TPi January 2013

Grooved mechanical pipe joining: the groove explained By Andy Carter, product engineer at Victaulic

Does grooving weaken the pipe end? A question that is often asked regarding a grooved joint has to do with the reliability of the groove under stress. When examining the stress on pipes created by internal pressure and the typical “overstress” failure mode, the cut groove has been shown to not be the weakest link. When under pressure, two basic stresses occur: longitudinal and hoop. Longitudinal stress is often described as “tensile” stress in material, the force pushing pipe apart in the axial direction. A failure due to longitudinal stress would cause a circumferential fracture around a pipe. Hoop stress can be described as ballooning or expansion of a pipe diameter in a radial manner. A failure due to hoop stress would cause the pipe to split lengthwise along its axis. Mathematically, the formulas for longitudinal and hoop stresses are: Hoop stress = (pressure x outside diameter) ÷ (2 x wall thickness) Longitudinal stress = (pressure x outside diameter) ÷ (4 x wall thickness) With pipe that is roll grooved, any potential increase in pipe hardness, reduction in tensile strength or reduction in elongation has no effect on the integrity of the joint, and pipe

Pioneered during World War I, mechanical pipe joining was invented by the company now known as Victaulic and since being made commercially available in 1925 has become a preferred joining method on many piping applications. These include, shipbuilding, mining, power, oil and gas, water systems treatment and building services. The grooved method has gained widespread use in its 85-year history, but many myths and misunderstandings still exist. Mechanical grooved pipe joining has been proven through research, testing and extensive evaluation. It has stood the test of time in some of the most impressive engineering feats including the Hoover Dam in the United States, the Grand Arche de La Défense in France, the Alexandria Library in Egypt, the Petronas Twin Towers in Malaysia, and the Burj Al Arab Hotel in the Middle East. For engineers, the benefits of the grooved system are many: the design versatility of the joint allows rigidity and flexibility throughout a system; a mechanical joint provides noise and vibration attenuation, seismic relief, and accommodates thermal expansion and contraction. Roll grooving and beyond In terms of pipe preparation, the most common method used today in mechanical piping systems is cold forming a roll groove onto the end of the pipe. Roll grooving was invented by Victaulic in the 1950s and is used on 90 per cent of grooving applications. It is used on a wide variety of pipe sizes and standard wall thicknesses because it is fast and clean. Roll grooving can be used on carbon steel, stainless steel, copper and aluminium pipe or tubing as well as PVC pipe. Most coupling manufacturers provide proper roll-grooving equipment based on customers’ requirements. The latest development in grooving technology is for medium- to large-diameter pipes between 350mm and 1,524mm (14 to 24"). The Advanced Groove System (AGS) developed by Victaulic offers enhanced strength and reliability through a more robust coupling housing and a patented wedge-shaped roll groove, resulting in a 40 per cent increase in end-load carrying capabilities compared to previous joining methods. The system also provides a union at every joint for ease of system maintenance and expansion.

The Victaulic developed Advanced Groove System (AGS) for medium- to large-diameter pipes offers enhanced strength and reliability

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January 2013 Tube Products International