When one is using tubing rather than piping, bending is the NORMAL thing to do. The use of orbitally welded or union fitting elbows is permitted, but so is bending which is much easier.

In piping, you are also permitted to use bends in place of elbows. However, the radius of a bend you can make cold, without internal guide tooling, and still retain something close to the full diameter through the bend, is pretty large. The bent component has residual stress in it, which may be of concern in corrosive services.

Is bending cheaper than adding elbow fittings? That depends on the method of attachment. For threading? No. For socket welding? Possibly. For butt welding? Probably.

ASME B31.3 section 304.2.1 gives the requirements for checking wall thinning on bends.

Are you talking piping or pipelines?

Piping rarely uses bends ( classified as anything more than 1.5D radius) as they cost a lot more than forged elbows.

tubing such as instruments normally used bends

Pipelines always use bends, usualy of 3D or 5D bends made by heating a thicker pipe than normal then bending and allowing for bend thinning on the outer radius or 40D bends made by cold bending pipe.

tightest bend I've ever used cold bent was 10D but it was a special.

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Also: If you get a response it's polite to respond to it.

Bent pipe IS used, frequently, in solids and slurry handling piping, where the bend radii required are beyond the normal long radius elbow. That's pipe, cold bent, with compensation as noted for wall thinning. We have, in past, also bent pipe primarily to avoid welds and NDE at elbows.

You can buy standard pipe elbows in standard diameters (1.0 D short radius, and 1.5 D long radius) much cheaper and much faster than trying to roll your own.

But, buying a pre-fabricated pipe elbow outside of those two ranges is difficult, and so rolling a pipe is necessary, but very expensive when you absolutely need a different radius bend.

Thank you everyone very much for the replies.
I am a beginner in this field so I do not know exactly what I need to ask but I think I have got the answer that I was looking initially.
I currently work in piping used for Waste to Energy Power Plant. Most of the bends were elbows but few were 3D and 5D.

We have customers that use bends exclusively, They will pre-bend lengths of pipe and then in the field all welds are on nice straight sections. They claim that bending in the shop is less expensive than buying elbows and making two welds.

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P.E. Metallurgy, Plymouth Tube

Bends and elbows cannot be used interchangably in piping system design ....

One of the important differences commonly ignored between bends and elbows is the change in "stiffness" of a system.

When a detailed stress analysis is performed on similar piping systems, a system with "all bends" is much stiffer than one with "all elbows". In accordance with the rules of ASME B31.1 and B31.3, the flexibility factors are significantly different

This means that the the calculated reaction forces on the system terminations (anchor points) are much higher with the system of bends than the one with elbows.

For piping systems with reaction sensitive terminations (like one with centrifugal pumps) stay away from the bends.

IMHO .....

MJCronin
Sr. Process Engineer

"...When a detailed stress analysis is performed on similar piping systems, a system with "all bends" is much stiffer than one with "all elbows"..." I am surprised at that statement; I would have thought that 90 degrees short and long radius bends would have more flexibility than with 90 degrees elbows.

Elbows are better than bends, because bends induce more stress than elbows. In the majority of the times, bends require pre and post weld heat treatment which is not all ways the case, when we are dealing with elbows.