Valve Cryogenic Testing
The low temperature test is an important means of verifying the performance of the low temperature valves and, as stated by the current standard, must be performed differently according to the working conditions. Performing the test improperly would cause distortion of the test results or even damage to the tube under test.
In this paper we will mainly deal with the special structure of the low temperature ball valve
, analyzing the problems that can easily occur in testing this valve and proposing some countermeasures based on actual operating experience.
Ball valves are widely used in low temperature piping systems due to their quick opening and closing characteristics, reliable sealing, simple structure, light weight and small flow resistance. Except for a small number of special and atypical designs, most ball valves used in industrial low temperature piping are non-metallic soft-seal valve seat designs. Due to harsh operating conditions and critical functions, they have high sealing requirements and rigorous performance evaluation. Performance testing is a key process in the manufacturing and use process. The low temperature test of ball valves has some peculiarities and it is believed that understanding and mastering the principles, methods and its characteristics can improve product quality and ensure safe and effective designs.
Low temperature test of the valve
The low temperature valve test is to test the performance of the valve at low temperature in the simulated low temperature working environment and can evaluate the performance of the valve at low temperature. Currently, the main standards implemented in low temperature valve testing are: GB / T24925-2010, BS 6364: 1984, etc.
The main contents of the low temperature test include:
inspection of the sealing condition of gaskets
packing, top seals
operating performance of the whole machine under pressure conditions Test parameters include:
cumulative loss and mean loss at the valve outlet
extreme variations of the mean test pressure
The test medium is generally helium.
2) Test device
The test device is mainly composed of three parts: low temperature system, pressure system and measurement and control system.
The low temperature system uses liquid nitrogen as a refrigerant to create a suitable low temperature environment. It should have two cooling modes: dipping and spraying. Among these, the spray cooling method should be able to reach an adjustable temperature from 0 to -196 °C. The pressure source required for the valuable test medium should be capable of being recycled and reused as much as possible; the measurement and control system is responsible for the collection, induction and evaluation of various physical quantities during the test process, for the management of auxiliary parameters
and test parameters and for the process control of the entire test device. Provide a complete human-machine interface.
3) Test process
Prior to the low temperature test, the valve under test must be completely dried to remove grease and debris in the valve. Install the low temperature valve in the low temperature test tank, connect all joints, make sure that the packing part of the valve is above the insulating cover and that the temperature is kept above 0 ° C. Immerse the valve in the low temperature fluid, cover the upper end of the connecting part between the valve body and the bonnet, or use a nozzle to spray the low temperature fluid evenly under the valve bonnet neck to cool the valve to temperature corresponding test. Keep it for some time until the temperature is stable everywhere and the temperature change should be within ± 5 ℃. Open and close the valve several times to check its operating performance at low temperature; close the valve, pressurize in the normal flow direction and perform a leak test. Then put the valve in the half-open state, close the needle valve at the outlet end and check the tightness of the valve gasket, the connection between the valve body and the bonnet. Compare the test results with the corresponding standards, judge the results and draw a conclusion.
Issues to watch out for in low temperature ball valve test
1) Differences from actual working conditions
The low-temperature test methods recommended by current standards and data almost all use the external cooling method, i.e. the use of refrigerant to extract heat from the outside of the valve under test to reduce the temperature of the valve. The actual working conditions of the low temperature valve are: the low temperature fluid flows from the inside of the valve and the outside comes into contact with the normal temperature or relatively high temperature environment.
The problem brought by the external cooling method is that the low temperature valve produces a temperature gradient opposite to the real working condition in the initial phase of the test. For the low temperature ball valve, the valve body and valve cover are rapidly cooled, resulting in volume contraction.
At this time, the ball and valve seat is still not completely cold, mainly due to the insulating effect of the non-metallic valve seat, which further delays the heat transfer process. At this point, the original coupling is changed and the non-metallic valve seat or the non-metallic sealing ring of the combined valve seat may be over-extruded, making it difficult for each component to move.
