In a piping system, it is not always possible to have straight runs of pipe. At the point where the piping beneath a gasoline dispenser ties into the pipe that runs to the storage tank, for example, the connection essentially consists of a 90-degree angle. Moreover, it is desirable to have some flexibility in the underground piping system to accommodate movement: frost heave, vibration, and the like.
The traditional method of providing angle turns, as well as flexibility, in underground piping systems has been through use of swing joints. A swing joint is a pipe connection made up of a series of pipe elbows and related fittings. When a swing joint is in use, ground movement is accommodated as one threaded end of an elbow turns and tightens, and another threaded end–somewhere in the swing joint assembly, turns and loosens.
As a result of this movement, swing joints often leak. In addition, they will not move unless subjected to significant force.
In response to the disadvantages of swing joints in underground applications, the flexible connector was developed. A typical connector is 2 feet or so in length and, because of its flexibility, resembles a short length of hose. Unlike ordinary hose, however, flex connectors are usually constructed of synthetic materials, typically teflon, and are covered by an outer stainless steel mesh.
Because stainless steel can corrode, flex connectors in contact with the soil must either be cathodically protected or isolated from the soil through use of an outer jacket of noncorrosive material, such as PVC.