Ball valves are characterized by rapid opening and closing, reliable sealing, simple structure, light weight, and low flow resistance. Currently, they are widely used in low temperature piping systems. Ball valves used in industrial cryogenic pipelines, except for a few special-purpose and atypical structures, are mostly non-metallic soft-seal valve seat structures. Because of their poor operating conditions and critical functions, they have high sealing requirements, strict performance evaluation, and low temperatures. Performance testing is a key process in production and use. The low temperature test of the ball valve has certain characteristics. The Vacuum Technology Network (http://) thinks that it is necessary to understand the principles, methods and characteristics of the low temperature test of the ball valve, and scientifically and reasonably use the means of low temperature test to promote the research and production of the low temperature ball valve. The continuous improvement of product quality and the protection of key projects are of great significance. 2.1 Purpose Valve low temperature test is to test the performance of cryogenic valve in the environment of low temperature simulation. It can evaluate the performance of low temperature valve. At present, the standards for valve low-temperature tests are: GB/T24925-2010, BS 6364:1984, etc. The main contents of the low-temperature test include: Inspection of seals at seals, packings, upper seals, etc.; operating performance of the whole machine with pressure conditions. Detection parameters include: valve body, valve cover, stem, valve disc, stuffing box, refrigerant and ambient temperature; instantaneous leakage at the outlet end of the valve, cumulative leakage, and average leakage; extreme changes in test medium pressure. The test medium is generally helium. 2.2, test device The low-temperature test device of the valve is shown in Fig. 1. The test device consists of three parts: a cryogenic system, a pressure system, and a measurement and control system. The cryogenic system uses liquid nitrogen as a refrigerant to create a suitable low-temperature environment. It should have two cooling modes, dipping and spraying, in which the spray cooling method must be able to adjust the temperature from 0 to -196°C; the pressure system controls the test medium pressure and provides tests. The required pressure source shall be capable of recycling as much as possible for the precious test medium; the measurement and control system shall be responsible for the collection, induction, evaluation of the physical quantities during the test, the management of the auxiliary parameters and test parameters, the process control of the entire test device, and Provide a complete man-machine interface. 2.3, the test process Prior to the low temperature test, the valve under test shall be fully dried to remove grease and debris from the valve. The cryogenic valve is installed in the low temperature test tank, and all the joints are connected to ensure that the valve packing part is located above the thermal insulation cover and the temperature is maintained above 0°C. The valve is immersed in a low-temperature medium. The low-temperature medium covers the upper end of the connecting portion between the valve body and the valve cap, or the nozzle is used to uniformly spray the low-temperature medium below the neck of the valve cover of the valve to cool the valve to the corresponding test temperature. Keep for a certain period of time until the temperature in each place is stable. The temperature change should be within ± 5°C. The valve was opened and closed several times to test its low temperature operating performance; the valve was closed and pressurised in the normal flow direction for the sealing test. Then the valve is in a half-open state, the needle valve at the outlet end is closed, and the seal of the valve packing, the valve body and the valve cap joint is checked. The test results were compared with corresponding standards and the results were judged to form a conclusion. 3.1. Differences from actual operating conditions The current standards, data recommended by the low-temperature test methods are almost always using the external cooling method, that is, the use of refrigerant outside the test valve to extract heat, reduce the valve temperature. The actual working conditions of the cryogenic valve are: The low-temperature medium flows through the inside of the valve, and the outside is exposed to a normal temperature or a relatively high temperature environment. The problem caused by the external cooling method is to make the cryogenic valve produce a temperature gradient opposite to the actual operating conditions at the initial stage of the test. For the cryogenic ball valve, the valve body and the valve cover are rapidly cooled to produce a volumetric shrinkage, while the sphere and the valve seat It has not been completely cold, especially due to the thermal insulation of non-metallic seats, further delaying the heat transfer process. At this point, the original fit has been changed, non-metallic valve seats or non-metallic seals of the combined valve seat may be over-compressed, resulting in difficulty in the movement of various parts, let's call this phenomenon: lock at low temperature. Cryogenic locking will cause permanent deformation of non-metallic seats, the so-called "cold flow" phenomenon, and non-metallic materials such as PTFE have a larger coefficient of thermal expansion than metal materials, with the gradual balance of internal and external temperatures. The internal parts shrink, the seal specific pressure decreases or disappears, and the seal pair fails. Even if the product is qualified in the low temperature test, the temperature gradient of the actual condition of the low temperature piping may always exist, the temperature level of the valve housing is higher than the internal parts, and the pre-added sealing specific pressure during assembly may be reduced, and the sealing effect may still be caused. decline. 