We call this phenomenon:
low temperature block.
Low temperature locking will cause permanent deformation of the non-metallic valve seat, which is the so-called “cold flow” phenomenon, and the thermal expansion coefficient of non-metallic materials such as PTFE is greater than that of metallic materials. The internal parts shrink, the specific pressure of the gasket decreases or disappears, and the pair of gaskets fails. Even if the product is qualified in the low-temperature test, the temperature gradient of the actual working conditions of the low-temperature pipeline can always exist, and the temperature level of the valve shell is higher than that of the internal parts, the pre-added the specific sealing pressure during assembly will be reduced and the sealing effect may still be caused.
2) Low temperature lock
The damage caused by low temperature lock is sometimes serious. In addition to the extrusion of the valve seat, the fasteners and sealing elements connecting the casing will also be damaged by abnormally increased stress, and the casing and the internals will hug each other. After tightening, the stress situation is complex, and in severe cases, it may cause permanent structural changes. When the low temperature ball valve is locked at low temperature, it is best not to perform the opening and closing operations immediately. The opening and closing operations at this time will easily produce a seriesof indentations on the sealing surface of the valve
seat under the action of great stress, and even cause the ball port. The phenomenon of “gnawing and cutting” on the valve seat makes the valve seat completely invalid.
The effective means to prevent low temperature lock 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 in the valve, maintain a certain temperature stabilization time, and try to do as much as possible before opening and closing operations.
3) Influence of low temperature properties of materials
Currently, the metal materials of cryogenic valves, especially ultra low temperature valves for LNG and other fluids, are mainly Ni-Cr austenitic stainless steels such as 304, 304L, 316, 316L, which can still maintain good strength and toughness at low temperatures. But these materials also have some shortcomings pouchè are metastable stainless steels, which will undergo a metallographic transformation into martensite at low temperatures. The density of the martensite of the cubic lattice centered on the body is lower than that of the superficial austenite with a centered cubic lattice entails, after the transformation at low temperature, an expansion of the volume and deformation of the parts.
Furthermore, the reduction of the temperature will also cause the shrinkage of the metal structure which in turn generates a thermal stress which, in turn, when it exceeds the yield point of the material produces an irreversible permanent deformation. Therefore, the cryogenic treatment process of the low temperature valve parts
is very critical. The purpose of cryogenic treatment is to ensure that these phase transitions and deformations occur completely before finishing, in order to ensure the structural stability of the finished parts and components.
The low temperature valve whose parts have not been subjected to cryogenic treatment can cause deterioration of the overall performance of the whole machine after entering the low temperature environment. The non-metallic valve seat of the low temperature ball valve is generally made of polytetrafluoroethylene (PTFE) or polychlorotrifluoroethylene (PCTFE). The theoretical embrittlement temperature of PTFE and PCTFE is -180 ~ -195 ℃, but in commercial reality the chemically purchased products cannot reach that temperature and the damage caused by the low temperature brittleness of the valve seat is sometimes severe. The embrittled valve seat has lost its elastic compensating ability. If ball accuracy is not high enough, tightness is difficult to achieve, especially due to the zero leakage requirements of soft seal valve seats specified in Chinese standards. In addition, the hardness of the embrittled valve seat increases significantly, which can cause damage to the ball surface or embrittlement of the valve seat.
4) Influence of cooling rate
The rate of cooling by dipping is actually very difficult to control, depending on the surface condition of the valve and the thermal conductivity of the material. But the spray method can adjust the cooling rate by controlling the amount of liquid nitrogen spray. In theory, a lower cooling rate can reduce the temperature difference between the inside and outside of the valve and reduce the temperature gradient, which is beneficial to the test process, but will increase the consumption of liquid nitrogen.
