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Ensuring safe, economical, and reasonable forming of end caps 1. Definition of thickness according to GB150-1998 standard (1) The calculated thickness δ is the thickness obtained by using the formulas provided in various chapters. If necessary, the thickness required for other loads should also be taken into account. (2) The design thickness δd is the sum of the calculated thickness δ and the corrosion margin C1. (3) The nominal thickness δn is the design thickness δd plus the negative deviation C1 of the steel thickness, rounded up to the standard specification thickness of the steel. That is, the thickness indicated on the drawing. (4) The effective thickness δe is the nominal thickness δn minus the corrosion allowance C2 and the negative deviation of the steel thickness C1. (5) The relationships between various thicknesses are shown in Figure (6). The starting thickness (i.e., the thickness of the blank) is determined according to Chapter 10 of GB150---1998 and the relevant thickness relationship charts: δs = δ + C1 + C2 + Δ1 (the first rounding value for the thickness) + C3 (the amount of thinning due to processing) + (the second rounding value for manufacturing). 2. Example of head design calculation: The inner diameter of the container is Di = 4000 mm, the calculated pressure is Pc = 0.4 MPa, the design temperature is t = 50°C; the head is a standard elliptical head, the material used is 16MnR (the allowable stress of this material at the design temperature is 170 MPa); the negative deviation of the steel thickness is not more than 0.25 mm and does not exceed 6% of the nominal thickness; the corrosion allowance C2 is 1 mm, and the welding joint factor for the welded joints of the head is k = 1. Calculate the calculated thickness, design thickness, and nominal thickness of an elliptical head. The calculated thickness δ for KpDi is =----------------= 4.73 mm. The calculated thickness δd for 2tΦ-0.5pc is δ + C2 = 4.73 + 1 = 5.73 mm. Considering that the effective thickness δe of a standard elliptical head should be no less than 0.15% of the inner diameter Di of the head, the effective thickness δe = 0.15% × Di = 6 mm. Since δe > δd, C1 = 0 and C2 = 1; thus the nominal thickness δn = δe + C1 + C2 = 6 + 0 + 1 = 7 mm. Taking into account that the standard thickness specifications for steel require an additional 1 mm, the first rounded value for design purposes is △1 = 1, so δn is set at 8 mm. According to the technical data from a professional head manufacturing factory, for Di=4000 and δn=8, the thickness reduction amount for head processing is C3=1.5 mm, and the second rounding value for the thickness is △2=0.5. If it is required that the thickness of the head after forming shall not be less than the nominal thickness δn minus the negative deviation C1 of the steel plate, then the thickness of the material to be used is: δs = δn + C1 + C3 + △2 = 8 + 0 + 1.5 + 0.5 = 10 mm; the minimum thickness after forming is 8.5 mm. If the thickness of the head after forming is not less than the design thickness δd (the value of δe should be used), then the thickness of the material to be fed is: δs = δd(δe) + C3 + △2 = 8 mm. The minimum thickness after forming is 6.5 mm, which is greater than the effective thickness δe, as well as greater than both the design thickness δd and the calculated thickness δ. As can be seen from the above, the two different requirements result in a difference of 2 mm in the material thickness of this end cap, with a weight difference of up to 300 kg. 3. The thickness definitions in GB150 and related head standards are not very reasonable. This is particularly evident in the thickness requirements for containers and heads after they have been formed; for convex heads and hot-rolled shells, the required forming thickness must be no less than the nominal thickness minus the negative deviation of the steel plate (δn–C1). As a result, it may be necessary to increase the thickness twice during design and manufacturing, once based on the design thickness and once again to ensure that the required forming thickness is achieved. To this end, the concept of minimum forming thickness was introduced: \"the thickness that must be ensured after processing a hot-rolled cylinder or convex head, with a value that is not less than the design thickness.\" In other words, the designer should indicate on the drawings the nominal thickness and the minimum formability thickness (i.e., the design thickness δd), so that the manufacturing unit can decide whether to apply an additional thinning amount based on the manufacturing process and the rounding considerations specified in the original design. The definition and marking of this thickness is the prevailing method in the international pressure vessel industry, and it has its rationale; however, there are two issues that need to be addressed in China’s current standards. (1) Regarding the issue of the variation in the straight edge height with thickness in the head standards, the straight edge height of a head is a variable determined by the head’s diameter and thickness; there is no need for such a requirement, as it only complicates the