(2) The structure design of the dripping board
Since the medium in the valve is a low-temperature medium, in order to avoid or reduce the transmission of the medium's temperature to the valve stem and the filling material at the upper end, and prevent these materials from failing due to freezing, a dripping plate can be added to the valve. Some research institutes have carried out experimental verification on this kind of valve with a drip plate, and proved that the upper end of the valve bonnet with a drip plate has a relatively high temperature. The temperature at the upper part of the extended valve bonnet is low, and the valve is usually exposed to the air. The water vapor in the air will liquefy into water droplets when encountering the low-temperature valve bonnet. The diameter of the dripping plate exceeds that of the middle flange, which can prevent water vapor caused by low temperatures from dripping on the middle flange's bolts and avoid affecting maintenance due to the rust of the bolt. In addition, the drip plate needs to be arranged outside the cold insulation layer to prevent condensed water from falling onto the cold insulation layer and upper part of the valve body, protect the cold insulation layer and prevent the loss of cold capacity.
(3) Structural design of pressure relief components
For cryogenic valves with a closed cavity structure, when used in flammable, explosive and easily vaporized media, there are special requirements for the valve's sealing structure. Some low-temperature media will increase in volume after vaporization. For example, the volume of LNG after vaporization is 600 times more than that of liquid. When the valve is closed and the ambient temperature is relatively high, the low-temperature medium in the valve body absorbs the heat in the environment. It vaporizes gradually due to the heat, and its volume rises rapidly, causing overpressure inside the valve, even threatening the safety of the valve, causing medium leakage and even fire accidents. To ensure the safety of the valve and the factory, this type of valve requires self-venting with a cavity The pressure structure enables automatic relief when the valve cavity pressure is abnormally overpressured. For example, there will be obvious differences in pressure relief design for low-temperature gate valves and ball valves due to different valve sealing principles. However, different manufacturers have their own different characteristics in the design of the pressure relief structure.
(4) Anti-static and fireproof structure design
Since cryogenic valves are generally used in flammable and explosive media, anti-static and fire protection design are particularly important. The anti-static design is mainly based on a kind of guide current method similar to a lightning rod, which conducts the valve stem and valve body to derive static electricity to eliminate potential safety hazards and ensure the safety of the entire system. For example, GB/T24925 clearly stipulates that valves with soft seats or soft closing inserts used for flammable vapor or liquids should be designed to ensure that the valve body and stem have conductive continuity, and the great resistance of the discharge path should not exceed 10Ω. The design of the fire protection structure is mainly aimed at the problem of leakages of media caused by the drastic temperature change. The design of the fire protection structure is similar to the design requirements for ordinary valves.
(5) Valve bodies
The valve body should be able to fully withstand the expansion and contraction caused by temperature changes. Moreover, the structure of the valve seat will not be deformed due to temperature changes.
(6) Valve discs
A gate valve adopts a flexible wedge or an open wedge. The flat seat of globe valves and needle valves adopt a plug-shaped disc. These structures can maintain a reliable seal no matter how the temperature changes.
(7) Valve stems
The valve stem needs to be chrome plated, nickel-phosphorus plated or nitrided to increase the surface hardness of the valve stem and prevent the stem packing and packing gland from seizing each other, damaging the packing and causing the packing box to leak.
(8) Gaskets
The low-temperature properties of the gasket's material should be considered for the selection of gaskets, such as compression resilience, pre-tightening force, tightening pressure distribution, and stress relaxation.
(9) Packing boxes and packing
The packing box cannot be in direct contact with the low-temperature section, but it is set at the top of the long-neck valve bonnet, so that the stuffing box is located far away from the low temperature and works in a temperature environment above 0℃. In this way, the sealing effect of the packing box is improved. In the event of leakages, or when the low-temperature fluid directly contacts the packing and the sealing effect is reduced, grease can be added from the middle of the packing box to form an oil seal layer to reduce the pressure difference of the packing box as an auxiliary sealing measure. The packing box mostly adopts a two-stage packing structure with a metal isolating ring in the middle. However, there are some other types such as general valves with a packing box structure and a self-tightening valve stem with a double packing box structure.
(10) Upper sealing
All cryogenic valves are equipped with a sealing seat structure. The upper sealing surface should be welded with cobalt-chromium-tungsten carbide and polished after finishing.
(11) Sealing surfaces of valve seats and plates
The closing part of the cryogenic valve adopts a cobalt-chromium-tungsten carbide surfacing structure. The soft sealing structure is only suitable for cryogenic valves with a temperature higher than -70℃ due to the great expansion coefficient of PTFE and brittleness at low temperatures. However, PTFE can be used for cryogenic valves with a temperature of -162℃.
(12) Bolts of the middle flanges
Bolts should have sufficient strength, because bolts work under repeated loads and often break due to fatigue. Because stress concentration easily happens at the thread roots of bolts, bolts with a full-thread structure are used.
