Vacuum equipment leak detection and flaw detection

- Jul 16, 2019-

Vacuum equipment leak detection and flaw detection

 

With the continuous development and update of technology, the types and structures of vacuum equipment are becoming more and more diversified. In order to ensure the safety and efficiency of production, different structures of equipment are combined to put forward different requirements for leakage detection and flaw detection of equipment. Analyze the traditional and latest testing techniques and means, and put forward the application scope and key points according to the actual application, so that the production and user units can choose the appropriate testing methods and instruments conveniently and quickly.

 

With the continuous progress of science and technology and the improvement of product quality requirements, the application of vacuum equipment in various industries has been unprecedented promotion. Vacuum related equipment is widely used in the fields of aerospace simulation test, nuclear industry, space ion collision test, vacuum heat treatment of mechanical parts, vacuum welding of new parts, vacuum metallurgy, biopharmaceutical and vacuum freeze-drying storage and preservation. No matter for the equipment production enterprises or the use of enterprises, the safety and reliability of equipment is to ensure the normal production of the early conditions. Early detection of equipment leakage point and defect location, become the key to eliminate hidden dangers. With the continuous update and progress of technology in recent years, more and more testing methods and technical equipment are available, and the fields and industries are more extensive.

 

Based on the production practice in recent years and the tracking of related technology development at home and abroad, this paper comprehensively analyzes the main leak detection/flaw detection technologies and principles at present, and summarizes the advantages and disadvantages of various leak detection/flaw detection technologies under different vacuum equipment and operating conditions, so as to provide theoretical basis for personnel with leak detection and flaw detection needs.

 

According to the different shell structure of vacuum equipment, vacuum equipment can be divided into two types: double-jacket structure (water-cooled) and single-layer structure (non-water-cooled). In this case, the defect may lead to water leakage or air leakage. Both water leakage and air leakage will bring great damage to the production test and the equipment itself, especially for the vacuum high-temperature heat treatment furnace, the occurrence of water leakage may lead to human safety accidents, which needs to be paid great attention to.

 

1. Types of defects
Equipment shell defects can be generally divided into several types
.According to the form of the above defects, the following will be elaborated from two aspects:
(1) leakage detection of leakage points through the body materials.
(2) flaw detection for internal defects of the body.

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Figure 1 leakage at welding seam

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FIG. 2 leakage of machining defects

 

2. Leak detection and flaw detection of vacuum equipment

 

Generally speaking, before leaving the factory, the vacuum equipment should be tested for tightness, and the formed leakage point can be found in time, which can be solved by welding. For pressure vessel (storage tanks, tank, high-pressure gas quenching furnace, etc.) must adopt the method of X-ray flaw detection, early to plate the flaws of the intermediate inspection, but for the pressure vessels of vacuum equipment, the sand inclusion and weak defect or not leaking defects such as weld pool too thin, don't usually conduct inspection, this is for the use of equipment after the safe hidden trouble, so treat this kind of defect should also test, eliminate as soon as possible.


The commonly used detection methods are positive pressure method, negative pressure method and X-ray flaw detection, magnetic powder flaw detection, ultrasonic flaw detection, permeability flaw detection, eddy current flaw detection, gamma ray flaw detection, kerosene test, thermal infrared imaging flaw detection, phased array instrument flaw detection, color inspection and flaw detection.

 

2.1 positive pressure method

The positive pressure method is to seal and inflate the detected equipment so that its internal pressure is higher than the external atmospheric pressure, which can be judged by the pressure drop method, water immersion detection and fixed-point detection.

A. Judgment by pressure drop method: under the condition of completely sealing all contact surfaces, the size of leakage amount can be determined by measuring pressure changes by pressure instrument, and then the leakage degree of the equipment can be inferred, but the leakage point cannot be clearly positioned.

     q =( P1-P2)/ΔT

Where P1 -- -- pressure value inside the equipment at the beginning of test after inflation, Pa; P2 -- pressure value inside the equipment at the end of the test, Pa; Δ T - test total time T2 - T1, h
In order to make a more accurate quantitative judgment of the leakage size of the equipment, it can also be converted into the leakage rate.

 qL = q×V/3600

Where, V -- volume of vacuum equipment, L

According to different equipment, the required leakage rate is not the same, which is suitable for qualitative judgment of leakage of large containers, and it is difficult to detect micro leakage.

 

B. Water immersion detection: after the pressure in the equipment reaches positive pressure, immerse it in water and judge the location and size of leakage by observing the bubble condition. It is suitable for the detection of small complex cavity seals and can determine the location of leakage point more accurately. Therefore, this method is also called bubble method. For large equipment that require large enough water tank, at the same time, the equipment of the lifting and handling is relatively difficult, staff observations is not easy to close to and operation, and the method to pay attention to the control of inflation pressure, low pressure with micro funnelled won't produce bubbles, too much pressure on the device itself and the operating personnel safety hidden trouble, should be according to the design of test container to determine appropriate pressure range. At the same time, for the double-layer jacketed structure, only to determine whether there is a leak, bubbles will overflow from the inlet and outlet pipe, can not be clear about the wall of the specific leakage point.

