48 Quick Answers on Pressure Vessel Design

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1. What properties of materials should be considered when selecting materials for pressure vessels? When selecting materials for pressure vessels, consideration should be given to the mechanical properties, chemical properties, physical properties, and processability of the materials. 2. What type of steel should be used for the pressure-bearing components of pressure vessels? Steel for pressure vessel pressure-bearing components should be killed steel produced by oxygen converters or electric furnaces. For low-alloy steel sheets and austenitic-ferritic stainless steel sheets with a minimum standard tensile strength of 540 MPa or more, as well as low-temperature steel sheets and low-temperature steel forgings intended for use at temperatures below -20°C, a secondary refining process shall also be employed. 3. What are the application limitations of cast iron materials for pressure vessels? Cast iron shall not be used for the pressure-bearing components of pressure vessels containing media with an extreme or high degree of toxicity or toxic hazards, as well as explosive media at a design pressure of 0.15 MPa or higher, nor shall it be used for the pressure-bearing components of shell-and-tube waste heat boilers. 4. What information must at least be indicated on the general design drawing of a pressure vessel? Name and category of the pressure vessel, as well as the main regulations and standards governing its design and manufacture ； Working conditions, including work pressure, working temperature, medium toxicity, and level of explosion hazard, etc ； Design conditions, including design temperature, design load (all loads to be considered, including pressure), medium (components), corrosion margin, weld joint factor, natural conditions, etc. For storage tanks used for storing liquefied gas, a filling coefficient should be specified; for storage containers prone to stress corrosion, a limit on the concentration of corrosive substances should be indicated ； Material grades and standards for the main stressed components ； Key characteristic parameters (such as the volume of pressure vessels, the heat exchange area and number of stages of heat exchangers, etc.) ； Service life of pressure vessel design (number of surface cycles for fatigue-resistant vessels) ； Special manufacturing requirements ； Heat treatment requirements ； Non-destructive testing requirements ； Requirements for voltage withstand test and leakage test ； Requirements for corrosion prevention ； Specifications of safety accessories and special ordering requirements (excluding those for which process factors have already been taken into account) ； Location of the pressure vessel nameplate ； Requirements for packaging, transportation, on-site welding, and installation. 5. What are the requirements regarding the location where inspection holes for pressure vessels should be made? The inspection holes should be positioned reasonably and appropriately to facilitate viewing or cleaning of the interior ； The handhole should be located on the head or on the shell near the head. 6. Which pressure vessels do not require inspection holes? Pressure vessels that meet one of the following conditions: those with an inner diameter of 300 mm or less ； Pressure vessels are equipped with removable end caps, covers, or other types of lids that can be opened and closed; the dimensions of these end caps, covers, or lids are not smaller than those of the designated inspection holes ； Pressure vessels with no corrosion or only mild corrosion, requiring no internal inspection or cleaning ； Pressure vessels for refrigeration units. 7. What requirements should level gauges for pressure vessels meet? Level gauges for pressure vessels shall comply with the provisions of relevant standards and meet the following requirements: They shall be selected appropriately based on the vessel’s diameter, maximum operating pressure, and temperature. Before installation and use, level gauges for low- and medium-pressure vessels shall undergo a hydraulic test at a pressure of 1.5 times the nominal pressure of the gauge ； The level gauge of high-pressure vessels shall be tested at 1.25 times the nominal pressure of the level gauge. For pressure vessels containing media at temperatures below 0°C, frost-proof level gauges should be used. For level gauges used outdoors in cold regions, those with a jacketed or insulated structure should be selected. Liquefied gas pressure vessels used for highly flammable media with an extremely high level of toxicity and high hazard should be equipped with protection devices to prevent leaks. If a stable indication from the level gauge is required, float-type level gauges should not be used. Glass plate level gauges shall not be used in mobile pressure vessels. 8. How many main types of welding grooves are there for the shell plates of pressure vessels? The main types of welding grooves for pressure vessel shell plates include single-sided V-grooves, double-sided V-grooves, single-sided U-grooves, double-sided U-grooves, and X-grooves, a total of five types. 9. What are the main types of welding defects in pressure vessels? The main welding defects in containers include seven types: porosity, slag inclusions, cracks, undercutting, lack of fusion, misalignment and undercutting, and deformation. 