Does the designer also need to pay attention to the calculation of head edge? The calculation formula of straight edge of head is equal to that of cylinder. This is clearly stipulated in ASME UG32 (L). Indeed, when the minimum thickness of the head is far less than the minimum calculated thickness of the cylinder, it is very necessary to calculate the straight edge of the head to avoid excessive primary membrane stress on the straight edge.
The welding joint of head and cylinder is specified in ASME, as shown in the figure below:
What do these regulations have in common? How do you remember these rules?
The first point is 1:3 trimming to reduce structural discontinuity and local stress.
The second point is: there should be “material” in the arc section of the head and the straight section of the cylinder. Material means that there can only be more material, not less. Taking tangent as the boundary, the calculation formulas of arc section and straight section are head formula and cylinder formula respectively. The values of the two formulas ensure that the local membrane stress does not exceed the standard. If you look at the figure above, the thickness of the arc section at the junction is thickened. In the straight section, the thickness of the cylinder is either unchanged or thickened.
The author has carried out the finite element analysis of plane82 element and axisymmetric model for edge cutting and “material”, as long as these two points are satisfied. The strength of the structure can be guaranteed.
Of course, if ASME Design is adopted, pvelite software has checked the straight edge of the head. SW6 doesn’t have this check. When there are special circumstances, designers should avoid mistakes.
From the second point, we should pay attention to such a situation in the tubesheet design of pressure vessels. The following is a real case in the project:
The cylinder is made of carbon steel plate, and the tube plate is made of carbon steel forgings and stainless steel cladding. Due to the higher allowable stress of plate and the lower allowable stress of forging, the straight edge of shoulder will be thicker than the cylinder according to the calculation formula of cylinder. So the thickness of the shoulder is thickened in the design. When the stainless steel tubesheet is connected with the carbon steel cylinder, there is also this problem, so it is necessary to recheck the strength of the straight section.