For the thickness δs of the blank for the drawing head, can the nominal thickness δnh of the head in SW6 be used?
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As shown in the figure below, the dimensions of the head as specified by a certain design firm are indicated there; the thickness of the blank is δs = 18 mm, while the minimum thickness after shaping is δmin = 15.36 mm. The corrosion margin specified on the overall layout diagram is 3 mm. Many design firms, in accordance with the provisions of GB25198, indicate the type as either “raw thickness δs (formed thickness δmin)” or “design thickness (design thickness – negative deviation of 0.3)”. To meet the requirements regarding pressure, temperature, and opening reinforcement for such containers, many design institutes choose the \"minimum forming thickness δmin of the vessel head\" to be equal to the \"nominal thickness δnh of the head in SW6\". However, the design firm used a value of “δs, the nominal thickness of the head” equal to 18 mm in the SW6 calculation sheet. Additionally, although the corrosion margin indicated on the overall layout is 3 mm, the corrosion margin specified in the calculations for this head is 5.34 mm, as shown in the figure below. I calculated based on a nominal thickness δnh of 15.8 for the head; if the minimum formability thickness δmin is less than 15.8, the effective thickness will be less than the calculated thickness, and it is considered unacceptable. As the manufacturing unit, we cut the materials to a thickness of 18 mm as specified by the design institute and send them to the head manufacturing factory for processing. If the factory processes them to a thickness of 15.8 mm, this is indeed greater than the minimum thickness after molding specified in the drawings, which is δmin = 15.36; thus it meets the requirements. But isn’t this thickness not even sufficient for the calculations to be valid? In light of the above, the following questions are raised: 1. Can the thickness δs of the blank for the drawing’s head be taken as the nominal thickness δnh of the head in SW6? Is the approach of this design firm appropriate? 2. Why are the corrosion margins in the calculation sheet and the overall layout different? What is the reason for this?
This is just my understanding; it’s not necessarily correct. 1. The minimum forming thickness of the head is the actual thickness, which is used as a basis for checking the strength of the head; there should be no problem with that. 2. During calculation, C1 is built into the system and cannot be adjusted; designers can only adjust C2. The value of C2=5.34 appears to be due to the designer including the forming thinning amount in the calculations. Another method is to use the minimum forming thickness for calculation; there is no need to subtract C1 again. Perhaps it is this excess subtraction of C1 that is the reason why the calculation fails.
When using SW6 to calculate elliptical heads, the nominal thickness is generally consistent with that specified on the drawings (entering the minimum formability thickness here will result in a lower calculated mass for the component and possible shifts in the allowable stress values); instead, enter the adjusted value at the corrosion allowance position to ensure that the effective thickness equals the minimum formability thickness minus the corrosion allowance. We usually modify the calculation sheet; for example, the nominal thickness field is changed to 18 mm (minimum formability thickness: 15.36 mm), the corrosion allowance field is changed to 3 mm, and an entry is added for effective thickness (the effective thickness is calculated as the minimum formability thickness minus the corrosion allowance).
1. It is fine to use the thickness of the end cap as the nominal thickness; this also helps to avoid issues related to changes in the allowable stress of the material caused by thickness variations. Since the software does not have an input for C3 (formation thinning amount), C3 is added to C2 when C2 is entered. This is also the reason why the corrosion margin in the calculation sheet does not match that in the drawing. 2. As long as the minimum thickness of the head after forming is not less than the calculated thickness plus the corrosion allowance (design thickness), the calculation is considered valid.
Yes, the minimum forming thickness of the head is the actual thickness. However, the design team calculates it based on the thickness of the raw material before any processing; if calculation is done using the minimum forming thickness, the result will be unsatisfactory. Shouldn’t shaping and thinning be something considered by the manufacturing unit? The design team should provide the minimum thickness required for molding; the problem is that the minimum thickness they specified, 15.36, does not meet the requirements related to pressure and temperature. If I choose a head manufacturing factory with relatively poor processing skills, the resulting thickness might be 15.5, which zwar meets the requirements specified in their drawings, but it still does not satisfy the calculations
The calculation of effective thickness needs further discussion; what is said may not be correct, please bear with me. 15.8-0.3-5.34=10.16, is this Mr. Li’s calculation method? First, for the C1(-0.3) issue, when calculating based on the minimum forming thickness, there is no need to use -0.3; this represents the actual thickness of the steel plate, not the nominal thickness. The nominal thickness is a theoretical value, while the actual thickness is the measured value, so there is no need to subtract another -0.3. Secondly, regarding issue C2 (-5.34), the design institute previously increased C2 from 3 to 5.34 in order to take the thickness reduction during molding into account; now that molding is complete, there is no need to increase C2 any further – 3 remains the appropriate value. Therefore, this effective thickness should be 15.8 – 3 = 12.8. As for how to adjust the program, there are too many details to cover in writing; it is recommended to consult an experienced designer.
Thank you for your guidance. How did you know my name is Engineer Li? :o
The calculation documents prepared by the design institute are fine; the reasons are as mentioned by everyone above
This post was last edited by zjq1962 on 2022-7-14 at 13:11. The original poster used a nominal thickness of 15.36 mm for the calculation; in this case, the corrosion margin should be set to C2 = 3.0 mm (instead of 5.34 mm). It is necessary to uncheck the option for a negative deviation in sheet thickness, as C1 is already included in the minimum thickness, with an additional thickness of 3.0 mm. Effective thickness: 15.36 – 3.0 = 12.36 mm. How could it be Unqualified?