Standards for hot and cold bending
July 13, 2004
Bending tube or pipe so the finished product conforms to one of two bending standards can help to reduce rejects and improve relations between fabricators and their customers. The standards can facilitate the use of bending terms, and promote an understanding of bending tolerances and acceptable defects before starting a bending project.
You recently acquired a pipe bender to enhance your fabrication capabilities, received an order, procured the pipe stock your customer requested, and bent it as specified. Your customer rejected the first lot. He called and complained about a hump on the extrados, and he said something about it being oval. You have no idea what he's talking about. You put a bend in a round pipe, not a hump on the extrados of an oval pipe. What is the customer talking about, and what went wrong?
The answer is that even though you bent the pipe the specified degree of bend, the finished pipe was flawed. The mandrel left an imperfection—specifically, a hump—on the outside of the bend, which in pipe bending jargon is called the extrados. A second bend, one made without a mandrel, resulted in a somewhat flattened section, so it was slightly oval rather than round.
The Recommended Standards for Cold Bending of Pipe and Tube, published by the Tube & Pipe Association, International® (TPA), was meant to prevent this type of situation. Another publication, Recommended Standards for Induction Bending of Pipe and Tube, also published by the TPA, addresses the main issues associated with hot bending.
Developed by two separate committees, one that developed the cold-bending standard and another that worked on the induction bending standard, the guidelines were the result of a push to decrease the costs associated with industrial pipe installations.
"In the early 1960s a large chemical producer commissioned a task force to find a more efficient way to erect and construct chemical processing plants," said Drew Kershaw, who was the chairman of the cold-bending standards committee.
The traditional approach to constructing this type of plant involves a lot of welding to install elbows, T's, and flanges. Every weld requires an X-ray inspection. The labor and the inspections can be expensive, Kershaw said. Other difficulties include the work environment—elbows, T's, and flanges are assembled at the job site, often where workers are exposed to the elements and work as high as 50 feet off the ground.
Replacing traditional components with bent pipe cuts costs because it reduces the number of welds and weld inspections required. Also, the bending environment is less harsh than a job site; pipes are bent in the controlled environment of a fabrication shop and then are transported to the job site.
Starting From Scratch. Bending pipe for a chemical processing plant wasn't as simple as using an existing machine to bend existing pipe. Typical pipe materials used in petrochemical plants, such as ASTM A53 and A106, are not very ductile and tend to flatten when bent, Kershaw said. The fluid processing industry's requirements are stringent and do not allow the use of flattened pipe. The committee found A587, a bendable pipe material that isn't prone to flattening when bent, and the committee worked with a pipe bender manufacturer to develop a bender for petrochemical applications.
That was only part of the battle. Bending knowledge in the fabrication industry was limited at the time. "Many contractors knew just basics of pipe fabrication—changing direction or adding a fitting—and that's it," said Tony Granelli, director of SWR America, who participated in the cold-bending committee. "Meanwhile, other contractors, those who had bending capabilities and knowledge, were willing to provide pipe bending services, but many potential customers weren't familiar enough with specifying pipe bends to use bent pipe."
A push to educate contractors and their customers followed, and the two committees were formed to develop the standards with the intent of spreading bending knowledge throughout all industries—not just petrochemical, but every industry involved in metal tube and pipe. The standards were intended both for companies that needed tube or pipe bent and contractors that provided bending services.
Add a Little Heat. The induction bending committee addressed similar topics, but focused on hot bending, said Frank Corgiat, who was chairman of the induction bending committee. Like cold bending, induction bending provides two chief advantages: It reduces labor and nondestructive testing, which reduces costs. The cost reduction is greatest for industries that rely most heavily on nondestructive testing, such as the power, nuclear, and gas transmission industries.
"Another advantage is that with induction bending, you're not tied to a specific bend radius," Corgiat said. Typical fittings have a 1.5D bend radius, but induction bending is flexible and allows much larger bend radii. "Power plants need 3D bends. Larger-radius bends are more efficient because they allow better flow characteristics." But induction bending isn't limited to 3D bends, of course. "With induction bending you're not locked into any specific tooling, so it's more versatile and can be used to make bends to a wide range of radii," Corgiat explained.
The standards cover basic bending terminology and include glossaries that define everything from A to Y (arc, which is the curved portion of a bend, to yield strength, which is the stress at which a material exhibits a specified deviation from proportionality as a result of stress and strain). The standards also have illustrations of many of the terms defined in the glossaries.
Bending Parameters. The standards provide a comprehensive list of parameters that customers can use to specify bends, including material specification and grade, material construction (such as seamless, welded, or DOM); OD and original wall thickness; minimum wall thickness after bending; ovality; quantity of bends; bend angle; bend radius; applicable dimensions and tolerances; end preparation; applicable codes and standards; heat treatment; and destructive and nondestructive testing.
This parameters section helps to facilitate communication between the customer and the fabricator so the project can be accomplished with a minimal amount of rework or rejects.
Acceptance Criteria. This is where all the preparation pays off—checking out the bent pipe or tube to be sure it conforms to the customer's specifications. Both standards recommend tolerances for bend radius, degree of bend, plane of bend, flat plane of each bend, and linear tolerances. At this point, the two standards part ways.
The cold-bending standard addresses how to identify and resolve surface defects and provides details on the maximum acceptable magnitude of flaws such as wrinkles or humps, ovality, wall thinning, fiber elongation, and ductility. While much of the standard applies to general manufacturing applications, it also has separate sections on acceptance criteria for the petrochemical industry and for the power boiler industry.
The induction bending standard provides more information on topics relevant to hot bending, including more thorough detail on wall thinning and ovality. It also lists specific codes for tensile strength and toughness testing and suggested procedures for hardness testing, and lists specific testing locations on the tube or pipe.
Although the standards were initiated from a fabrication standpoint, they were written from a much broader perspective. The cold-forming committee comprised 27 people from several manufacturing sectors.
"The value of the committees came from their structure," Kershaw said. "The committees were vertically integrated. They included steel producers, machine toolmakers, fabricators, and end users. This structure kept them objective."
And the standards aren't limited to a specific industry; they apply to pipe and tube, not an application, so any industry that uses bent pipe or tube can rely on these standards to get the pipe or tube bent right.