Non-destructive Testing of Pressure Vessel——Non-destructive test of Automobile and Railway Tanker

Abstract: Summarize the defects and possible non-destructive testing methods used in the manufacture and use of automobiles and railway tankers, including radiation, ultrasound, magnetic particles, penetration, magnetic leakage, acoustic emission, and magnetic memory testing, etc., and introduce each non-destructive Characteristics of detection methods.

Keywords: pressure vessel; tanker; ultrasonic testing; magnetic particle testing; radiographic testing; penetration testing; magnetic leakage testing; acoustic emission testing; magnetic memory testing.

The Pressure Vessel Safety Technical Supervision Regulations include automobile tankers (liquefied gas transport (semi-trailer) vehicles, low-temperature liquid transport (semi-trailer) vehicles), railway tankers (the medium is liquefied gas and low-temperature liquid) and tank containers (the medium is liquefied gases and cryogenic liquids) are collectively referred to as mobile pressure vessels and are available in the normal temperature, low temperature, and cryogenic types. They are widely used in the fields of petroleum and chemical, aerospace, electronics and machinery, food and tobacco, and medical treatment.In particular, in recent years, China has accelerated the pace of urban gasification and increased environmental protection. The amount increased rapidly. Because most of the media stored and transported by mobile pressure vessels are flammable, explosive and harmful liquefied gases and low-temperature liquids, such as sudden safety accidents, it will bring great harm to national property and people's lives. Therefore, the mobile pressure vessel must be tested to ensure its safe use. The characteristics and requirements of the non-destructive testing technology used at different stages will be summarized according to the characteristics of its manufacture and use. In addition, non-destructive testing technology for low-temperature and deep-cooled containers will be specifically discussed in subsequent topics.

Automobile tank trucks and railway tank trucks involve professional fields such as traffic management, public security fire protection, special equipment, and transportation of hazardous chemicals. China's existing non-destructive testing technologies are required regulations or technical specifications of piezo ceramic crystals for these two types of mobile pressure vessels mainly have the original national quality. Regulations on Pressure Vessel Safety Technical Supervision issued by the Technical Supervision Bureau, Regulations on Periodic Inspection of Pressure Vessel issued by the General Administration of Quality Supervision, Inspection and Quarantine, Regulations on the Safety Supervision of Liquefied Gas Vehicles and Tankers promulgated by the former Ministry of Labor, and the Ministry of Chemical Industry "Liquefied Gas Railway Tanker Safety Management Regulations"; The relevant standards related to the two types of non-destructive testing of mobile pressure vessels are JB / T 6897 "Cryogenic Liquid Tanker", JB / TQ782 "Liquefied Petroleum Gas Tanker", HG 2599 Technical Specifications for Liquid Ammonia Tank Trucks, GB 10478 Technical Specifications for Liquefied Gas Railway Tank Trucks, and GB 10479 Technical Specifications for Aluminum Rail Tank Trucks.

Non-destructive testing technology in the manufacturing process

The requirements for the manufacture of automobile tank trucks and railway tank trucks are respectively stipulated in the "Liquefied Gas Vehicle Tank Truck Safety Supervision Regulations" and "Liquefied Gas Railway Tank Truck Safety Management Regulations" (two regulations for short), and the "Pressure Vessel Safety Technical Supervision Regulations" ( (Referred to as "capacity regulations") also put forward non-destructive testing requirements for the manufacture of these two types of mobile pressure vessels. Due to the different departments in which the regulations are formulated and the implementation dates are different, the manufacturing requirements and non-destructive inspection acceptance standards for the two types of mobile pressure vessels are also different. For the non-destructive testing in manufacturing, in addition to meeting the design standards and documents, the requirements of the “Capacity Regulations” should also be generally met. If the non-destructive testing should be carried out in accordance with the provisions of JB 4730 —2005 “Non-destructive Testing of Pressure Equipment”. The other special requirements of the two types of vehicles not covered by the Regulations should also meet the requirements of the above two regulations.

