1, Overview Low-temperature shut-off valve is a kind of work in low temperature gas hydrogen or liquid hydrogen medium, used to connect or cut off the pipeline medium device. In this paper, a cryogenic shut-off valve for oxyhydrogen engine test bench is introduced. Aiming at the problem that the valve can not be actuated during the operation, a fault analysis is carried out. By improving the structure, the valve runs normally in the test system. 2, structural features 2.1, technical parameters Low-temperature valve technical parameters are as follows. Working medium Low temperature gas Hydrogen, liquid hydrogen Nominal diameter 65mm High use pressure 25MPa Minimum temperature Hydrogen temperature drive Pneumatic (cylinder bi-directional ventilation) Operating gas source Nitrogen gas, 5 ± 012MPa Connection flange 2.2, Structure Low temperature globe valve Valve body, disc, stem and cylinder and other components. Body, bonnet and stem are made of stainless steel, the valve with copper. Upper and lower stems with epoxy phenolic laminated glass fiber reinforced plastic panels to achieve insulation, flange sealing surface with aluminum shims, PTFE packing used, the use of dual-cylinder seal to reduce friction, the valve as a whole with polyurethane Foam insulation. 3, design 3.1, material selection Aerospace rockets with low temperature propellants are generally liquid hydrogen, liquid oxygen and fluorine, etc. Therefore, the valve material required low temperature resistance, corrosion resistance and low temperature media compatible and has a lower thermal conductivity. At present, the use of more metal materials are austenitic steel, copper or aluminum, non-metallic materials such as glass, PTFE and so on. Considered from the metallographic, austenitic steel with face-centered cubic lattice in metallic materials, copper and aluminum do not exhibit low temperature brittleness at low temperature, but due to the hardness of aluminum and aluminum alloy is not high, wear-resistant aluminum sealing surface , Scratch resistance is poor, so the use of low-temperature valves have some restrictions, only for low-pressure and small-caliber valves. Cryogenic valve gasket must be at room temperature, low temperature and temperature changes with reliable sealing and recovery, it is generally the performance of small choice gasket material. Such as impregnated PTFE asbestos filler or molded plastic filler, and FRP due to the thermal conductivity is very small, mostly used as thermal bridge components. 3.2, the sealing structure in the closure of the design, the seat with harder material --- stainless steel, valve with a softer material --- copper, when the austenite phase transition occurs at low temperatures, the sealing surface rugged, The soft material is deformed by the operation force to make it close to the valve seat to compensate the material deformation caused by the thermal stress and the tissue stress and solves the sealing problem under the high pressure and low temperature conditions. 3.3, heat insulation structure In order to better achieve the low temperature blocking valve insulation, the bonnet design into a slender structure, increase the length of the heat bridge to prevent part of the stuffing box due to subcooling so that the stuffing box in the valve stem and the upper part of the valve cover Parts frost, affect the use. The use of insulation pads (epoxy phenolic material for the glass cloth) will be divided into the stem on the next two parts, reducing heat flow, to achieve adiabatic. The overall use of external insulation valve form, the use of polyurethane foam insulation, Paul thickness 200mm. 4, failure analysis After the cryogenic shut-off valve is applied to the test system, there is a malfunction that can not be operated. After the disintegration of the valve found that the valve stem and cover appeared occlusion, the centerline of the stem surface symmetry of the location there are two deep scratches, matching the corresponding position under the valve cover flange there are two Scratches. After analysis, the cause of the failure has two aspects. (1) Storage Period The valve is placed horizontally and in an open state during storage. The valve stem and the valve flap form a cantilever structure with a cantilever length of 147 mm and a cantilever weight of 418 kg. As a result of gravity under the valve stem bending, making the next valve stem and flange flange hole deviation increases. In the original design, the maximum deviation of coaxiality is 0.1mm, and the deviation of measured coaxiality after demolition is about 0.4mm. As the two parts of the material are OCr18Ni9 stainless steel, can not be lubricated at low temperatures, the action due to large coaxial deviation occlusion phenomenon. (2) Processing period After inspection, the pin groove on the bonnet is not according to drawing processing (Figure 1). The square groove in the drawing is processed into a circular arc groove, which affects the normal cooperation of the pin and the pin groove, and the lower stem receives the lateral force, so as to cause interference between the lower stem and the bonnet. 5, improve the analysis of the existing problems, the structure of the valve has been improved cryogenic. (1) Stem and bonnet mating part Lower stem (2) Stem anti-rotation device The pin and dowel groove cooperate to prevent the stem from rotating. Due to the flap, stem, cylinder and piston of the valve, the rotation of the valve stem will not affect the sealing and opening / closing performance of the valve. To avoid lateral forces on the lower stem due to improper mating of the pins and dowel slots, the pin was eliminated. 6, the conclusion of the improved valve in the actual use of good performance. Under normal circumstances, the low temperature valve movement in the appropriate gap, you can use stainless steel, but long-term use in the lateral force or shaft deformation conditions prone to bite, so try not to use. In addition, the valve to be placed and used vertically, long-term non-use, the valve should be closed.
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