A technique for providing flexibility at various points in an underground piping system by making up joints composed of steel 90-degree elbows and nipples.
Consider, for example, the point at which the horizontal pipe from an underground storage tank must turn upward to enter a pump island dispenser. If you made this connection simply by installing a right-angle elbow connector between the pipe and the riser leading into the dispenser, your connection would work well enough. It would, however, be absolutely rigid. If, later, there was a slight shift in the position of the dispenser, or a heaving motion caused by ground frost, the connection could not “give.” Instead, it might break or crack.
To prevent this, tank system designers have long recognized the need for providing a certain amount of flexibility in the connections where underground piping changes directions. The traditional technique for creating this flexibility has been the swing joint. A swing joint is made up of a series of from two to five 90-degree pipe elbows and nipples (short pieces of pipe, threaded on each end). These fittings are all threaded together, in a series, to provide a change of direction in the piping. If, later, there is some ground movement, the swing joint will “give.” One elbow section will rotate in one direction, while compensating movement occurs in another section. Rupture of the piping is thus avoided but, unfortunately, leakage often occurs.
Although swing joints are still used, in the U.S. they largely have been supplanted by flexible connectors. These “flex” connectors, 30 inches or so in length, are made of impervious plastic covered with a mesh of stainless steel or other material. They will bend freely in any direction, and are generally considered much more flexible and less likely to leak than swing joints.