Low temperature valve(Valve Guardian) is a valve that works in low temperature medium. Common low temperature valves(1250) are divided into low temperature butterfly valve, low temperature gate valve, low temperature ball valve, low temperature globe valve, low temperature check valve, low temperature throttle valve, etc., mainly used in ethylene, liquefied natural gas installation, natural gas LPG LNG storage tank, acceptance base and satellite station, air separation equipment, petrochemical exhaust gas separation equipment, etc. Liquid oxygen, liquid nitrogen, liquid argon, carbon dioxide low temperature storage tank and tank truck, pressure swing adsorption oxygen production equipment. The output liquid low-temperature media such as ethylene, liquid oxygen, liquid hydrogen, liquefied natural gas, liquefied petroleum products, etc., are not only flammable and explosive, but also gasification when heating up, and the volume expands hundreds of times when gasification. Foreign countries are generally divided into six temperature levels according to the liquefaction temperature of various gases under normal pressure.
The first stage is 0~-46\u2103(1640);
Secondary is -47~-60\u2103(1250);
The third stage is -61~-70 \u2103;
Level 4 is -70~-101 \u2103;
Grade 5 is -102~-196\u2103;
Level 6 is below -253 \u00b0 C.
Generally, -46~-150 \u00b0 C is called low temperature, and below -150 \u00b0 C is called ultra-low temperature.
With the development of modern science and technology, the production scale of cryogenic engineering products has been expanding, and liquid oxygen, liquid nitrogen and liquefied petroleum gas have been widely used. Especially liquefied natural gas is getting more and more attention in the world. The temperature of liquefied ammonia is -269 \u00b0 C, the temperature of liquid hydrogen is -254 \u00b0 C, the temperature of liquefied helium is -196 \u00b0 C, the temperature of liquid oxygen is -183 \u00b0 C, the temperature of liquefied natural gas is -162 \u00b0 C, and the liquefied fractionation, transportation and storage of the above substances require the use of a large number of low temperature valves. The use of low temperature valve is more and more wide, the demand is more and more large, and the requirements for the technical performance and working characteristics of low temperature valve are also constantly improving.
First, the temperature level of the low temperature valve:
Table 1 Material selection of cryogenic valve:
Second, the general design requirements of low temperature valve:
The working conditions of the low-temperature valve are harsh, and most of the working media are flammable, explosive and highly permeable substances. The minimum working temperature can reach -269 \u00b0 C and the maximum working pressure can reach 10MPa. Therefore, the design, manufacture and inspection of low-temperature valves are very different from those of universal valves. In general, according to the use of low-temperature valve conditions, the design work put forward the following requirements:
1) The valve and its components should have the ability to work for a long time under low temperature media and ambient temperature (generally 10 years or 3,500 to 5,000 cycles);
2) Relative to the low-temperature medium, the valve should not become a significant heat source, because the inflow of heat will reduce the thermal efficiency, and the heat inflow is too much, but also may make the low-temperature medium inside the valve vaporization, resulting in abnormal pressure boost, causing danger;
3) The low temperature medium should not have a harmful effect on the operating performance of the handwheel and the sealing performance of the packing;
4) The structure of the valve assembly directly in contact with the low-temperature medium should meet the relevant explosion-proof and fire-proof requirements;
5) Valve assemblies operating at low temperatures cannot be lubricated, so measures need to be taken to prevent chafing of friction parts.
The above requirements should be throughout the low-temperature valve design process, in addition, it should be noted that the above requirements are unique to the low-temperature valve requirements, in the design process of low-temperature valve should also comply with the relevant general valve requirements.
Third, the evaluation indicators of the technical level of low temperature valves:
In the design process of low temperature valve, in addition to the need to consider the flow capacity of low temperature valve and flow resistance and other general requirements, but also need to consider some other indicators, in order to better evaluate the technical level of low temperature valve. Usually by measuring whether the energy consumption is reasonable to evaluate the technical level of low-temperature valves, the main evaluation indicators are the following.<\/p>\n\n\n\n

