Difference between pipe Bend and Elbow

Bends and its details

1. Pipe Elbow is a standard fitting but bends are custom fabricated.
2. Bends are generally made or fabricated as per the requirement; however elbows are pre-fabricated and standard.  
3. The major difference is the radius of curvature.
4.  Elbows generally have radius of curvature between one to twice the diameter of the pipe.
5. Bends have a radius of curvature more than twice the diameter. 
6. Pipe bends are generally made or fabricated as per the need of the piping; however elbows are pre fabricated and standard, and are available off the shelf.
   
   
7.  Elbows and bends Code requirements
   The code pressure design requirements for elbows and bends that are not manufactured
   in accordance with the listed standards are as follows:
   
   
   (1) Bends can be made by either a hot or cold process, provided the material is suitable for the bending process (332.1).
   (2) The finished surface shall be free of cracks and substantially free from buckling.
   
   
   (3) The wall thickness after bending shall not be less than the minimum wall, considering corrosion/erosion and mill under-run tolerance.
   
   
   (4) In bend flattening, the difference between maximum and minimum diameters at any cross section shall not exceed 8% of nominal outside diameter for internal pressure and 3% for external pressure (332.2.1).
   
   
   The major concern with respect to an elbow in external pressure service is the potential
   for a structural collapse of the elbow. 
   
   
   Elbows in external pressure service more than
   3% out of round may be well on the way to structural collapse. 
   
   
   Elbows out of round as
   much as 8% in internal pressure service are permitted because the pressure will
   straighten the out of roundness to values less than 8%.
   The degree to which an elbow wall might be reduced on the extrados by the manufacturing process.
   
   Figure 2.19 shows the nomenclature for bends in pipe.