1.1 Nondestructive testing of raw materials

Carbon steel and low-alloy steel steel plates are used for manufacturing tanks shall be subjected to ultrasonic testing one by one. The ultrasonic testing of steel plates shall be carried out in accordance with the provisions of JB 4730. The qualified grade of steel plates shall not be lower than Grade II. The main purpose of ultrasonic testing is to find defects such as delamination, white spots, folded heavy skins and cracks generated during smelting and rolling of the sheet. Inspection can be performed on any rolling plane of the steel plate, and the probe is scanned along parallel lines perpendicular to the rolling direction of the steel plate with a distance of 100 mm. Other types of scanning can also be performed according to the requirements of the contract, technical agreement or drawing. When a defect is found, it should be carefully inspected and measured around the defect area. The nature of the defect should be comprehensively judged according to the waveform characteristics and the steel plate manufacturing process.For example, the white point defect waveform is sharp and active, the repeatability is poor, the bottom wave is significantly reduced, and the number of times is reduced. When the probe is moved, the echo fluctuations are large, one after the other, and the thickness direction symmetry and so on.

2.1.2 Nondestructive testing during manufacturing

Automotive and railway tank trucks are prone to a variety of welding defects, such as pores, slag inclusions, non-fusion, unwelded, and cracks, so it is important to use non-destructive testing to control welding quality. Generally, the internal defects of the weld are inspected by radiography or ultrasound. For corner joints and T-joints that require non-destructive inspection, magnetic powder or penetration inspection should be performed when radiography or ultrasound inspection is not possible.

The above-mentioned "Capacity Regulations" and the two regulations impose the following requirements on non-destructive testing in the manufacturing process of tanks: ① Non-destructive testing personnel shall be evaluated in accordance with the "Boiler Pressure Vessel Non-destructive Testing Personnel Qualification Evaluation Rules", and only after obtaining a qualification certificate can they undertake Nondestructive testing work which is corresponding to the type and technical level of the qualification certificate. ② The welded joints of the tank should be checked for shape, size and appearance quality, and non-destructive testing can be carried out after passing. For materials with a tendency to delayed cracking, non-destructive testing shall be performed 24 hours after welding is completed; for materials with a tendency to reheat cracking, non-destructive testing shall be added after heat treatment. ③ The butt joints of the tank and manhole must be inspected by radiography or ultrasound. ④ The surface of the corner joints, such as manholes, reinforcement plates, and piping of the tank, should be tested by magnetic powder or penetration.
The "Liquefied Gas Railway Tanker Safety Management Regulations" also stipulates that when the 100% ultrasonic inspection is used for the butt joints of the tank, at least 20% of the radiographic review should be added. The re-examination site should include the intersection of the weld and the suspicious site of ultrasonic inspection. When the re-inspection finds that there is an over-standard defect, the re-inspection length of 10% (the total length of the corresponding weld) should be increased. If the over-standard defect is still found, a 100-% re-inspection should be performed. The test standard is carried out according to JB 4730. The quality requirements of radiographs should not be lower than AB grade. For butt joints that are 100% tested, the test result is not lower than Grade II to be qualified. For butt joints with 100% ultrasonic testing, GradeⅠ is acceptable. Before magnetic particle or penetration testing, the tested surface should be polished to expose the metallic luster, and the weld and the base metal should be smoothly transitioned. The test is carried out according to the JB4730 standard, and the test results are gradeⅠqualified.