C.Fixed-point test: after the positive pressure is reached inside the equipment, all interfaces and welds will be coated with soapy water. When there is leakage, obvious bubbles will be found. The size and frequency of bubbles can be used to locate the leakage point and determine the size of leakage. This method is particularly suited to the structure of the double jacketed monitoring (as shown in figure 3), generally only the inlet outlet, due to the structure by blocking an opening, to another mouth filling the positive pressure, can to equipment inside and outside all interfaces in the weld and plate funnelled to accurate judgement, and this kind of structure can be inflation pressure is relatively high, more convenient for observation, and the operation is simple, no other purchased components and testing equipment.

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FIG. 3 double jacket furnace body

2.2. Negative pressure method

The negative pressure method is to vacuum the equipment under the condition of sealing, which is just the opposite of the positive pressure method. The leakage judgment and determination can be completed through the pressure rise rate method, the acetone leak detection method, and the helium GMS leak detector.

A. pressure rise rate method. Qualitatively judge the leakage amount by calculating the rate of pressure rise

   q = (P2-P1)/ΔT

In the formula P1 -- vacuum degree at 15 min after stopping the extraction of vacuum chamber, Pa; P2 -- vacuum degree inside the equipment at the end of the test, Pa; Δ T - test total time T2 - T1, h

Due to the different operating conditions of equipment and requirements for vacuum degree, P1 may be several orders of magnitude different, which determines the form of vacuum extraction system. At the same time, for different forms of furnace should fully consider the effect of outgassing (especially graphite carbon felt and aluminum silicate fiber porous thermal insulation material), suggest to take limit and baking furnace, the rate of pressure rise test again, can judge the leakage, and by calculation but can't make a clear positioning for funnelled position.

B. Acetone leak detection method: the acetone method can be used to determine the location of the leak point when the vacuum degree is less than 10 Pa. In order to facilitate observation, usually after a long time of pumping air into vacuum state, when the pressure inside the equipment down slowly (has reached the equipment can be thought of ultimate vacuum), use the needle to acetone to spray the interface and the weld position, at this point, the vacuum degree were observed at any time, if there are funnelled, reach funnelled position in acetone, due to the effect of isolation of the liquid, prevent the continued leak into the atmosphere, vacuum degree will rise rapidly, but with the acetone inhaled liquid equipment inside the cavity, volatilizes quickly gasification, the equipment of vacuum degree will immediately drop, then by this method can be more clearly identified funnelled position. Since acetone is a liquid, when spraying, the welding seam on the vertical ground should be sprayed from the bottom up to prevent the rapid flow of liquid to the bottom of the welding seam, affecting the determination of the leakage point. This method for vacuum equipment, the testing cost is low, but considering the acetone may bring certain corrosion effect on vacuum system, a small amount of using this method, need to be careful, especially in the case of a diffusion pump, acetone can produce pollution to the oil body, affect the diffusion pump oil extraction ability and service life, operators also want to do a good job in their own protection to prevent the acetone in contact with the human body.

C. Temperament of helium spectrum analyzer: the method is based on negative pressure method, using a mass spectrometer to leak detection, there are many types of its connection mode, the author according to the actual use, for the most general and leak detection, the lowest way can the method to determine the location of funnelled and judgment of the leakage, the detection principle diagram as shown in figure 4 (saves for small cavity body, the vacuum system, the temperament of helium spectrum analyzer directly for the cavity are pumping gas leak detection).

 

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FIG. 4 schematic diagram of typical leakage detection

As shown in figure 4 typical vacuum equipment, and its working principle is: start mainly consists of the vacuum system vacuum furnace, vacuum to leak detector when measuring point, after open leak detector, leak detector is itself a group made up of small molecular pump and a mechanical pump vacuum system, after its opening is also involved in the extraction of furnace body, at this point, the inspection personnel, helium gas can be used to blow all interfaces in the furnace body and weld, if there are funnelled, when helium after this position, there will be a small amount of helium gas along with air into the furnace, again by the vacuum tube road away, at this point, there will be a small amount of helium gas into the leak detector, The helium mass spectrometer inside the detector will detect the presence of helium and send an alarm to alert the detector that there is a leak near the location.Due to the low density of helium gas, it will float upward in the atmosphere. Therefore, during the gas blowing test, the upper part of the equipment should be blown to determine the location of the leakage point. When conducting leakage detection of double jacket, the jacket can be used as an independent cavity for vacuum detection, so as to realize one-time detection of internal and external walls. Since modern leak detector technology is sufficient to make the air leakage of the vacuum system much smaller than the air discharge of the system, the size of vacuum pump required by the vacuum system mainly depends on the size of the air discharge rate of the material used in the vacuum system. Therefore, vacuum pump pumping speed is different for different types of liner materials.

