Find once again the paper, quoted the Shanghai Yiling company's technology news, just also is the difference between cryogenic ball valves and ordinary ball valves, but the site does not allow copying, I typed out one by one, please adopt.
Low-temperature ball valves, most of the working medium for the flammable, explosive, permeability of the material, the lowest working temperature of up to -269 ℃, the highest use of pressure up to 10MPa, the conditions of work are relatively harsh. Therefore, the low-temperature valve design, manufacturing, testing and room temperature valves compared to a big difference.
Low-temperature valves are valves that can be used in low-temperature operating conditions, usually the operating temperature is lower than -40 ℃ valves called low-temperature valves. Cryogenic ball valves are one of the indispensable and important equipments for petrochemical, air separation, natural gas and other industries, and their quality determines whether they can be produced safely, economically and continuously. With the development of modern science and technology, the use of cryogenic valves is becoming more and more extensive, and the demand is also growing.
From the material point of view, for example, the U.S. VITON VITON cryogenic ball valves CF8, LCB, LF1, F304 and so on more than a dozen kinds, can be applied to different temperatures and media, and have formulated the relevant standards, not only provides for the castings and forgings of the dimensions and the appearance of the quality requirements, but also on the castings and forgings of the chemical composition, heat treatment, mechanical properties, physical properties, weld, post-weld heat treatment, flaw detection, intergranular corrosion test (austenitic corrosion test), and the quality of the castings and forgings of the castings and forgings.
According to the U.S. VTON cryogenic ball valves manufacturing standards and requirements, cryogenic ball valves have different characteristics from ordinary ball valves:
1 cryogenic valves of the general design requirements
(1) valves and their components in the low-temperature media and the surrounding ambient temperature should be able to work for long periods of time (generally 10 years or 3,500 ~ 5,000 times cycle)
(2) the ball valve relative to the low-temperature media, should not be a significant heat source, this is because the inflow of heat will reduce the thermal efficiency, and the inflow of heat is too much, but also may make the valve inside the low-temperature media vaporization, resulting in an abnormal pressure, resulting in danger;
(3) low-temperature media should not be handwheel operating performance and packing sealing performance of the harmful effects;
(4) directly with the low-temperature media and the surrounding environment temperature should have long working ability (generally 10 years or 3500 ~ cycle)
(4) the structure of the valve assembly in direct contact with cryogenic media should meet the relevant explosion-proof and fire requirements;
(5) the valve assembly working at low temperatures can not be lubricated, so measures need to be taken to prevent the friction parts from being scratched.
The above requirements should be throughout the design process of cryogenic valves, in addition, it should be noted that the above requirements are unique to cryogenic valves, cryogenic valves should also be designed to comply with the relevant requirements of the general valve.
2 cryogenic valve cooling performance
Cooling performance of cryogenic valves refers to cryogenic ball valves from room temperature cooling to the working temperature of the ability. This performance can be measured by using the valve in the above process of energy consumption, that is, the valve in the above process of heat transfer to the cryogenic medium Q2. For the cyclical work of low-temperature valves for cooling performance indicators have an extremely important significance. But only Q2 to measure the cooling performance of cryogenic ball valves is not enough, the following indicators can be used:
3.1 Valve body, bonnet, seat, opening and closing parts, such as the choice of materials
Temperature higher than -100 ° C can be selected ferrite stainless steel, the temperature is lower than -100 ° C selection of austenitic stainless steel, low-pressure and small-diameter valves can be selected from copper alloy or aluminum alloy.
3.2 Valve stem material selection
Made of austenitic stainless acid-resistant steel, need to undergo appropriate heat treatment to improve the tensile strength, and must be plated with hard chrome (plating thickness of 0.04 ~ 0.06mm), or nitriding treatment to improve surface hardness.
3.3 fastener material selection
Temperature higher than -100 ℃, bolt material Ni, Cr-Mo and other alloy steels, need to be properly heat-treated to prevent thread bite; low temperature sub-100 ℃, bolt material can be used austenitic stainless steel. Nut material is generally used Mo steel or Ni steel, at the same time the thread surface and coated with disulfide key.
3.4 Gasket Material Selection
When the temperature is higher than -196℃ and the maximum pressure is 3MPa, the asbestos rubber sheet made of long-fiber asbestos can be used; when the temperature is higher than -196℃ and the maximum pressure is 5MPa, the asbestos wound gasket with stainless steel belt, PTFE wound gasket with stainless steel belt, or expanded graphite wound gasket with stainless steel belt, or expanded graphite wound gasket with stainless steel belt, or expanded graphite wound gasket with stainless steel belt, or expanded graphite wound gasket with stainless steel belt, or expanded graphite as stainless steel belt. Expanded graphite wound gasket.
It should be emphasized here that all cryogenic material parts must be deep-cooled before finishing to reduce the shrinkage and deformation of cryogenic valves under low-temperature conditions.
4 low-temperature valve structure design
Low-temperature valve structure and general-purpose valve there are some differences in the process of structural design of low-temperature valves, in addition to considering the general requirements of the valve structure, but also need to focus on solving the following problems:
(1) after the closure of the valve, residual low-temperature medium in the valve body due to the temperature and rapid vaporization, resulting in an abnormal pressure rise inside the valve body. Valve body internal abnormal pressurization of the problem; (2) low temperature on the packing box sealing performance of the adverse effects; (3) cold deformation of parts and components of the valve of the harmful effects; (4) low-temperature media on the parts of the explosion-proof requirements, and so on.
