Bending forming of pipes in pressure vessels

Thread Content

Methods of bending pipes: There are various methods for bending pipes; generally, they can be categorized into manual bending and mechanical bending. There are also various methods for mechanical tube bending, such as press bending, roll bending, reverse bending, and extrusion bending. Regardless of the bending method used, the main challenge throughout the bending process is how to overcome local deformation. The most widely used bending methods in engineering are manual bending and bend-back bending techniques. The return bending method involves bending pipes on a rotary pipe bender; it can be further divided into mold-assisted bending and moldless bending. Manual pipe bending: Manual pipe bending does not require specialized equipment or complex processing tools; it is capable of bending pipes with various radii, angles, and spatial orientations. However, this bending method requires high labor intensity, has low productivity, and results in unstable quality. Manual bending of steel pipes mostly involves hot bending; for stainless steel and non-ferrous metals, cold bending is preferred. Before bending the pipe, a filler is first inserted into its interior; for steel pipes, the filler is usually pure, dry fine sand. For stainless steel and non-ferrous metal pipes, low-melting-point substances such as rosin and lead are suitable as fillers, in order to prevent wrinkles and reduce ovality. There are die-bending machines; all types of such bending machines use dies for bending. The main molds are of two types: disc-shaped sheaves and horn-core heads. When bending pipes using a disc-shaped grooved pulley, the bending die is located on the outside of the pipe. Half of the bent portion lies within the groove, while the other half of the pipe in the bent area is pressed down by a small grooved roller (also known as a pressure roller). The end of the pipe is fixed to a disc-shaped bending die using a chuck; if the pressing roller does not move, the disc-shaped bending die rotates on its own to complete the bending of the pipe, which is known as pull-bending ； If the tube is pushed, causing the disc-shaped bending die to rotate accordingly in order to bend the tube, this is called push-bending ； If the disc-shaped bending die remains stationary and the compression roller presses the tube to rotate around it in order to achieve bending, this method is known as compression bending. When bending tubes using a ram’s horn-shaped mandrel, the mandrel is located on the inside of the tube; the ram’s horn shape resembles that of a ram’s horn, with an axis length equal to 1/4 of the circumference. The bending radius is the same as that of the tube being bent, and the maximum bending angle is 180°. The thin end of the horn-shaped core is slightly thinner than the inner diameter of the tube blank; the tube blank is slipped over this thin end. The thick end serves as the outlet and is slightly thicker than the inner diameter of the tube blank. When bending pipes, the pipe billet is first heated and then inserted into the mold; under thrust, the pipe undergoes both bending and expansion processes, and it takes on its bent shape once it slides out of the end of the mandrel. Dieless bending: In this method of bending tubes, no special bending die designed for a specific bending radius is required. It can be divided into two types: pull-bending and push-bending. The pipe bender has a rotating arm whose length can be adjusted. During bending, the pipe is secured to the rotating arm; the distance from the pipe’s central axis to the rotation center of the arm is the bend radius. During the process, a copper induction coil is placed around the pipe. The coil is supplied with medium-frequency (for thicker pipes) or high-frequency (for thinner pipes) electricity, which locally heats the pipe to 900–950°C; the pipe is then bent. If the rotating arm is used to actively bend the tube back into shape, it is called stretch bending ； When the end of the tube is subjected to a thrust force, the tube pushes the rotating arm, thereby causing the tube to bend – this process is known as push-bending. Immediately thereafter, water is sprayed from a ring of small holes on the inner side of the induction coil onto the already bent pipe section to cool it rapidly. The purpose of spraying water is to keep the heating section and the deformation zone within a very small area, thereby preventing the tube from wrinkling or getting flattened.