3.2 Nondestructive testing technology for in-use cars and railway tankers

The "Regulations for Periodic Inspection of Pressure Vessels" has formulated a special annex "Regular Requirements for Periodic Inspection of Mobile Pressure Vessels" for the inspection of tankers. It integrates the "Capacity Regulations" and the two regulations, and proposes a more standardized, specific and operational strong periodic inspection requirements. The unified specifications such as the intermediate and overhaul of railway tankers in mobile pressure vessels and the intermediate inspection of tank containers are called annual inspections and comprehensive inspections, and the content of non-destructive testing of welds in some original annual inspections is divided into comprehensive inspections. The requirements for the in-service inspection of the tanker, "Regulations for Periodic Inspection of Pressure Vessels" are that 100% surface inspection of fillet welds and butt welds on the inner surface of the tank should be carried out. When one of the following conditions exists, the welder should also be rayed or ultrasound random inspection, that is, ① the tanker has been out of service for more than 10 years and re-commissioned. ② Repair welding parts during use. ③ where the amount of misalignment and edge angle exceed the standard. ④ leakage of welded joints and extensions at both ends. ⑤ Due to the accident, the welded joints of the tank or the nearby welds are seriously damaged and deformed. ⑥ The place where the last buried defect inspection was suspected, and a follow-up inspection was required. ⑦It has the tendency of stress corrosion such as hydrogen bulging. ⑧Required by the user or deemed necessary by the inspector. For those who have undergone random inspection by radiography or ultrasound, the next comprehensive inspection, if no defects are found by appearance or surface inspection, are generally not inspected, but the tank should be subjected to radiography or ultrasound after two comprehensive inspection cycles. With the enrichment of detection methods, for the in-use detection of tank trucks, acoustic emission can also be used to determine the activity of defects, or magnetic memory detection methods can be used to determine whether fatigue damage exists in certain high-stress concentrated areas, and magnetic leakage detection methods can be used. Monitoring the corrosion status of the inner surface of the tank from the outer surface of the tank, these technologies are particularly suitable for the daily in-use detection of tank trucks.

4.2.1 Surface inspection

During the full inspection of the tanker, fillet welds such as manholes, reinforcement plates, and nozzles on the tank and the butt welds on the inner surface are all tested on the surface. The surface inspection of the butt welds of ferromagnetic materials generally adopts magnetic particle inspection. When magnetic powder inspection of piezoelectric ceramic transducers cannot be used for fillet welds, penetration inspection methods can also be used. The surface inspection of non-ferromagnetic materials uses the penetration detection method. During the non-destructive testing of the surface of tank trucks, the following parts should be emphasized: ① Welding and fatigue cracks often exist in the weld and the heat affected zone of the weld, especially in the T-shaped mouth of the weld, fillet weld, butt weld and weld Surface defects. ② Fatigue and stress corrosion cracking are easy to occur in places with high local stress. The higher stress in the tanker tank is mainly generated in the place where the structure is discontinuous, such as around the manhole opening of the tank, the safety valve installation hole, the tank bottom in the middle of the tank is located at the saddle bearing plate, and the combination of the tank and the cassette parts, the transition part of the head and its vicinity, the end of the cylinder and the welding accessories. Magnetic particle detection generally uses a portable yoke flaw detector, which is simple in the structure, light in weight, and easy to use. The yoke method uses single-joint and multi-joint yokes to longitudinally magnetize the workpiece, which is suitable for local inspection of tank welds, such as butt welds and fillet welds. Its characteristics are simple equipment and convenient operation, but because it is necessary to make at least two independent tests perpendicular to each other at the same location, the efficiency is low and it is easy to cause missed detection. The cross yoke method uses a rotating magnetic field to magnetize the workpiece, which is suitable for the local inspection of the butt weld of the tank. Because the crossed magnetic yoke can be used to obtain the rotating magnetic field, it is sensitive and reliable and the flaw detection efficiency is high, so it is widely used in tank inspection. A fully continuous magnetization method is commonly used to magnetize a workpiece using a rotating magnetic field. The JB 4730 standard specifies that the magnetization specification of the yoke method can be determined based on the sensitivity test strip or lifting force. For the yoke, when the distance between the poles of the electromagnetic yoke is 200 mm, the AC electromagnetic yoke should have a lifting force of at least 44 N, and the DC electromagnetic yoke should have a lifting force of at least 177 N. The magnetic pole spacing should be controlled between 50 and 200 mm, but The effective magnetization area is defined as the area of ± 50 mm on both sides of the two-pole connection, which should overlap by 15 mm between two tests; for a cross yoke, the lifting force requirement is ≮88 N. When magnetic particle testing cannot be performed due to structure or material, penetration testing is used. Automotive and railway tanker penetration testing mainly uses color penetration inspection. Before testing, the surface of the weld must be cleaned with tools such as grinders and iron brushes to remove debris such as welding slag, rust and oily oxide layers, and prevent surface defects from being blocked during cleaning. Under the condition of 15-50 ℃, the penetration time of penetrant is generally ≮10 min. The solvent is usually removed by wiping. Generally, first wipe with a clean non-hairless cloth until most of the excess penetrant is removed, and then wipe with a clean non-hairless cloth or paper dipped in a cleaning agent, until all the excess penetrant is removed from the test surface. Wipe clean, do not wipe back and forth, and do not rinse directly with the cleaning agent on the surface being inspected. Welding imaging is usually sprayed. After spraying the imaging agent for 3 to 5 minutes, the displayed image can be observed with the naked eye or with the help of a 3 to 5 times magnifying glass. The observation should be performed under the condition that the visible light on the surface to be inspected is> 500 lx.

