Discussion on design of long neck flange of vessel

Long weld neck flange is the most commonly used type of equipment flange. Its reasonable design can not only improve the safety performance of equipment, but also reduce the manufacturing cost. In this paper, the influence of flange thickness and cone neck height on various stresses of flange is studied through example calculation, and some suggestions for optimal design of long neck flange are given, so as to make full use of various properties of materials and improve the economy of flange design.

0. Introduction

In the design of pressure vessel, flange is an important part of pressure vessel, and its reasonable design is a key link. Generally, we select standard flanges according to design parameters and flange service conditions.
There are many factors affecting flange design, such as bolt material, number, diameter of distribution circle, gasket material, size, thickness, flange thickness and cone neck size. Among many influencing factors, flange thickness and cone neck size are the key to whether the flange design is reasonable. Therefore, this paper takes the control of various stresses in the long neck flange (axial stress, radial stress, circumferential stress) as the design basis, and discusses the influence of flange thickness and cone neck height on the flange through example calculation, in order to optimize the design of the long neck flange.

The structural diagram of long neck flange is shown in Figure 1.

Figure.1 Structural diagram of long neck flange

1. Influence of structure size on flange stress

Now we will illustrate the influence of the main structural dimensions of the flange on the various stresses of the flange through an example calculation.
Design parameters: nominal diameter DN1000, design pressure 2.2MPa, design temperature 200 ℃, corrosion allowance 3mm; Flange parameters: long neck butt welding flange, metal material: 16Mn forging; Cylinder parameters: material Q345R, thickness 14mm; Fastener and gasket: stud nut 35CrMoA/30CrMoA, gasket is spiral wound gasket.
The basic dimensions of flange shall be determined according to FM1000-2.5 standard flange in NB/T47023-2012.

1.1 Influence of flange thickness on stress

In order to discuss the influence of flange thickness on various stresses of flange, it is assumed that other dimensions of flange remain unchanged and only the thickness change is considered. The influence of flange thickness on stress is shown in Figure 2.

Figure.2. Effect of thickness on flange stress
It can be seen from Figure 2 that with the increase of flange thickness, the axial stress and radial stress decrease significantly, while the circumferential stress increases slightly. When the flange thickness increases from 68mm to 82mm, the axial stress decreases by 17.59%, the radial stress decreases by 38.93%, and the circumferential stress increases by 4.55%.

1.2 Effect of cone neck height on stress

Similarly, in order to discuss the influence of flange cone neck height on various stresses of flange, it is assumed that other dimensions of flange remain unchanged, and only the change of its cone neck height is considered. The influence of cone neck height on stress is shown in Figure 3.

Figure.3. Effect of cone neck height on flange stress
It can be seen from Figure 3 that with the increase of flange cone neck height, the axial stress and circumferential stress decrease, while the radial stress increases slightly. When the cone neck height increases from 42 mm to 56 mm, the axial stress decreases by 22.66%, the circumferential stress decreases by 10.63%, and the radial stress increases by 2.29%.
From the above analysis, it can be seen that the flange thickness and cone neck height have an impact on the various stresses of the flange, and this impact is interrelated. Therefore, in the actual design process, it is necessary to comprehensively consider the flange thickness, cone neck height and other structural dimensions so that the material can give full play to various strength properties to achieve optimal design.

2. Discussion on flange optimization design

According to the above analysis, the flange thickness has the greatest impact on the radial stress, and the flange cone neck height has the greatest impact on the axial stress. Therefore, when the flange strength check is unqualified, we should analyze the reason of unqualified flange strength according to the calculation results to optimize the design.
If the radial stress is unacceptable, the flange thickness needs to be increased, and the cone neck height remains unchanged or decreases slightly; If the axial stress is unqualified, it is preferred to increase the cone neck height, and then increase the flange thickness; If the circumferential stress is unacceptable, the cone neck height shall be increased, and the flange thickness shall be unchanged or slightly reduced.
Figure 4 shows the flange design results that can meet the strength requirements under the same design conditions.

It can be seen from Figure 4 that the thickness of flanges from 1 # to 6 # gradually decreases, and the height of the cone neck gradually increases. For 1 # flange with large thickness and small cone neck, which is similar to plate flange, the flange is prone to circumferential deflection, so the circumferential stress is large and the radial stress is small; 6 # flange has large cone neck and small thickness, which is easy to produce radial bending, large radial stress and small circumferential stress; The structure proportion of 3 # and 4 # flanges is relatively reasonable. The radial stress and circumferential stress are basically the same. The material can exert its strength performance in all directions.

Figure.4 Flange design results under the same design conditions

3. Conclusion

The change of flange thickness can significantly affect the radial stress, followed by the axial stress, and has little effect on the circumferential stress; The change of flange cone neck height can significantly affect the axial stress, followed by the circumferential stress, and has little impact on the radial stress.
Considering the flange thickness and cone neck height reasonably, the flange can give full play to its various strength properties, and the flange tends to full stress state, which is conducive to making the flange obtain smaller volume and weight, and improving the economy of the flange.
The optimal design of flange is a complex problem. This paper only discusses the influence of flange thickness and cone neck height, and the influence of other factors needs to be further discussed.
Author: Li Qian
Source: China Long Neck Flange Manufacturer – Yaang Pipe Industry (www.epowermetals.com)