1. The structural characteristics of the double cut-off plug valve
The double cut-off plug valve (Figure 1) is designed for special applications with strict sealing and cut-off requirements. The valve design is very compact; it is light and takes up small space. Also, the advantage of this valve design is that it is shorter than the conventional two shut-off valve and discharge valve. In most cases, the structural length of the double cut-off valve is the same as a common single plug valve or ball valve. The double cut-off plug valve is designed based on the structure of the pressure balanced inverted cone plug valve, so the operation and maintenance methods are the same as those of pressure balanced inverted cone plug valves. Because the only moving parts are the plug and valve stem, the valve is easy to operate. When the plug rotates 90° clockwise, the valve completes the action from opening to closing, and vice versa. The taper of the plug is 1:6, which is completely tightly assembled with the valve body, and a metal-to-metal hard seal is adopted, that is, a soft seal that can be damaged by the flowing fluid is not used. The double cut-off plug valve is designed and manufactured according to the principle of "pressure balance", which means the plug is equipped with a pressure balance hole to ensure that the plug is always balanced in the axial direction, thereby preventing the plug from being jammed. In addition, in order to reduce the torque of the valve, the surface of the plug is sprayed with a PTFE protective layer by thermal spraying technology. The plug and valve stem are two independent parts, which are connected by the balance ring of the universal connecting piece. The valve stem cannot be removed freely, and it can only be taken from the bottom when it is reassembled.
As a secondary seal, the valve provides an oil seal system. When the valve is activated, special sealing grease is allowed to enter the valve's oil seal circuit. In addition to the sealing function, it can also play a role of lubrication to prevent corrosion and wear of internal parts, and at the same time reduce the operating torque of the valve. The valve has three independent sealing methods, which are its unique advantages. It is composed of a reinforced plate R-PTFE and a 100% pure graphite seal composed of a stainless steel backup ring. It is very effective to use graphite to seal in ultra-high temperature conditions and occasions that meet various demanding requirements for fire protection design. The top of the valve stem is the place for the primary sealing. This is a PTFE sealing ring (4) with an embedded special alloy spring. The sealing ring can be replaced from the outside. It is used to compress the valve stem bearing (2) and the opening or locking ring. In addition to the sealing of the valve stem mentioned above, the manually operated valve has a waterproof sealing function to prevent water penetration and dirt from entering the valve stem.
Figure 1 Double cut-off plug valves (Partial parts)
2. Valve stem bearing 4. Sealing rings 5. Gaskets 6. Thrust washers 7. Valve bodies 8. Extending pieces 9. Bottom covers 11. Pressure adjusting screws 12. Bottom screws 13. Nuts 13a. Bolts 14. Fixing rings 15. Adjusting gaskets 16. Plug bodies 17. Balance rings 18. Valve stems 21. One-way valves 22. Keys 24. Grease injection valves 25. Elastic clamp rings 26. Springs 27. Steel balls 29. Driving devices 41. Fireproof gaskets 70. Graphite sealing rings
The bottom cover (9) is fixed on the valve body with bolts (13a) and nuts (13). Two soft plates or diaphragms are placed in the groove between the valve body and bottom cover. They mainly play the role of metal sealing and preventing the medium from leaking when the pressure adjustment screw (11), fixing ring ( 14) and bottom screw (12) are adjusted.
The valve also has a new feature, that is, a protection interface is provided between the two valve spools to prevent internal cavitation, which is called a dedicated pressure release port. This feature is an interface leading from the sealing surface of the valve body against the diaphragm to the outside of each valve spool. When the operation is normal, the interface is closed by the pressure of the bottom cover, and the bottom cover is fixed by bolts and nuts against the direction of the diaphragm. In the situation of the two valve spools being in the closed position, once overpressure occurs, if the valve is easily affected by thermal stress, the pressure against the bottom cover of the valve will make the bolt deformed (stretched), thus opening the release valve, and the pressure will be released in the pipeline. The pipeline's pressure is released to the atmosphere in a short time, and the deformation of the bottom screw exceeds the allowable limit value defined by the ASME or API specification in a short time. When the pressure drops, the bolt returns to the bottom cover, and then closes this interface. In addition to the metal seal, a pure graphite sealing ring (70) is also placed between the valve body and bottom cover.
According to the needs of different users, the exhaust interface will be provided accordingly. This interface allows the sealed inlet of the tested valve, and any leakage through the first spool can be detected through it. The valve spool is adjusted by the pressure adjusting screw (11) inside the valve body. When the bottom screw (12) is tightened, the valve spool is fixed by the fixing ring (14). All adjustments to the valve spool are achieved through an adjustment washer composed of two diaphragms.
As mentioned above, the plug valve provides an oil sealing system, which penetrates the allowable special sealing grease into the valve through the grease injection valve (24) and one-way valve (21). The sealing grease is sprayed into the groove through a high-pressure oil gun. This system ensures that all sealing surfaces have a thin layer of lubricant, and an effective secondary seal can be achieved by doing this. In addition, the connecting plate of the driving device of the double cut-off plug valve is designed according to the ISO standard, which can be matched with various actuators. The valve has completely two-way flows, no restricted installation directions, and it can be installed in any position.
2. Flow rates of double cut-off plug valves
The valve is designed according to the requirement for keeping the minimum flow rate change to achieve the purpose of the maximum flow area when the minimum flow rate changes. The result of this design makes the CV value of the valve the highest among valves of similar specifications. The double cut-off plug valve has greater flow rates and smaller pressure losses than the two single independent plug valve with the same diameter.