size series of heads. Under both the 1992 version and the latest 1997 version of the JIS B8247 standard, the height of the straight edges is 3 times the thickness, with a minimum of 20 mm and a maximum of 38 mm, making it relatively straightforward. This issue can be resolved when the standards are revised. (2) Regarding the calculation for reinforcement due to openings, the effective thickness δe (δn – C1 – C2) is used in this calculation; it consumes the rounded value △1 selected by the designer. If the design-related rounded value △1 is used again in manufacturing, it may result in insufficient strength. Opening reinforcement is merely a local structural calculation method, and the most commonly used equal-area method is also based on experience and conservatism; therefore, it is possible to ignore rounding values when determining the reinforcement area, and to carry out the reinforcement calculations based on the design thickness (δ+C2). 4. Defining the minimum forming thickness of the head using the design thickness is in line with the principles of safety and economy. (1) As explained above, the requirement that the forming thickness of the head be not less than the nominal thickness minus the negative deviation of the steel plate (δn–C1) may lead to unnecessary waste, as both the design and manufacturing processes involve adding thicknesses (△1+△2) based on the design thickness (δd). This is also an approach to addressing such issues. (2) In fact, the \"Standard Interpretation\" GB150-89 already states that \"the minimum thickness of formed heads should meet the requirements for strength (i.e., the calculated thickness δ) and service life (i.e., the corrosion allowance C2)\", and this is the consensus among most experts. (3) JB4732-95 \"Steel Pressure Vessels – Code for Analytical Design\" specifies in 11.2.1: \"Based on the manufacturing process conditions, determine the machining allowance to ensure that the actual thickness of each part of the finished product is not less than the designed thickness for that part.\" (4) In Paragraph 2 of Article 69 of the new Code, \"the weld scars remaining after the removal of the shims for temporary lifting lugs and struts must be polished smooth.\" . . . . . The thickness after grinding should not be less than the designed thickness of that area (δ+C2). "In other words, the overall design thickness of the container (δ+C2) can be used to determine the minimum safety thickness required for various parts of it. (5) To ensure that the forming thickness of the head is not less than the nominal thickness minus the negative deviation of the steel plate thickness, the current practice requires adding the thinning amount C3 (usually 2 mm) to the nominal thickness δn, and then rounding △2 to the thickness specified by the standard steel plate specifications in order to determine the thickness δs of the raw material. This is not only wasteful but also leads to the situation where, due to the thickening of the straight edges of the end caps after pressing, the misalignment when assembling them with the cylindrical sections exceeds the allowable limits, forcing the need for additional thinning of those straight edges. Sometimes, an increase in the material thickness δs can cause a change in thickness levels, leading to a decrease in the strength parameters (σb, σs) and the allowable stress t, which necessitates a reevaluation of the strength. (6) The forming methods for end caps include hot stamping and cold stamping, as well as cold spinning and hot spinning. Different sizes and processing methods result in varying degrees of thinning; professional end cap manufacturers have detailed technical data on this. By providing the design thickness (δ+C2) and adding the actual thinning amount specified by the manufacturer, and then rounding it to the standard thickness for steel plates, it is possible to avoid waste resulting from additional rounding steps in design and manufacturing (△1+△2). This approach yields a safe, economical, and reasonable thickness for the end cap. This is also why foreign industry peers use a minimum guaranteed thickness (i.e., the design thickness of δ+C2). 5. Conclusion (1) Using the design thickness (δ+C2) rather than the nominal thickness (δn) minus the negative steel plate deviation (C1) as the minimum guaranteed thickness for head forming is both reasonable and most economical while meeting safety requirements. (2) The problem is that the design thickness (δ+C2) must be clearly indicated on the design drawings; the design thickness primarily refers to the calculated thickness δ, as C2 is usually specified in the overall layout. Otherwise, although regulations such as the Allowable Deviations Standard and GB150 stipulate that the thickness after polishing should be no less than the design thickness, this remains unknown and unworkable for manufacturers. (3) To determine the head forming thickness in a safe, economical, and rational manner, cooperation among pressure vessel standards, designers, manufacturers, head users, and specialized head manufacturers is required, as well as a unified understanding of these standards. This post was last edited by echo7 on 2008-6-10 20:43]