Pipe fabrication has historically been a conservative market segment, and the basic manufacturing methods have satisfactorily met the industry needs. As the market becomes more global, however, changes are taking place to meet the needs for reduced cost and labor and improved quality. Sometimes, these changes involve implementing different processes.

This article covers two processes used by industry for cost effective pipe fabrication: branching (extruded outlets) and flanging.

(figure 1)

The drawing shows the components of a typical extruded outlet.

Branching

An extruded outlet forms a branch connection from the main pipe (see figure 1). Also known as a mechanically formed or ball-pulled connection, the extruded outlet has been around for close to 100 years. However, new technology has made it a viable alternative to fittings, weld outlets, and nozzle welds.

Benefits

Using extruded outlets eliminates joints in the piping that could cause cavitation. It also eliminates weld connections, allowing automatic orbital welding on the remaining joint.

X-raying of welding pipe connections is simplified when extruded outlets are used. Their characteristics are comparable to those of standard tee fittings, and the process lends itself to simple pipe manifolding.

Compliance.

Benefits are meaningless, however, if the formed connection does not meet the working pressures required of modern piping systems.

Over the years, extruded outlets have been subjected to many tests to determine their joint limits and compliance to industry standards. Following is a partial list of their compliances in the U.S. and worldwide:

1. Testing laboratories: Underwriters Laboratories (UL), Detroit Test Laboratories, Universal Testing Laboratories, and many other in Europe

2. Governmental standards: U.S. Navy, U.S. Army Corps of Engineers, U.S. Coast Guard, American Bureau of Shipping (ABS), U.S. Department of Energy

3. Standards organization: American National Standards Institute (ANSI), American Society of Mechanical Engineers (AMSE), ect.

4. National, state, and local plumbing codes.

5. Worldwide approvals: ITUF, Det Norski Veritas, Lloyds, ect.

Several fabricators using extruded outlets in their products satisfy their customers' concerns by subjecting the fabricated connection to a burst test witnessed by the customer.

Recently, a large chemical company evaluated the actual results of destructive testing on an extruded outlet against the theoretical calculations made in compliance with the ANSI standards (B31.1). The results were:

2 x 2 x 2 inch schedule 10 Stainless connection

Calculated pressure = 824 pounds per square inch (PSI)

Actual tube deformation = 7,500 PSI

4 x 4 x 4 inch schedule 10 stainless connection

Calculated pressure = 466 PSI

Actual tube deformation = 3,800 PSI

In the test of the 2 inch connection, the test rig blew apart at 6,000 PSI. A larger system was purchased to reach the final test results.

Both the ANSI standard and the ASME Boiler and Pressure Vessel Code included mechanically formed (extruded) outlets as an acceptable method of pipe fabrication.

Another comparison was made between a nozzle weld and a mechanically formed connection. In all cases, the mechanically formed connection met or exceeded the nozzle weld connection (see Figure 2).

Limitations. The limitations of the branching process vary depending on the end product requirements. General parameters are outlined here:

1. Applicable materials: These include copper, aluminum, low - carbon steel, stainless steel, INCONEL® alloy, titanium alloy, copper nickel, brass, bronze, and admiralty brass. Other materials have been formed, but they can only be considered after testing.

2. Elongation: The minimum should be 23 percent or higher for best results. Materials with elongations of less than 23 percent have been formed, but the ratio of outlet diameters to main pipe diameter is reduced.

3. Mild steel: Carbon content should be less than .22 percent for best results. Higher carbon steels can be used if the pipe is heated before forming.

4. Stainless steel: Stainless steels with a chrome content of 13 to 18 percent and a carbon content of .03 to .08 percent can be formed into outlets of up to "size on size." Commonly formed stainless steels includes 304, 304L, 316, 316L, 321, and 409.

5. INCONEL® alloy: 625 can be formed, but care must be taken, since it work-hardens rapidly. The pilot hole edge must be polished to eliminate roughness that coul result in cracking.

6. Titanium: This must be commercially pure (99.9 percent) and requires a double pull to ensure tolerances.

7. Wall thickness: As the "T" ratio (the outlet diameter divided by the main pipe diameter) increases, the resulting wall thinning increases. The wall thinning on a size-on-size outlet can exceed 20 percent. While this may appear significant, the thinning occurs in the "stirrup" area. This amount of thinning does not exceed the limits imposed by the ANSI standard formula since it stays within the 35 percent that is allowed.

Conversely, the wall in the "saddle" area increases, a point the standards calculations indicate is the most critical for pressure applications.

Flanging

Flanging (also called Van Stone, lapjoint, or loose flanging) formed a flanged pipe connection. Depending on the application, the flanges are classified A, B, or C, reflecting the inside radius of the formed section.

A and B flanges are used for more critical installations in which interanl pipe smoothness is required to limit contamination such as the food industry. This article concentrates on C flanges (see Figure 3), principally used by the chemical maritime industries.

Figure 4 illustrates the typical design criteria for a flange. Flanging has been used extensively in the following industries:

1. Chemical

2. Pulp and paper

3. Shipbuilding

4. Power plants

5. Offshore oil rigs

6. Petroleum

7. District heating

Benefits. Flanging eliminates the need for welding, since the flange is mounted loosely behind the formed end configuration. It allows carbon steel flanges to be used on stainless pipe applications and also allows simpler pipe installations, since the need for pipe/flange alignment is eliminated. Flanging also eliminates the need to X-ray the weld.

Limitations. The limitations of the flanging process parallel those of the branching process in terms of materials and forming characteristics.

Van Stone flanges comply with ANSI, the ASME code, and international standards in low pressure applications (225 PSI).

Most countries allow the flanges to be used for similar applications. Recent German tests (HDW Shipyard) have demonstrated that pressures up to 3,000 PSI are possible using specially designed gaskets.

A recent test by an independant laboratory simulated a 40 year life cycle of the flanged assembly with varying pressures from 220 to 500 PSI oscillating at 1,840 cycles per minute. The system ran 20,976,000 cycles without failure.

The inside diameter (ID) radius of the formed section has been established to mate with the loose flange dimensions. However, the development of plastic-lined pipe in the chemical industry has brought on the development of flanges with increased ID radii, made possible by machinery that provides for radius forming adjustments.

Recent expiration of patent number 3,451,243 for flange serrations allows the resultant grooves to be applied on all applicable formed pipe flanges to provide better gasket sealing for flange connections. Another recent development is the ability to form finished elbows, complete with the loose flanges.

Summary

For applications in which either branching or flanging can be used, these methods warrant evaluation. They can provide quality and economic benefits for certain applications, particularly those that allow the user to convert a current Schedule 40 carbon steel system to one using lighter, longer lasting stainless pipe.

William Holyoak is President of T-Drill Industries, Norcross Georgia. Origionally presented at Pipe Fabricating '97, February 5-6 1997, Mobile Alabama, sponsered by Tube & Pipe Association, International (TPA). INCONEL® is a registered trademark of the Inco family of companies.