The cooling rate should be determined according to the specific parameters of the tested valve, such as diameter, wall thickness
, structural conditions, internal components and so on. An excessively fast cooling rate will aggravate the low-temperature locking phenomenon, and an excessively large temperature gradient will cause high internal stress, resulting in component damage.
Consumption control of test refrigerant and medium
1) Refrigerant consumption and its control
The ideal consumption of the refrigerant in the low temperature test should be:
Heat required to reduce the test valve
Auxiliary facilities in the test tank from room temperature to the
Total amount of latent heat of vaporization of the consumed refrigerant
Reducing refrigerant consumption is an important measure to reduce the test cost. To reduce refrigerant consumption, we can start from the following aspects:
- Select a low-temperature test tank with appropriate volume
- Minimize the excess space in the test tank as much as possible, and use the heat capacity (use smaller materials to fill the excess space)
- For tests that require temperature adjustment, try to adjust the temperature by spraying to avoid immersion after diluting the refrigerant with alcohol
- Arrange the tests in a centralized manner, and products of the same specification can be tested continuously, in order to rationally utilize the residual liquid
- Strengthen the thermal insulation measures to reduce the additional loss of cooling capacity
2) Test medium consumption and its control
The medium of the low temperature test is generally specified as helium, which is an inert gas, and the liquefaction temperature under standard atmospheric pressure is -269 ℃, which is a more suitable medium for the low temperature test of the valve. However, the market price of helium is expensive, and the consumption of helium in the low-temperature test of large-diameter and high-pressure valves is huge. Therefore, the consumption control and recovery and reuse of helium are of great significance. In principle, the recovery technology of helium is not complicated, the important thing is the operability of
its process and the safety design of operation. Some people have worried about the purity of helium after recovery. In fact, the low temperature test of the valve does not require high purity of helium. Moreover, during the low temperature test of the valve, the valve cavity is first purged with helium to remove
the air in the inner cavity. . The cryogenic high pressure environment during the low temperature test of the valve has exceeded the liquefaction and condensation point of most substances, and the purity of the recovered helium has little change, and has little effect on the repeated use of the low temperature test.
The low temperature test of the valve is a dangerous work and safety precautions are very important. Its dangers have very important consequences especially in:
- local hypoxia caused by a large amount of volatilization of liquid nitrogen (which will cause personnel suffocation)
- possible low-temperature “scalding” to personnel
- refrigerant splash caused by a large amount of leakage of test medium
- low temperature of instruments and meters. damage
Therefore, the workplace of the low temperature test should maintain good ventilation, and if necessary, artificial forced ventilation should be carried out. It is necessary to scientifically and reasonably formulate the operating procedures for the low temperature valve test of the enterprise on the basis of relevant standards. Operators must hold certificates and be equipped with necessary safety protection equipment. Special attention should be paid to the protection of exposed body parts such as face and hands. During low-temperature operation, one person should be strictly prohibited from working on site.
The emergency stop function should be set on the test equipment, and its operating parts should be placed in the most conspicuous and easy-to-operate position. For high-pressure, large-diameter and other high-parameter tests, remote operations should be carried out as far as possible. The valve under test shall have fixing and clamping measures in the low temperature test tank. The selection of instruments and meters should pay attention to their low temperature resistance characteristics and performance stability at low temperature, so as to prevent their non-low temperature resistant parts from contacting the low temperature environment. For exposed low-temperature equipment or low-temperature parts of the equipment, warning signs should be set up and isolated to avoid accidental injury to unrelated personnel.
There is a difference between the low temperature valve test method required by the current standard and the actual operating conditions of the low temperature valve. The cooling rate, the cooling method and the holding time should be reasonably controlled to make 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 the materials that compose it to avoid damage to the tested valve such as low temperature blocking and cold flow. The refrigerant and test medium in the low temperature valve test are expensive and highly consuming, so attention should be paid to controlling consumption, or they should be recycled and reused. In addition, safety precautions must be strengthened and closely monitored at all times to prevent damage to personnel and equipment.