1. Thermal insulation performance of low temperature valve
   The adiabatic capacity of the valve can be measured by the ratio of the heat flow rate Q1 into the low-temperature medium and the mass of the low-temperature medium through which it passes. However, when the type of medium is unchanged and only the flow rate changes, its value will change, so it is obviously inappropriate to use this method as an indicator to evaluate the adiabatic capacity of low-temperature valves. The index KT can be used for unified comparison.
   Where: P– the heat entering the low-temperature medium in unit time, W, P=Q1\/T;
   \u03b1T- ratio coefficient, considering that the low temperature valve is generally tested in liquid nitrogen, so \u03b1T=0.0216;
   \u0394T- the difference between the ambient temperature (20 \u00b0 C) and the temperature of the low-temperature medium, \u00b0 C;
   DN- Nominal diameter of valve, m.
   The KT value is an approximate constant for the same medium, only related to the properties of the valve itself.<\/li>
2. Cooling performance of low temperature valve
   The cooling performance of the low-temperature valve refers to the ability of the low-temperature valve to cool from room temperature to the working temperature. This performance can be measured by the energy consumed by the valve in the above process, that is, the heat Q2 transmitted by the valve to the low temperature medium in the above process. The cooling performance index is of great significance for the periodic low-temperature valve. However, it is not enough to only use Q2 to measure the cooling performance of low-temperature valves, and the following indicators can be used:<\/li>
3. Working performance of low-temperature valve opening and closing seals
   Under low temperature conditions, the performance of the seal is often destroyed, in order to achieve reliable sealing, it is necessary to use a reasonable sealing structure or increase the specific pressure of the seal. Therefore, it is necessary to evaluate the sealing effect.
   Parameters related to leakage can be used to measure the sealing capacity of low-temperature valves, namely:
   Where \u0394V is the average leakage of gas during the working life, m3\/s.<\/li>
4. The condition that the surface of the low-temperature valve does not freeze
   When the low temperature valve is working, its surface should not be dew, let alone ice. Whether the outer surface of the valve freezes depends first on the difference between the ambient air temperature and the surface temperature of the component \u0394T1, and then on the dew point temperature of the air. In fact, in all-weather conditions, it is difficult to completely eliminate dew formation. However, if \u0394T1 meets certain conditions, the possibility of freezing will be greatly reduced, and the condition for the valve surface not to freeze is \u0394T1\u22645 \u00b0 C.
   Table 1 shows some technical level indicators of low temperature valves. It can be seen that with the increase of nominal diameter, the performance of the valve has improved. Therefore, the requirements for Kr and Km indicators should be changed according to the nominal diameter. The value of Km also depends on factors such as the type of valve, the material of the valve housing and the degree of structural perfection of its components. Generally speaking, the Km index of the butterfly valve ****, but the low temperature sealing performance of the butterfly valve is not very good, so it is only used to adjust the medium flow rate; The performance of the globe valve and gate valve is second, and the low temperature sealing performance is better, and it is commonly used to cut off the medium in the industrial field.
   Table 2 Technical level index of pneumatic vacuum insulated cryogenic valve
   Four, valve body, valve cover, valve seat disc (gate) material selection:
   The principle of selection of materials for these main parts is roughly: when the temperature is higher than -100\u2103, ferritic steel is selected; Choose austenitic stainless steel when the temperature is below -100\u2103; Select the appropriate material according to the lowest operating temperature during design.<\/li>
5. Material selection of valve stem and fasteners:
   When the temperature is higher than -100\u2103, the valve stem and bolt materials are made of Ni, Cr-Mo and other alloy steel, and are subjected to appropriate heat treatment to improve the tensile strength and prevent thread bites. When the temperature is below -100\u2103, it is made of austenitic stainless acid-resistant steel. However, the low hardness of 18-8 acid-resistant steel will cause friction between the valve stem and the packing(1250), resulting in leakage of the packing(686). Therefore, the stem surface must be chrome plated, or nitriding and nickel phosphorus plated treatment to improve the surface hardness. It can also use thermal spraying process to spray molybdenum, STL and other wear-resistant and corrosion-resistant alloys on the surface. In order to prevent the nut and bolt from biting(HT-1800), the nut is generally made of Mo steel or Ni steel, and the thread surface is coated with molybdenum disulfide.
   Six, low temperature valve gasket, packing(636) material selection:
   With the decrease of temperature, the shrinkage of fluorine plastics is very large, which will reduce the sealing performance and easily cause leakage. Asbestos fillers do not prevent seepage. Rubber has a blistering property to natural gas and can not be used at low temperatures. In the design of low temperature valve, on the one hand, the structural design is used to ensure that the packing is working at close to the ambient temperature, for example, the long neck valve cover structure is used to make the packing box as far away from the low temperature medium as possible, and on the other hand, the low temperature characteristics of the packing should be considered when selecting the packing. Low temperature valves generally use asbestos packing impregnated with tetrafluoroethylene.
   Flexible graphite is an excellent sealing material. This material is impervious to gas and liquid, and has an elasticity of 10% to 15% in the thickness direction, and lower fastener pressure can achieve seal. It has self-lubricating properties, and is used as a valve filler to prevent the wear of the filler and the valve. The use temperature range of flexible graphite filler is -200~870\u2103.
   Low temperature valve installation requirements
   Because of the special structure of the low-temperature valve, the installation of the low-temperature valve also has its special requirements. Because of the structural characteristics of the long neck valve cover of the low-temperature valve, the stem stem direction of the low-temperature valve must be in the range of 45 degrees vertically upward when it is installed, and it should be avoided as far as possible on the vertical pipeline. Otherwise, the low temperature medium will fill the extended part of the valve cover, causing the failure of the valve packing, and will pass the cold amount to the valve handle, which will bring personal injury to the operator. For low-temperature valves with pressure relief structure, special attention should be paid to the requirements of the pressure relief direction of the valve when installing the valve. The direction of the valve pressure relief should be marked on the process flow chart, and reflected in the pipeline axometric diagram (this is extremely important, extremely important, extremely important), if it is necessary to set the pressure relief hole and not set, after the valve is closed, the liquid in the valve chamber is heated and vaporized, which is easy to cause the valve body to burst. If the direction of pressure relief is installed incorrectly, flammable or toxic process media may be discharged to the operation and maintenance side, causing casualties!
   Cryogenic valve manufacturing
   The production of the low-temperature valve(Valve Guardian) has developed a strict manufacturing process and the use of special equipment, the processing of parts for strict quality control. After special low temperature treatment, the roughed parts are placed in the cooling medium for a few hours (2-6 hours) to release stress, ensure the low temperature performance of the material, ensure the finishing size, and prevent leakage caused by temperature change caused by the deformation of the valve in low temperature conditions. Valve assembly and ordinary valves are also different, parts need to be strictly cleaned, remove any oil, to ensure performance.
   Cryogenic valve inspection
   In addition to normal temperature and low temperature tests, the following tests should also be carried out
   1, the main parts of the ultra-low temperature valve should be cryogenic treatment (how to cryogenic treatment later we have the opportunity to talk about, the following is the cryogenic treatment of the valve).
   2, the main parts and welds should be low-temperature impact test to ensure that the valve is not brittle at low temperature conditions;
   3, the valve must be tested at normal temperature, first 1.5 times the nominal pressure water pressure strength test, and then 1.1 times the nominal pressure seal test;
   4, after the hydraulic test, the valve must remove water, no grease, and keep dry;
   5, each batch of low-temperature valves should be sampled according to a certain proportion of low-temperature test, low-temperature test must be carried out after 1.5 times the water pressure strength test;
   6, after the valve low temperature test, air should be prevented from entering the valve;
   7, after the low temperature test of the valve, it should be heated naturally or blow with a fan, and it is strictly prohibited to accelerate the valve temperature by heating and other methods;
   8, the valve low temperature test after the natural temperature rise, the valve fasteners tighten again;
   9, when the valve is stored, be sure to check the valve switch to ensure that the valve is in a reasonable state;
   10, valve storage and transportation process, should pay attention to stainless steel and carbon steel valves separate, and do dustproof, waterproof, oil and anti-collision;
   11, the flow direction of the valve, pressure relief direction and nameplate, etc., should be checked according to the requirements of the design unit.
   (2023-10-22)<\/li><\/ol>\n\n\n\n