Since the medium in the valve is a low-temperature medium, in order to avoid or reduce the transmission of the medium's temperature to the valve stem and the filling material at the upper end, and prevent these materials from failing due to freezing, a dripping plate can be added to the valve. Some research institutes have carried out experimental verification on this kind of valve with a drip plate, and proved that the upper end of the valve bonnet with a drip plate has a relatively high temperature. The temperature at the upper part of the extended valve bonnet is low, and the valve is usually exposed to the air. The water vapor in the air will liquefy into water droplets when encountering the low-temperature valve bonnet. The diameter of the dripping plate exceeds that of the middle flange, which can prevent water vapor caused by low temperatures from dripping on the middle flange's bolts and avoid affecting maintenance due to the rust of the bolt. In addition, the drip plate needs to be arranged outside the cold insulation layer to prevent condensed water from falling onto the cold insulation layer and upper part of the valve body, protect the cold insulation layer and prevent the loss of cold capacity.
(3) Structural design of pressure relief components
For cryogenic valves with a closed cavity structure, when used in flammable, explosive and easily vaporized media, there are special requirements for the valve's sealing structure. Some low-temperature media will increase in volume after vaporization. For example, the volume of LNG after vaporization is 600 times more than that of liquid. When the valve is closed and the ambient temperature is relatively high, the low-temperature medium in the valve body absorbs the heat in the environment. It vaporizes gradually due to the heat, and its volume rises rapidly, causing overpressure inside the valve, even threatening the safety of the valve, causing medium leakage and even fire accidents. To ensure the safety of the valve and the factory, this type of valve requires self-venting with a cavity The pressure structure enables automatic relief when the valve cavity pressure is abnormally overpressured. For example, there will be obvious differences in pressure relief design for low-temperature gate valves and ball valves due to different valve sealing principles. However, different manufacturers have their own different characteristics in the design of the pressure relief structure.
(4) Anti-static and fireproof structure design
Since cryogenic valves are generally used in flammable and explosive media, anti-static and fire protection design are particularly important. The anti-static design is mainly based on a kind of guide current method similar to a lightning rod, which conducts the valve stem and valve body to derive static electricity to eliminate potential safety hazards and ensure the safety of the entire system. For example, GB/T24925 clearly stipulates that valves with soft seats or soft closing inserts used for flammable vapor or liquids should be designed to ensure that the valve body and stem have conductive continuity, and the great resistance of the discharge path should not exceed 10Ω. The design of the fire protection structure is mainly aimed at the problem of leakages of media caused by the drastic temperature change. The design of the fire protection structure is similar to the design requirements for ordinary valves.
(5) Valve bodies
The valve body should be able to fully withstand the expansion and contraction caused by temperature changes. Moreover, the structure of the valve seat will not be deformed due to temperature changes.
(6) Valve discs
A gate valve adopts a flexible wedge or an open wedge. The flat seat of globe valves and needle valves adopt a plug-shaped disc. These structures can maintain a reliable seal no matter how the temperature changes.
(7) Valve stems
The valve stem needs to be chrome plated, nickel-phosphorus plated or nitrided to increase the surface hardness of the valve stem and prevent the stem packing and packing gland from seizing each other, damaging the packing and causing the packing box to leak.
(8) Gaskets
The low-temperature properties of the gasket's material should be considered for the selection of gaskets, such as compression resilience, pre-tightening force, tightening pressure distribution, and stress relaxation.
(9) Packing boxes and packing
The packing box cannot be in direct contact with the low-temperature section, but it is set at the top of the long-neck valve bonnet, so that the stuffing box is located far away from the low temperature and works in a temperature environment above 0℃. In this way, the sealing effect of the packing box is improved. In the event of leakages, or when the low-temperature fluid directly contacts the packing and the sealing effect is reduced, grease can be added from the middle of the packing box to form an oil seal layer to reduce the pressure difference of the packing box as an auxiliary sealing measure. The packing box mostly adopts a two-stage packing structure with a metal isolating ring in the middle. However, there are some other types such as general valves with a packing box structure and a self-tightening valve stem with a double packing box structure.
(10) Upper sealing
All cryogenic valves are equipped with a sealing seat structure. The upper sealing surface should be welded with cobalt-chromium-tungsten carbide and polished after finishing.
(11) Sealing surfaces of valve seats and plates
The closing part of the cryogenic valve adopts a cobalt-chromium-tungsten carbide surfacing structure. The soft sealing structure is only suitable for cryogenic valves with a temperature higher than -70℃ due to the great expansion coefficient of PTFE and brittleness at low temperatures. However, PTFE can be used for cryogenic valves with a temperature of -162℃.
(12) Bolts of the middle flanges
Bolts should have sufficient strength, because bolts work under repeated loads and often break due to fatigue. Because stress concentration easily happens at the thread roots of bolts, bolts with a full-thread structure are used.
Next: The Installation Requirements for Cryogenic Valves
Previous: The Design of Petrochemical Cryogenic Valves (Part One)
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