The issue of end cap thinning
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As the title suggests, I have a distillation tower with a cylinder length of approximately 30 meters, a skirt length of about 6 meters, and an inner diameter of 3.8 meters. The cylinder is made of stainless steel. I’m facing an issue: when entering the parameters for the upper and lower heads in SW6, if the nominal thickness is set to 12 and the corrosion allowance is set to 0, the individual calculations for the upper and lower heads yield acceptable results, with calculated thicknesses of 10 and 7 respectively (the amount of thinning indicated by PS is correct), and the overall calculation report is also satisfactory. However, if the nominal thickness is set to 12 and the corrosion allowance is set to 1.8 (as a reduction in thickness), the individual calculations for the upper and lower heads are still acceptable, but the calculation report indicates that the axial stress check for the lower head is not satisfactory. How can this be resolved?
This post was last edited by fzujunru on 2019-3-27 at 20:59. When calculating the head separately, the program takes only the internal pressure into account; when calculating the entire structure, it considers the axial stress generated by wind loads. Pay attention to the cross-section at the junction between the cylinder and the head. The original poster should take a look at NB/T 47041 section 7.9, as well as the information regarding the “critical sections” of towers
What was said on the 2nd floor is correct. When calculating separately, only the internal pressure is taken into account; for a proper assessment of the tower, stresses caused by wind and seismic combined bending moments must also be considered
Agree with the second floor! Axial stress has a significant impact on tower calculations. It doesn’t make much sense to calculate each component separately.
Thank you. I understand now – there was an issue with the input values. When entering the nominal thickness of stainless steel end caps, we should directly enter the value of thickness minus the amount of thinning; it’s not very accurate to enter the thinning amount as part of the corrosion allowance. The problem with the latter approach is what I mentioned earlier regarding axial stress – it should be 7.9.1. The denominator in this formula represents the effective thickness, and the effective thickness is defined as the nominal thickness minus the corrosion allowance. For stainless steel, the corrosion allowance is generally 0, yet we entered the amount of thinning as the corrosion allowance
Another issue when calculating towers is the welding joint coefficient; when checking the axial stresses caused by wind loads and seismic loads, it is necessary to take into account the welding joint coefficient of the circumferential joints.
For stainless steel as well, the amount of thinning must be taken into account. It is more appropriate to use a corrosion margin rather than directly using the minimum forming thickness. Although this stainless steel part does not have a corrosion margin, in the case of stamped heads, the top portion of the head and the transition areas at the corners are where thinning is most likely to occur. The minimum forming thickness of the head must still meet the requirements of strength calculations. If increasing the overall thickness is not desired, it is recommended to set the minimum forming thickness as \"the nominal thickness minus the negative deviation of the steel plate\"“
The amount of thinning can be calculated by oneself after the calculation sheet is prepared; as long as the effective thickness listed on the sheet, plus the negative deviation and the amount of thinning, is less than the nominal thickness specified, then everything is fine. Generally, it’s acceptable to include the amount of thinning in the corrosion allowance for the caps, but in the formula for the axial stress of this tower, there is a parameter representing the effective thickness, which is determined by subtracting the corrosion allowance from the original thickness. If the amount of thinning is mistaken for the corrosion allowance, then the calculated axial stress will not be accurate
The last edit to this post was made by fzujunru on 2019-3-28 at 11:18. The minimum forming thickness is the lowest thickness permitted in the design; if the actual thickness of the head is less than this value, it is considered unacceptable. Manufacturers use this value to determine the material to be used. It can be said that the minimum forming thickness corresponds to the actual thickness of the head. In my opinion, it is still necessary to carry out calculations using this value. If such calculations are not done, the amount of excess metal in the head as calculated for reinforcement purposes might be higher than the actual amount, which is not safe
You probably didn’t understand what I meant: L