A long radius (LR) elbow has a centre-line radius of 1.5 times the nominal pipe size. A short radius (SR) elbow has a radius of 1.0 times the nominal pipe size, so it turns the line in a tighter space.
Long radius is the default choice. The gentler bend means lower pressure drop, less turbulence and less erosion, which matters a great deal in slurry, catalyst and other abrasive service.
Short radius is used where space is genuinely tight — congested pipe racks, skid-mounted packages and equipment nozzles. The trade-off is higher pressure drop and faster wall thinning at the extrados of the bend.
Whichever you select, the fitting wall thickness must match the schedule of the connecting pipe so the butt weld has a matching bore and no internal step.
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