the sealing surface, some manufacturers also surface hard alloys in a certain range of the lower part of the wedge and valve seat to prevent erosion corrosion of the non-sealing surface. This is not related to the sealing of the valve, but it is beneficial to the valve as a whole.
② Inlets of globe valves
First of all, it is necessary to ensure the processing accuracy of the valve disc and valve seat to obtain a good initial seal. In terms of anti erosion and corrosion, a foreign manufacturer adopts a stepped disc structure, that is, when the valve is opened or closed, the non sealed steps on the disc firstly form a smaller flow path to withstand the erosion corrosion; the sealing surface of the valve disc and valve seat is no longer directly subjected to erosion corrosion, or the degree of erosion corrosion is alleviated.
③ Inlets of ball valves
Eccentric or other frictionless spherical structure and metal-to-metal sealing should be adopted. This is because the structure has good sealing reliability, good fire safety, no friction, prolonging the effective life of the valve.
④ Plug valves
In foreign countries, an inverted cone balanced valve core is generally adopted. Generally, a mechanical balance structure is adopted for valves with pressure less than and equal to CL300, and a pressure balanced structure for valves with pressure greater than and equal to CL600, which can not only ensure a good seal, but also reduce the wear of the sealing surface metal. A foreign manufacturer sprayed the plug on this basis, thus saving the workload of maintenance of regular sealant injection.
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