With the rapid development of modern petrochemical industry, coal chemical industry and refrigeration industry, the use of cryogenic valves is becoming more and more widespread and the dosage is also increasing. Therefore, the proper and reasonable use of cryogenic valves arouse people's attention. The use of cryogenic valves in addition to the general requirements of conventional valve use, there are some special requirements need attention.

Cryogenic valves generally refer to valves that operate at temperatures below -29 ° C. Some cryogenic valves deliver liquefied gases (such as nitrogen, oxygen and natural gas) when the temperature drops to -196 ° C. Can guarantee normal work.

With the reduction of the working medium temperature, the material and structure of the cryogenic valve are also different from the conventional common valve, especially the cryogenic valve (t

Cryogenic valves receive considerable restrictions in opening the closure. Although thermal insulation is used, there is a limited range of heat flow, resulting in the production of a liquid phase in individual parts of the system. Therefore, when the valve is rapidly opened, the liquid may gain a great deal of speed and a water hammer will be generated in the event of a barrier or resistance such as a valve, which is to be noticed in use.

Commonly used in refrigeration technology, the media for the nitrogen or Freon refrigerant, cast iron valves used in the past, the applicable temperature is between -28 ~ 150 ℃, now less, but the valve with ferrite nodular cast iron has an increasing trend. This is because ferritic ductile iron has better impact toughness, wear resistance and corrosion resistance.

The general use of carbon steel (WCB) valve material, the use of temperature -29 ~ 150 ℃; also have low-temperature carbon steel (LCB, LC1, LC2), the lowest temperature of -46 ~ -73 ℃; austenitic stainless steel , 316) is a good performance of low temperature steel, the lowest temperature of -196 ℃ below, some of -254 ℃.

General low-temperature valve closing pieces, the valve body on the use of metal seal seat, the valve flap on the choice of fluorine plastic (F4), there is no special requirements in the structure and conventional common valve. According to the temperature and medium reasonable selection.

Cryogenic valve, only consider the working characteristics of ultra-low temperature valve. Can be installed inside the incubator anywhere, but the stuffing box, operating mechanism (hand wheel, wrench), valve lift position indicator should be placed outside the box. Easy to operate and change packing. Most valves are installed with the stem horizontal. For ultra-low temperature electric valve stem requires horizontal placement.

Low temperature packing device working conditions are very complex, due to the filling material is easy to lose elasticity, as a result of the evaporation medium leakage, the stem is frozen, it will lead to stuck phenomenon, the valve can not be normally opened and closed. In order to improve the working conditions of the packing device, a heat shield can be installed to ensure the insulation of the packing. This is mainly in the design, increase the stem length. The baffles are made of non-metallic materials (cloth, tape, etc.) with a low thermal conductivity so as to significantly reduce the heat loss of the stuffing box. Alternatively, the packing device may be jacketed and the media having a temperature sufficient to maintain the working capacity of the packing device may be driven into the resulting space. Its role is to make the valve from the low temperature to the operating point has a temperature gradient, at a certain point of operation and over the operating point, the pressure will be at a liquid that can make liquefied gas is no longer a liquid and returned to the gaseous temperature.

The gas pressure is always on at all times and normally communicates with one end of the valve or with the other so that no dangerous high pressures are created if the temperature rises within the gas pressure.

As the valve parts are usually manufactured and tested at room temperature, working at low temperature, different parts with different temperature will have different expansion and contraction, it will directly affect the normal operation of the valve, which is the use must pay attention to And understand.

Cryogenic valves use a wider variety of materials. Common materials such as carbon steel (WCB) will become brittle as the temperature decreases. The most commonly used choice of materials are: austenitic stainless steel, bronze and copper-nickel alloy, these materials will not occur brittle fracture, to ensure normal use. Experience shows that austenitic stainless steel, bronze, copper-nickel alloy is the best toughness and strength of the material. Therefore, some important parts of the cryogenic valves, such as: globe valves, safety valves, control valves, check valves and so choose this type of material.

Production experience tells us that, even austenitic stainless steel, bronze and copper-nickel alloy, the finished products without proper low-temperature treatment, when the valve is operating at low temperatures, the inner part of the material will be transformed due to low temperature deformation, resulting Valve leak.

Low temperature valve in the assembly process, due to the flange gasket, connecting bolts and connectors of different materials, resulting in shrinkage between various materials, do not generate relaxation and lead to leakage. Therefore, low-temperature valves, especially the -196 ℃ below the use of low-temperature valve, the connection method is best to use the welding connection.

Cryogenic valve packing gasket, the general majority of F4, because of its good self-lubricating, low friction coefficient, but also has a unique chemical stability. Therefore, it is widely used, but F4 also has some shortcomings. First, the tendency of cold flow is large. Second, the coefficient of linear expansion is large, causing shrinkage at low temperature to cause leakage, resulting in a large amount of icing at the valve stem, thus causing the valve to open. So only in general low temperature conditions. In case of ultra-low temperature conditions, should choose flexible graphite braided packing, or stainless steel and flexible graphite wound litter. Some low-temperature valves in operation, often found that the valve drive parts sticky. Biting phenomenon occurs, the main reason is: matching material selection unreasonable to reserve too cold clearance, and processing accuracy and other reasons. In the process of using a similar problem found in the timely submission to the supplier, to improve the structural design to replace the appropriate bearing (sleeve) material.

Other requirements of cryogenic valves and conventional valves the same.

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