Create a spark to reduce labor, welding costs
For 70 years, factory-made, wrought butt-welding fittings were the choice for pressure piping systems, in shapes defined by ASME B16.9. However, in recent years, new metal forming processes have enabled the development of wrought socket-welding fittings.
By Ray Stubbs Jr., Bestweld Inc. -- Plant Engineering, 6/15/2008
For 70 years, factory-made, wrought butt-welding fittings were the choice for pressure piping systems, in shapes defined by ASME B16.9. However, in recent years, new metal forming processes have enabled the development of wrought socket-welding fittings. In 1996, those fittings were standardized in “MSS Standard Practice SP-119, Belled End Socket Welding Fittings, Stainless Steel and Copper Nickel” – more familiarly known as “belled-end pipe fittings.” The bodies of these are essentially the same as those in ASME B16.9, but the welding skill, materials and labor time to join them are far less extensive.
According to the American Welding Society and the Bureau of Export Administration, in their May 2002 report entitled Welding Related Expenditures, Investments and Productivity Measurement in U.S. Manufacturing, Construction and Mining Industries, labor typically accounts for 76% of total welding cost. Given the amount of welding involved in a typical piping system, simplifying the process can amount to considerable savings.
This simple change in pipe fitting specification can save 50% to 70% of the labor time needed in joint preparation and welding. Multiply that by each joint throughout the piping system, and this can amount to huge savings to the plant budget. And while significant economic advantages are realized, no sacrifice is made in piping system performance – and in some cases system reliability is improved.
Easing the fit and the weld
Cold-formed, wrought belled-end pipe fittings have expanded ends, creating a socket to receive the connecting pipe. This design allows them to be joined by fillet welds rather than the butt welds needed to join traditional pipe fittings. Both the type of weld and the shape of the parts make good welds easier to achieve.
Fillet welds can be done four to seven times faster and require fewer steps than butt welds. Much less joint preparation goes into a fillet weld, with no machining of parts onsite needed to ensure fit. Butt-welded joints require both pieces to be beveled at the point of installation for a precise fit of root geometry.
Pre-weld fit time is virtually eliminated with belled-end fittings, where butt-welded joints take a significant amount of time to fit. Belled-end fittings joined with fillet welds are more forgiving: where the shape and alignment of the two pieces may vary just a slight amount, welds can still be done successfully. With butt-welding, “out of round” situations, misalignment and mismatched wall thicknesses can cause problems in achieving a good weld.
In butt-welding, an interior backing ring may sometimes be needed to support the welded seam and provide a good surface on which to weld the two beveled edges together. The backing ring is tacked in place, and then weld material is deposited into the groove created by the two machined parts. Where a backing ring is not used, the two parts still must be fit into a jig and tack-weld before being final-welded into place. Joining belled-end fittings with fillet welds eliminates these preliminary steps. In addition, back-side weld joint gas inerting is often required for butt joints but is seldom needed for socket welds.
The ability to use a fillet weld at a lap joint between the fitting and the pipe instead of a butt weld also reduces the chance for burn through – a contributor to internal deformities such as craters, fissures and icicles that can affect process flow. Fillet welds are much easier to do and much less expensive to inspect. Most fillet welds are accomplished in one root pass and one finish pass, whereas comparable strength butt-welded joints require multiple passes. Fillet welds are inspected visually for size and slope, but butt welds are inspected radiographically in order to ensure proper joint preparation and root pass penetration.
Belled-end fittings perform
Today, B31.3, the piping designer’s most significant specification, recognizes the MSS SP119 fillet weld fittings as a cost-reducing alternative to standard butt welding fittings. The current edition of ASME B31 Code for Pressure Piping lays out design requirements for effective, safe and insurable systems. B31.3 Process Piping is “piping typically found in petroleum refineries, chemical, pharmaceutical, textile, paper, semiconductor and cryogenic plants, and related processing plants and terminals.”
But how does performance stack up? Fillet welds in themselves are strong, reliable joints; in piping systems using belled-end pipe fittings, the performance meets or exceeds standard pipe fittings. The fittings provide the same pressure and temperature limits as the corresponding butt-weld fittings. Manufacturers’ design-proof burst testing confirms that MSS-SP119 fittings have burst capacities matching those of ASME B16.9 rated butt-welded fittings.
The fillet-welded joint is stronger than the pipe alone. The cold-formed wrought fittings also better match the wall thicknesses of piping systems than cast or forged fittings, which tend to be rigid and oversized. That properly enables systems with belled-end fittings to flex more uniformly, distributing the stress into the sidewalls rather than the joints. This extends system life where fatigue is a concern.
U.S. Navy testing of the fittings discovered that the fitting bell contributed a significant reinforcement value to elbows. In fatigue testing of angular displacement large enough to produce B16.9 elbow failures in 1,000 cycles, belled-end elbow fittings lasted two to four times longer, the testing found.
Belled end fittings have Piping Code recognition: the current standard MSS SP-119 is referenced by B31.3, Code for Chemical Piping. Standard Practice SP-119 currently is being revised to include belled-end fittings in more materials and with thicker walls, broadening the application possibilities.
Belled end fittings can be used with standard wall and light wall pipe, and commonly are supplied in several alloys of stainless steel, copper nickel, titanium and aluminum. In today’s economy, labor cost outweighs component cost; even where special materials are used, installation and performance issues still make belled-end fittings a preferred choice.
Consideration of welding requirements during piping design will yield impressive benefits. Using belled-end fittings can help a manufacturing facility cut welding labor costs, reduce inspection costs and welding rework and build stronger piping systems.
| Author Information |
| Ray Stubbs has been in the welded piping industry for more than 30 years, serving since 1984 as a founding partner and vice president of sales at Bestweld Inc. A producer of stainless steel and higher nickel alloy welding fittings for high-pressure, high-temperature and severe corrosion applications, Bestweld is a U.S. Navy ship parts supplier. Bestweld was named 2004 Supplier of the Year by Northrop Grumman Newport News and Northrop Grumman Ship Systems. |
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