Common defects in pressure vessels and preventive measures

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This post was last edited by Lantian on 2019-10-16 at 15:48. Defects in pressure vessels have a significant impact on the performance and safety of equipment; it’s too late to regret anything once an accident occurs. What are the defects of pressure vessels? Do you know how to avoid it? Common defects in pressure vessels: 1. Cracks CRACKS 2. Welding defects WELDING DEFECTS 3. Other defects OTHER DEFECTS (1) Delamination defects ； (2) Surface opening-type defect ； (3) Erosion defects ； (4) Corrosion defects ； (5) Deformation defects. The impact of manufacturing defects in pressure vessels on their safety: Another type of defect that can occur during the manufacturing process is one that results in discontinuities in the vessel’s geometric shape, such as irregularities and deformation at the joints. The stress on various rotating shells under internal pressure is related to their radius of curvature. When two shells with different radii of curvature are connected, the deformations resulting from the different stresses also vary. But they also constrain each other, thereby generating shear and bending moments at the junctions, which cause additional bending stress in the shell and result in excessively high local stresses. Generally speaking, depressions with a large diameter and small depth exhibit a more gradual change in geometric shape, and the resulting impact is also minor. The unevenness of the heads produced during container manufacturing is generally quite gradual. Effect of internal stress: Generally speaking, the greater the degree of cold deformation, the greater the internal stress generated. The residual internal stresses in the container shell, even if they do not cause cracks, will exacerbate fatigue cracking and stress corrosion cracking of the pressure vessel. How is the test pressure determined for airtightness tests of chemical containers? Airtightness tests on chemical containers are conducted primarily to verify the integrity of these containers. Containers that have undergone pressure strength testing and passed the inspection do not need to undergo additional airtightness testing. The airtightness test must be conducted after the hydraulic test is successful, with the test pressure being 1.05 times the design pressure. During the test, the pressure should be increased gradually; once the specified test pressure is reached, it should be maintained for 10 minutes, after which it should be reduced to the design pressure. Leaks should then be checked at the welds and connection points. Small containers can also be submerged in water for inspection. In the event of a leak, carry out hydraulic and airtightness tests again after repair. Contents of inspection for internal and external condition grades in low, medium, and high-pressure vessels 1. External inspection (1) Cracks, overheating, deformation, leakage, etc. in the vessel body, connection parts, weld joints, etc ； Leak detection in holes and signal holes, leakage of liquid or air, and cleaning of leak detection tubes. (2) Corrosion of the outer surface, damage, detachment, moisture, and heat loss in the insulation layer ； Abnormal vibration, noise, and mutual friction between adjacent pipes or components. (3) Damage to supports or bearings, settlement, tilting, or cracking of the foundation, and the condition of the fastening bolts. (4) Check and confirm that the safety accessories meet the specified requirements. 2. Structural inspection (focus on inspecting the following areas): (1) Welds at the connection between the cylinder and the head, as well as those at corners and overlaps, where the arrangement is unreasonable ； (2) Square holes, manholes, inspection holes, and their reinforcement ； (3) Heads, supports, bearings ； (4) Flanges and drain ports. 3. Geometric dimension inspection can be carried out based on the original data to check the following: (1) the misalignment amount and edge angle of longitudinal and circumferential welds, the weld excess height, the weld thickness of fillet welds as well as the dimensions of the fillet angles, as well as those welds with improper layout ； (2) Deviations related to the maximum and minimum diameters on the same cross-section, the surface of the head, the height of the straight edges and longitudinal wrinkles, as well as excessive thinning in the butt joints of dissimilar-thickness (forged) parts ； (3) Verticality of the supports for vertical and spherical pressure vessels ； (4) Gap between adjacent steel strips in a coiled pressure vessel. 4. Surface defect inspection: (1) Corrosion and mechanical damage – determine their depth, diameter, length, and distribution, and record them graphically. For abnormal corrosion, the cause should be identified. (2) Surface cracks 1) The welds on the inner surface (including the area near the weld) should be inspected for cracks using the naked eye or a 5–10x magnifying glass. In any of the following situations, surface flaw detection covering at least 20% of the weld length shall be carried out ； Material strength grade b > 540 MPa ； Made of Cr-Mo steel ； With an austenitic stainless steel surfacing layer ； The medium has a tendency to stress corrosion ； Other suspicious welds. If cracks are detected, the inspector shall determine the percentage increase in surface inspection based on the potential defects present ； If cracks are still found, surface flaw detection of all welds should be carried out. At the same time, further inspection is required for any crack defects that may exist in the welds on the outer surface. In areas where the welds on the inner surface have cracks, spot checks should be conducted on the welds on the corresponding outer surface. 2) Special attention should be paid to stress concentration areas, deformation areas, welds between different steel types, weld marks from tooling, areas damaged by arcs, and areas prone to cracking. 