The long weld neck flange of pressure vessel is optimized through two specific examples, and the effects of cone neck height and flange thickness on the three main stresses of flange axial stress, radial stress and tangential stress are analyzed. The calculation results show that when the flange thickness and cone neck height are adjusted to be similar, the three main stress values are close to the full stress value. This optimized design makes the flange compact in structure, reasonable in force, reduces the weight, and can significantly reduce the flange cost. For long weld neck flanges with small diameter and low pressure, on the premise of ensuring that the slope of flange cone neck section is less than 1:3, the flange can not have straight edge section.
Long weld neck flange is the most commonly used equipment flange in pressure vessels. Although NB/T 47023-2012 standard  gives the long weld neck flange of carbon steel and low alloy steel pressure vessels with nominal pressure of 0.6 ~ 6.4MPa and working temperature of – 70 ° C ~ 450 ° C, the flange, stud, nut and backing sheet materials need to be fully implemented according to the matching table and correction table in the standard, which is subject to many restrictions, Moreover, in engineering practice, many long weld neck flanges are beyond the scope of NB/T 47023-2012 standard, such as stainless steel flange or long weld neck flange with working temperature exceeding 450C, which shall be in accordance with GB/T 150 3-2011 design and calculation of non-standard flange. Through two specific calculation examples, the author optimizes the design of UNS S30408 long weld neck flange and 15crmo long weld neck flange, analyzes the effects of cone neck height and flange thickness on the three main stresses of flange axial stress, radial stress and tangential stress, and gives suggestions on the optimal design of long weld neck flange, which can be used as a reference for relevant designers of pressure vessels. In addition, the author also draws lessons from the flange design schemes of some large design institutes and engineering coMPanies. For the long weld neck flange with small diameter and low pressure, it is recommended that the flange design should not have straight edge section, but according to GB/T 150 3-2011 it is required to ensure that the slope of flange cone neck section is 31:3, which can significantly reduce the flange cost.
The flange material of a vessel is s30408 and the design temperature is 300C; Design pressure: 2.6MPa; The specification of butt barrel is dn1000x14mm, and the winding pad is selected: M = 3.0, y = 69MPa; The allowable stress of flange under normal temperature  = 137MPa, the allowable stress of flange under design temperature  / = 85MPa, the material of stud is 35CrMoA, the specification is M30, the quantity is 48, and the corrosion allowance is not considered.
Firstly, the author designs and calculates with reference to the overall dimensions of NB/T 47023-2012 standard equipment flange. The outer diameter of the flange is 1215mm, the inner diameter of the flange is $1000mm, the diameter of the bolt circle is 1155mm, the effective thickness of the flange is 100mm, the height of the cone neck is h = 42mm, the effective thickness of the large end of the neck is G1 = 36mm, and the effective thickness of the small end of the neck is G0, which is taken as the thickness of the butt cylinder 14mm. After preliminary calculation, the axial stress 0h = 199.25MPa > 1.5 [called /, comprehensive stress max (0.5 (0h + 0r), 0.5 (0h + 0t)) = 136.01MPa > [0h] /, and the flange strength is unqualified. At this time, some designers will blindly increase the flange thickness until it is qualified, which is not desirable. Blindly thickening the flange will cause material waste and unreasonable stress on the flange.
When the axial stress is too large or too small, the method of adjusting the size of the cone neck shall be adopted, and the thickness or height of the cone neck can be adjusted. The maximum value of the axial stress is usually located on the section of the small end of the cone neck, which can be judged from the coefficient F. the coefficient f is the stress at the small end of the cone neck.
When f is greater than 1, the maximum stress is at the small end of the cone neck. When f is less than or equal to 1, the maximum stress is at the large end of the cone neck. In this example, in order to facilitate docking with the cylinder, the thickness of the small end of the cone neck is taken as the thickness of the cylinder without adjustment; the effective thickness of the large end of the cone neck shall be in accordance with the provisions of the minimum value of La in table.1 in GB/T 150.3.
| Cone neck heighth/ mm|| Axial stress is calculated/MPa|| Allowable axial stress value/MPa|| Radial stress is calculated/MPa|| Allowable radial stress value/MPa|| The tangential stress is calculated/MPa|| Allowable tangential stress value/MPa|| Comprehensive stress calculation value/MPa|| Combined allowable stress value/MPa|| Check the results|
| 55|| 167.82|| 127.5|| 18.29|| 85|| 64.85|| 85|| 116.34|| 85|| Unqualified|
| 65|| 143.75|| 127.5|| 19.27|| 85|| 59.71|| 85|| 101.73|| 85|| Unqualified|
| 75|| 120.8|| 127.5|| 20.11|| 85|| 55.34|| 85|| 88.07|| 85|| Unqualified|
| 78|| 114.22|| 127.5|| 20.34|| 85|| 54.16|| 85|| 84.19|| 85|| Qualified|
It can be seen from the stress calculation results in Table.1 that after increasing the cone neck height, the axial stress value decreases significantly, the tangential stress value also decreases, and the radial stress value increases slightly. When the cone neck height increases to 78mm, the flange is checked and qualified, but is this the optimal design? In the above calculation process, the author only increases the cone neck height, and the flange thickness is not adjusted. The flange design should follow According to the full stress design principle, the axial stress and radial stress in the above calculation are close to the full stress value. Through further adjustment and calculation of the flange thickness and cone neck height, the author concludes that when the flange thickness is 90mm and the cone neck height is 84mm, the axial stress 0h = 116.04MPa, the tangential stress 0t = 52.25MPa, the axial stress and tangential stress value and the flange thickness are 100mm and the cone neck height is 78m M is basically the same, the radial stress 0r = 27.92MPa, and the radial stress value increases slightly. The flange thickness is reduced, the flange weight is significantly reduced, the flange cost can be significantly reduced, and the stress is reasonable.
In the process of pressure vessel design and verification of several projects, the author found that some large design institutes and engineering coMPanies do not have straight edge segments in flange design, especially for long weld neck flanges with small diameter and low pressure. According to the provisions of GB/T 150.3 and JB4732 standards, long weld neck flanges can not have straight edge segments on the premise of ensuring the slope of flange cone neck section of 2:3, which is difficult to calculate The effective thickness of the small end of the flange neck is the thickness of the butt cylinder. The author also understands that the main purpose of the design institute’s design is to save flange materials, which is particularly important for the pressure vessel manufacturer. If the flange structure can be optimized, the flange cost can be significantly reduced. Moreover, in the actual production and manufacturing process, the groove type of the flange is often determined by the welding process , if there is no straight edge section, the thickness of the small end of the flange neck can be the same as that of the butt cylinder. In this way, whether the outer slope or the inner groove is adopted, it can ensure the inner flush during assembly, which is conducive to welding and does not need thinning treatment, which greatly improves the production efficiency.
Flange optimization design is a complex and tedious process, and different designers often have different calculation results, but flange design must follow the principle of full stress, and give full play to the strength performance of flange materials through full stress optimization design. Through the above two examples, the author analyzes the three main stress values of flange, and adjusts the cone neck size and flange thickness respectively The design results that each stress value is close to the full stress are obtained. This optimal design makes the flange coMPact, reasonable stress and light weight. Therefore, the optimal design of the flange has obvious economic benefits.