Straightening Process of Longitudinal Welded Pipes

The straightening of longitudinal welded pipes is a crucial step in their production. This process is especially critical for high-quality pipes that meet API standards, such as petroleum casings, oil and gas pipes, and pipes for specialized machinery. These types of pipes not only have strict requirements for the steel grade and weld quality but also demand high precision in straightness. Any deviation in straightness can directly impact the threading and coupling of pipe ends, as well as cause twisting and deformation during pipeline use.

Causes of Pipe Bending

Various factors can lead to the bending of steel pipes. These include the thermal effects during welding, eccentricity during forming, and imbalances in compressive and bending forces. Fundamentally, bending occurs due to internal stress within the pipe. Simply put, bending results from uneven stress distribution. However, even a straight pipe retains some internal stress, albeit to a lesser degree than a bent pipe.

Understanding Internal Stress

Internal stress arises from molecular interactions during deformation caused by temperature and external forces. During the forming and welding of steel pipes, welding temperatures and bending forces contribute to the development of internal stress. A pipe's cross-section is annular, which generates two basic types of stress: forces parallel to the annulus and forces perpendicular to it. Parallel stresses can cause the pipe to become out-of-round, while perpendicular stresses lead to bending. Therefore, a cold expansion process is employed in the production of longitudinal welded pipes to eliminate internal stress and enhance the pipe's strength.

The Straightening Procedure

The straightening process involves feeding the longitudinal welded pipes from the left (or right) end of the machine onto the lower roller of the straightener. The upper roller descends to press the pipe and stops at the appropriate position. The upper and lower rollers are set at a specific angle relative to the pipe's axis. The hyperbolic generating line of the rollers matches the pipe's outer diameter, creating an enveloping effect.

Three upper rollers press down on the longitudinal welded pipes under the action of their respective hydraulic cylinders, while two lower rollers are driven by their hydraulic motors. This setup causes the pipe to rotate around its axis and move axially. By changing the rotational direction of the hydraulic motors, the rotation direction and axial movement of the pipe can be reversed, allowing for bidirectional straightening. Once straightened, the pipe is conveyed through the discharge device at the other end to a material rack. For pipes with significant local bends, local straightening is first performed using punches at both ends before the entire pipe is straightened.