Stress analysis of 45° forged lateral tee

Stress analysis of 45° forged lateral tee

The 45° forged lateral tee is commonly used in the convergence of superheated steam and reheat steam piping, and the stress situation is complicated. The stress situation of the 45° forged lateral tee of the reheat steam pipe of a 660MW supercritical boiler is calculated by the finite element method, and the design of the 45° lateral tee is optimized for comparison.

The main steam and reheat steam of large-capacity, high-parameter boilers are mostly led out bilaterally, and it is necessary to utilize the tee to converge the steam conduits to one place. 45° lateral tee is used in increasing numbers due to its advantages, such as low resistance. At present, the lateral tee is mostly a 45° forged lateral tee, which has a special shape and complex stress situation. According to the current standards, there is no clear formula for its strength calculation, and it is necessary to use the finite element method for stress analysis to check its strength.

This paper takes a 660MW supercritical boiler reheat steam pipe 45° forged lateral tee as an example, adopts the finite unit method, establishes the relevant calculation model by using ANSYS software to carry out the stress analysis, determines the weak position of the 45° forged lateral tee’s stress, and optimizes the structure of the tee according to the calculation results.

1. Research content

1.1 Basic parameters

The material of the reheated steam pipe lateral tee is SA-182 F92, the design temperature is 618℃, design pressure is 7.2MPa. According to ASME-related data, the permissible stress of SA-182 F92 under design temperature is S=62.2MPa, and the elasticity modulus is E=165.5GPa. According to the design requirements of the flow rate, the main pipe’s inner diameter is 829mm, and the branch pipe’s inner diameter is 581mm. The inner diameter of the branch pipe is 581mm.

1.2 Preliminary design program

According to the above design pressure and other parameters to calculate the lateral tee wall thickness and consider the margin, the preliminary design of lateral tee main pipe specification ID829 × 90mm and branch pipe specification ID581 × 85mm. The main pipe length of 1800mm, branch pipe length of 1400mm. tee coherent line inside chamfer R = 20mm, outside coherent line chamfer from R = 85mm to R = 350mm. Transition to R=350mm. Design drawings are shown in Figure 1.

Figure.1 Preliminary design drawing of lateral tee

According to Fig.1, three-dimensional modeling software INVENTOR is used to establish a three-dimensional model; tee is a symmetric structure, one-half of the model is used in the calculation, and the three-dimensional model is shown in Fig.2.

Figure.2 Preliminary design of three-dimensional model of lateral tee

The three-dimensional model is imported into ANSYS software for pre-processing, and 8NODE SOLID 185 cells are used for calculation. The model is meshed, the internal pressure and the axial tensile stress at the pipe mouth generated by the internal pressure are applied, and the symmetry constraint is added to the symmetry plane and calculated; the calculated stress cloud is shown in Figure 3.

Figure.3 Preliminary design of lateral tee stress map

According to the relevant standards on the finite unit method calculation of stress classification requirements and stress control principles, and combined with Figure 3 stress map, the results of the different regions of the stress classification, linearization and determination. The lateral tee general location (except for the intersection of the main pipe and branch pipe location) belongs to the primary stress area. Selected a general location for linearization; the stress values are listed in Table 1 for the overall film and “film + bending” stress. The lateral tee stress highest point is located in the branch pipe and the main pipe coherent line of the acute angle side, with a peak stress value of 351.9MPa. This is a structural discontinuity and belongs to the secondary stress zone. The linearization of this area along the tee wall thickness direction, the same linearized stress values are listed in Table 1 for the local film and “film + bending” stress.

According to Table 1 of the linearization results of the calculated value and the standard permitted value of the comparison, it can be seen that the general location of the tee of the overall film stress and “film + bending” stress are qualified, and there is a certain amount of margin; and the tee of the main pipe and the branch pipe coherent acute angle part of the more serious stress concentration, “film plus bending” stress does not meet the specification requirements, need to optimize and improve.

Table.1 Stress linearization results of the preliminary design of lateral tee

1.3 Optimization Scheme 1

Optimization option 1 is to thicken the wall thickness of the main and branch pipes of the tee. The optimized main pipe specification is ID829×120mm, and the branch pipe specification is ID581×115mm; the rest of the parameters are the same as the preliminary design scheme.

Fig.4 Stress cloud of lateral tee in optimized scheme 1

According to the above information, to establish the calculation model and import it into ANSYS to divide the grid, apply constraints, load and calculate, the calculated stress cloud diagram is shown in Figure 4. According to the stress cloud diagram, it can be seen that after the increase of the main pipe and the branch pipe wall thickness, the highest stress value of the secondary stress zone of the coherent line has decreased, and the peak stress is reduced to 275.4MPa.

The primary and secondary stress zones are linearized separately, and the results are listed in Table 2. From the results in Table 2, it can be seen that the use of increased wall thickness reduces the stress in the secondary stress zone, and this design meets the code requirements. However, this design method leads to the tee wall thickness being thicker; the weight is large, limited by factors such as equipment production capacity, larger weight forgings material and higher manufacturing costs are higher.

