1 Overview

With the rapid development of the petroleum, chemical and gas industries, and especially the widespread use of liquefied natural gas (LNG) as an emerging energy source, the demand for cryogenic valves for LNG has greatly increased. According to the national energy strategy, China will actively participate in the development of the world oil and gas market. Currently, the NDRC has planned to build 11 natural gas receiving stations in Guangdong, Fujian, Shanghai, Shandong, Zhejiang, Jiangsu, Tianjin and Liaoning, and to import large quantities of natural gas from abroad through LNG carriers. LNG receiving stations, LNG carriers and delivery lines to the customer end require a large number of valves. Due to the LNG atmospheric pressure of -162 ℃, and flammable, so in the design of LNG cryogenic valves, the sealing performance of its higher and more stringent requirements.

Valve sealing performance is one of the main indicators to assess the quality of the valve, which mainly includes two aspects, namely the inner sealing performance and outer sealing performance. The inner seal is the degree of sealing achieved by the media between the seat and the closure. Such as ball valve between the ball and the seal between the butterfly valve butterfly valve seat and the seal between the valve and valve seat closing valve seal between the gate and valve seat seal. These seals are mainly flat seal, spherical seal and cone seal. Sealing materials can be divided into metal and nonmetallic material seal and metal to metal seal. Outer seal refers to the seal of the stem packing and the seal of the flange gasket. In some media does not allow the discharge into the atmosphere under special conditions, the outer seal than the inner seal is more important.

2 low temperature on the valve sealing performance

2.1 non-metallic seal pair

Ball valves and butterfly valves that work at room temperature generally use metal on the non-metallic seal material. Due to the elasticity of the non-metallic material, the pressure required to obtain the seal is low and therefore the seal is good. However, at low temperature, due to the non-metallic materials, the expansion coefficient is much larger than the metal material, so that the amount of shrinkage at low temperature and the metal seal, valve body and other parts of the contraction amount of a larger difference, resulting in serious pressure seal Reduce the result can not be sealed. Most non-metallic materials harden and become brittle at cryogenic temperatures, losing toughness, resulting in cold flow and stress relaxation. Such as rubber at temperatures below its glass transition temperature, it will completely lose its elasticity, into a glassy state, losing its sealing. In addition, the presence of rubber in the LNG media, swelling, it can not be used for LNG valves. Therefore, the current design of cryogenic valves, the general temperature is lower than -70 ℃ no longer use non-metallic seal deputy material, or non-metallic materials through special processing into a metal and non-metallic composite structure type.

According to foreign data records, there are some non-metallic materials can be very good application in the cryogenic state. In the 1970s, Irish Alloy Co., Ltd. a new plastic "slip shod" is an ultra-high molecular weight polyethylene, at -269 ℃ temperature is still very good toughness, to withstand a certain impact stress without breaking, and can Maintain a fair amount of abrasion resistance. Mylar plastic developed in France is still quite flexible at liquid hydrogen (-253 ° C). Former Soviet Union HT Lomaninke polycarbonate sealing seat in liquid nitrogen (-196 ℃) temperature sealing test, the data show that the polycarbonate has a good low temperature sealing effect.

2.2 metal seal pair

Under the condition of low temperature, the strength and hardness of the metal material are increased, the plasticity and toughness are reduced, the cold cold and brittle phenomenon appears to varying degrees, seriously affecting the performance and safety of the valve. In order to prevent low stress brittle fracture at low temperature, the design temperature of the valve, the general temperature is higher than -100 ℃ with ferritic stainless steel, and the temperature is lower than -100 ℃, the body, bonnet, stem, Sealing seat, etc. Most of the use of face-centered cubic lattice of austenitic stainless steel, copper and copper alloys, aluminum and aluminum alloys. However, due to the hardness of aluminum and aluminum alloy is not high, the wear-resistant sealing surface, scratch resistance is poor, so very few applications in the cryogenic valve. Generally used austenitic stainless steel materials are mostly 0Cr18Ni9,00Cr17Ni12Mo2 (304,316L) commonly used, these materials do not have low temperature cold brittle critical temperature, at low temperatures, can still maintain a high degree of toughness.

