GB150.1~150.4-2011 “Pressure vessel” is the authoritative design standard for pressure vessel design in China. How to properly select the thickness of the pressure vessel according to the standard to meet the strength requirements of the equipment is of great significance in the safe operation of the pressure vessel. Since the pressure-receiving element such as the head has a certain amount of processing and thinning during the molding process, GB150 stipulates that the nominal thickness and the minimum forming thickness of the container element should generally be marked on the design drawing.

1, The conventional method of determining the minimum thickness of the head

In general, the conventional method of determining the minimum thickness of the head = the design thickness, that the thickness + corrosion margin = design thickness, but this method only considers the effect of the pressure load on the head, without considering other loads and restrictions. The influence of the conditions causes the minimum thickness of the head to fall short of the design working condition, and there is a safety hazard in the operation of the pressure vessel.

Factors affecting the determination of the minimum thickness of the head

The effect of stiffness

The head should have a certain rigidity while being under pressure to avoid deformation and damage. Therefore, GB150 stipulates that the minimum thickness of carbon steel and low alloy steel containers excluding corrosion allowance shall be not less than 3 mm; the minimum thickness of high alloy steel containers excluding corrosion allowance shall not be less than 2 mm.

Influence of pressure load

The head is a pressure component that bears a certain pressure. The first condition that must be met is the pressure load. The head that bears the internal pressure or the external pressure load can be calculated by the calculation formula of GB150, Chapter 3, where the thickness is calculated. Thickness + Corrosion Margin = Design Thickness, this design thickness is the minimum thickness required for the head obtained from the pressure load alone.

Influence of internal pressure instability

Under the action of internal pressure, circumferential compressive stress is generated at the bottom edge of the head, where the head is prone to circumferential instability. In order to prevent the elliptical head from being unstable under internal pressure, GB150 stipulates that the effective thickness of the elliptical head of Di/2hi ≤ 2 shall not be less than 0.15% of the inner diameter of the head, for the ellipse of Di/2hi>2 The effective thickness of the head shall not be less than 0.30% of the inner diameter of the head. Although the above limitation is the effective thickness, it also indirectly limits the minimum thickness of the head forming.

2.4 The effect of opening reinforcement

Common opening reinforcement methods for pressure vessels include: forged pipe reinforcement, reinforcing ring reinforcement, and overall reinforcement. The so-called overall reinforcement is to increase the thickness of the shell to reinforce the strength of the shell after the container is opened. When the opening on the head adopts the integral reinforcing structure, the minimum thickness of the head is the thickness required to meet the opening reinforcement.

2.5 The local stress produced by the pipe load

In pressure vessel design, some important technical nozzle sealing head need to check by the control of local stress load and the outside shell when take over is quite harsh external load, increase the thickness of the head to reduce the effects of pipe load stress, in order to achieve the purpose of checking by, then the minimum thickness of the head is to satisfy local stress checking thickness of qualified.

2.6 The effect of the horizontal container head on the reinforcement

In the horizontal container design, the circumferential compression stress is generated on the shell body in which the reaction force of the container support contacts the saddle. At the lowest position of the shell of the support cross section, the circumferential compressive stress is the largest, and in the absence of the reinforcement ring, this The shell is prone to instability, and the tangential shear force at the cross section of the shell produces a circumferential bending distance in the radial section of the shell. In the design of the saddle, the distance between the center of the saddle floor and the tangential line of the head should be as much as A≤0.5Ra, so that the head can strengthen the shell and balance the circumferential compression force and bending distance of the shell. The minimum thickness of the head is the thickness when the saddle stress is checked.

In the above Ra is the average radius of the shell

2.7 Impact of stress test stress check

GB150 stipulates that if the pressure test is carried out with test pressures greater than those specified in 4.6.2.2 and 4.6.2.3, the stress levels of the pressure-bearing components under the test conditions shall be checked before the withstand voltage test to meet the calibration conditions. When the pressure vessel needs to increase the test pressure and carry out the stress test stress check, the stress of the head test working condition may exceed the allowable stress value. At this time, the effective thickness of the seal head is increased to meet the test stress check condition. The minimum thickness is the thickness when the stress test stress is checked.

2.8 Effect at the critical value of material thickness

GB150 stipulates that the calculated thickness refers to the thickness calculated according to the formula of each chapter; the design thickness refers to the thickness of the sum of the thickness and the corrosion margin; the nominal thickness refers to the design thickness plus the negative deviation of the thickness of the steel plate and then rounds up to the steel standard. The thickness of the specification, that is, the thickness marked on the pattern; the effective thickness is the thickness of the nominal thickness minus the corrosion margin and the negative deviation of the thickness of the steel sheet.

For the head, its blank thickness = design thickness + forming thinning amount + rounding amount to the material standard thickness, the minimum thickness after forming shall not be less than the design thickness. Among them, design thickness = calculated thickness + corrosion margin. When rounding to the thickness of the blank after the standard specification of the material and calculating the thickness of the head, the thickness of the allowable stress selected is not in the same thickness range as the allowable stress table in GB150 ( jump), such as the thickness of the blank The allowable stress of the file is calculated to increase the thickness of the head, because the allowable stress of different thicknesses of the same material is different according to the allowable stress table of each material in GB150, and as the thickness increases, the allowable stress follows. reduce. At this time, the minimum thickness of the head is the design thickness obtained by calculating the allowable stress value after the jump (the thickness of the head blank).

3, The conclusion

The minimum thickness of the head can not be taken simply by the design thickness. According to the specific design conditions, combined with the various influencing factors described in this paper, and taking the maximum of each influencing factor as the minimum thickness of the head, the safe operation of the pressure vessel can be guaranteed.