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Gate valves and globe valves are two common types of valves in industrial pipeline systems. They are widely used in the petroleum, chemical, and electric power industries, among others. The upper sealing structure is an important part to ensure the operation of valves under harsh working conditions such as high pressure and high temperature.
Upper sealing structure
The upper sealing structure of gate valves and globe valves usually consists of valve bonnets, valve stems, packing boxes, packing, and packing glands. Its main purpose is to prevent the medium from leaking through the gap between the valve stem and the valve bonnet. The upper sealing structure of the gate valve can be divided into packing seal, bellows seal, and O-ring seal. The packing seal uses flexible graphite, polytetrafluoroethylene (PTFE), and other materials as packing, compressed by the packing gland to form a seal. The bellows are connected to the valve stem, and the sealing is achieved through the elasticity of the bellows. It is suitable for occasions with high sealing requirements. An O-ring is installed on the valve stem, and the sealing is achieved through the elastic deformation of the O-ring.
During the pressure test of gate valves and globe valves, according to the requirements of GB/T13927 and JB/T9092, an upper sealing test must be carried out. Why are there no requirements for ball valves, butterfly valves, check valves, and plug valves? Because gate valves and globe valves have a unique valve stem sealing structure.
The upper seal refers to the conical or spherical upper sealing surface on the valve stem, forming an effective seal with the upper sealing seat of the valve bonnet when the valve is fully opened. Its structure is shown in the figure.
When the valve is fully opened, the upper seal can reduce the force of the medium on the packing, reducing the extrusion deformation of the packing, extending the service life of the packing, and reducing the possibility of leakages at the valve stem. Therefore, when the gate valve and globe valve are opened, they must be fully opened so that the sealing surface on the valve stem and upper sealing seat can fit well to form an effective seal. When the globe valve is closed, the general flow direction is from the bottom of the valve disc, and the pressure is isolated by the valve disc. At this time, the valve stem packing is not affected by the medium pressure. When the flow is from above the valve disc, the packing is under pressure from the medium pressure. Therefore, the flow direction of the globe valve is mostly from below the valve disc.
When the gate valve is closed, for a gate valve with double-side sealing, the pressure in the closed cavity of the valve body is less than the inlet pressure. For a gate valve with single-side sealing, such as a slab gate valve, the medium pressure pushes the inlet end seal and enters the valve body cavity, so the packing is always under pressure. Not all gate valves have upper seals; for example, knife gate valves. The Z45 non-rising stem gate valve and bellows gate valve also have no upper seal. Similarly, not all globe valves have upper seals, such as bellows globe valves.
According to JB/T5210, for cast carbon steel gate valves and globe valves, the upper sealing seat can be processed separately and installed in the upper valve bonnet. Generally, 1Cr13, 304 stainless steel, and 316 stainless steel are used.
For austenitic stainless steel valve bonnets, the upper sealing surface can be directly processed on the valve bonnet without the need for a sealing seat. Moreover, austenitic stainless steel and hard surface materials can be welded at the corresponding positions of the carbon steel valve bonnet, and then the sealing surface can be processed. The thickness of the weld layer should not be less than 1.6 mm after processing. It is recommended to directly weld and then process the sealing surface for globe valves less than DN50. According to GB/T13927 and JB/T9092 requirements, when the upper seal pressure test is performed, a high-pressure upper seal test should be conducted. The pressure is 1.1 times the maximum allowable working pressure at 38°C. The high-pressure upper seal test medium can be water, air, kerosene, etc. The low-pressure upper seal test pressure is 0.4-0.7 MPa, and the low-pressure upper seal test medium can be air or inert gas.
During the upper seal pressure test, the valve should be fully opened, the packing gland should be loosened, and no visible leakage is allowed, including dripping or bubble leakage. Valves that have passed the upper seal pressure test are not allowed to replace the packing under pressure to prevent danger. Because the media in the pipeline are diverse, it can be water, high-temperature steam, acid, and alkali media, and any leakage will cause an accident. Therefore, valves with upper seals are not allowed to replace the packing under pressure.
During the pressure test of gate valves and globe valves, according to the requirements of GB/T13927 and JB/T9092, an upper sealing test must be carried out. Why are there no requirements for ball valves, butterfly valves, check valves, and plug valves? Because gate valves and globe valves have a unique valve stem sealing structure.
The upper seal refers to the conical or spherical upper sealing surface on the valve stem, forming an effective seal with the upper sealing seat of the valve bonnet when the valve is fully opened. Its structure is shown in the figure.
When the valve is fully opened, the upper seal can reduce the force of the medium on the packing, reducing the extrusion deformation of the packing, extending the service life of the packing, and reducing the possibility of leakages at the valve stem. Therefore, when the gate valve and globe valve are opened, they must be fully opened so that the sealing surface on the valve stem and upper sealing seat can fit well to form an effective seal. When the globe valve is closed, the general flow direction is from the bottom of the valve disc, and the pressure is isolated by the valve disc. At this time, the valve stem packing is not affected by the medium pressure. When the flow is from above the valve disc, the packing is under pressure from the medium pressure. Therefore, the flow direction of the globe valve is mostly from below the valve disc.
When the gate valve is closed, for a gate valve with double-side sealing, the pressure in the closed cavity of the valve body is less than the inlet pressure. For a gate valve with single-side sealing, such as a slab gate valve, the medium pressure pushes the inlet end seal and enters the valve body cavity, so the packing is always under pressure. Not all gate valves have upper seals; for example, knife gate valves. The Z45 non-rising stem gate valve and bellows gate valve also have no upper seal. Similarly, not all globe valves have upper seals, such as bellows globe valves.
According to JB/T5210, for cast carbon steel gate valves and globe valves, the upper sealing seat can be processed separately and installed in the upper valve bonnet. Generally, 1Cr13, 304 stainless steel, and 316 stainless steel are used.
For austenitic stainless steel valve bonnets, the upper sealing surface can be directly processed on the valve bonnet without the need for a sealing seat. Moreover, austenitic stainless steel and hard surface materials can be welded at the corresponding positions of the carbon steel valve bonnet, and then the sealing surface can be processed. The thickness of the weld layer should not be less than 1.6 mm after processing. It is recommended to directly weld and then process the sealing surface for globe valves less than DN50. According to GB/T13927 and JB/T9092 requirements, when the upper seal pressure test is performed, a high-pressure upper seal test should be conducted. The pressure is 1.1 times the maximum allowable working pressure at 38°C. The high-pressure upper seal test medium can be water, air, kerosene, etc. The low-pressure upper seal test pressure is 0.4-0.7 MPa, and the low-pressure upper seal test medium can be air or inert gas.
During the upper seal pressure test, the valve should be fully opened, the packing gland should be loosened, and no visible leakage is allowed, including dripping or bubble leakage. Valves that have passed the upper seal pressure test are not allowed to replace the packing under pressure to prevent danger. Because the media in the pipeline are diverse, it can be water, high-temperature steam, acid, and alkali media, and any leakage will cause an accident. Therefore, valves with upper seals are not allowed to replace the packing under pressure.
Conclusion
The upper sealing structure of gate valves and globe valves plays a vital role in the normal operation of the valve. Selecting suitable sealing materials, ensuring appropriate clamping force, ensuring the surface finish of the valve stem and stuffing box, and designing a compact sealing structure are the keys to achieving long-term and stable operation of the valve. Through reasonable design and manufacturing processes, the sealing performance of gate valves and globe valves can be effectively improved to ensure the safe and reliable operation of industrial pipeline systems.
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