2. 2 Ray Inspection
The advantage of radiographic inspection is that the defects are qualitatively accurate and intuitive, which is convenient for many people to analyze the defects. For a comprehensive inspection of the internal and external tank trucks during use, the choice of ray energy depends on the thickness of the transillumination workpiece and the type of material, and sometimes depending on the equipment conditions. Generally, as the energy of the ray decreases, the contrast of the transillumination image increases. Therefore, when the exposure time permits, the lower ray energy should be used as much as possible. In addition, over-standard defects found by ultrasonic testing are usually re-examined with radiographic testing to further determine the nature and specific location of the defect and provide a basis for defect repair.

2. 3 ultrasonic testing
Due to the low cost and high speed of ultrasonic inspection, the high detection rate of area defects is suitable for butt welds and fillet welds. The detection instrument is small in size, light in weight, convenient to use on the site, and compared with radiation. No harm to people, so it is widely used in the comprehensive inspection of tanks. With the development of technology, the ultrasonic detection method has been able to accurately quantify defects, which provides the basic conditions for the fracture mechanics evaluation of defects. For buried defects that exceed the standard, there are many cases where the safety of their use is determined by the method of safety assessment. The "Liquefied Gas Railway Tanker Safety Management Regulations" proposes comprehensive technical assessment requirements for liquid chlorine, liquid sulfur dioxide tankers with a lifespan of more than 15 years, and tankers with other mediums that have a lifespan of more than 20 years, but does not specify the content of the assessment. Some inspection units use the method of safety assessment to find out whether the tanks with defects exceeding the standard can be used. The newly revised JB 4730 stipulates that the method for measuring the height of defects in ultrasonic detection is flawed endpoint diffraction wave method, end maximum echo method and 6dB method, etc., but the method with the highest measurement accuracy at present is the flaw endpoint diffraction wave method, with an accurate range of 0. 5 to 1 mm. Ultrasound detection resolution is an important indicator of ultrasonic detection, which is divided into time resolution, space resolution and contrast resolution. Time resolution reflects the reliability of the test results in time, ensuring that there is no missing test during the test. Spatial resolution refers to the ability to distinguish two adjacent reflectors under certain conditions. Contrast resolution is a measurement of the degree to which two adjacent structures in an ultrasound image can be distinguished. In order to improve the detection resolution, the theoretical research of TOFD method, phased array method and holographic imaging method has been mature, and the corresponding ultrasonic testing instruments have begun to be popularized and applied. These methods can obtain more intuitive display and more accurate data of internal defects.

Magnetic memory test
Due to structural discontinuities such as heads, manholes, and nozzles, residual stresses in the weld, support of the container, residual stresses produced by processing of pressure components, and discontinuities in the internal structure of the material, stress concentrations inevitably exist. These stress-concentrated parts of piezoelectric disc transducer are prone to stress corrosion cracking and fatigue cracks under the combination of factors such as media, temperature, pressure, and bumps during tank transportation. Therefore, finding out the stress concentration part on the tank, determining the size of the stress concentration, and analyzing its impact on the safety performance of the tank truck have become the key in testing. Conventional non-destructive testing methods (such as radiation, ultrasound, magnetic particle, and penetration testing) can only detect macro defects of a certain size, and it is difficult to find micro defects. The metal magnetic memory detection technology can detect the stress concentration parts that may induce damage or destruction, which provides a basis for the early diagnosis of the tank. Magnetic memory testing is mainly used for on-line monitoring and periodic inspection of tanks. The purpose is to quickly scan the overall stress concentration of the equipment, conduct early diagnosis of possible damage, and focus on reviewing the parts that may have problems after diagnosis to ensure the safety of use and prevent accidents. Because the magnetic memory test does not require high cleanliness of the weld surface, it is not necessary.