The influence of local thinning under internal pressure and bending moment on the ultimate bearing capacity of elbows and its solutions
In piping systems, elbows are fittings that change the direction of the pipeline. According to the angle, there are three most commonly used types: 45 °, 90 °, and 180 °. In addition, other abnormal angle elbows such as 60 ° are also included according to engineering needs. The materials for elbows include cast iron, stainless steel, alloy steel, malleable cast iron, carbon steel, non-ferrous metals, and plastics.
The methods of connecting with pipes include direct welding (the most commonly used method) flange connection, hot melt connection, electric melt connection, threaded connection, and socket connection. According to the production process, it can be divided into welded elbows, stamped elbows, push made elbows, cast elbows, welded elbows, etc. Other names: 90 degree bend, right angle bend, etc
Elbow is a commonly used connecting pipe fitting in plumbing installation, used for connecting pipes at bends and changing the direction of pipes.
Other names: 90 ° elbow, right angle elbow, Love bend, stamped elbow, pressed elbow, mechanism elbow, welded elbow, etc.
Purpose: To connect two pipes with the same or different nominal diameters, allowing the pipeline to make turns of 90 °, 45 °, 180 °, and various degrees.
A bending radius less than or equal to 1.5 times the pipe diameter is considered an elbow, while a radius greater than 1.5 times the pipe diameter is considered a bend
A commonly used connecting pipe fitting in pipeline installation, which connects two pipes with the same or different nominal diameters to make a certain angle turn in the pipeline, with a nominal pressure of 1-1.6 Mpa.
Local thinning is a common defect in the production of elbows, but research on this type of defect mainly focuses on straight pipes at home and abroad, and there are few literature reports on local thinning of elbows. Tianjin Baode Iron and Steel Company conducted detailed finite element calculations and theoretical analysis to study the influence of local thinning under internal pressure and bending moment on the ultimate bearing capacity of elbows, as well as the mutual interference effect of multiple local thinning under internal pressure and the strengthening effect of straight pipes on the ultimate load of elbows under bending moment. Partial experimental verification was carried out, and the following research results were obtained:
1. The finite element method was used to systematically analyze and calculate the ultimate load of locally thinned elbows under internal pressure. It was found that the ultimate pressure of locally thinned elbows is different from that of locally thinned straight pipes. The ultimate pressure of elbows depends not only on the size of local thinning, but also on the location and bending radius of local thinning. If the calculation method of locally thinned straight pipes is used to evaluate elbows, it will result in unsafe or overly conservative results; The influence of thinning width on the ultimate load cannot be ignored. On the basis of finite element analysis, a calculation formula for the ultimate pressure of locally thinned elbows is provided. The calculated results of the formula are consistent with the finite element calculation and experimental results, and are relatively safe. The calculation formula can be practically applied to the safety assessment of locally thinned elbows, filling the gap in this research.
2. Through finite element analysis, the mutual interference effect between multiple local thinning under internal pressure was studied. The study showed that the mutual influence of multiple local thinning is not only related to the spacing, but also to the depth of thinning. When the thinning depth is shallow, the axial local thinning spacing is greater than twice the wall thickness, and the ultimate load of double local thinning is basically the same as that of single local thinning; When the thinning depth is deep and the axial local thinning spacing is greater than 4 times the wall thickness, the ultimate load of double local thinning is basically the same as that of single local thinning, which supplements the shortcomings of existing research.
3. Through finite element calculations, the strengthening effect of connected straight pipes on the ultimate bending moment of elbows was studied. It was pointed out that connecting straight pipes to elbows would increase the ultimate bending moment of elbows, and the increase in ultimate load of elbows would vary with different bending radii. When the length of the connected straight pipe is greater than 3 times the pipe diameter, the strengthening effect of the straight pipe on the elbow no longer increases. This study supplements the insufficient research on the strengthening effect of straight pipes on elbows.
4. The influence of local thinning on the ultimate bending moment of elbows was studied in detail through finite element analysis, and it was found that the magnitude of the ultimate bending moment of locally thinned elbows under in-plane bending moment is related to the thinning position, thinning size, and bending radius. Research has shown that the influence of geometric nonlinearity is significant under the action of bending moment. Based on the finite element analysis of local thinning and large deformation of the inner wall, a calculation formula for the ultimate bending moment of the locally thinned elbow under the action of in-plane bending moment is given. The calculation results can accurately and conservatively reflect the finite element calculation results, and are consistent with the experimental results. This study fills a gap in this field.
