By Sara Heger, Ph.D.
Using primer and glue is the most common method to join together two pieces of PVC pipe, but there are other connections and joints used and needed for specific applications.
Threaded connections are often needed when installing components in a septic pipeline. Male and female adapters are used to transition to threaded fittings. There are two types of threaded connections: those made in a manufacturing facility and those made in the field. When the threads are made during the manufacturing process these connections are usable at the full pressure rating.
When making the threads on site, Schedule 40 and SDR pipe should not be threaded. Threading this material reduces the wall thickness and the pressure rating of the pipe is reduced by half. If pipe nipples are needed, Schedule 80 has a thicker wall and can be threaded without significant loss of strength, although you will lose 15% of the pressure rating.
All pipe threads need to be sealed. Teflon tape is commonly used for wastewater piping systems. Pipe joint compounds, pastes or other thread lubricants must be approved for use with PVC.
Threaded connections should be hand-tightened. At most, a strap wrench can be used for tightening. Over-tightening should be avoided as this can cause thread or fitting damage. When combining plastic and metallic threaded systems, it is recommended that plastic male threads be screwed into metallic female threads to prevent the plastic from being cracked by the expanding metal threads.
Mechanical joints are used with PVC, ductile iron and gasketed pipe. Mechanical joints have four parts: a flange cast with a bell; a rubber gasket that fits in the bell socket; a gland, or follower ring, to compress the gasket; and tee head bolts and nuts for tightening the joint. Joint assembly is labor-intensive but very simple and requires only one tool — an ordinary ratchet wrench. Mechanical joints are flexible, with the amount of deflection dependent on pipe diameter. The mechanical joint is used mainly with fittings rather than pipe. This is due to the predominant use of push-on joints, which are more economical, faster to install, more trouble-free, and offer better service than mechanical joints.
Lubrication of the plain end, socket and gasket during assembly of mechanical joints is typically recommended. Use soapy water or approved pipe lubricant during mechanical joint assembly to improve gasket sealing and long-term performance. Do not use PVC glue or petroleum-based lubricants.
Some types of PVC pipe are connected with gasket joints. These gaskets do not provide a mechanical bond but instead use the weight of the backfill to hold the connection in place. Sometimes the gaskets are part of the pipe (bell-end) while other times they are a separate fitting. The gaskets must be clean and dry before installing. Both the gasket groove and the spigot should be clean and dry prior to making the connection. A lubricant is needed to make the connection based on the manufacturer's recommendations. The lubricant is applied to the spigot end of the pipe. A good joint is made when the insertion line of the spigot end of the pipe is lined up with the edge of the bell that the spigot is inserted into. This insertion should be done carefully with the pipes lined up horizontally and vertically or you can roll the gasket.
Thrust blocks and mechanical connections
Gasketed pipe connections do not provide a mechanical bond between the pipe and the fittings. This style of pipe connection depends on the weight of the backfill to hold the connections in place. In pressurized applications, the initial fluid velocity (caused by starting the pump) can stress pipe connections. When effluent reaches an elbow, the elbow must force the flow to change directions — this change in direction has a resultant force of trying to push the elbow off of the pipe. With time, the soil around the elbow will yield to the force in the elbow and allow the fitting to fail. Thrust blocks and mechanical connectors are used to reinforce connections to prevent the hydraulic forces from pushing the pipe apart.
Thrust blocks are a rough pour of concrete that provides a greater bearing surface against the undisturbed soil wall. The concrete is poured on the outside of an angled fitting and extends back to the native soil. It is recommended that thrust blocks be placed around tees, crosses, elbows and valves. Mechanical connectors physically hold the fittings together. Clamps are placed around both fittings to prevent any movement between the pipe and fitting. The size and location of thrust blocks and/or mechanical fittings should be provided in the design. Consult with system designers if you are unsure as to the need or location for joint reinforcement.