 

This method has a high detection accuracy and is suitable for the detection of equipment with strict requirements on air tightness. The one-time investment of helium leak detector is relatively high, the operating cost of detection is relatively low, and there is no physical harm to the detection personnel. Therefore, it is an ideal leak detection method. The current leak detector has been manufactured in China, which can be used for start-up detection with low vacuum, and the minimum leak detection rate has reached 10-12 cc/ SEC. The alarm point can also be manually selected. Through the comparison of the two leak detectors used by the author, with the progress of technology, the vacuum degree of the current starting point has reached 2000Pa, which greatly saves the vacuum extraction time before detection. Meanwhile, the volume of the leak detector is getting smaller and lighter, making it easier to carry.

 

2.3, X, Y ray, neutron rays, three, the first two are widely used in boiler pressure vessel weld defect detection, and other industrial products, structure material, and used only for special occasions, neutron rays x-rays through the irradiation objects will have a loss, after different thickness material to their absorption rate, put the film on the other side of the object to be light, can produce corresponding because radiation intensity and different graphics, review sheet personnel can according to the image to judge whether objects inside the defective and the nature of the defect, this method requires a relatively large initial investment, evaluation of people also need certain experience, It takes a long time to train, and the operators need to do their own protection. For the sandwich-type equipment, the ray attenuation in the secondary penetration is very large, can not image, the specific location of the defect is difficult to determine.

 

2.4 the principle of magnetic particle flaw detection is: when the workpiece is magnetized, if there is a defect on the surface of the workpiece, the magnetic flux leakage will be generated due to the increase of magnetoresistance at the defect, and the local magnetic field will be formed. The magnetic powder will show the shape and position of the defect here, so as to judge the existence of the defect. The advantages of magnetic particle inspection are as follows: it is very effective for the inspection of defects such as steel material or workpiece surface cracks; Simple equipment and operation; Fast inspection speed is convenient for inspection of large equipment and workpiece on site; The cost of inspection is also lower. Disadvantages: only applicable to ferromagnetic materials; Only the length and shape of the defect can be shown, but it is difficult to determine its depth; Some work pieces that have influence on remanence magnetism still need demagnetization and cleaning after magnetic particle inspection. Therefore, this method is not suitable for non-magnetized metal and structure with double jacket structure.

 

2.5, the basic principle of ultrasonic testing is the use of ultrasonic deep penetration of metal materials, by a cross section into another section, on the edge of the interface reflection characteristics of flaws, a method to check the parts when the ultrasonic beam from the surface by the probe through the metal parts inside, encountered defects with parts respectively when bottom reflection wave, on the screen form pulse waveform, according to the pulse wave Shape to determine defect location and size. The current ultrasonic flaw detector has been able to accurately indicate the position of the defect, judge the defect type of the weld seam by judging the continuous state of the defect position, and configure different types of probes according to the different surface shapes of the detected workpiece (see FIG. 5).

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FIG. 5 ultrasonic detector

Compared with X-ray inspection, ultrasonic inspection has the advantages of high sensitivity, short cycle, low cost, flexibility, convenience, high efficiency and harmless to human body. The disadvantage is that it requires smooth working surface and experienced inspection personnel to distinguish the types of defects and have no visual perception of defects. Ultrasonic testing is suitable for inspection of parts with large thickness. For smooth plane or detect larger radius of curvature, ordinary probe can be used to weld testing, surface for the opposite sex can be made according to the customer's test artifacts of special sensor to meet the requirements of all kinds of working condition, for the commonly used material for testing ultrasonic features of the software has realized the basic material prefabrication, through to the defect size and alarm limit can be set.

 

2.6 phased array detector is also a kind of ultrasonic wave. Parameters of main ultrasonic beam, such as Angle, focus range and focus size, can be controlled by software. Moreover, the beam can be multiplexed over a very long array (similar to phased radar technology). These features add a number of new applications to phased array technology, such as the ability to quickly change the beam Angle without moving the probe itself while scanning the workpiece. Phased array can also replace multiple probes and mechanical parts. Through internal software calculation and analysis, it can intuitively display the location and size of internal defects in the graphical interface (as shown in FIG. 6). The imaging accuracy and position accuracy are greatly improved, and the operation is relatively simple. The equipment can be moved at will, and the probe can be placed at any position and Angle. At present, the purchase price of this kind of equipment is relatively high, and the operation cost is low.