10. Why do some container sheets need to be preheated before welding? Preheating of the container sheet before welding is necessary due to the properties of the materials used for the container, in order to reduce welding defects such as cracks and residual stresses. 11. Why cannot pressure vessels be changed in purpose? Pressure vessels are devices that are designed, manufactured, and used in strict accordance with **laws and regulations. In general, it is not allowed to change their intended use; doing so will result in changes to the operating temperature, pressure, and the type of medium used, which can lead to accidents. Only after submission and approval by the local quality and technical supervision bureau can it be modified for use. 12. What should be done for single-sided welds on containers whose thickness is no more than 80 mm and cannot be inspected? For the final circumferential sealing weld of cylinders and heads with a diameter not exceeding 800, when a single-sided butt weld without gussets is used and radiographic or ultrasonic testing is not possible, testing may be omitted, provided that gas shielded welding is used for the root pass. 13. What does a voltage withstand test include? Hydraulic test, pneumatic test. Gas-liquid combined test. 14. For hydraulic testing, what are the requirements for the test fluid? Water is generally used as the test liquid; other liquids that do not pose any risks can also be used if necessary ； During testing, the temperature of the liquid should be below its flash point or boiling point. After performing hydraulic testing on containers made of austenitic stainless steel with water, any residual water must be removed completely; if this is not possible, the chloride content in the water should be kept at no more than 25 mg/L. During the hydraulic testing of steel containers such as carbon steel Q345R, the liquid temperature must not be lower than 5°C; for other low-alloy steels, it must not be lower than 15°C. If factors such as plate thickness cause an increase in the material’s ductile transition temperature, this limit still applies. The temperature of the test liquid then needs to be increased accordingly. 15. What are the acceptance criteria for hydraulic testing? No leakage ； No visible deformation ； No abnormal noises were heard during the test. 16. Under what conditions can a pneumatic pressure test be conducted? The volume is too large; it cannot bear the weight of water ； The structure is complex, and hydrostatic testing is insufficient to fully verify the pressure testing requirements for all components ； Due to the design structure, water is not suitable for use (e.g., it’s not permitted for test liquid to remain in the container) ； Other insurmountable difficulties, such as those who face difficulties in obtaining water for large containers. 17. What are the acceptance criteria for pneumatic testing and gas-liquid combined pressure testing? During the pressure test, the pressure vessel should produce no abnormal noises; no leaks should be detected using soap solution or other leak detection fluids, and there should be no visible deformation, in which case it is considered qualified ； For the gas-liquid combined pressure test, the outer wall of the container must be kept dry; after checking to ensure there is no liquid leakage, it should then be inspected with soapy water or another leak detection fluid to confirm that there is no air leakage, no abnormal noises, and no visible deformation. 18. What are the purposes of conducting pressure tests on pressure vessels? Evaluate the overall strength, stiffness, and stability of the container ； Check the density of the welded joint ； Verify the sealing performance of the sealing structure ； Eliminate or reduce welding residual stresses and the peak stresses in local discontinuities ； It exerts a closing effect on microcracks, passivating their tips. 19. What does a leakage test include? When to do it? Airtightness tests, as well as ammonia leak detection tests, halogen leak detection tests, and ammonia leak detection tests, etc. For containers with a medium toxicity level that constitutes extreme or high hazard, or those in which even trace leaks are not permitted, a leakage test should be conducted after the pressure resistance test is passed. 20. Under what circumstances is a leak test required? How is it carried out? For containers with a medium toxicity level that constitutes extreme or high hazard, or those in which even trace leaks are not permitted, a leakage test should be conducted after the pressure resistance test is passed. The airtightness test should be conducted after the hydraulic test is successful. For pressure vessels specified in the design drawings to undergo a pneumatic test, it shall be specified in the design drawings whether an airtightness test is also required. For pressure vessels made of carbon steel and low-alloy steel, the temperature of the test gas shall be not lower than 5°C. When conducting a airtightness test on pressure vessels, all safety accessories must be installed. The gas used for the airtightness test should be dry, clean air, nitrogen, or other inert gases. The airtightness test pressure shall be specified on the drawing. The test pressure should be increased gradually, and once the specified test pressure is reached, the pressure should be maintained for 30 minutes. Conduct leak checks on all welds and joints; small containers can also be submerged in water for inspection. In case of leakage. After repair, conduct hydraulic testing and airtightness testing again. It is considered qualified if no leaks are detected upon inspection. 