Design and research of valves for low temperature fields
The rapid development of science and technology in contemporary times, cryogenic technology is widely used in the field of cold storage and preservation, cryogenic transportation, liquefied natural gas LNG, manned spaceflight and superconductivity. In the wave of the rapid changes in cryogenic technology, how to improve the quality of cryogenic valves is particularly important. This paper analyzes and studies from valve standards, manufacturing processes, structural design and materials to provide reference for cryogenic valve selection, design and manufacturing.
Domestic and international standards for cryogenic valves
Domestic and international standards for cryogenic valves are: “Cryogenic Valve Technical Conditions” (GB/T 24925-2019), “LNG Valves” (JB/T 12621-2016), “Cryogenic Valves” (BS 6364-1998), “Industrial Valves. Isolation Valves for Low Temperature Applications” (BS EN ISO28921-2017), “Industrial Valves. Isolation valves for liquefied natural gas. Suitability Verification and Test Specification” (BS EN 12567-2000), “Cryogenic Valve Specification” (MESC SPE 77-200-2012). A comparison of commonly used standards is shown in Table 1.
Table.1 Applicable types of low-temperature valve standards
|GB/T 24925-2019||BS EN 12567-2000||BS 6364-1998|
|-196 ～ -29℃||-196 ～ 60℃||-196 ～ -50℃|
|Applicable to flange, clamping connection and welding end of low-temperature gate valve, globe valve, check valve, ball valve, butterfly valve, and other valves can be used for reference.||
1. Isolation valves (including gate valve, globe valve, ball valve and butterfly valves) for the production, storage and transportation (pipeline transport, rail transport, road transport and ship transport) of liquefied natural gas.
2. Liquefied natural gas filling valves for vehicles, ships and other transportation systems are not included in the scope of use of this standard.
1. For steel ball valves, wedge gate valves, check valves; globe valves, stop check valves.
2. For steel wedge gate valve globe and check valves with nominal throughput ≤ DN50.
3. Copper alloy gate valve, globe valve, globe check valve, check valve and general purpose butterfly valve.
|JB/T 12621-2016||BS EN ISO 28921-2017||MESC SPE 77-200-2012|
|＜ -162℃||-196 ～ -50℃||-196 ～ -20℃|
1. Valves with flanged and welded connections suitable for media of LNG.
2. T≥-162℃ low temperature valve reference use.
|Cryogenic (isolation valve) gate valve, globe valve, check valve, ball valve, butterfly valve, and other low-temperature valves.||For use with MESC SPE noted “extended bonnet” requirements for cryogenic valves.|
Analysis of low-temperature valve materials
Selection of metal materials
Cryogenic state, many metals and polymer materials, as the temperature decreases, there is an increase in hardness and strength, plastic toughness decreases, resulting in materials prone to fracture and deformation. From microscopic analysis, the ultra-low temperature makes the body-centered cubic gap impurity atoms and dislocation and intergranular interaction increases, hindering the dislocation movement, blocking the role of slip intensified, making its adaptability to deformation weakened, manifested as low-temperature brittleness. The face-centered cubic structure, low temperature conditions can maintain a high degree of toughness and plasticity.
Austenitic stainless steel is face-centered cubic structure materials, representative of 304/316L/316/316L/310/310L, in -200 ℃ can still maintain good toughness, of which 316L and 310L low-temperature structural stability is particularly outstanding. Austenitic stainless steel also has high temperature resistance, oxidation resistance, corrosion resistance and other characteristics.
The choice of non-metallic materials
Low temperature state, many polymer materials will become glassy, brittle and hard. PCTFE is a polymer of trifluorochloroethylene, good low temperature resistance, can be used for a long time at -200 ℃, chemical stability is second only to polytetrafluoroethylene. PCTFE molecular structure of the C-Cl bond, so PCTFE hardness, rigidity and creep resistance are good performance, and of all the non-metallic materials PCTFE has the lowest water-vapor permeability, does not permeate any gas, and is non-combustible.
Flexible graphite has low coefficient of friction, good lubricity, low coefficient of thermal expansion, compression rate greater than 40%, resilience greater than 15%, and no hardening and brittleness under low temperature conditions. Flexible graphite has good chemical stability, resistant to acid, alkali and organic solvent corrosion, flexible graphite can be used as sealing filler for low temperature valves and the preferred choice for winding gasket sealing.