However, austenitic stainless steel as a low temperature valve metal seal material there are some deficiencies. Because much of this material is metastable at room temperature, the austenite in the material will transform into martensite when the temperature falls below the phase change point (MS). The martensite density of the body-centered cubic lattice is lower than that of the face-centered cubic lattice, and since some carbon atoms regularly occupy the position of the body centered cubic lattice, the lattice grows along the C-axis Changes in volume caused by an increase in internal stress, so that the original seal surface after grinding to achieve warping deformation, resulting in seal failure.

In addition to deformation caused by low temperature deformation sealing surface failure, due to temperature differences between parts or due to differences in the physical properties of different materials, causing uneven contraction, will produce temperature stress. When the stress is lower than the elastic limit of the material, reversible elastic deformation occurs in the sealing surface. When a part of the thermal stress exceeds the yield point of the material, the parts will be irreversible distortion, the same will cause the sealing surface failure, affecting the sealing effect.

To deal with the influence of low temperature on the metal seal pair, corresponding measures must be taken to minimize the deformation of the metal seal surface or the deformation of the seal surface. First of all in the material as far as possible the choice of materials with high stability of the metallurgical structure (such as 316L but higher cost). Second, the valve body, valve cover, valve stem, seals and other parts made of austenitic materials must be low-temperature processing, in order to make the material martensitic transformation and deformation is fully carried out before finishing. Low temperature processing temperature should be lower than the material phase transition temperature (MS) and lower than the actual working temperature of the valve, the processing time to 2 ~ 4h is appropriate, if necessary, can be carried out a number of low-temperature treatment or appropriate aging treatment. In addition to the above measures, structural design should also be considered to reduce the sealing surface deformation on the sealing performance, such as the gate valve, ball valve and butterfly valve design can be considered flexible sealing structure, so that low temperature deformation to be partially compensated. For the closing valve should be tapered sealing structure, so that low temperature deformation of the sealing surface less affected.

3 low temperature on the valve sealing performance

3.1 stem packing

Due to the defects of low temperature rubber material and the cold and brittle phenomenon of most non-metallic materials, the seal design between the low-temperature valve stem and the valve body can not adopt the form of a sealing ring, and only the stuffing box sealing structure and Bellows seal structure. The general application of the bellows seal medium is not allowed to allow micro-leakage and inappropriate packing occasions, the single-layer structure of a very short life expectancy, multi-layer structure of high cost, difficult processing, it is generally not used.

Stuffing box sealing structure manufacturing and processing simple, easy maintenance and replacement, quite common in practical applications. However, the general working temperature of packing can not be less than -40 ℃. In order to ensure the sealing performance of packing, the stuffing box of low-temperature valve should be operated under the condition of close to ambient temperature. At low temperatures, as the temperature decreases, the elasticity of the filler gradually disappears and the leakproofness decreases. Due to media leakage caused by packing and stem ice, it will affect the normal operation of the stem, but also due to the stem movement of the filler will scratch, causing serious leakage. Therefore, under normal circumstances, require cryogenic valve packing temperature above 0 ℃, which requires the design through the long neck valve cover structure, so that the stuffing box away from the low temperature medium, while selecting the filler with low temperature characteristics. Commonly used fillers are PTFE, asbestos, PTFE impregnated asbestos ropes and flexible graphite, etc., of which asbestos can not be avoided permeability leakage, PTFE linear expansion coefficient is large, serious cold flow, it is generally not used . Flexible graphite is an excellent sealing material that is impermeable to gases and liquids with a compression rate of more than 40%, a rebound of more than 15% and a stress relaxation of less than 5%, which allows for tight sealing at lower tightening pressures. It is also self-lubricating, as a valve packing can effectively prevent the filler and stem wear, the sealing performance is significantly better than the traditional asbestos materials, it is one of the best sealing material.

As the filler is generally non-metallic materials, the linear expansion coefficient than the metal packing box and valve stem is much larger. Therefore, at room temperature assembly of the packing, down to a certain temperature, the shrinkage of the filler hole and the valve stem is greater than the amount of contraction, may result in reduced preload pressure caused by leakage. When designing, the packing gland bolt can be preloaded with multiple sets of disc spring washers so that the pre-tightening force of the packing at low temperature can be continuously compensated to ensure the packing sealing effect.

