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Keywords: Valve; Non-destructive; Defects; Detection 1 Valves First, a simple understanding of the lower valve, the valve is used to control the components of the medium in the pipeline, the overall mechanical product with a movable mechanism. It belongs to the pipeline accessory product. Where there are pipes, there are valves, and the valve plays a role of cut-off and adjustment. The media used in the valve are gas, steam, oil, chemical, flammable and explosive. In some pipelines, if the valve fails to operate, it will cause the entire device to stop production, pollute the environment, and seriously endanger the safety of people and equipment. There are four kinds of valves that are commonly seen in life: check valves, ball valves, emergency shut-off valves, and pull-off valves. The most basic function of the valve is to cut off or switch the circulation of the medium existing in the pipeline and change the flow direction of the medium in the pipeline, and change the flow and pressure of the medium in the pipeline. This is not only to protect the pipeline but also to Let the device run normally.
2 Valve defects and formation causes The manufacture of valve products involves roughcasting and forging, machining, surfacing or welding, heat treatment, non-destructive testing, assembly and test and inspection, because of the composition of the steel casting materials, the design of the castings, and the refining The process of steel, type selection, sand, finishing, welding repair and heat treatment and many other factors, after the casting shrinks after cooling, the casting will be subjected to resistance, and then stress will occur; generally speaking, the liquid steel is also “Liquid†has ultimate strength and plastic limit at a certain temperature. If the stress on the casting exceeds the limit at that temperature, cracks will be generated; at this time, the casting will undergo a series of solidification and Shrinking reaction, but at the same time it can not get enough molten liquid of steel, that is, the timely addition of "liquid steel", and "shrink hole" and "shrinkage" will occur; if there is no way to control the original material of the casting In the process of casting and refining steel, or in the casting process, the gas reacts with the molten steel and gravel type and reacts with each other. The metal contaminants left behind are unfortunately left in the interior of the steel castings; or because the composition of the water in the molding sand is too high, but the coating for the cold-coated iron is not well handled, and the poor gas permeability of the sand type causes the casting to be cast. The control system and the cavity in the form of inhaled or mixed gas during the pouring process can not discharge in good time, so in the process of casting iron and steel castings, there will be air holes, etc., which are all from the current pressure valve body. The defects caused by steel castings, and currently there is no standard process for the inspection of the valve through the radiation penetration of the valve, the inspection personnel can not be qualified inspection of the valve, so the use of the valve to the existence of the safety of personnel The hidden danger is not small.
3 modern commonly used detection methods - non-destructive testing valve detection methods are many, there are mechanical properties testing, analysis of chemical composition, non-destructive testing. Non-destructive testing, in short, is a method of detecting a detected item without damaging the detected item. It is a method that has existed as a basic application technology discipline since the development of science and technology. Generally, if a material changes its internal organization and structure, it will cause physical changes, so that we can Use this principle to carry out non-destructive testing of objects, which is also the basic principle of non-destructive testing. Nowadays, we often use ray, ultrasonic, magnetic powder and so-called penetration to detect objects, namely RT, UT, MT and PT detection. During the process of casting the valve body, there will always be a series of defects such as shrinkage in various volume states, and the surface of the valve body is often not so smooth. The sensitivity of ray detection is just very high and can directly see these defects. At the same time, this technology is influenced by human factors is minimal, so this technology is very suitable for accurate detection of the body of the valve Production requirements.
3.1 ComputerTomogrdphy technology Radiology, one of the five major non-destructive testing methods, has a very wide range of applications in industry. There is no essential difference between X-rays and natural light. They are electromagnetic waves, but the energy of X-ray photons is much larger than that of visible light. It can penetrate objects that cannot be penetrated by visible light, and when it penetrates objects, it will undergo complex physical and chemical interactions with the substance. It can cause atoms to ionize, make certain substances fluoresce, and also make certain substances produce photochemistry. reaction. If there is a defect in the local area of ​​the workpiece, it will change the attenuation of the radiation on the object and cause the intensity of the transmitted radiation to change. In this way, a top-of-form computer tomography technique, CT technology, is now a common detection method in the country and is also used by the industry. Recognized as a better detection method. It can quantitatively detect the mechanical and physical properties of the detected object and can determine the shape and exact size of the defect. This detection method even has an intuitive representation of the orientation of the defect and its distribution. In recent years, this method has been widely used in the automotive industry in foreign countries. Although this technology has been very useful, but each technology has its limitations, CT technology is no exception. First of all, this technique is only applicable to the detection of one or several types of parts. Therefore, when the detected objects undergo some changes, their detailed resolution capability will be weakened. In practical applications, this technology is shown to be relatively large. The resolving power of objects is not so high, so this technology is not widely used because it is expensive.
3.2 Ultrasonic Testing Ultrasonic Testing Technology is also known as Ultrasonic Testing, Ultrasonic Testing abbreviation UT, ultrasonic testing, is one of five conventional non-destructive testing methods. We all know that sound waves will refract and reflect at the interface between two or more different materials. This is what we already learned in junior high school physics. So we can use this principle to carry out ultrasonic testing. When there are some defects in the interior of the valve body, two different material interfaces will be generated, so that we will get different reflected waves or refracted waves. You know that there are flaws inside the valve but we can't accurately know the size of the defects and their distribution, so this method is simple but cheap but it is not accurate.
3.3 Compton scattering imaging detection technology Compton scattering imaging technology, that is, Compton scattering imaging detection technology, this technology is not widely used in our country, only a small part of the unit research and use Compton scattering imaging Detection technology but its advantages are still very obvious. First of all, in this technology, the position of the detector and the radiation source is very free. That is to say, they can freely choose their position and are not limited. Such a feature makes them widely applicable to ductile objects in industry. Non-destructive testing; and the sensitivity of this method is very high and the amount of radiation he needs is relatively small can be said to be much smaller than the traditional, this feature makes this technology shine in the field of medicine.
3.4 penetrant testing technology peterrant testing, the abbreviation for PT, also known as penetration testing, this technology is based on a principle called "capillary action" to take effect, is the use of a liquid dye on the surface of the object being detected The dyes are washed off after a preset period of time, but because of the capillary action of the dyes, professional inspectors can then analyze the traces left by the object under inspection and subsequently obtain the defect state of the object under inspection. This method is mostly used for items that do not absorb dyes such as steel, ceramics or non-ferrous metals. This detection method is particularly suitable for field testing in the field.
4 Summary In view of the characteristics of the valve casting forging process, there may be many defects such as slag inclusions, pores, shrinkage holes and the like in the casting body, and the surface of the valve is also not smooth. If an appropriate detection method is selected, it is possible to accurately detect the internal defects of the valve body, so that it is possible to effectively judge whether the valve is in conformity with the use requirements.
references:
[1] Wang Xingxin. "Study on acoustic leakage detection method for pressure bearing valve" [D]. Daqing Petroleum Institute.
[2] (United States) J. Lyons. "Valve Technology Manual" [M]. Beijing: Mechanical Industry Press, 1990.
[3] Li Jiawei, Chen Jiyu. "Nondestructive testing manual" [M]. Beijing: Mechanical Industry Press, 2002.
Study on Nondestructive Testing Method of Valve Defect Detection
Abstract: As a component-valve of special equipment, developed countries have paid great attention to the quality of valve manufacturing, especially in terms of safety and quality. For various reasons, our country’s safety monitoring and inspection measures for the safety of valves are later than those for boilers and pressure vessels. Therefore, the detection of valve defects is even more important. Non-destructive testing is also One of the most effective detection methods, the following preliminary study and discussion on the non-destructive testing of the valve.