National Pipe Tapered (NPT) is common in plumbing and some industrial uses. Sometimes referred to as “pipe thread” because pvc pipe and schedule 40 pipe are typically threaded in NPT. As its name implies, this is a tapered thread so that the outside diameter of the male (ODM) gets smaller towards the end of the fitting and the inside diameter of the female narrows with depth. As the two fittings are threaded together the opposite tapers force thread friction and makes the seal without the use of a gasket. The taper is 1/16" or .0625" per 1" of thread length.
NPTF threads are used when the application is such that pipe sealing compounds may fail due to higher heat or pressure than normal NPT threads can withstand. The threads are designed to seal mechanically by slightly, but sufficiently, crushing the threads when tightened with a wrench. This allows for joining the pipe and fitting without sealants.
The NPT and NPTF threads can be interchanged if sealants such as PTFE tape or suitable pipe joint compounds are used. Female NPT threads can be designated as FPT, FNPT, NPT(F) or FIP and male NPT threads can be designated as MPT, MNPT, NPT(M) or MIP.
British Standard Pipe (BSP) can be most commonly found in Canada, UK, Europe, Asia and Australia. It is a finer thread than National Pipe Thread (NPT), and will not seal properly if used in conjunction with it.
Two types of threads are used: British Standard Pipe Taper (BSPT) and British Standard Pipe Parallel (BSPP).
British Standard Pipe Taper uses a tapered thread system, and sealant is provided to help secure the male and female connection.
British Standard Pipe Parallel uses a parallel thread system, and a rubber gasket is often included to secure the male and female connection.
NH / NST Threads
The most common thread type in use by United States Fire Departments today is National Hose (NH). It is also called National Standard Thread (NST) or National Hose Thread (NHT). It is often referred to as “fire hose thread”. NH, NST and NHT are different abbreviations for the exact same thread.
The female and male side of a fire hose coupling with NH threads will only connect to fittings, nozzles or hoses with the same threads. For example, a fire hose with NH threaded couplings will require a nozzle with NH threads.
National Hose (NH) is considered a “straight” thread which means the threads are consistent from tip to collar and the seal is made with the gasket inside the female coupling mating against the square face of the male coupling. The threads do not make the seal, they merely hold the two fittings tightly together. The size of an NH thread refers to the inside diameter of the waterway on a male coupling. For example 2 1/2" NH male fitting has a 2 1/2" waterway.
Some fire departments in the United States have special threaded fire hoses and fire hydrants.
Garden Hose Threads (GHT)
Garden Hose Threads (GHT), just like NST and NPSH, seals with a gasket in the female coupling. A typical garden hose may be 5/8" or 3/4" inside diameter but the fitting on the ends will be GHT. GHT is not compatible with NPT. For example, 3/4" GHT is not the same as 3/4" NPT.