The function of the elbow is to change the direction or flow in the piping system. By default, there are 5 opportunities, 45°, 90°, and 180° elbows, three are in the "long radius" version, and the other 90° and 180° elbows are in the "short radius" version.
The elbows are divided into two groups, which define the distance they change direction; one end to the opposite centerline. This is called the "center-to-face" distance, which is equivalent to the radius of the elbow.
The center-to-face distance of the “long” radius elbow (abbreviated as LR) is always “1½x nominal pipe size (NPS) (1½D)”, while the center-to-face distance of the “short”radius elbow (abbreviated SR even) is nominal pipe size.
Here below, for example, you will find the center to face distance of NPS 2 elbows. And below you can find the 45 degree elbow center formula
(the A distance on the image)
1. 90°-LR : = 1½ x 2(NPS) x 25.4 A=76.2 mm
2. 180°-LR : = 2 times the 90° LR elbow A=152.4 mm
3. 90°-SR : = 2(NPS) x 25.4 A=50.8 mm
4. 180°-SR : = 2 times the 90° SR elbow A=101.6 mm
3D elbows as an example, are calculated with:
3(D) X 2(NPS) X 25.4
45° elbow and 90° elbow have the same function, but the size measurement is different from 90° elbow.
The radius of the 45° elbow is the same as the radius of the 90°LR (1½D). However, the center-to-face dimension is different from the radius of the 90°LR elbow. This is the intersection point from each face to the mutually perpendicular center lines, which is the distance B on the image. This is because the curvature is small. Short radius 45° elbows are not available.
90° long radius and 45° elbows are most used. If the space is too small, 90° short radius elbows are used. The function of the 180° elbow is to change the direction of water flow through 180°. The center-to-center dimensions of the LR and SR types are both twice that of the matched 90° elbow. These accessories are usually used in furniture or other heating or cooling devices.
In addition to the defined elbow, there are also reducing elbows, that is, elbows with different diameters at both ends. Because for many suppliers, this elbow is not a standard part, so the price may be high and the delivery time is longer. If the situation permits, you can choose to use the "normal" elbow with a separate reducer.
Elbows with other angles can be processed with standard elbows. Types with a longer radius, the center-to-face size is three times the nominal size (3D), and even available.
ASME B16.9 specifies the size and dimensional tolerances of long radius and short radius elbows.
Elbow wall thickness
The weakest part of the elbow is the inner radius. ASME B16.9 only standardizes center-to-face dimensions and some "squareness" dimensional tolerances. The wall thickness at the weld position is even standardized, but does not include the rest of the elbow. The standard stipulates that the minimum tolerance should be within 12.5% of the minimum order wall thickness of the pipe. The maximum tolerance is only specified at the end of the pipe.
Many butt welding elbows (and tees) suppliers provide a larger thickness table to maintain adequate wall thickness after forming.
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