Discussion on nondestructive testing of pressure pipeline
With the development of the times and the progress of science and technology, higher and higher requirements are put forward for the safety, quality and performance of products. Because of the advantages of nondestructive testing technology, it is widely used in the manufacture and use of pressure pipeline products.
Common nondestructive testing technology of pressure pipeline
At present, according to the definition of American Society for testing and materials, radiographic testing can be divided into four categories: Photographic testing, real-time imaging testing, tomographic testing and other radiographic testing technologies. Among them, radiographic testing is the most widely used radiographic testing method in pressure pipeline.
Radiographic method refers to the nondestructive testing method of using X-ray or γ – ray to penetrate the test piece. The strength difference of the test piece is caused by the influence of the defect on the absorption of the ray. The defect is detected by measuring the difference, and the film is used as the equipment to record the information. Radiographic equipment can be divided into: X-ray testing machine, high-energy ray testing equipment and gamma ray testing machine.
Characteristics of radiography:
- (1) it can obtain the visual image of the defect and locate accurately.
- (2) the test results can be recorded directly and stored for a long time.
- (3) the detection rate of volume type defects (such as pores, slag inclusions, etc.) is very high, and the detection rate of area type defects (such as cracks, lack of fusion, etc.) is easy to be missed if the camera angle is not appropriate.
- (4) the butt weld is suitable for inspection, the fillet weld is poor, and the plate, bar, forging, etc. are not suitable for inspection.
Ultrasonic testing is mainly used to detect the internal buried defects of butt weld and internal surface cracks of pressure pipe weld, as well as the possible cracks of pressure pipe forgings and high-pressure bolts. Ultrasonic inspection can be divided into ultrasonic inspection, ultrasonic thickness measurement, ultrasonic grain size measurement, stress measurement, etc. there is a pulse reflection method based on the echo of defects and the echo of the bottom
There are penetration method to judge the defect condition according to the shadow of the defect, and resonance method to judge the defect condition or the thickness of the plate by the standing wave generated by the tested object. At present, the most commonly used method is pulse reflection method.
Features of ultrasonic testing:
- (1) the area type defect detection rate is higher, while the volume type defect detection rate is lower.
- (2) it is suitable to inspect the workpiece with thick thickness.
- (3) applicable to all kinds of test pieces, including butt weld, fillet weld, t-weld, plate, pipe, bar, forging, composite material, etc.
- (4) the inspection cost is low, the speed is fast, the volume of the inspection instrument is small, the weight is light, and the field use is convenient.
- (5) it is difficult to get the direct image of the defect, and the quantitative accuracy is not high.
- (6) there is no direct witness record for the inspection results.
Magnetic particle testing
Magnetic particle testing is a non-destructive testing method based on the interaction of magnetic leakage field and magnetic particle at the defect to show the surface and near surface defects of ferromagnetic materials. The principle is that the ferromagnetic materials produce strong magnetic induction strength inside after magnetization, and the density of magnetic field line increases hundreds to thousands of times. If there is discontinuity in the materials (mainly including discontinuity caused by defects and discontinuity caused by structure, shape, material and other reasons), the magnetic field line will be distorted, and some of the magnetic field lines may overflow the surface of the material and penetrate from space The local magnetic pole of the leakage magnetic field can attract ferromagnetic materials.
Features of magnetic particle testing:
- (1) it is suitable for testing of ferromagnetic materials and cannot be used for testing of non ferromagnetic materials.
- (2) surface and near surface defects can be detected, which cannot be used to check internal defects.
- (3) the detection sensitivity is very high, and very small cracks and other defects can be found.
- (4) low cost and fast detection.
- (5) the shape and size of the workpiece sometimes affect the detection, so it is difficult to magnetize and cannot be detected.
After the element surface is coated with the penetrant containing fluorescent dye or dye, the penetrant can penetrate into the defect of the surface opening under the action of capillary tube for a certain period of time; after the excess penetrant on the element surface is removed, the developer is applied on the element surface, similarly, under the action of capillary tube, the developer will absorb the penetrant retained in the defect, and the penetrant will infiltrate back into the defect In the imaging agent, under a certain light source, the trace of penetrant at the defect is displayed, so as to detect the morphology and distribution of the defect.
