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1.0 SCOPE
This specification covers machine 1½D and 3D pipe bends (made at ambient temperatures). Substituting pipe bends for butt welded elbows will provide piping with higher integrity, faster fabrication, and lower cost.

2.0 BEND RADIUS

2.1 General
Pipe bends are classified according to the centerline radius (CLR) of the bend as a ratio to the nominal pipe diameter. For example, 4" N.P.S. pipe which is bent on a 6" CLR is classified as a 1½D Bend (1½ times the nominal pipe diameter). When bent on a 12" CLR, the bend is classified as 3D.

2.2 1½D Bends
Most cold bends are made on a 1½D radius. The major reason for choosing 1½D is that it is interchangeable with long radius butt weld elbows (a 4" Long Radius 90°Elbow is 6" center-to-face). As a result, drawings do not need to be changed to permit bending. When modifying an existing pre-formed system, a standard weld fitting can be used or vice versa.

2.3 3D Bends
As an alternative to 1½D bending, when flow restriction may be a concern, 3D bend radii may be utilized. Studies have also indicated potential energy savings using larger bend radii. 3D Bending may require special design consideration.

3.0 SIZE LIMITS
Cold machine formed close radius bends are currently available as follows:

½ - 6" N.P.S
Sch 5 - Sch 80

Cold close radius bending of Schedule 5 pipe is only possible with carefully selected pipe and tooIing.

4.0 MATERIALS

4.1 Carbon Steels
Carbon steel pipe is suitable for machine cold bending without heat treatment to the bend radius limits indicated in the following table.

4.2 Alloys
Alloy pipe suitable for machine cold bending without heat treatment to 1½D or 3D bend radius. All other materials should be inquired.

Monel®, Inconel®, and Incoloy® are trademarks of Inco Alloys International, Inc. Hastelloy® is the trademark of Haynes International

5.0 BEND SPECIFICATIONS

5.1 Bends
A pipe bend made to this specification and verified for pressure design in accordance with ASME B31.3 shall be suitable for the same service as the pipe from which it is made.

5.2 Out of Roundness
Flattening of a bend, is measured by the difference between the maximum and minimum outside diameter at any cross section. For 1½D bends, out of roundness shall not exceed 5 percent of the nominal outside diameter for internal pressure, and 3 percent for external pressure. For 3D bends, out of roundness shall not exceed 3 percent. (See FIG. 1)

5.3 Dimensional Tolerances
Degree of bend is to be held to plus or minus 1 degree. Overall spool length will be held to plus or minus 1/8 inch. All centerline-to-centerline or centerline-to-end face dimensions will be plus or minus 1/8 inch. (See FIG. 2)

5.4 Wall Thinning
Wall thinning of 1½D and 3D bends, as measured by the difference between the actual thickness of the pipe "A" and the minimum thickness "B" on the outside of the bend shall not exceed 18 percent of the actual thickness for 1½D bends and 12 percent for 3D bends. (See FIG. 3) Maximum allowable working pressure should be calculated using the following formula:

For 1½D Bending     1P= 2SE[.82t -c]
                                                    D

For 3D Bending     1P= 2SE[.88t -c]
                                                    D

P - Maximum Allowable Working Pressure
SE - Allowable Stress
t - Actual Pipe Wall Before Bending
c - Corrosion Allowance
D - Outside Diameter of the Pipe

5.5 Wrinkling Wrinkling
Light wall stainless coupled with pipe wall variation near the outer limits of mill specifications may occasionally wrinkle. (See FIG. 4) In these cases, peak to valley depth will be limited to: