Threaded Fittings

NPT versus BSP Threads

NPT and BSP threads are not interchangeable, and mixing them creates a joint that leaks. NPT, or National Pipe Thread, is the American standard defined in ASME B1.20.1, with a 60 degree thread angle and a taper of 1 in 16. BSP comes in two forms: BSPT, the tapered form, with a 55 degree thread angle and a taper of 1 in 16, and BSPP, the parallel form, with a 55 degree thread angle and no taper, which seals on a gasket or an O ring at the face rather than on the thread. Because the thread angles differ, an NPT male will engage a BSPT female only partially and will leak under pressure even when tightened hard. Always confirm which thread standard the project uses, and specify it explicitly on the order, since the two look similar to the eye.

When to Use Threaded Fittings

Threaded fittings are the practical choice where welding is impractical or undesirable: for small bore instrument and utility lines, for connections that must be assembled and disassembled in the field, for maintenance tie-ins, and for lines in hazardous areas where hot work is not permitted. Their limitation is the thread itself, which is the sealing element and the weakest point of the joint. Threaded joints are therefore not used above the size and pressure limits set by the applicable code, and are generally restricted by ASME B31.3 to sizes of 2 inch and smaller for severe cyclic service. In sour service, threaded joints are additionally restricted because the thread root acts as a stress concentration, and the number and size of threaded joints is often limited by the project specification.