Pipe threads allow pipes to be joined together, with male threads on the outside and female threads on the inside. Industry standards include NPT and NPSM, with GHT and NST for specific applications. PTFE tape is recommended for leak-proof seals. Pipe sizes are measured by OD and TPI. Building codes require different materials for different applications and may require licensed contractors for gas lines.
A pipe thread is a spiral ridge on the end of a pipe that allows pipes to be joined together. For male fittings, this thread appears on the outside diameter of the tube; if female, it appears on the internal diameter. By twisting one end of the male pipe into a female thread, the two fittings come together.
Because male and female threads must line up correctly to form a connection, manufacturers follow industry standards. The two main standards are the National Pipe Thread (NPT) and the National Standard Free-Fitting Straight Mechanical Pipe Thread (NPSM). NPT is a tapered thread which can realize a tight wet seal. NPSM is straight and forms a mechanical seal only; it is not used for liquid applications. In the US, garden hoses have their own standard (GHT), as do fire hose fittings, which use the National Standard Thread (NST).
When joining NPT tapered pipe threads to make a leak-proof seal, professionals recommend using a sealant such as polytetrafluoroethylene (PTFE) tape. PTFE, more commonly known as Teflon®, is also the name of the best known brand of sealing tapes. The tape should be wrapped tightly around the male wires, running in the same direction as the lead wire so that the twisting motion of joining the pipes follows the winding direction of the tape. The sealing tape makes it easier to insert the male tube deeper by allowing the threads to slide over each other, while filling small spaces to prevent seepage. Pipe thread tape also facilitates later disassembly of the joint, if necessary, by reducing thread seizure or the tendency for certain thread types to stick together over time.
Modern pipe sizes are established by a measurement of the outside diameter (OD), which is measured at the pipe thread in the case of a male pipe. To make sure the threads match, you can also count threads per inch (TPI). The inside diameter (ID) is sometimes used when purchasing polyvinyl chloride or PVC pipe.
Building codes require piping to meet certain standards depending on the application. The pipes for pressurized gas or liquid, natural gas, drinking water and underground discharges will be made of different materials. Keep in mind that local laws may require that only licensed contractors work on gas lines. Before undertaking any home plumbing project, check local building codes and take proper safety precautions.
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