3) For those prone to intergranular corrosion, metallographic examination or hammer testing can be employed. During the hammer inspection, use a hand hammer weighing 0.5–1.0 kg to strike both sides of the weld or other areas. 4) The weld joints at the beginning and end of the steel strip in coiled pressure vessels shall be inspected for surface cracks. (3) Inspection for weld undercutting (4) For other materials sensitive to welding, attention should also be paid to checking for potential crack formation at the weld toe. Deformation and measurement of deformation dimensions, other defects that may arise, and analysis of the causes of deformation. 5. Wall thickness measurement: (1) The locations of the measurement points should be representative, and there should be a sufficient number of such points. After measurement, it should be recorded graphically. The location of the measurement point should generally be selected at the following areas: 1) Areas where the liquid level fluctuates frequently ； 2) Corrosion-prone and erosion-prone areas ； 3) Areas where the wall thickness is reduced during manufacturing and forming, and areas that deform during use ； 4) Suspected areas identified when checking surface defects. (2) When using an ultrasonic thickness gauge to measure the wall thickness, if delamination defects are present in the base material, more measurement points should be taken or an ultrasonic flaw detector should be used to determine the distribution of the delaminations as well as their inclination relative to the surface of the base material. When measuring the wall thickness of pressure vessels in hydrogen-containing environments, if an increase in wall thickness is observed, the possibility of hydrogen corrosion should be considered. 6. Material: (1) The type and grade of the material used for the main compressive components should generally be determined. For those with unknown material properties, for steel pressure vessels without special requirements, it is permissible to conduct strength verification using the lower limit of the strength value for the Q235 steel grade ； For tanks, tank trucks, and pressure vessels with special requirements, the material must be identified. For those who have already undergone this examination and have received a clear resolution, no further examination is required. (2) Whether the material of the main stressed components has deteriorated can be determined, depending on the specific circumstances, through methods such as chemical analysis, hardness testing, spectral analysis, or metallographic examination. 7. For pressure vessels with a covering layer, whether to remove the insulation layer should be determined based on the operating conditions and external environmental factors. The insulation layer may not need to be removed in any of the following situations. 1) During manufacturing, all weld surfaces were inspected and found to be qualified ； 2) Local spot checks were conducted on representative areas, and no defects such as cracks were found ； 3) The external environment has not invaded or caused cooling down ； 4) The external environment has reliable anti-corrosion measures ； 5) Those with similar usage experience ； 6) The inspector deems it unnecessary. (2) In pressure vessels with a metal lining, if penetrating corrosion, cracks, local bulging, or dents are found in the lining, and the medium is leaking from the inspection holes, the lining layer should be removed partially or entirely to determine the degree of corrosion or other defects in the vessel’s structure. (3) Those lined with austenitic stainless steel weld-on linings, in case of damage, cracking, peeling, or delamination of the lining. In the case of linings made of non-metallic materials, if damage, cracking, or peeling of the lining is detected, or if abnormal wall temperatures occur during operation, the lining should be removed partially or entirely to determine the degree of corrosion or other defects in the main structure. (4) For those with coatings on both the inner and outer surfaces, the inner surface should first be inspected in accordance with points 2 and 4 of this question; if serious defects such as cracks are found, the coating on the outer surface should be removed partially or entirely for inspection. 8. Inspection of buried defects in welds: (1) In any of the following situations, radiographic or ultrasonic testing is generally required; when necessary, cross-verification using these methods is also needed. 1) Areas where the weld has been repaired more than twice during manufacturing, or where the weld has been rewelded during use ； 2) Cracks were found on the surface of the weld during inspection, and it was determined that a check for buried defects in the weld was necessary ； 3) Weld areas where the misalignment and edge angle exceed acceptable limits significantly ； 4) Areas where weld leakage occurs during use and the extended sections at both ends thereof ； 5) Parts requested by the user or deemed necessary by the inspector. This check has already been performed; if no abnormalities are found upon rechecking, it is generally not necessary to conduct another examination. (2) The inspection method and the number of samples to be inspected shall be determined by the inspector based on the specific circumstances. 9. Inspection of safety accessories shall be carried out in accordance with the relevant regulations for safety accessories. 