It should also be noted that low-temperature ball valves in addition to low-temperature media, the same work in the surrounding ambient temperature, that is, in the temperature of about 20 ℃ work, in the design of the valve components, especially in the design of the opening and closing seals must take this into account.
According to the actual needs of the work site, the structural design of low-temperature valves to put forward the following baseline requirements:
(1) the valve body should be able to fully withstand the expansion and contraction caused by temperature changes, and the structure of the valve seat part of the structure will not be permanently deformed due to temperature changes;
(2) the use of long-necked bonnet structure that can protect the stuffing box;
(3) Use no matter how the temperature changes can maintain a reliable seal of the valve flap, such as gate valve with elastic gate and open gate, globe valve with conical valve flap, etc.
(4) the use of the upper sealing structure;
(5) the use of drilled chrome tungsten carbide surfacing structure of the valve seat, flap sealing surface;
(6) the use of the pressure relief holes to prevent abnormal pressurization, the opening of the relief hole position depends on the valve structure, can be located in the valve body. Valve structure, can be located in the valve body, can also be located in the gate.
4.1 low-temperature ball valve valve body design
Valve body is the main pressure parts of the valve, must have a certain degree of strength in order to ensure the normal operation of the valve. In low-temperature conditions, the valve body is subjected to low-temperature stress, expansion and contraction of additional stress are very large, to maintain the valve sealing vice does not deform, the valve body must have a certain degree of rigidity. At the same time, to prevent damage caused by low-temperature stress concentration, should try to avoid sharp corners, grooves, etc. in the valve body.
4.2 low-temperature valve long neck bonnet design
Low-temperature valves need to use the long neck bonnet structure, its day is to reduce the outside world into the device in the heat; to ensure that the stuffing box part of the temperature above 0 ℃, so that the filler can work properly; to prevent the filler box part of the overcooled in the filler box part of the stem as well as the bonnet of the upper part of the valve parts of the frost or freeze.
Long neck bonnet design is mainly the design of the neck length L, L refers to the bottom of the stuffing box to the upper surface of the upper surface of the sealing seat (Figure 1), which and the thermal conductivity of the material, thermal conductivity of the area and the surface of the coefficient of thermal conductivity, heat dissipation area and other factors, the calculation of the more cumbersome, generally by the experimental method of obtaining. Usually, it can be determined according to Table 2.
In industrial applications, according to the actual situation on site (such as insulation, operating space, location, etc.) needs, appropriate lengthening of the neck size.
4.3 Design of Pressure Relief Components
The problem of abnormal pressure rise generally exists only in low-temperature gate valves. When the gate valve gate closed, residual in the valve body cavity of the low-temperature medium from the surrounding environment to absorb a large amount of heat, rapid vaporization, in the valve body to produce a very high pressure. Abnormal pressurization is very harmful, it may be the gate tightly pressed on the valve seat, resulting in the gate jammed, so that the valve can not work properly, may also be washed out packing and flange gaskets, and even cause the valve body explosion. Therefore, measures must be taken to avoid.
Commonly used measures are the design of pressure relief holes and set up a bypass system. For small diameter valves (DN ≤ 300mm) can be directly in the gate near the high pressure side (i.e., the inlet side) design a pressure relief hole, for large diameter valves need to increase the bypass system. For the increase of pressure relief holes or bypass system for cryogenic valves must be labeled media flow direction.
4.4 on the design of the sealing device
In the valve is fully open, to prevent leakage of the working medium to the packing box of a device called L sealing device.
The upper sealing device has two functions. First, the upper sealing device can reduce the working medium on the packing damage. Industrial valves in the vast majority of working time in the open state, such as no upper sealing device, the medium pressure directly on the packing. Packing in a long-term pressure state, easy to aging. Secondly, when there is a leak at the packing, fully open the valve, so that the upper sealing device is in working condition, you can replace the packing with pressure. Therefore, for gate and globe valves are required to have the upper sealing device.
The upper sealing surface can be used in the bonnet surfacing drilling chrome tungsten carbide, and then finishing, grinding and made of technology (for austenitic stainless steel material bonnet, can be directly in the bonnet on the processing of the upper sealing surface), but also in the specialized upper sealing seat grinding and made of.
In short, in the design process of low-temperature valves should be comprehensive consideration of the various effects of low temperature on the valve, the use of reasonable structure, to avoid the adverse effects of low temperature on the normal operation of the valve.
5 low-temperature valve inspection
Low-temperature valves in addition to room temperature test, but also must do low-temperature test.
Normal temperature inspection mainly includes shell hydraulic strength test, water pressure, air pressure sealing test, on the sealing test, as well as opening and closing and torque test.
The main purpose of the low-temperature test is to check the low-temperature valve in the low-temperature state of the operating performance and sealing performance. Operational performance requirements of the valve opening and closing flexible, moving parts and sealing vice shall not occur abrasion and bite. Sealing performance requirements of the valve sealing surface leakage is less than the allowable leakage.
In short, cryogenic ball valves require low-temperature deep-cooling treatment of stainless steel as the valve material, must be lengthened stem processing, can be used for -196 ℃ LNG, liquid nitrogen, liquid oxygen, liquefied natural gas media occasions.