Seamless steel pipes, straight-seam welded steel pipes, forged steel pipes, precision steel pipes, and high-precision steel pipes, among other steel pipes, all exhibit varying degrees of curvature in the longitudinal direction (i.e., lengthwise) during the production process. While some finished pipes exhibit a degree of curvature that can be visually detected, these products are considered substandard and will not be permitted to leave the factory by any reputable manufacturer. Others require specialized instrumentation to measure their straightness. This degree of curvature is expressed numerically as the pipe’s curvature or straightness.
For general fluid transport applications, the standard for straightness is not stringent. The primary requirement for this type of pipe is concentricity at the pipe ends to facilitate welding. However, for pipes used in processing machinery, particularly rubber rollers and center shafts, the requirements for straightness or curvature become higher, and this becomes a crucial performance indicator.
This involves a significant amount of processing, which directly impacts production costs. If the process fails, the pipe will be rendered useless. Therefore, manufacturers like those producing roller equipment pay particular attention to straightness testing when purchasing seamless or straight-seam steel pipes.
Of the three types of seamless, straight-seam, and forged steel pipes, forged steel pipes offer the best straightness. Because their outer diameter is machined incrementally using machining equipment, straightness is generally impeccable. Seamless steel pipes fare second best. The production process for these pipes typically includes a three-roll straightening step (for higher precision requirements, there are 6-roll, 9-roll, and 12-roll straightening machines, and for pipes requiring extremely high straightness, imported computer-controlled multi-function hydraulic straightening machines are used), which can generally meet customer straightness requirements. The worst performance is straight-seam steel pipe. While this step is also part of the basic process, these pipes are typically rolled from steel plates or coils. If internal stress within the steel isn’t properly released, it can cause deformation after processing.
There are generally two ways to calculate the straightness, or curvature, of a steel pipe:
First, the local curvature of a seamless or straight-seam steel pipe: This is done by using a one-meter-long ruler to measure the chord height (in millimeters) at the maximum bend of the pipe. This is the local curvature value, expressed in mm/m, and expressed as 2.5 mm/m. This method also applies to the curvature of the pipe ends.
Second, the total curvature of the pipe along its entire length: Using a thin string, pull it taut from both ends of the pipe. Measure the maximum chord height (in millimeters) at the bend, then convert it into a percentage of the pipe’s length (in meters). This is the total curvature along the pipe’s entire length.
For example, if a pipe is 10 meters long and the maximum chord height is 30 mm, the total curvature of the pipe along its entire length should be: 0.03 ÷ 10m × 100% = 0.3%
Post time: Sep-17-2025