Grooved mechanical pipe joining is a technique pioneered during World War I as a fast, reliable way to transport fuel and water to the Allied troops. It has become a viable joining method on utility and process piping applications when compared with welding, threading and flanging. The mechanical joint, or coupling (Figure 1), has four elements: the grooved pipe, the gasket, the coupling housings and the fasteners. The pipe ends have a cold formed or machined groove that provides an engagement point for the coupling and a sealing surface for the gasket. The gasket seals the pipe ends, trapping fluid within as it is enclosed by the coupling housings. The housings are held together by bolts and nuts tightened with a socket wrench or impact wrench. The coupling housing encases the gasket and engages the circumferential pipe groove to produce a leak-tight seal in a self-restrained pipe joint.
Figure 1. Four elements comprise a mechanical joint: the grooved pipe, the gasket, the coupling housings and the fasteners.
The method has gained widespread use since its invention 85 years ago, but many myths and misunderstandings regarding the grooved joint’s engineering and performance still exist.
The benefits of using a grooved system are plentiful. Rigid and flexible coupling options meet system designers’ needs, a union at every joint allows for ease in system maintenance and retrofit, noise is attenuated as vibrations are reduced at each pipe joint, and the strategic use of sway bracing and rigid/flexible couplings accommodate seismic movements. However, the bottom-line question remains: Is it reliable?
Let’s explore some misconceptions about the grooved mechanical pipe joining method:
- The indent from roll grooving introduces a pressure drop
- The grooving process weakens a pipe and leads to stress failures
- Couplings can’t produce a truly rigid joint and, therefore, the piping system sags over time and requires additional support compared to a welded system.
Cut and roll grooving
Some ask how the pipe groove is formed and how that affects the pipe’s performance in different applications. There are two types of grooving: roll grooving and cut grooving (Figure 2).
Used in approximately 90% of grooved piping applications, roll grooving is the most common method. Roll grooving was developed more than 50 years ago for light or thin-wall pipe, which opened the door to a fast, economical method for mechanically joining such pipe. Roll grooving is now used on a wide variety of pipe sizes and wall thicknesses because it’s a fast, efficient and clean pipe end preparation technique without sacrificing pipe joint performance.
Roll grooving displaces a small portion of the pipe wall radially in a cold forming process. The pipe end is placed between the roll set of a grooving machine. The roll set closes, the pipe is compressed and rotated, and a groove is formed around the outside diameter, leaving a circumferential recess on the outside and an indent on the inside.
Roll grooving can be used on Schedule 5 through ANSI standard wall thickness carbon steel and stainless steel, copper, and aluminum pipe or tubing, and up to schedule 80 PVC plastic pipe. Roll-grooved systems range in diameter from 3/4-in. to 60 in. in some materials.
Figure 3. The groove, depicted by the black bar, removes less pipe wall material that threading.
Cut grooving, on the other hand, is commonly used on pipes of ANSI standard wall or greater and for abrasive applications, where the inside of the pipe wall needs to be as smooth as possible to allow materials to pass without obstruction. The smooth, continuous inside surface remains after the groove is cut, making cut grooving suitable for systems that require plastic coating, cement lining or other protective linings for corrosion or abrasion reasons.
Cut grooving involves removal of one-third of the total pipe wall thickness to provide the lip that engages the coupling housing. Cutting a groove removes less material, to less depth, than threading the pipe (Figure 3). Cut grooving can be completed manually or by using motorized tools.