The double cut-off plug valve (Figure 1) is designed for special applications with strict sealing and cut-off requirements. The valve design is very compact; it is light and takes up small space. Also, the advantage of this valve design is that it is shorter than the conventional two shut-off valve and discharge valve. In most cases, the structural length of the double cut-off valve is the same as a common single plug valve or ball valve. The double cut-off plug valve is designed based on the structure of the pressure balanced inverted cone plug valve, so the operation and maintenance methods are the same as those of pressure balanced inverted cone plug valves. Because the only moving parts are the plug and valve stem, the valve is easy to operate. When the plug rotates 90° clockwise, the valve completes the action from opening to closing, and vice versa. The taper of the plug is 1:6, which is completely tightly assembled with the valve body, and a metal-to-metal hard seal is adopted, that is, a soft seal that can be damaged by the flowing fluid is not used. The double cut-off plug valve is designed and manufactured according to the principle of "pressure balance", which means the plug is equipped with a pressure balance hole to ensure that the plug is always balanced in the axial direction, thereby preventing the plug from being jammed. In addition, in order to reduce the torque of the valve, the surface of the plug is sprayed with a PTFE protective layer by thermal spraying technology. The plug and valve stem are two independent parts, which are connected by the balance ring of the universal connecting piece. The valve stem cannot be removed freely, and it can only be taken from the bottom when it is reassembled.
As a secondary seal, the valve provides an oil seal system. When the valve is activated, special sealing grease is allowed to enter the valve's oil seal circuit. In addition to the sealing function, it can also play a role of lubrication to prevent corrosion and wear of internal parts, and at the same time reduce the operating torque of the valve. The valve has three independent sealing methods, which are its unique advantages. It is composed of a reinforced plate R-PTFE and a 100% pure graphite seal composed of a stainless steel backup ring. It is very effective to use graphite to seal in ultra-high temperature conditions and occasions that meet various demanding requirements for fire protection design. The top of the valve stem is the place for the primary sealing. This is a PTFE sealing ring (4) with an embedded special alloy spring. The sealing ring can be replaced from the outside. It is used to compress the valve stem bearing (2) and the opening or locking ring. In addition to the sealing of the valve stem mentioned above, the manually operated valve has a waterproof sealing function to prevent water penetration and dirt from entering the valve stem.
Figure 1 Double cut-off plug valves (Partial parts)
2. Valve stem bearing 4. Sealing rings 5. Gaskets 6. Thrust washers 7. Valve bodies 8. Extending pieces 9. Bottom covers 11. Pressure adjusting screws 12. Bottom screws 13. Nuts 13a. Bolts 14. Fixing rings 15. Adjusting gaskets 16. Plug bodies 17. Balance rings 18. Valve stems 21. One-way valves 22. Keys 24. Grease injection valves 25. Elastic clamp rings 26. Springs 27. Steel balls 29. Driving devices 41. Fireproof gaskets 70. Graphite sealing rings
The bottom cover (9) is fixed on the valve body with bolts (13a) and nuts (13). Two soft plates or diaphragms are placed in the groove between the valve body and bottom cover. They mainly play the role of metal sealing and preventing the medium from leaking when the pressure adjustment screw (11), fixing ring ( 14) and bottom screw (12) are adjusted.
The valve also has a new feature, that is, a protection interface is provided between the two valve spools to prevent internal cavitation, which is called a dedicated pressure release port. This feature is an interface leading from the sealing surface of the valve body against the diaphragm to the outside of each valve spool. When the operation is normal, the interface is closed by the pressure of the bottom cover, and the bottom cover is fixed by bolts and nuts against the direction of the diaphragm. In the situation of the two valve spools being in the closed position, once overpressure occurs, if the valve is easily affected by thermal stress, the pressure against the bottom cover of the valve will make the bolt deformed (stretched), thus opening the release valve, and the pressure will be released in the pipeline. The pipeline's pressure is released to the atmosphere in a short time, and the deformation of the bottom screw exceeds the allowable limit value defined by the ASME or API specification in a short time. When the pressure drops, the bolt returns to the bottom cover, and then closes this interface. In addition to the metal seal, a pure graphite sealing ring (70) is also placed between the valve body and bottom cover.
According to the needs of different users, the exhaust interface will be provided accordingly. This interface allows the sealed inlet of the tested valve, and any leakage through the first spool can be detected through it. The valve spool is adjusted by the pressure adjusting screw (11) inside the valve body. When the bottom screw (12) is tightened, the valve spool is fixed by the fixing ring (14). All adjustments to the valve spool are achieved through an adjustment washer composed of two diaphragms.
As mentioned above, the plug valve provides an oil sealing system, which penetrates the allowable special sealing grease into the valve through the grease injection valve (24) and one-way valve (21). The sealing grease is sprayed into the groove through a high-pressure oil gun. This system ensures that all sealing surfaces have a thin layer of lubricant, and an effective secondary seal can be achieved by doing this. In addition, the connecting plate of the driving device of the double cut-off plug valve is designed according to the ISO standard, which can be matched with various actuators. The valve has completely two-way flows, no restricted installation directions, and it can be installed in any position.
2. Flow rates of double cut-off plug valves
The valve is designed according to the requirement for keeping the minimum flow rate change to achieve the purpose of the maximum flow area when the minimum flow rate changes. The result of this design makes the CV value of the valve the highest among valves of similar specifications. The double cut-off plug valve has greater flow rates and smaller pressure losses than the two single independent plug valve with the same diameter.
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