What are the Bends?

In piping systems, elbows are fittings that change the direction of the piping. According to the angle, there are three most commonly used angles of 45 ° and 90 ° and 180 °. In addition, according to the needs of the project, it includes other abnormal angle elbows such as 60 °. Elbow materials are cast iron, stainless steel, alloy steel, malleable cast iron, carbon steel, non-ferrous metals and plastics.

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Elbow is a kind of connection fittings commonly used in plumbing installations. It is used to connect the bends of pipes to change the direction of the pipes.

Other names: 90 ° elbow, right-angle bend, love and bend,

1. due to

Elbow

Reducing elbow

Elbow with stand base elbow

Because the elbow has good comprehensive performance, it is widely used in chemical engineering, construction, water supply, drainage, petroleum, light and heavy industry, refrigeration, sanitation, plumbing, fire protection, power, aerospace, shipbuilding and other basic projects.

It is required to control the radius of curvature. If the radius length is 1.5D, then the radius of curvature must be within the required tolerance range. Since most of these pipe fittings are used for welding, in order to improve the welding quality, the ends are turned into bevels, leaving a certain angle and a certain edge. This is also a strict requirement, how thick the side is, how much the angle is and the deviation range. There are regulations, and there are many more items in geometrical dimensions than pipe fittings. The surface quality and mechanical properties of the elbow are basically the same as those of the pipe. For welding convenience, the material of the steel to be connected to the pipe must be the same.

Stainless steel elbow

1,

1. The austenitic stainless steel matrix is mainly austenite structure ( phase) with face-centered cubic structure, non-magnetic, stainless steel that is mainly strengthened by cold working (and may cause certain magnetic properties).

2. Austenitic-ferritic (dual-phase) stainless steel matrix has both austenite and ferrite two-phase structure (the content of less phases is generally greater than 15%), it is magnetic and can be strengthened by cold working. .

3. The ferritic stainless steel matrix is mainly composed of a body-centered cubic crystal structure of a ferrite structure ( phase), which is magnetic and generally cannot be hardened by heat treatment, but cold-working stainless steel can be slightly strengthened.

4. The matrix of martensitic stainless steel is a martensitic structure, which is magnetic, and its mechanical properties can be adjusted by heat treatment.

5. Precipitation hardening stainless steel is an austenite or martensite structure, and can be hardened (strengthened) by precipitation hardening (also known as aging hardening).

6. Stainless steel 1Cr18Ni9Ti 0Cr18Ni9 00Cr19Ni10 0Cr17Ni12Mo2Ti 00Cr17Ni14Mo2 304 304L 316 316L etc. 3. Stainless steel elbow has certain corrosion resistance (oxidizing acid, organic acid, cavitation), heat resistance and wear resistance. Usually used for power station, chemical, petroleum and other equipment materials. The weldability of stainless steel elbow is poor. Pay attention to the welding process, heat treatment conditions and use of suitable welding electrodes.

Local thinning is a common defect in elbows. However, researches on this kind of defects at home and abroad mainly focus on straight pipes, and few studies on the local thinning of elbows have been reported in the literature. In this paper, through detailed finite element calculations and theoretical analysis, the effects of local thinning on the ultimate load carrying capacity of the elbow under internal pressure and bending moment are studied, as well as the mutual interference effects and bending moment effects of multiple local thinning under internal pressure. The strengthening effect of the lower straight pipe on the ultimate load of the elbow, and some experimental verifications have been performed, and the following research results have been obtained:

1. The finite element method was used to systematically analyze and calculate the limit load of the locally thinned elbow under the internal pressure. It was found that the limit pressure of the locally thinned elbow is different from that of a straight pipe with locally thinned. The pressure depends not only on the size of the local thinning, but also on the location of the local thinning and the bending radius. If the elbow is evaluated using the calculation method of the local thinning straight pipe, an unsafe or too conservative result will be obtained; at the same time, the width of the thinning will be reduced. The effect on the ultimate load cannot be ignored. Based on the finite element analysis, a formula for calculating the ultimate pressure of a partially thinned elbow is given. The calculation result of the formula is quite consistent with the finite element calculation and experimental results. It is safe to use. The calculation formula can be actually applied to the partially thinned elbow. Safety assessment complements the gap in this study.

2. Through the finite element analysis, the effect of mutual interference between multiple local thinnings under internal pressure is studied. The research shows that the mutual effects of multiple local thinnings are not only related to the distance, but also to the depth of thinning. It is pointed out that when the depth of the thinning is shallow, the axial partial thinning interval is greater than 2 times the wall thickness. The limit load of the double partial thinning is basically the same as the limit load of the single partial thinning. When the thinning depth is deep, the axial local thinning is When the distance is more than 4 times the wall thickness, the limit load of dual partial thinning is basically the same as the limit load of single local thinning, which supplements the shortcomings of the existing research.

3. Through the finite element calculation, the reinforcing effect of the connected straight pipe on the ultimate bending moment of the elbow is studied. It is pointed out that the straight pipe connected to the elbow will increase the ultimate bending moment of the elbow. When the bending radius is different, the ultimate load of the elbow The increase is different. When the length of the connected straight pipe is more than 3 times the pipe diameter, the strengthening effect of the straight pipe on the elbow will no longer increase. This study complements the lack of research on the strengthening effect of straight pipes on elbows.

4. The effect of local thinning on the ultimate bending moment of the elbow was studied in detail through finite element analysis, and the magnitude of the ultimate bending moment of the locally thinned elbow under the action of the in-plane bending moment was related to the thinning position, thinning size and bending radius. . Studies have shown that the effect of geometric nonlinearity is significant under the action of bending moments. Based on the finite element analysis of local thinning and large deformation of the inner wall, the calculation formula of the ultimate bending moment of the locally thinning elbow under the in-plane bending moment is given. The calculation result can reflect the finite element calculation result more accurately and conservatively. And in line with the experimental results. The study fills a gap in this area.