21 Common types of end caps? What are the advantages and disadvantages of various end caps? Convex end caps (elliptical end caps, butterfly end caps, dome-shaped end caps, and hemispherical end caps), conical end caps, and flat covers. In terms of stress distribution, the order is: hemispherical, elliptical, disc-shaped, conical; the flat cover has the worst performance ； In terms of manufacturing, flat lids are the easiest to produce, followed by conical, disc-shaped, oval, and hemispherical lids ； Although conical end caps do not provide good stress distribution, they facilitate the discharge of fluids. 22. What does the weld joint coefficient in the thickness calculation formulas for internal pressure cylinders and spherical shells refer to? What is this weld joint coefficient? The weld coefficient in the cylinder formula is the weld coefficient for longitudinal welds (i.e., Class A welds). The weld coefficient in the shell formula is the minimum weld coefficient among all the welds on the shell, including the weld coefficient of the circumferential welds that connect the shell to the cylinder (i.e., Class A welds). 23. What are the two types of failure modes for pressure vessels subjected to external pressures? What are the two aspects that should be included in the design of pressure vessels subjected to external pressures? Failure of externally pressurized vessels mainly occurs in two forms: failure due to insufficient strength and instability failure due to insufficient stiffness ； The design should include strength calculations and stability checks. Since instability often occurs before strength failure, stability calculations are a key consideration in the design of pressure vessels. 24. What are the failure modes of pressure vessels? When a pressure vessel loses its ability to function properly due to excessive mechanical or thermal loads, it is referred to as failure. Forms include: strength failure: caused by material yield or fracture ； Stiffness failure: Excessive elastic deformation of the container, resulting in difficulties in transportation and installation or loss of operational capability ； Stable failure: Sudden change in shape under load leads to loss of functionality ； Leakage failure. 25. What are the main differences between the conventional design method and the analytical design method for pressure vessels? Conventional design method: It is based on elastic failure as a criterion, using film stress to determine the component thickness so that the maximum stress does not exceed a certain allowable value (usually twice the allowable stress). Local stresses such as the relatively large edge stresses present in the container are represented in the form of an enhancement factor, and the maximum allowable stress after accounting for these local stresses is taken to be the same as the allowable strength value for membrane stresses. Analytical design method: Based on plastic and elastic failure criteria, it takes into account various stresses within the container to perform accurate calculations; the stresses are classified, and different strength constraints are applied according to the various forms of failure caused by each type of stress, thereby enabling the calculation of the component’s thickness. 26. How are thin-walled containers and thick-walled containers classified? Based on the ratio of thickness t to its curvature radius R, generally, when (t/R)MAX ≤ 0.1 it is considered a thin shell; otherwise, it is a thick shell. For cylindrical shells, if the wall thickness ratio K = (Do/Di)MAX ≤ 1.1–1.2, they are considered thin-walled cylinders. 27. What are the similarities and differences in membrane stress for cylinders and spherical shells under internal pressure? Same: Both generate biaxial film stress, and it is consistent everywhere. Difference: The circumferential film stress in the cylinder is twice the axial stress. The biaxial membrane stress in the spherical shell is equal, and its value is equal to the axial stress in the equidiameter cylinder. For this reason, at the same diameter and pressure, the wall thickness required for a spherical shell is only half that of a cylinder. 28. What are the characteristics of edge stress? Locality – In the edge stress, longitudinal bending stress predominates, but its range of influence is limited; it decreases rapidly in the longitudinal direction away from the edge. Self-limiting nature – Edge stress arises as a result of the need for deformation compatibility among adjacent elements; when this stress reaches the material’s yield point, plastic flow occurs in the material, thereby ensuring such deformation compatibility. Once the deformation requirement is met, the plastic flow of the material automatically ceases. Therefore, its stress and strain energy are automatically constrained. 29. Why do oval end caps and butterfly nozzles have straight edge sections? Avoid overlapping the connection weld between the head and the cylinder with the stress concentration area at the edge. 30. What are the stability and critical pressure of a container? What are its features? When a container is subjected to compressive stress, its shape changes suddenly, resulting in collapse – a failure mode known as loss of stability. The stress acting on the container walls shifts from a membrane stress state to a bending stress state; the minimum external compressive stress at which the container collapses is referred to as the critical pressure. As long as compressive stress exists in a thin-walled container, instability is possible. Standard elliptical heads subjected to internal pressure also face stability issues due to the circumferential membrane compressive stress in their transition zones; therefore, limits are imposed on their minimum thickness. 