 

2.7. The basic principle of color (penetration) flaw detection is to use capillarity to make permeation fluid penetrate into defects, remove surface permeation fluid through cleaning, and then use the capillary effect of imaging agent to adsorb the residual permeation fluid in defects to achieve the purpose of testing defects. It can only be used to detect defects on the surface, and can not be detected for defects with too thin welding inside the surface or pores in the middle of the plate.

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FIG. 6 phase controlled flaw detector

 

2.8 eddy current detection is to apply the principle of electromagnetic induction to add excitation signal to the probe coil. When the probe is close to the metal surface, the alternating magnetic field around the coil generates induced current on the metal surface. For plate metal, the flow direction of induced current is a concentric coil circle, shaped like a vortex, called vortex. The size, phase and flow pattern of the eddy current are affected by the conductivity of the specimen. The eddy current also creates a magnetic field, which in turn changes the impedance of the test coil (see figure 7).

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FIG. 7 eddy current testing principle

So when the conductor surface or near surface defects or measurement of metal material changes, will affect the intensity and distribution of eddy current, and caused by changes in eddy current testing coil voltage and the change of impedance, according to the change, can indirectly knew of conductors defects if there is a change and the performance of metal materials. At the same time, the object of eddy current testing must be conductive materials, and it is not suitable to detect deep internal defects of metal materials, which is the limitation of eddy current testing in application. Secondly, eddy current testing is still in the equivalent comparison stage, and the accurate qualitative and quantitative determination of defects is still to be developed and studied.

 

This method can be used to detect the defects in the plate of equipment. It can also be used for the welding seams that are automatically welded and the welding pool is more uniform. However, for the welding seams that are manually welded, the measurement deviation of this method is large, which affects the judgment of the quality of the welding seam.

 

2.9 thermal infrared imaging flaw detection: the part whose wavelength is 2.0ms ~ 1000ms is called thermal infrared ray. All the objects around us that are above absolute zero (-273 ° c) constantly emit hot infrared rays. Therefore, thermal infrared (or thermal radiation) is the most widespread radiation in nature. In addition to the universality of existence, thermal radiation has two other important properties.

(1) thermal infrared "atmospheric window". Because of this feature, thermal infrared imaging technology provides advanced night vision equipment for the military.

 

 

(2) the amount of thermal radiation energy of an object is directly related to the temperature of its surface. This characteristic of thermal radiation enables people to use it to measure the non-contact temperature and analyze the thermal state of objects, thus providing an important detection method and diagnostic tool for industrial production, energy conservation, environmental protection and other aspects. Modern thermal imaging devices work in the mid-infrared region (wavelength 3 micron ~5 micron) or the far-infrared region (wavelength 8 micron ~12 micron). By detecting infrared radiation from an object, the thermal imager produces a real-time image that provides a thermal image of the scene. And transform the invisible radiation image into the visible and clear image of human eyes. The thermal imager is very sensitive and can detect temperature differences of less than 0.1℃. Based on this, we can understand the tested equipment as a heat source, and judge the defects inside the material by analyzing the thermal imaging of the surface.

According to the thermal infrared imaging technology, we can be on the inner surface of equipment and weld thermal infrared detection and imaging of weld and all the plates in the plate thickness or composition changes, from the infrared wave is also with the surrounding, by heating the inner surface, the weld molten pool is weak parts of the temperature will be higher than the surrounding temperature, can appear in the infrared imager.

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Figure 8 welding defective parts

 

When observing the workpiece defects as shown in FIG. 8, due to the influence of weld surface flatness and workpiece surface, it is not easy to observe the leakage point in FIG. 9. When detecting the furnace body as shown in FIG. 10 below, the red area shown can be used as the key location of hidden dangers. Through the actual imaging inspection of different types of workpiece, the effect is obvious when the workpiece with larger curvature or better flatness is inspected. The defect position can be observed obviously in the interface of the detector, and it is not easy to use this method for complex surfaces. In terms of thermal imager itself, the relatively small size, simple operation, easy to carry, the operator can detect handheld directly, more important is the advantage of the test are contact, so that it can realize on-line detection, can be arbitrarily placed Angle, high detection efficiency, can also video detection process, provide real-time field detection. However, the abnormal points provided by the thermal infrared imager also need to be further examined by the detector. Meanwhile, the price of the thermal infrared imager is relatively high, and the resolution and precision of the domestic products need to be improved. In the monitoring of electrical components, the low efficiency and large error of the point temperature measuring gun can realize the constant monitoring away from dangerous sources.

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FIG. 9 thermal imaging results

 

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FIG. 10 thermal imaging results of furnace body

 

Based on the above various flaw detection and leakage detection methods and instruments, the user selects appropriate and economical detection means and instruments mainly according to the characteristics and requirements of their own equipment.