3.2, low temperature locking The damage caused by cryogenic locking can sometimes be very serious. In addition to the extrusion of the valve seat, the fasteners and sealing elements that connect the housing can also be damaged by an abnormally high stress. The housing and the inner parts hold each other. Immediately afterwards, the situation of stress is complex and, in serious cases, it may cause permanent structural changes. The cryogenic ball valve is most likely to open and close immediately after it is locked at a low temperature. At this time, the opening and closing operations can easily generate a series of indentations on the sealing surface of the valve seat under extreme stress, and may even result in a ball port. The "cut" phenomenon of the valve seat completely disables the valve seat. The effective means to prevent low-temperature locking and damage is to control the cooling rate, keep the valve in the fully open or fully closed position during the cooling process, try to measure the temperature inside the valve, maintain a certain temperature stabilization time, before opening and closing operations as much as possible Keep the valve inside and outside temperature balance. 3.3, the impact of low temperature properties of the material At present, cryogenic valves, especially cryogenic valves for mediums such as LNG, are mainly made of Ni-Cr austenitic stainless steels such as 304, 304L, 316, 316L, and these materials can maintain good strength and toughness at low temperatures. However, these materials also have some deficiencies. These materials belong to metastable stainless steels, which will undergo metallographic transformation to martensite at low temperatures, due to the lower density of martensite in the body-centered cubic lattice. In the austenite of the cubic lattice, the low-temperature phase transformation causes volume expansion and deformation of the part. In addition, the temperature will reduce the shrinkage of the metal structure. Due to the non-uniform shrinkage of the parts, the temperature stress is generated. When the temperature stress exceeds the yield limit of the material, the part will produce irreversible permanent deformation. Therefore, the cryogenic treatment process of cryogenic valve components is critical, and the purpose of cryogenic treatment is to allow these phase transformations and deformations to fully occur before the finishing process, so as to ensure the structural stability of the finished product parts and components. Cryogenic valves without cryogenic components may cause complete failure of the overall performance after entering a cryogenic environment. The non-metallic valve seats of cryogenic ball valves are generally made of polytetrafluoroethylene (PTFE), polychlorotrifluoroethylene (PCTFE), etc. The theoretical embrittlement temperature of PTFE and PCTFE are both -180 to -195°C, but in fact, commercial The procurement of products far below this temperature, the damage caused by the low temperature brittleness of the valve seat is sometimes serious, the valve seat after embrittlement has lost the elastic compensation ability, if the ball accuracy is not high enough, it is difficult to achieve sealing Requirements, especially for the zero-leakage requirements of the soft seal seats specified by the Chinese standard. In addition, the hardness of the valve seat after embrittlement increases sharply, which may cause damage to the ball surface or embrittlement of the valve seat. 3.4, the effect of cooling rate The rate of cooling by dipping is actually very difficult to control, depending on the surface conditions of the valve and the thermal conductivity of the material. However, the spray rate can be adjusted by controlling the amount of liquid nitrogen spray. In theory, a lower cooling rate can reduce the temperature difference between the valve and the outside and reduce the temperature gradient, which is beneficial to the test process, but it will increase the consumption of liquid nitrogen. The cooling rate should be determined by the specific parameters of the valve under test, such as diameter, wall thickness, structural conditions, and internal parts. Excessive cooling rate will aggravate the phenomenon of low temperature locking, and excessive temperature gradient will cause high internal stress and damage to components. 