Characteristics of penetrant testing:
- (1) it can be used for any kind of materials except porous materials.
- (2) there are defects in several directions at the same time, and comprehensive detection can be achieved by one operation.
- (3) the surface finish of components has great influence, and the test results are easily affected by the level of operators.
- (4) the defects of surface opening can be detected, but the buried defects or closed surface defects cannot be detected.
- (5) The testing sensitivity is lower than that of magnetic particle testing.
TOFD detection technology is the conversion of wave form at the tip of discontinuous defect. When it is converted, diffraction wave will be generated. This diffraction wave covers a large angle range, then diffraction wave will detect the existing defects. The over time of recorded signal can measure the height of defects, so defects can be quantified. Defect size is usually defined as diffraction signal The signal amplitude has nothing to do with defect quantification.
TOFD detection technology uses two pulse probes, one for sending and one for receiving, which are symmetrically arranged relative to the weld center line. When the acoustic pulse is transmitted to one end of the receiving probe, the first signal reaches one end of the receiving probe, which is the lateral wave. The lateral wave propagates under the surface of the inspected workpiece.
If there is no defect, the second signal to the receiving probe will be the bottom echo. The receiving probe determines the position and height of the defect by receiving the diffraction signal of the defect tip and its time difference.
TOFD detection features:
- (1) one scan can almost cover the whole weld area, which can achieve very high detection speed.
- (2) the detection rate is very high, and it is easy to detect defects with poor directionality.
- (3) be able to find various types of defects, not sensitive to the direction of defects.
- (4) defects extending to the surface can be identified.
- (5) the quantitative and positioning of the vertical direction of defects is very accurate, and the accuracy error is less than 1mm.
- (6) when combined with pulse reflection method, the detection effect is better, and the coverage rate is 100%.
Image from en.wikipedia.org
Application characteristics of nondestructive testing on pressure pipeline
NDT should be combined with destructive testing
Nondestructive testing technology has many advantages, but it has some limitations, which can not replace destructive testing. In the evaluation of pressure pipeline, the results of nondestructive testing and destructive testing should be compared and verified to make a correct judgment.
For example, in addition to the nondestructive testing of LPG steel cylinder, the blasting test should also be carried out. In order to evaluate the quality of welded joints, in addition to nondestructive testing, it is necessary to cut samples for mechanical property analysis, sometimes metallographic and fracture inspection.
Choose the right time for NDT implementation
During nondestructive testing of pressure pipeline, the implementation time of nondestructive testing shall be correctly selected according to the purpose of testing, combined with the working condition of equipment, material and manufacturing process characteristics. For example, the splicing head shall be subject to NDT after forming; the materials with delayed crack tendency shall be subject to NDT at least 24 hours after welding;
For materials with reheat crack tendency, one NDT shall be added after heat treatment; for low alloy steel pressure pipes with lower limit value of standard tensile strength greater than or equal to 540Mpa, surface NDT shall also be conducted for welded joints after pressure test.
Selection of appropriate NDT methods
In nondestructive testing of pressure pipeline, due to the certain characteristics of various testing methods, it can not be applied to all workpieces and all defects. In order to improve the reliability of testing results, the most appropriate nondestructive testing method should be selected according to the material, manufacturing method, working medium, failure mode of the equipment and the types, shapes, positions and orientations of the defects that may occur.
Comprehensive application of various NDT methods
In NDT, we must realize that any NDT method is not omnipotent, and each method has its own advantages and disadvantages. Therefore, in nondestructive testing, if possible, do not only use one nondestructive testing method, but also use several testing methods as much as possible, learn from each other’s strengths and make up for their weaknesses, and obtain more defect information, so as to have a clearer understanding of the actual situation, so as to ensure the safe and long-term operation of pressure pipeline.
The above briefly introduces several methods and characteristics of non-destructive testing commonly used in pressure pipeline, and in practical application, we should combine the characteristics of various non-destructive testing methods, select suitable non-destructive testing methods, so that non-destructive testing technology can be better applied in pressure pipeline products.
Source: China Pressure Pipeline Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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