Most PVC fittings go outside pipe. This makes sense because, regardless of schedule, all PVC pipe of the same nominal size have the same outer diameter. That means any outer fittings can go on any pipe that is the same size. But sometimes you need to extend an already-existing pipeline. Other times you need to couple pipe but there's no room for a regular outer coupling. In both of these instances, you will need fittings that go inside pipe. With interior fittings, the schedule of pipe is important, as this affects the inner diameter of the pipe.
Pipe Extenders
Pipe extenders fit inside schedule 40 PVC pipe and extend it another couple inches, which can be helpful in many different situations. They consist of a small end that fits inside the pipe and a larger end that matches the outer diameter (OD) of the pipe it's connecting to. This means that, from the outside, the surface of the pipe will be the same size the whole way down. An extended pipe can be treated the same as a regular pipe. Fittings can be placed on the pipe extender the same as they would be placed on regular PVC pipe.
PVC pipe extenders are typically used in repair scenarios when an extra piece of pipe is needed to connect a fitting, valve, or other piece of hardware. They only work for schedule 40 pipe because the inner diameter (ID) is different for other kinds of pipe. For example, a 2" PVC schedule 40 pipe extender will only fit with a 2" nominal size schedule 40 PVC pipe. Fittings that fit on the outside of pipe are interchangeable, but interior fittings can only be used with their designated schedule of PVC pipe.
Inside Connectors
This part does exactly what it sounds like: connect from the inside. It consists of two pipe ID spigot ends that fit inside of pipe with the same nominal size and schedule. These do the same job as a regular PVC coupling, but do the work from the inside instead of from the outside. The reason you would choose an inside connector instead of a coupling is because you lack the space around the pipe to have a fitting around it. Another possible reason to want an inside connector is that it makes the surface of the pipe continuous; a coupling causes a bump along the surface of the pipe.
The problem with these inside connectors is that they can reduce flow through a pipeline, as they lessen the space inside of the pipe. This could cause issues in some applications where maintaining a certain rate of flow is important. To measure flow and see how much an inside connector affects it, try using a flow meter! Just like with the pipe extenders, the schedule of inside connectors and PVC pipe have to match or else they will be incompatible.
Installing Interior Fittings
The installation of interior fittings is mostly common sense, but there are a few important things to remember. First, the smaller part of the fitting (usually referred to as a pipe ID spigot) goes inside the pipe and should fit snugly. You may need to use a mallet to get them completely into position. Second, to handle any liquid or gas transportation, interior fittings will need to be installed with PVC cement or some other sealant to assure there are no leaks. Below are a few illustrations to show how these fittings will look once installed. Clear PVC pipe has been used so you can see how the fitting sits inside the pipe.
In the world of PVC, fittings that go inside pipe are rare, so it can be easy to buy the wrong part. Make sure you do your research before purchasing a pipe extender or inside connector!