The United States Garlock production of low-dispersion composite stem packing, the end of the ring with carbon fiber braided packing, sealing ring using high purity diamond textured graphite tape molding through the cup taper structure and radial expansion properties, so that the sealing performance Greatly improve.

Low temperature deformation of the stem material, the sealing performance of the filler will also have a certain impact. Therefore, with the valve body, bonnet, seal vice material, the valve stem must also be cryogenic processing and then finishing, in order to minimize the deformation at low temperature. In addition, the low temperature stem material used in austenitic stainless steel can not be heat treated to improve the surface hardness, making the stem and filler joints are more likely to scratch each other, resulting in leakage at the filler. Therefore, the stem surface must be hard chrome plated or nitrided to improve the surface hardness.

3.2 flange gasket

Whether it is the valve flange flange connection or flange connection valve external connection, are generally used in the form of gaskets. Gaskets require harder and lower plasticity at low temperatures and therefore require more spacers for cryogenic valves, which must have reliable sealing and restorative properties at ambient, low temperature and temperature changes, taking into account the combination of low temperature Gasket sealing performance.

According to the commonly used gaskets sealing form, as the temperature decreases, the bolt length, the thickness of the gasket and flange will shrink smaller, in order to ensure a reliable gasket at low temperatures, must meet

ΔHT3ΔHT-ΔHT1-ΔH1

In the formula

ΔH1 --- bolt assembly tensile deformation, mm

ΔH1 = σ1 / E1H

ΔHT1 --- bolt in the temperature range ΔT shrinkage, mm

ΔHT1 = Hα1ΔT

ΔHT --- seal gasket shrinkage in the ΔT temperature range, mm

ΔHT = hα2ΔT

ΔHT3 --- upper and lower flange shrinkage in the ΔT temperature range, mm

ΔHT3 = (Hh) α3ΔT1

σ1 --- bolt preload, N / mm

E1 --- modulus of elasticity of the bolt, N / mm

α1, α2, α3 --- bolt, gasket and flange materials, respectively, the linear expansion coefficient, mm / m

H, h --- mm

Gasket seal from the room temperature to reach the design of the work of low temperature, the amount of contraction of the upper and lower flange and gasket shrinkage and the amount must be less than the amount of bolt shrinkage and bolt assembly, the amount of tensile deformation, so as to ensure the seal Part of the working temperature is still some preload there, to maintain the sealing ability.

Accordingly, in the design should be considered from four aspects. ① bolt with a larger linear expansion coefficient of material, have a larger amount of shrinkage at low temperatures. ② flange with a smaller linear expansion coefficient of material to reduce ΔHT3. ③ reduce the thickness of gaskets, with a small coefficient of linear expansion coefficient of gaskets. ④ increase the tensile deformation of the bolt.

For low temperature valves below -100 ℃, the body material and bolt materials are generally used austenitic stainless steel, the linear expansion coefficient of the same, so the choice of suitable gasket material and increase the amount of bolt tensile deformation is more important. The ideal low temperature gasket material has low hardness at normal temperature, good rebound performance at low temperature, low linear expansion coefficient and certain mechanical strength. In practical applications, generally used stainless steel filled asbestos or PTFE wrapped flexible graphite spiral wound gasket, which is made of flexible graphite and stainless steel winding wound gasket sealing effect is the most ideal. To increase the amount of tensile deformation of the bolt, due to the bolt preload setting restrictions, the added margin is not enough, consider the provision of disc spring washer to compensate.

4 Conclusion

Cryogenic valves and general-purpose valves in the design, manufacture and testing are very different, especially the selection of low-temperature valve sealing material and the design of the sealing structure is more complicated. Therefore, in the design of low-temperature valve, in addition to the general design rules to comply with the valve, but also must be based on the characteristics of the media and the minimum operating temperature selection of suitable sealing materials, and as far as possible the use of flexible sealing structure so that adverse effects caused by low temperature are effectively compensated. Before finishing, all cryogenic metal materials must be cryogenic treatment, the minimum deformation of the material at low temperature. The newly designed cryogenic valve must be tested at room temperature and low temperature test.

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