3D elbows are commonly used in the curved parts of pipelines to change the direction of the pipeline. Commonly used are 45° elbows and 90° elbows. Elbows with a bending radius approximately 1.0 times the pipe's outer diameter are called short-radius elbows, while those approximately 1.5 times the pipe's outer diameter are called long-radius elbows. When arranging a piping system, it is generally advisable to use long-radius elbows for connections. Short-radius elbows are typically used in situations where the pipe system installation space is compact or to reduce costs. When using short-radius elbows, their maximum working pressure should generally not exceed 0.8 times that of similarly specified long-radius elbows. Oblique connection elbows are usually used on large-diameter pipelines under low-pressure conditions (design pressure ≤ 2.0 MPa) with water and similar fluid media where conditions are relatively mild. When the single-section change of direction angle of an oblique connection elbow exceeds 45°, it should not be used on pipelines carrying toxic, flammable media, or on pipelines subjected to vibration, pressure pulsation, or alternating loads due to temperature changes.
Source: Zhuohui 3D Elbow (http://www.hbzhuohui.com/wtgj/3dwt.html)