When buying threaded pipes and fittings, many customers often don't know how to choose. NPT or BSPT? Are they the same? Which one can meet my need? What the hell are the differences? Today let us tell you.
The Difference between NPT and BSPT
Pressure systems in the process industry use different ways of sealing depending on many factors like geographic region, size of the pressure system and environment. Threaded pipes can provide an effective seal for pipes transporting liquids, gases, steam and hydraulic fluid. These threads are now used in materials other than steel and brass including PTFE, PVC, nylon, bronze and cast iron. There are regional differences in sealing types as well as industry preferences so different kinds of thread are always used in different regions.
Both of NPT and BSPT threads belong to taper thread. The taper can allow the threads to form a seal when torqued as the flanks of them compress against each other. Taper thread fittings and compression fittings are opposed to parallel/straight ones in which the threads merely hold the pieces together and do not provide the seal. As the thread body is tapered(0.75 in/ft or 62.5mm/m) a larger diameter keeps compressing into a smaller diameter and finally forms a seal(no clearance remains between the crests and roots of the threads because of the taper). This means that taper thread fittings should be burr-free and lubricated using a lubricating material like lubricating paste or tape. The use of tape also helps to limit corrosion on the threads, which otherwise can make future disassembly nearly impossible.
NPT(National(American) Pipe Thread) is defined by ANSI/ASME standard while BSPT(British Standrad Pipe Thread) by British standard. They are similar except for one important difference. The angle across the flanks of threads(if you sliced the fitting in half long-ways and measured the angle from root to crest to root) for NPT is 60 degrees instead of 55 degrees as it is for BSPT. So NPT threads have flattened peaks and valleys while BSPT threads have round ones. But commonly used sizes of NPT and BSPT pipes and fittings both are 1/8", 1/4", 3/8", 1/2", 3/4", 1", 11/4", 11/2" and 2".