By Sara Heger
Fitting and joints are a critical aspect of PVC piping installation. Overlapping joints are used to connect pipe lengths and fittings. Whether using threaded or solvent-welded connections, there is an expanded-diameter section that allows entry of the previous pipe. The expanded-diameter portion is the female end and it is often called a socket, bell, rip or hub. The male end is frequently called a slip, mip or spigot.
An advantage of PVC pipe is the ease of connections. In onsite wastewater applications, the most common means of connecting pipe is with solvent-welded fittings. A strong solvent is used to chemically weld pipe sections. Sometimes called gluing or cementing, solvent welding provides a mechanically strong and watertight seal.
PVC pipe can also be jointed with gasket fittings. This connection style uses a bell end that contains one or more molded gaskets or O-rings. The spigot end is pushed into the bell, and the O-rings provide a watertight connection. This is not a rigid connection; if the pipes and fittings are not supported by bedding material, thrust blocks or mechanical fasteners, then water pressure within the pipe will force the fittings apart.
Among other uses, fittings are used to transition between pipe diameters, provide directional changes, and convert between solvent-welded and threaded connections. The most common fitting is a coupling, which is used to connect two sections of pipe. When many lengths of pipe are assembled, pipe should be specified that has an integral bell on one end. This eliminates the need for a coupling between the pipe lengths and only necessitates one glued connection. A coupling adds two connections in a line that provides additional potential locations for leaks and clogs.
Couplings can be specified that transition into different pipe diameters. A bushing fits into a pipe hub to reduce the diameter for a smaller pipe spigot. A reducer connects the spigot ends of pipes with different diameters. An elbow or “ell” is an angled joint that allows the pipeline to change directions. Fittings are available that provide 22.5-, 45- and 90-degree directional changes.
Most fittings have a hub (or socket) on each end. A street fitting contains both a hub on one end and a spigot on the other end. This fitting style takes up less space than two regular fittings connected with a short length of pipe. Place the spigot end of a street fitting on the downstream side whenever possible to limit catching debris.
Unions are fittings that provide a means of pipe-system disassembly. These fittings are easy to break apart. By placing a union on each side of a valve or pump, that device can be removed without damaging the pipeline. This union connection is most appropriate made with a screwed or threaded connection, but is sometime made with a flexible rubber connection and tightened by stainless steel clamps or bands at both ends. This method is not recommended due to its flexibility, which may result in a sag and non-watertight connection over time. The bands can deteriorate over the years and finely break which could cause a wastewater leak.
All fittings increase the friction in a hydraulic system due to the added roughness in the transitions. The number and types of fittings can impact the required pump size. Many resources provide the equivalent pipe length for various fittings and valves with one such example found in the Engineering ToolBox. These equivalent pipe lengths are an indication of the friction that various pipe fittings can produce. Friction loss is the reduction in pressure of liquid flowing through pipe and associated devices as a result of contact between the liquid and the pipe walls, valves and fittings. As the length of pipe and number of fittings increases, friction losses increase. Using smaller diameter pipes or a different type or number of fittings results in more friction losses because flow is restricted.
Proper application of fitting and joints is critical to a watertight piping installation.