   <\/figure>\n\n\n\n

   <\/figure>\n","protected":false},"excerpt":{"rendered":"

   Low temperature valve(Valve Guardian) is a valve that works in low temperature medium. Common low temperature valves(1250) are divided into low temperature butterfly valve, low temperature gate valve, low temperature ball valve, low temperature globe valve, low temperature check valve, low temperature throttle valve, etc., mainly used in ethylene, liquefied natural gas installation, natural gas …<\/p>\n

   Low temperature valve performance index, material selection and manufacturing installation<\/span> \u66f4\u591a \u00bb<\/a><\/p>\n","protected":false},"":2,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"site-sidebar-layout":"default","site-content-layout":"default","ast-main-header-display":"","ast-hfb-above-header-display":"","ast-hfb-below-header-display":"","ast-hfb-mobile-header-display":"","site-post-title":"","ast-breadcrumbs-content":"","ast-featured-img":"","footer-sml-layout":"","theme-transparent-header-meta":"","adv-header-id-meta":"","stick-header-meta":"","header-above-stick-meta":"","header-main-stick-meta":"","header-below-stick-meta":""},"categories":[67],"tags":[],"_links":{"self":[{"href":"https:\/\/www.oktcz.com\/en\/wp-json\/wp\/v2\/posts\/7306"}],"collection":[{"href":"https:\/\/www.oktcz.com\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.oktcz.com\/en\/wp-json\/wp\/v2\/types\/post"}],"":[{"embeddable":true,"href":"https:\/\/www.oktcz.com\/en\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.oktcz.com\/en\/wp-json\/wp\/v2\/comments?post=7306"}],"version-history":[{"count":1,"href":"https:\/\/www.oktcz.com\/en\/wp-json\/wp\/v2\/posts\/7306\/revisions"}],"predecessor-version":[{"id":7309,"href":"https:\/\/www.oktcz.com\/en\/wp-json\/wp\/v2\/posts\/7306\/revisions\/7309"}],"wp:attachment":[{"href":"https:\/\/www.oktcz.com\/en\/wp-json\/wp\/v2\/media?parent=7306"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.oktcz.com\/en\/wp-json\/wp\/v2\/categories?post=7306"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.oktcz.com\/en\/wp-json\/wp\/v2\/tags?post=7306"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}