10. For fastener inspection, high-pressure bolts should be cleaned one by one. Inspect for damage and cracks; surface non-destructive testing should be carried out if necessary. Special attention should be paid to checking for circumferential cracks in the threads and transition areas. Inspection cycle for internal and external safety condition grades in low, medium, and high-pressure vessels (1) External inspection. It refers to the regular on-line inspections carried out by professionals during the operation of pressure vessels, at least once a year. (2) Internal and external inspection. It refers to the inspection carried out by qualified inspectors when pressure vessels are shut down. The frequency of such inspections is as follows: for vessels with a safety status rating of 1 to 3, the inspection must be conducted at least once every 6 years ； For those with a safety status level of 3 to 4, it should be conducted at least once every 3 years. (3) Withstand voltage test. It refers to the hydraulic or pneumatic test conducted during the shutdown inspection of pressure vessels, at a pressure higher than the maximum operating pressure, and such tests must be carried out at least once every 10 years. The provisions regarding external inspections, internal and external inspection contents, and safety condition grades are outlined in the \"Inspection Regulations for In-Use Pressure Vessels\". For pressure vessels that fall under any of the following conditions, the inspection intervals for both the internal and external parts should be reduced accordingly: (1) when it is unknown how corrosive the medium is to the material of the pressure vessel, when the corrosion rate of the medium on the material is greater than 0.25 mm/year, or when the corrosion data provided by the designer are significantly inaccurate ； (2) The material has poor weldability and required multiple revisions during manufacturing ； (3) First inspection ； (4) Poor operating conditions and low management level ； (5) Those with a service life of over 15 years, which have been determined through technical assessment to be unable to be used according to the normal inspection schedule ； (6) Those that the inspector deems should be shortened. For pressure vessels that fall under any of the following conditions, the inspection period for both the internal and external parts should be extended appropriately: (1) When the non-metallic lining is in good condition, its inspection cycle shall not exceed 9 years ； (2) Pressure vessels in which the corrosion rate of the medium on the material is below 0.1 mm/a, or those equipped with a reliable corrosion-resistant metal lining, may be deemed to meet the original requirements after one or two internal and external inspections, provided that this period does not exceed 10 years ； (3) The periodic inspection cycle for reactors equipped with catalysts and large pressure vessels filled with packing is determined by the user based on the design drawings and actual operating conditions. Reasons for the formation of thermal cracks in pressure vessel welding: Thermal cracks occur and develop along the boundaries of the dendritic crystals in the weld metal. The most common scenario is cracking along the length of the weld in its middle, sometimes between two dendritic grains within the weld. Thermal cracks all occur at grain boundaries, which indicates that grain boundaries are weak areas during the solidification of the weld. The cause of thermal cracking is the presence of a liquid interlayer in the weld, along with tensile stress acting on the weld during the crystallization process. The presence of a liquid interlayer is the fundamental cause of thermal cracking, while tensile stress is a necessary condition for it. It is not the later stages of the entire crystallization process; it is only in the vicinity of the solidus line that the temperature range posing a risk of thermal cracking exists. The metal in the vicinity of the weld seam is also heated to a very high temperature (slightly below the melting point). If there are impurities or low-melting-point eutectics at the grain boundaries of the base material in this area, they will melt under the influence of heat, forming a liquid interlayer at those grain boundaries. The tensile stresses that occur during cooling can then cause thermal cracks in the area near the weld seam. There is a interdependent relationship between weld heat cracks and heat cracks in the area near the weld; heat cracks in this nearby area may be a continuation of the weld heat cracks, or they may be the origin of such cracks. Reasons for the effect of notches on brittle failure in high-pressure vessels: The presence of notches and the condition of the small area of material at their ends are important factors determining whether a component will fail in a brittle manner; many cases of brittle failure start at the ends of notches or cracks. The reason is that when a stretched member has notches (cracks), due to this discontinuity, tensile stress cannot be transmitted to the cross-section of the crack. The load applied to the cracked area is instead transferred to a small region at the end of the crack, resulting in high local concentrations of stress and strain in that area. Of course, the longer the crack, the greater the local stress and strain. The local stress at the crack tip increases as the applied stress rises, quickly reaching the material’s yield strength, which in turn causes the cross-sectional area to decrease. The material adjacent to the crack surface experiences no stress in the tensile direction; this material prevents the deformation of the small volume of metal at the crack tip. Such restraint generates secondary stresses in two other directions, resulting in triaxial tensile stresses within that small volume of material at the crack tip. This triaxial stress state does not allow any contraction or deformation in the thickness direction of the material, which is what is known as a plane strain condition. According to the third strength theory, when both the maximum and minimum of the three principal stresses are tensile stresses, the maximum principal stress can exceed the material’s yield stress under uniaxial tension; that is, due to the presence of a notch, a triaxial tensile stress state is generated in the small volume of material at the notch tip. It increases the yield stress, but at the same time reduces ductility, thus leading to brittle failure under certain conditions. This article is from the Design Institute website www.shejiyuan.com