Bending & Limitations on Piping System

Bending
Economics. The use of bends versus welding fittings for changes in direction should be carefully evaluated from an economic viewpoint. Bends whose radii range from 3 to 5 times the nominal pipe diameter will offer the least pressure drop while still affording adequate flexibility to the system. Since each bend eliminates a welding fitting and at least one weld with its attendant examination, bending is very often the economic choice. In the case of special pipe sizes which are frequently used for main steam, reheat, and feed water lines in large central power generating units, bending may be the only option available.

Limitations. The metal being bent should preferably exhibit good ductility and a low rate of strain hardening. Most metals used in piping systems fulfill these requirements. A successful bend is also a function of its diameter, thickness, and bending radius. As the diameter-to-thickness ratio increases and the bending radius decreases, there is greater probability of flattening and buckling. Each bending process has differing capabilities, so the selection of a bending process rests on the availability of equipment and/or practices capable of handling the material, diameter, thickness, and bending radius involved.

FIGURE A6.5 Bend ovality

Accept and Reject Criteria. The codes have certain requirements for the acceptability of finished bends:
1. Thinning: In every bending operation the outer portion of the bend (extrados) stretches and the inner portion (intrados) compresses. This results in a thinning of the extrados and a thickening of the intrados. Because of uncertainties introduced by the pipe-manufacturing method, by the pipe tolerances, and by those introduced by the pipe-bending operation itself, it is not possible to exactly predetermine the degree of thinning. However, it can be approximated by multiplying the thickness before bending by the ratio:
The codes require that the wall thickness at the extrados after bending be at least equal to the minimum wall thickness required for straight pipe. Accordingly, the fabricator must assure that the wall thickness ordered has sufficient margin for this effect.

Although the codes do not comment on the resulting increased thickness of the intrados, this thickness does serve to offset a portion of the increased stresses caused by internal pressure which are found at this location. (See Theory and Design of Modern Pressure Vessels.17)

FIGURE A6.6 Suggested pipe buckling tolerance. (Pipe Fabrication Institute PFI ES-24)

2. Ovality: A second acceptance criteria is ovality. During the bending operation, the cross section of the bend arc frequently assumes an oval shape whose major axis is perpendicular to the plane of the bend. The degree of ovality is determined by the difference between the major and minor axes divided by the nominal diameter of the pipe.
Where the bend is subject to internal pressure, the pressure tries to reround the cross section by creating secondary stresses in the hoop direction. Some codes consider an ovality of 8 percent acceptable in this case. Where the bend is subject to external pressure, the pressure tries to collapse the cross section. The ASME B31.3 Code18 recommends a 3 percent maximum ovality when the bend is subject to external pressure.

3. Buckling: Bending of pipe with large diameter-to-thickness ratios often results in buckling rather than thickening of the intrados, even where internal mandrels or other devices are employed to minimize it. The codes do not address this subject. It is, however, often the subject of ‘‘good workmanship’’ debates. The PFI gives a criterion which has been generally accepted. This appears in PFI ES-24.19 An acceptable
buckle is one where the ratio of the distance between two crests divided by the depth of the average crest to valley is equal to or greater than 12. See Fig. A6.6

Fabrication Practices and Forming

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Fabrication Practices
Cutting and Beveling. The methods of cutting plate or pipe to length can be classed as mechanical or thermal. Mechanical methods involve the use of saws, abrasive discs, lathes, and pipecutting machines or tools.

FIGURE A6.4 Pipe-cutting machine. (Pullman Power Products Corporation)

Thermal methods are oxyfuel gas cutting or electric arc cutting. Oxyfuel gas cutting is a process wherein severing of the metal is effected by the chemical reaction of the base metal with oxygen at an elevated temperature. In the cutting torch, a fuel such as acetylene, propane, or natural gas is used to preheat the base metal to cutting temperature. A high-velocity stream of oxygen is then directed at the heated area resulting in an exothermic reaction and severing of the material. Oxyfuel gas cutting is widely used for cutting carbon steels and low alloys. It does, however, lose its effectiveness with increasing alloy content.

For higher alloy materials, some form of arc cutting is required. Plasma arc cutting is the process most frequently employed. It involves an extremely high temperature (30,000 to 50,000_K), a constricted arc, and a high-velocity gas. The torch generates an arc which is forced to pass through a small-diameter orifice and concentrate its energy on a small area to melt the metal. At the same time a gas such as argon, hydrogen, or a nitrogen-hydrogen mixture is also introduced at the orifice where it expands and is accelerated through the orifice. The melted metal is removed by the jetlike action of the gas stream. Because oxyfuel gas and arc cutting involve the application of heat, preheating may be advisable in some cases.

A very detailed description of oxyfuel gas and arc cutting is presented in The Welding Handbook.16 Weld end bevels can also be prepared by the mechanical or thermal methods just described. Both mechanical and thermal methods are used to apply the V bevel, which is used in the vast majority of piping applications. For compound and U bevels or those which may involve a counterboring requirement, horizontal boring mills are most appropriate. Various factors to be considered in selecting a weld end bevel are discussed in the section, ‘‘Welding Joint Design.’’

Forming. The term forming as it relates to piping fabrication encompasses bending, extruding, swaging, lapping, and expanding. All of these operations entail the use of equipment normally only available in pipe fabrication shops. Although the availability of welding fittings in the form of elbows, tees, reducers, and lappedjoint stub ends may reduce the need for certain of these operations, economics may dictate their use, especially where special pipe sizes are involved.