The use and sealing requirements of low temperature valve how to choose the material of low temperature valve

The use and sealing requirements of low temperature valve how to choose the material of low temperature valve

2. The influence of low temperature on the sealing performance of valve
2.1 Non-metallic sealing pairs
Ball valves and butterfly valves working at room temperature generally use metal to non-metallic material seal pairs. Due to the high elasticity of non-metallic materials, the specific pressure required for sealing is small, so the sealing is good. However, at low temperature, because the expansion coefficient of non-metallic materials is much larger than that of metal materials, the shrinkage of the low temperature and the shrinkage of metal seals, valve bodies and other parts are much different, which leads to the serious reduction of sealing specific pressure and the result of sealing can not be sealed. Most non-metallic materials stiffen and become brittle at cryogenic temperatures, losing toughness, resulting in cold flow and stress relaxation. Such as rubber at a temperature lower than its glass temperature, will completely lose elasticity, become glassy, lose its tightness. In addition, rubber can not be used for LNG valves because it has bubble expansion in LNG medium. Therefore, at present in the design of low temperature valve, the general temperature is lower than -70℃ no longer use non-metallic sealing auxiliary materials, or non-metallic materials through a special process into metal and non-metallic composite structure type.
According to foreign records, some non-metallic materials can be used well in cryogenic state. In the 1970s, “slip shod”, a new plastic from Irish Alloy Co., LTD., was a kind of ultra-high molecular weight polyethylene, which had good toughness at -269 ° C, did not break under certain impact stress, and maintained considerable wear resistance. The Mylar plastic developed in France is still quite elastic at the temperature of liquid hydrogen (-253℃). The polycarbonate seal holder of H.T. Lomanenko of the former Soviet Union was tested in liquid nitrogen (-196℃). The data show that the polycarbonate has a good sealing effect at low temperature.
2.2 Metal seal pair
Under the condition of low temperature, the strength and hardness of metal materials increase, plasticity and toughness decrease, showing different degrees of low temperature cold brittle phenomenon, seriously affect the performance and safety of the valve. In order to prevent low stress brittle fracture of materials at low temperature, when designing low temperature valves, ferritic stainless steel materials are generally used when the temperature is higher than -100℃, while when the temperature is lower than -100℃, the valve body, valve cover, valve stem and sealing seat are mostly used with face-centered cubic lattice austenitic stainless steel, copper and copper alloy, aluminum and aluminum alloy, etc. But because the hardness of aluminum and aluminum alloy is not high, the abrasion resistance and abrasion resistance of the sealing surface is poor, so it is rarely used in the low temperature valve. Generally use austenitic stainless steel materials, commonly used 0Cr18Ni9, 00Cr17Ni12Mo2(304, 316L), etc., these materials have no low temperature cold brittle critical temperature, under low temperature conditions, can still maintain high toughness.
However, austenitic stainless steel as a low temperature valve metal seal auxiliary material also has some shortcomings. Because most of these materials are in a metastable state at room temperature, the austenite in the material transforms into martensite when the temperature is lowered below the phase transition point (MS). For body centered cubic lattice of martensite density is lower than that of face-centered cubic lattice of austenite, and because some carbon atoms arrange body centered cubic lattice position regulation, make the lattice along the C axis growth, thus the increase of the volume changes caused by internal stress, make originally after grinding meet the sealing requirements of sealing surface buckling deformation, resulting in seal failure.
In addition to the deformation failure of sealing surface caused by low temperature phase transformation, due to the temperature difference of each part or the difference of physical properties between different materials, resulting in uneven shrinkage, temperature variation stress will also occur. When the stress is below the elastic limit of the material, a reversible elastic distortion is produced in the sealing surface. When the temperature stress of a part exceeds the yield limit of the material, the parts will have irreversible distortion and deformation, which will also cause the failure of the sealing surface and affect the sealing effect.
In view of the influence of low temperature on the metal sealing pair, corresponding measures must be taken to make the deformation of the metal sealing surface small or the deformation of the sealing surface has little influence on the sealing performance. Firstly, in terms of materials, we should try to choose materials with high stability of metallographic structure (such as 316L but with high cost). Secondly, for the body, cover, stem, seal and other austenitic materials made of parts must be processed at low temperature, so that the martensite transformation and deformation of the material is fully carried out before finishing. The temperature of low temperature treatment should be lower than the material phase change temperature (MS) and lower than the actual working temperature of the valve, and the treatment time should be 2 ~ 4h. If necessary, multiple low temperature treatment or appropriate aging treatment can be carried out. In addition to the above measures, the structural design should also be considered to reduce the impact of sealing surface deformation on sealing performance, such as in the design of gate valves, ball valves and butterfly valves can consider the use of elastic sealing structure, so that the low temperature deformation can be partially compensated. For the globe valve should be conical seal structure, so that low temperature deformation on the sealing surface of the small impact.
3. Influence of low temperature on the sealing performance of valve
3.1 Stem packing
Due to the defects of rubber material at low temperature and the cold brittle and serious cold flow phenomenon of most non-metallic materials, the sealing design between the stem and the valve body of low temperature valve can not use the form of sealing ring, can only use the packing box sealing structure and bellows sealing structure. General bellows seal is used in the medium does not allow trace leakage and is not suitable for packing occasions, the life of its single-layer structure is very short, the cost of multi-layer structure is high, processing is difficult, so generally not used.
The sealing structure of stuffing box is easy to manufacture and process, easy to maintain and replace, and is quite common in practical application. However, the general working temperature of the packing can not be lower than -40℃. In order to ensure the sealing performance of the packing, the packing box device of the low-temperature valve should be operated under the condition of close to the ambient temperature. At low temperature, with the decrease of temperature, the elasticity of the filler gradually disappears, and the leak-proof performance decreases. Due to media leakage caused by packing and valve stem ice, will affect the normal operation of the valve stem, but also due to the movement of the valve stem will be packing scratch, causing serious leakage. Therefore, under normal circumstances, the low temperature valve packing is required to work at a temperature above 0℃, which requires the design of the long neck valve cover structure, so that the packing box away from low temperature medium, and the selection of packing with low temperature characteristics. Commonly used fillers are polytetrafluoroethylene, asbestos, impregnated polytetrafluoroethylene asbestos rope and flexible graphite, among which, because asbestos can not avoid permeability leakage, polytetrafluoroethylene linear expansion coefficient is very large, cold flow phenomenon is serious, so generally not used. Flexible graphite is an excellent sealing material, gas, liquid are impermeable, compression rate is greater than 40%, resilience is greater than 15%, stress relaxation is less than 5%, the lower fastening pressure can be sealed. It also has self lubricity, used as valve packing can effectively prevent packing and valve stem wear, its sealing performance is obviously better than the traditional asbestos material, so it is one of the most excellent sealing materials.
Because the filler is generally non-metallic material, the linear expansion coefficient is much larger than the metal filler box and valve stem. Therefore, when the packing assembled at room temperature drops to a certain temperature, its shrinkage is greater than that of the packing hole and the valve stem, which may cause leakage due to the decrease of the preloading pressure. In the design, the packing gland bolt can be preloaded with multiple groups of disc spring gaskets, so that the preloading force of the packing at low temperature can be continuously compensated to ensure the sealing effect of the packing.
The low leakage combined stem packing produced by Garlock Company in the United States, the end ring is made of carbon fiber braided disc root, the sealing ring is made of high purity diamond texture graphite strip molding, through the cup and cone structure and radial expansion characteristics, so that the sealing performance is improved.
The low temperature deformation of the stem material will also affect the sealing performance of the packing. Therefore, the same as the valve body, valve cover, sealing accessory materials, the stem must also be cryogenic processing after finishing, in order to make the low temperature deformation is small. In addition, because the austenitic stainless steel used in the cryogenic stem material cannot be heat-treated to improve the surface hardness, the joint between the stem and the packing is more likely to bruise each other, resulting in leakage at the packing. Therefore, the stem surface must be plated with hard chromium or nitride to improve the surface hardness.
3.2 Middle flange gasket
Both the middle flange seal of the valve and the external connection of the flange connection valve are generally in the form of gaskets. Since gasket material will harden and reduce plasticity at low temperature, the gasket for low temperature valves has higher requirements. It must have reliable sealing and recovery at normal temperature, low temperature and temperature changes. The influence of low temperature on gasket sealing performance should be considered comprehensively.
According to the commonly used gasket sealing forms, the bolt length, gasket and flange thickness will shrink as the temperature decreases. In order to ensure the reliable gasket sealing at low temperature, it must be met
Δ HT3 Δ HT – Δ HT1 – Δ H1 < 0
Type in the
ΔH1 — Tensile deformation of bolt assembly, mm
Δ H1 = 1 / E1H sigma
ΔHT1 — Bolt shrinkage in temperature range of ΔT, mm
Δ HT1 Δ T = H alpha 1
ΔHT — shrinkage of gasket in ΔT temperature zone, mm
Δ HT = alpha 2 Δ h T
ΔHT3 — Shrinkage of upper and lower flanges in ΔT temperature zone, mm
Δ HT3 = alpha 3 Δ T1 (H – H)
σ1 — Bolt preload, N/mm
E1 — elastic modulus of bolt, N/mm
α1, α2, α3 — are the linear expansion coefficient of bolt, gasket and flange materials, respectively, mm/m
H, H – mm
When the gasket seal reaches the designed working low temperature from room temperature, the sum of the shrinkage of the upper and lower flanges and the shrinkage of the gasket must be less than the sum of the shrinkage of the bolt and the tensile deformation of the bolt assembly, so as to ensure that the gasket still has part of the preload at the working temperature and maintain the sealing ability.
Accordingly, four aspects should be considered in the design. ① The bolt is made of material with larger linear expansion coefficient, which has larger shrinkage at low temperature. ② The flange is made of material with smaller linear expansion coefficient to reduce ΔHT3. ③ Reduce the thickness of the gasket, and use the material with small linear expansion coefficient as the gasket. (4) Increase the tensile deformation of bolts.
For low temperature valves below -100℃, the body material and bolt material are generally made of austenitic stainless steel, the linear expansion coefficient is the same, so it is more important to choose the appropriate gasket material and increase the tensile deformation of bolts. Ideal low temperature gasket material, at room temperature its hardness is low, at low temperature resilience can be good, linear expansion coefficient is small and has a certain mechanical strength. In practical applications, winding gaskets made of stainless steel tape filled with asbestos or polytetrafluorethylene or flexible graphite are generally used, and the sealing effect of winding gaskets made of flexible graphite and stainless steel is ideal. As for the increased tensile deformation of the bolt, due to the limit of the bolt installation preload, the increased margin is not much, so it can be considered to set the disc spring gasket to compensate.