Table.2 Linearization evaluation results in optimization scheme 1

1.4 Optimization Scheme 2

Due to the high-stress area being concentrated in the inner diameter of the coherent line formed by the chamfer part, increasing the entire tee wall thickness method to make up for the local is not economical enough in the appropriate increase in the tee wall thickness at the same time; the tee will be the inner diameter of the coherent line chamfer to increase to R = 50mm, OD coherent line locally thickened, the formation of the transition of the shoulder platform more rounded and smoother. Optimization scheme 2 of the main pipe specification ID829 × 100mm, branch pipe specification ID581 × 95mm, see Figure 5; the optimized tee three-dimensional model is shown in Figure 6.

Fig.5 Design drawing of lateral tee in optimized scheme 3

Fig.6 Model of tee in optimized scheme 2

Fig.7 Stress cloud of lateral tee in optimized scheme 2

The 3D model of optimization scheme 2 is imported into ANSYS software, the mesh is divided and loaded for calculation, and the stress cloud is shown in Fig.7.

The primary stress area and secondary stress area of the tee are linearized, respectively, and the results are listed in Table 3. It can be seen that by modifying the inner and outer coherent chamfer, the linearized stress value of the secondary stress area can be effectively reduced so that the lateral tee meets the relevant standard requirements. Compared with scheme 1, the wall thickness of the main pipe decreases from 120mm to 100mm, and the wall thickness of the branch pipe decreases from 115mm to 95mm. The overall weight of the lateral tee has a relatively large reduction, which saves the amount of metal and improves the economy of the forged tee.

Table.3 Linearization evaluation results in optimization scheme 2

1.5 Optimization Scheme 3

Further, the branch pipe is changed to a tapered pipe, tapered branch pipe lateral tee specifications for the main pipe specifications ID829 × 90mm, branch pipe specifications ID581 × 85mm; the branch pipe taper is 5 °. So that the branch pipe is closer to the connection part, the branch pipe wall thickness is greater, the better the reinforcing effect. The acute angle part of the outer diameter coherent line is a shoulder platform with a rounded transition, and the chamfer of the inner diameter coherent line is R=50mm. The design drawing of this type of tee is shown in Fig.8, and the corresponding three-dimensional model is shown in Fig.9.

Fig.8 Design drawing of lateral tee in optimization scheme 3

Fig.9 3D model of lateral tee for a tapered branch pipe in optimization scheme 3

The three-dimensional model of optimization scheme 3 is imported into ANSYS for calculation. The calculated stress cloud diagram is shown in Fig.10. The overall wall thickness of the conical branch lateral tee is smaller than that of the lateral tee designed in scheme 2. Still, the peak stress in the stress concentration area is 272.8MPa, which is smaller than that of 292.0MPa in scheme 2, which shows that the reinforcing effect of conical branch lateral tee is better than that of tee in the main tube and the branch tube. It can be seen that the reinforcing effect of the conical branch pipe on the main pipe and branch pipe is better than the overall thickening of the tee.

The primary and secondary stress areas of the tee are linearized, and the results are listed in Table 4. According to the linearization results in Table 4, the local and overall stresses of the lateral tee scheme with a tapered branch pipe meet the specification requirements. With the tapered branch pipe, the wall thickness of the main and branch pipe of the tee can be decreased to 90mm for the main pipe and 85mm for the branch pipe opening, which further reduces the overall weight of the tee.

Fig.10 Stress cloud of lateral tee in optimization scheme 3

Table.4 Linearization evaluation results in optimization scheme 3

2. Conclusion

• (1) The main pipe specification ID829×90mm and branch pipe specification ID581×85mm of lateral tee in the original design. After calculation, the stress concentration occurs in the acute angle part of the main pipe and branch pipe coherent line of the tee, and the value of the secondary stress in this place is higher, which doesn’t conform to the specification requirements and needs to be reinforced.
• (2) Optimize the main pipe specification ID829×120mm and branch pipe specification ID581×115mm of lateral tee in scheme 1. This scheme meets the specification requirements. It can be seen that the overall strengthening of the main pipe and branch pipe wall thickness can play a reinforcing effect, but the tee weight is heavy, and not economical.
• (3) Optimize the main pipe specification ID829×100mm and branch pipe specification ID581×95mm of lateral tee in Scheme 2, and at the same time, appropriately increase the R-value of the coherent chamfer of the inner wall of the branch pipe and the main pipe (R=50mm), as well as thickening the acute part of the coherent line of the outer wall and designing it as a rounded and smooth transition of the shoulder. The design can effectively reduce the stress concentration effect, tee all parts of the strength to meet the relevant specification requirements while reducing the weight of the tee.
• (4) Optimization scheme 3 in the use of tapered branch pipe, specifications for the main pipe ID829 × 90mm, branch pipe ID581 × 85mm, branch pipe taper 5 °. This design can further strengthen the intersection area of the main pipe and branch pipe while further reducing the overall weight of the tee, which can take into account the strength and economy and is a more reasonable design scheme.

Author: Wang Hongsheng