Example of head design calculation

Example of head design calculation

The inner diameter of the vessel Di = 4000 mm, the design pressure Pc = 0.4 MPa, the design temperature t = 50℃, the end cap is a standard elliptical head, the material is 16MnR (the allowable stress of the material at the design temperature is 170 MPa), the negative tolerance of the steel plate is no more than 0.25 mm and does not exceed 6% of the nominal thickness, the corrosion allowance C2 = 1 mm, and the welding joint coefficient of the end cap is η = 1.

Calculate the required thickness, design thickness, and nominal thickness of the elliptical head. The calculated thickness δ = KpDi / [2[σ]tΦ - 0.5pc] = 4.73 mm, and the design thickness δd = δ + C2 = 4.73 + 1 = 5.73 mm.

Considering the standard elliptical head, the effective thickness δe should be no less than 0.15% of the inner diameter Di of the head. The effective thickness δe = 0.15%Di = 6mm. Since δe > δd, and with C1=0 and C2=1, the nominal thickness δn = δe + C1 + C2 = 6 + 0 + 1 = 7mm. Considering the standard steel plate thickness specifications, a 1mm increase was applied for the first design rounding value △1=1, therefore δn = 8mm was chosen.

According to technical data from a professional head manufacturer, for a head with Di=4000 and δn=8, the thinning allowance during processing is C3=1.5mm, and the second rounding value for thickness is △2=0.5. The formed head thickness must not be less than the nominal thickness δn minus the steel plate negative tolerance C1.

The initial material thickness is: δs = δn + C1 + C3 + △2 = 8 + 0 + 1.5 + 0.5 = 10 mm, and the minimum thickness after forming is 8.5 mm. If the head forming thickness is not less than the design thickness δd (δe should be used), then the initial material thickness is: δs = δd (δe) + C3 + △2 = 8 mm, and the minimum thickness after forming is 6.5 mm, which is greater than the effective thickness δe and even greater than the design thickness δd and the calculated thickness δ.