Issues regarding the determination of the nominal thickness of EHA end caps
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I would like to ask everyone: There is a medium and low pressure vessel of category III. For the EHA head, the minimum forming thickness is calculated as the design thickness plus the corrosion allowance, which equals 24.47 + 2 = 26.47 mm; the nominal thickness is set at 30 mm, and the drawing indicates 30 (26.5). Query GB/T25198: For pressure vessel heads made of EHA2000 steel, the thickness should be ≥20 and <34; the thickness reduction rate is 13%. Based on this calculation, the minimum forming thickness = (nominal thickness – negative deviation) × (1 – reduction rate) = (30 – 0.3) × (1 – 13%) = 25.84 mm. Since 25.84 < 26.5, it is necessary to increase the nominal thickness of the head; however, is it appropriate to use a nominal thickness of 30 for the cylinder?
Each head manufacturing factory has different production capacities, as well as varying degrees of thickness reduction during shaping. It is possible to consult the head manufacturer to find out whether, with an input material thickness of 30, it is possible to ensure a minimum shaped thickness of 26.5. It is fine to use a nominal thickness of 30 for the cylinder. To determine the thickness, it is also necessary to consider whether the opening reinforcement has been approved.
The thickness reduction rate specified in GB/T25198 is a recommended value; the nominal thickness of the head should be determined based on the actual reduction rate. Most head manufacturers have empirical data in this area.
This post was last edited by prt0683 on 2020-11-25 at 11:06. Should the reinforcement for the header openings be calculated based on a nominal thickness of 30 mm, or based on the minimum formability thickness?
The nominal thickness for reinforcing the openings in the head should be based on the thickness of the material after cutting; it gets thinner as one moves toward the center. We calculate it using the minimum thickness after molding, to be more conservative.
This post was last edited by zjq1962 on 2020-11-25 at 11:16. If the allowable stress values for these two thicknesses do not change, then both thicknesses can be used in the calculations; if there is a change in those values, then the actual thickness of the material to be used must be entered. When calculating the input, if the minimum forming thickness is used, there is no need to consider the negative deviation of the sheet metal or the forming thinning amount (which is set to 0) ； If the feed thickness is used, the negative material deviation and the thinning due to molding must be taken into account (the thinning amount can be added to the corrosion allowance).
The opening reinforcement is calculated based on the minimum forming thickness of the head, with C1=0. Label 30 (26.5) in the diagram is appropriate; nowadays some head manufacturers can control the forming thinning rate at 5–6%, and even if this target is not achieved, equipment manufacturers will purchase sheet material with a higher nominal thickness for the heads.
The confirmation letter sent to the head manufacturer will clearly specify the minimum forming thickness, so that the manufacturer can carry out inspections on the heads upon receiving the order. Alternatively, consult the head factory in advance to ask whether it can ensure a thinning rate of <10%.
You can mark 28 (26.5) on the drawing; let the manufacturer consider the material thickness to be used； Today, manufacturers of heads with mature technology can keep the thickness reduction below 6%.
The value in the parentheses in your diagram represents the minimum thickness of the head after it has been formed; if the head manufacturing factory is unable to achieve this thickness, then it will not be possible for you to obtain such a product.