Structural design analysis of low-temperature valves
Bonnet heightened structure
The actual working temperature of low-temperature valves is extremely low, so the bonnet heightened design is needed to keep the packing part away from the flowing medium in the valve, to ensure that the temperature of the valve packing part is not lower than 0℃, to avoid freezing of the parts above the valve stem and bonnet, to ensure the performance of the seal and to extend the life of the seal.
Drip tray structure
Drip tray is set on the bonnet to further ease the transfer of temperature in the valve body cavity and ensure the working environment of the packing. The drip tray area is larger than the bonnet bolt position to prevent the corrosion of liquefied water droplets on the middle flange bolt.
Pressure relief hole design
Media such as liquid oxygen, liquefied natural gas LNG, liquid hydrogen, liquid helium, etc., will increase hundreds of times in volume when they are heated and vaporized. When the cryogenic valve is closed, the residual medium inside the valve cavity absorbs heat and rapidly vaporizes and expands in volume, resulting in a rapid increase in pressure inside the valve and damaging the internal components of the valve. Therefore, the cryogenic valve needs to add a pressure relief hole at the entrance to connect the cavity and the entrance pipe to prevent abnormal pressure rise inside the cavity.
Soft seal vice cryogenic valves, the opening and closing pieces rub against each other causing the gathering of static electricity, static electricity may cause sparks. Liquid oxygen, liquefied natural gas LNG, liquid hydrogen and other media may cause an explosion when encountering an open flame. Therefore, it is necessary to set up a conduction device between the stem and the body of the cryogenic valve and between the stem and the opening and closing parts to export the static electricity in time to avoid safety hazards.
Cryogenic valves in the channel seal to take multiple seals, when a fire occurs, PTFE gasket or O-ring burned, high temperature resistant graphite packing or graphite winding gasket seal to ensure that the valve will not produce external leakage; for soft seal cryogenic ball valves, the seat position needs to have a fireproof structure, when a fire occurs, the soft seal valve seat melted after failure, the spring preload will continue to push the seat to the ball, so that the second seal continues to Seal to prevent internal leakage and avoid damage to the back-end piping.
Inverted Seal Up Structure
The inverted seal protects the packing seal and facilitates online maintenance. The life of the inverted seal will be affected if the inverted seal is in ultra-low temperature for a long time. Moving the inverted seal upward ensures that the inverted seal works at room temperature, thus extending the life of the inverted seal.
Manufacturing Process Control
Deep cooling treatment
The production process of ultra-low temperature valves requires deep cooling treatment of austenitic materials. Austenitic materials after heat treatment still exist part of the unstable austenite, deep cooling treatment can make the unstable austenite fully transformed into martensite, so that the internal redistribution of the material to reach a balanced state. The current way of deep cooling treatment using -196 ℃ liquid nitrogen impregnation parts, according to the maximum wall thickness of the workpiece to calculate the actual cooling time, after the cooling temperature stability, need to keep warm for 2 ~ 4 hours. In accordance with the “Technical Conditions of Low Temperature Valves” (GB/T 24925-2019), low temperature valves need two deep cooling treatment, that is, at least two cycles of room temperature to low temperature treatment to ensure that the internal tissue conversion is adequate.
De-oiling and degreasing
When liquefied natural gas LNG, liquid hydrogen, liquid oxygen as the medium, cryogenic valves need to degrease and degrease cleaning. In the first step, cleaning is carried out by ultrasonic machine cleaning method, and the brush should be made of stainless steel and nylon. The second step, dipping and degreasing process, divided into a rinse, secondary cleaning, three degreasing process. The third step, degreasing inspection. Wipe the surface of degreased parts with clean and dry white filter paper, no grease traces on the paper is qualified, also can use ultraviolet light and grease concentration analyzer detection.
With the development of LNG, liquid oxygen, liquid hydrogen and other cryogenic fields, the storage, transportation and control processes in low-temperature environments will have greater demand for cryogenic valves, as well as higher requirements. In the design and manufacture of cryogenic valves, fully analyze the impact of ultra-low temperature environment on the material, mechanical structure and the product as a whole, and through continuous research and testing, in order to improve the safety and reliability of cryogenic valves.
Source: China Low Temperature Valve Manufacturer – Yaang Pipe Industry Co., Limited (www.sfutube.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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