Elbows are split into 2 groups which define the distance over which they change direction; the centerline of 1 end to the other opposite face. This is also known as the “center to face” distance and is similar to the radius through which the elbow is bent.
The center to face distance for a “short” radius elbow, abbreviated SR even is to nominal pipe size, while the center to face distance for a “long” radius elbow, abbreviated Long Radius always is “1½ x Nominal Pipe Size (NPS) (1½D)”.
An elbow is related among 2 lengths of pipe or tubing to allow a variation of way, usually a 45°or 90° angle; 22.5° elbows are also available. The radius of curvature of a short-radius (SR) elbow has a radius equal to the pipe diameter, but a long-radius (LR) elbow is 1.5 times the pipe diameter.
The 90° elbow is utilized in spaces that need a major turn. Whereas this might be a necessity, the flow rate and pressure within the pipe is precious. Just as with any piping system, the material under pressure hits the elbow and would like to continue in an equal direction. When it meets the resistance of the elbow wall, it is required to change direction (deflected) at a cost to the pressure.