4.1. Refrigerant Consumption and Control The ideal consumption of refrigerant in low temperature test should be: The amount of heat that the tested valve, together with the auxiliary facilities in the test tank, need to take away from the room temperature to the specified temperature is equal to the total latent heat of vaporization of the consumed refrigerant. Reducing the consumption of refrigerant is an important measure to reduce the test cost. Reducing the consumption of refrigerant can be started from the following aspects: (1) Choose a suitable low-temperature test tank; (2) Reduce the extra space in the test tank as much as possible, and use heat capacity. Smaller materials fill excess space; (3) For tests that require temperature regulation, try to use spraying to adjust the temperature and avoid impregnation after diluting the refrigerant with alcohol; (4) Concentrate the test, and the same specification product can be tested continuously , In order to rationally use the residue; (5) Strengthen the insulation measures to reduce the additional loss of cold. 4.2, Test medium consumption and its control Low-temperature test medium is generally defined as helium gas, helium gas is an inert gas, the standard atmospheric pressure liquefaction temperature is -269 °C, is a suitable medium for valve low temperature test. However, the market price of helium is expensive, and the helium consumption of large-caliber, high-pressure valve low-temperature test is huge. Therefore, helium consumption control and recycling and reuse are of great significance. In principle, the recovery technology of helium is not complicated. The important thing is the operability of the process and the safety design of the operation. There was concern about the purity of helium after recovery. In fact, the low temperature test of the valve does not require high purity of helium. In addition, during the low temperature test, the valve first performs helium purging on the valve chamber to eliminate the air in the chamber. . The cold high pressure environment during the low temperature test of the valve has exceeded the liquefaction condensation point of most of the substances, and the purity of the recycled helium has not changed much, and the repeated use of the low temperature test also has little effect. The low-temperature test of the valve is a work that has certain risks, and safety precautions are very important. The dangers are mainly reflected in: Partial lack of oxygen caused by the massive evaporation of liquid nitrogen, which can cause people to suffocate; There may be “scald†to the low temperature of the personnel; The splash of refrigerant caused by large leakage of the test medium; and The low temperature of instruments and meters damage. Therefore, the work place in the low-temperature test should be kept well ventilated. If necessary, manual forced ventilation is also required. It is necessary to scientifically and reasonably formulate the company's cryogenic valve test operating procedures based on relevant standards. The operator must hold a certificate and be equipped with the necessary safety protection equipment. Special attention should be paid to the protection of exposed parts of the body such as face and hands. A person must be strictly prohibited from working in the field at low temperatures. The emergency stop function should be set on the test equipment, and its operating parts should be in the most eye-catching and easy-to-operate position. For high-parameter, large-caliber, high-parameter tests, long-distance operation should be performed as far as possible. The tested valve shall have fixed and clamping measures in the low temperature test tank. The selection of instruments and meters should focus on their low temperature resistance and performance stability at low temperatures to prevent their non-temperature resistant parts from reaching low temperature environments. Warning signs shall be set up for the low-temperature parts of exposed cryogenic equipment or equipment, and isolated to avoid accidental injury to unrelated personnel. The current standards for valve low-temperature test method and the actual operating conditions of the cryogenic valve environment have certain differences, should be reasonable control of the cooling rate, cooling method, holding time, so that the test conditions as close as possible to the actual working conditions. For the low temperature test of the ball valve, attention should be paid to the low-temperature characteristics of its component materials to prevent damage to the valve under test, such as low temperature locking and cold flow. The refrigerant and test medium in the low-temperature test of the valve are expensive and consume a lot, so attention should be paid to the control of consumption, or the recycling thereof should be reused. Strengthen safety precautions to prevent personnel and equipment from being damaged by low temperatures. Wuxi Zijing Purification Engineering Co., Ltd , https://www.zijingjh.com
Low-temperature test is an important means to verify the performance of low-temperature valves. The current standards stipulate that there are certain differences between low-temperature test methods and actual conditions. Improper operation can easily cause distortion of test results and even damage to the tested valves. This article mainly focuses on the special structure of the cryogenic ball valve, analyzes the problems that are likely to occur in the low temperature test of the valve, and combines some practical experience with some countermeasures.
Cryogenic ball valve structural performance verification