31. What are the types of container instability? What are its characteristics? It is divided into circumferential instability and longitudinal instability. Circumferential instability is caused by the circumferential compressive film stress in the container ； Transverse instability is caused by the film stress resulting from axial compression of the container. In circumferential instability of the container, its cross-section changes from circular to wavy; in longitudinal instability, the cross-section remains circular while its meridians change from straight lines to wavy lines. 32. What are elastic instability and inelastic instability? Can replacing low-strength steel with high-strength steel improve the elastic stability of the container? When instability occurs, the compressive stress in the wall of the vessel is less than the material’s proportional limit; when stress is proportional to strain according to Hooke’s law, it is referred to as elastic instability. Since the critical pressure for instability at this stage is independent of the material’s yield limit and depends only on its elastic modulus and Poisson’s ratio, and since the elastic moduli and Poisson’s ratios of different steel types differ little, using high-strength steel in place of low-strength steel has little effect. If the compressive stress in the wall of the vessel at the time of instability exceeds the proportional limit of the material, and there is a very linear relationship between stress and strain, this is referred to as inelastic instability. The critical pressure for inelastic instability is related to the yield limit of the material; in such cases, high-strength steel can be used in place of low-strength steel. 33. Why are all externally pressurized convex air nozzles designed for stability as externally pressurized shells? Under internal pressure, the elliptical head exhibits a tendency to become circular, while under external pressure it tends to become flatter. This results in circumferential tensile membrane stresses in the transition zone of the head, with no instability issues; however, compressive membrane stresses exist in its spherical portion, just like in a shell under external pressure. Therefore, stability calculations must be performed using the principles applicable to spherical shells. For elliptical heads, it is necessary to calculate the equivalent shell radius of their \"spherical part\". 34. How should thermal stress under internal pressure be considered? Measures to eliminate thermal stress? Internal pressure and internal heating: the stress on the inner wall decreases, while the stress on the outer wall increases, which may lead to a deterioration of the stress condition. When △t ≤ 1.1P, the effect of thermal stress can be neglected. Internal pressure with external heating: the combined stress on the inner wall increases, while the stress on the outer wall decreases, resulting in a deteriorated stress condition. Thermal stress must be considered. Measures to eliminate thermal stress: Control the heating and cooling rates of the equipment ； Constraints for controlling and reducing thermal deformation of components ； Install expansion joints ； Use a good insulation layer. 35. What is the difference between the equal area method and the pressure-area method? The equal area method is based on the compensation of the stress-bearing area of the film, and is unrelated to force balance. The pressure area method is based on force balance at the opening (to ensure that the film stress remains below the allowable value through calculation). Generally, when the opening is larger, a larger area is required using the pressure area method. 36. What does the design of bolted flange connections include? Determine gasket material and dimensional specifications ； Determine bolt material, specifications, and quantity ； Determine the flange material, seal surface type, and structural dimensions ； Stress verification is performed (all dimensions in the calculations do not include a corrosion allowance). 37. What is a narrow-face flange? What is a wide-flange? A flange whose contact surface is within the area enclosed by the flange bolt holes is called a narrow-face flange ； Flanges whose gasket contact surfaces are distributed on both the inside and outside of the bolt’s central circle are called wide-face flanges. 38. How many types of flanges are there based on their degree of integrity? What are their respective features? Loose flange: The flange fails to be effectively integrated with the container or pipe fitting. In the calculations, it is assumed that the container or nozzle does not share the flange torque with the flange. Integral flange: The flange, the flange neck, and the container or pipe fitting are effectively connected to form a single unit, working together to bear the flange torque. Adaptive flanges: These are some welded flanges that are calculated as integral flanges. 39. What strength criteria need to be checked when verifying flange strength? Axial stress, hoop stress, radial stress, combined stress, shear stress. 