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The Ultrasonic Test Problem of In-service Super-high Pressure Crystal AutoclaveDong Shang Yuan
Institute of Boiler & Pressure Vessel Inspection
182# Xudong Road
Hubei Province, CHINA 430077.
In-service crystal autoclave works at high-temperature and high-pressure in long years and gets in directly touch with alkaline medium, so cracks are easilyappeared in the autoclave body. The dangerous defects in the body must be checked out by ultrasonic test (UT) to eliminate the hidden trouble of the accidents.
The article analysis the technical difficulties in checking defects of the autoclave body by UT. It points that The technology of adopting ultrasonic shear wave test to the cylindrical vessel with thick wall needs to be improved, a series of procedure parameters of ultrasonic shear wave test, such as selection of originate sensitivity, determination of probe, contrary testing block and standardized reflecting body must be test, and the appropriate countermeasure is made.
Keywords: Ultrasonic Test , Ultrasonic Shear Wave Test, NDT, Crystal Autoclave,Thick Wall Cylindrical Vessel
Man-made crystal is worthy. It grows in the crystal autoclave which bear the high temperature of 400° and the high pressure of 150 Mpa . The working medium in the autoclave is usually the solution of NaOH or NaNO3 with the thickness of 1~1.25N.
Crystal autoclave is a kind of single-layer cylindrical forging vessel with thick wall. Its internal diameter is generally between 180~400mm, and appropriate external diameter is 370~800mm. The proportion of the former two diameters is below 0.6. There are two kinds of autoclave structure. One is air-vent style, the other is blind-vent style. The main suffering pressure components are: the autoclave body, head, flange plate, hoop, sealing ring & pressure ring, high pressure pipe seat, screw plug, thread link pieces and so on. The material of the autoclave is high intensity alloy steel that has fracture tenacity and brittleness transformation temperature. At present, NiCrMoV steel is mostly used, and some of the crystal factories use the crystal autoclave reformed from abolishing gun pipes.
The regular test of the crystal autoclave is the important measure to guarantee its safe using. According to the relating information, many accidents had happened in many countries: the in-service autoclave body has cracked, leaked and fractured. Through the accident analysis, the main reason is mostly the defects which evolved from the autoclave body, such as stress corrosion crack (S.C.C), the other defects are corrosion fatigue and hydrogen white point. Therefore, the dangerous defects in the autoclave body must be checked out in the regular technical test of the autoclave to eliminate the hidden trouble of the accidents. The testing item usually contains: checking both interior and exterior, measuring to the wall thickness, NDT to the autoclave body, checking tight pieces, chemical ingredient analysis to the material of the autoclave, metallographic test and hardness measuring, safe attachment checking and pressure-resisting test. Among them, the ultrasonic testing (UT) of the autoclave body must be especially taken to care.
Since in-service crystal autoclave works at high-temperature and high-pressure in long years and gets in directly touch with alkaline medium, stress corrosive cracks are easily appeared in the inside wall. The directions of these cracks are stochastic. Therefore, it is necessary to adopt NDT method to stress on checking all kinds of the cracks on the inside surface of the autoclave wall and the supernormal defects in the wall, according to the in-service features. The super-high pressure crystal autoclave is cylindrical forging made of casting, heat treatment, mechanical processing, and the process of cylindrical forging consists of forging, punching and rolling, so its defect directions are much complex than axle forging and cake forging. For forging autoclave with thicker wall, it is much difficult by NDT technology (especially the ultrasonic shear wave test technology) to find defects of the autoclave body. For UT procedure, the test block form, probe type & its moving method, acceptance requirement should have be united and standardized.
2.1 Test to the surface cracks of both sides of the autoclave wall.
To the super-high pressure vessel with thick wall filling corrosive medium, cracks and other kinds of defects shall be appeared on the inside surface more easily than outside. Using the method of magnetic powder testing (MT), penetrate testing (PT), it is very simple to check the cracks on the outside surface. Since the diameter of he autoclave body is minor and there is a thin layer of adherence made of crystal film on the inside surface, so MT and PT can't be used. For checking the cracks on the inside surface, there are two methods: one is to adopt eddy current testing (ET) to check the cracks on the surface and near the surface through the inside surface. Since there is the serious influence of collecting signal which caused by the crystal adherence on the inside surface, the strict requirement of burnishing procedure, the expensive price of the equipment, so it's not easy to do by ET. The other is to adopt UT to check through the outside surface of the autoclave. It can be worked out with checking the inner defects of the autoclave wall after processing test.
2.2 Test to the inner supernormal defects of autoclave wall.
UT through outside surface of the autoclave wall can be used to resolve the two problems for checking inner defects of autoclave wall and inside surface defects. The straight probe can be used to check out the defects that are parallel to the outside round surface of the autoclave. Checking through the outside round surface, the different angle probe can be used to check out the inner annular and axial defects of autoclave wall, and the inside surface defects in directions. For the purpose, a series of UT processing test must be carried on to get the suitable procedure.
2.3 Test to the position of the crew plug
To the crystal autoclave of venting structure, its weak point is an R position of the plug step of autoclave bottom. Since the inside surface of the body is not continuous, and there is leak-free pre-tight stress in the plug, so the little cracks here has been produced before using. The cracks will extend because of the alkaline solution leakage, such vicious circle will lead to accidents. Before checking, contrasting test block must be cut off from the plug part of the real crystal autoclave and carry on relating experiment to determine the test procedure.
3.1 Ultrasonic testing procedure
At present, through the tentative research and test, the procedure important point for UT of the inside defects of thick wall cylindrical vessel with r/R<80% follows:
The contrary testing block used in UT must be manufactured by the geometric-shape and material that are similar to the test products.
The standardized reflecting body for checking the inside surface defects of the autoclave is best to use thread-cut. The standardized reflecting body for checking the inner axial defects of the autoclave is best to use short-transverse hole.
Use four probes to check the defects of the autoclave through the outside round surface:
Use once-wave testing and the shortest sound distance testing as possible. Straight probe adjusts the sensitivity by the bottom surface of workpieces, using j2 flat-bottom hole as originate sensitivity . Check the annular and axial inside surface the defects of the autoclave, the 25×1.0mm thread-cut on the surface inside the testing block should be used as the originate sensitivity.
3.2 Improve the technology of ultrasonic shear wave test.
The technology of adopting ultrasonic shear wave test to the cylindrical vessel with thick wall needs to be improved. A series of procedure parameters of ultrasonic shear wave test, such as selection of originate sensitivity, regulation of probe, contrary testing block and standardized reflecting body still need to test. The suggestions for improvement supplied by the writer follow:
According to the reality, work out the " ultrasonic testing technical regulation of super-high pressure crystal autoclave".
Research and manufacture the angle-adjustable focusing probe that should have good direction. Its direction precision must be at 1°±0.5°and the adjustable angle must be at 20°~ 35°to resolve more deviation of the main sound sheaf ,testing to the thick-wall high pressure vessel.
Research the special testing block that can determine the composite sensitivity (>0.5mm) and direction. The block is required to be small and multi-functional and can be calibrated in time when being test, so the defect ration and location will be determined.
Select the unit using crystal Autoclave and try to test, so to determinate the applicability of probe, testing block and the means of testing .It is required that the composite sensitivity of the probe, testing block and instruments should be suitable for being able to find out 0.5mm crack on the inside surface of cylindrical wall.
Based on the foundation, work out the special ultrasonic testing procedure to guide in actual test of the crystal autoclaves.
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