40. What are the requirements regarding gasket load in the rational design of flange connections? To minimize the flange torque on the flange, the gasket load should be kept as low as possible in the gasket design. To this end, it is required that the bolt load Wa determined by the compressive load Fa of the gasket during pre-tightening be close to the bolt load Wp determined by the compressive load Fp of the gasket during operation. 41. In the rational design of flange connections, what considerations are there for determining the diameter of the bolt’s center circle? To minimize the flange torque on the flange, the diameter of the bolt’s center circle should be kept as small as possible in the bolt design; for this reason, it is necessary that the diameter of the bolt’s center circle determined by the flange’s radial structure be close to the diameter determined by the flange’s circumferential structure. 42. When designing a flange, what are the requirements regarding the stresses on the flange in order to achieve a as compact a design as possible? The three stresses on the flange should be made as close as possible to the corresponding allowable stresses; by doing so, the flange stress reaches a state of full stress, which allows the strength properties of the material to be utilized to the fullest extent. 43. What is the effect of increasing the flange cone thickness on the stress in the flange? By increasing the size of the flange cone neck, its rotational stiffness is enhanced, which increases its load-bearing capacity. As a result, the boundary forces and boundary moments between the cone neck and the flange ring increase; in other words, the boundary forces and moments at the end of the cone neck increase, and this leads to an increase in axial bending stress. However, since the bending resistance cross-sectional area of the neck increases in proportion to the square of the neck thickness**, this ultimately results in a decrease in the axial bending stress of the neck. Boundary forces: an increase in torque leads to an increase in the radial bending moment acting on the inner edge of the flange, thereby increasing the radial bending stress in the flange ring. As the radial moment on the inner edge of the flange increases, the support required for the flange ring grows, which reduces deflection; consequently, its hoop stress decreases. 44. There are several types of overpressure relief devices; what are their advantages and disadvantages, and for which types of pressure vessels are they not suitable? There are three types: safety valves, burst disc devices, and a combination of safety valves and burst disc devices. A safety valve is an automatic pressure-relief valve that opens under the effect of the static pressure at the inlet; it discharges fluid by relying on the pressure of the medium itself in order to prevent the internal pressure from exceeding safe levels. Once the internal pressure returns to normal, the valve closes automatically, preventing further discharge of the medium. A burst disc is a non-self-sealing pressure relief device that is pressed and ruptured by the static pressure at the inlet to release the medium, thereby preventing the internal pressure from exceeding a predetermined value. A new burst disc must be installed again once the pressure returns to normal. A burst disc device must be used in cases where the medium inside the container could cause the safety valve to fail ； Containers that do not allow any material leakage ； The pressure inside the container rises too rapidly, so that the safety valve cannot keep up ； Other situations in which the safety valve cannot be used. Pressure vessels in which a sudden increase in pressure may occur during operation and the reaction rate reaches the level of detonation are not suitable for these overpressure relief devices. 45. What are the special requirements for weld inspection of low-temperature pressure vessels? For containers that are required to undergo 100% inspection in accordance with regulations, their T-joints, butt welds, and fillet welds must all be subjected to 100% magnetic particle or penetrant testing. The connection welds of non-pressure components that are welded to pressure components shall also be inspected in accordance with the requirements of this clause. 46. What is a low-temperature and low-stress operating condition? Are vessels under low-temperature ground stress conditions considered low-temperature pressure vessels? A low-temperature geostress condition refers to a situation in which the design temperature of the container or its stressed components is below or equal to -20°C, but the hoop stress is less than or equal to the yield strength of the steel at normal temperature, and not greater than 50 MPa. When a vessel or its pressure-bearing components are used under \"low-temperature geostress conditions\", if the design temperature increases by 50°C to above -20°C, the regulations for low-temperature pressure vessels need not be followed. 47. What are the methods for testing the mechanical properties of the weld joints in welding test plates? Tensile, bending, impact tests. 48. Under what circumstances should pressure vessels and their components be pickled and passivated? The surfaces of austenitic stainless steel and composite steel plate containers that require corrosion resistance should be subjected to pickling and passivation treatment. Austenitic stainless steel components that require corrosion resistance must undergo pickling and passivation treatment after heat treatment in accordance with the drawing specifications.