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What is the difference between “NH”, “NST”, “NPSH”, “IPT”, and “NPT” threads?

“NH”, or National Hose, thread is the thread that has become the standard thread used in firefighting today. The threads specified in NFPA 1963 are “NH”. This thread was previously known as “NST” or National Standard Thread. “NPSH”, or National Pipe Straight Hose, is slightly smaller in diameter and has more threads per inch than the same nominal diameter “NH” thread. “NPSH” thread is sometimes referred to as “IPT”, or Iron Pipe Thread. “NH” and “NPSH” threads are not interchangeable. “NH” and “NPSH” threads are straight threads and require a gasket for sealing between mating threads. “NPT”, or National Pipe Tapered, is the thread most commonly found in plumbing applications. Being tapered, the seal between mating threads is made by squeezing the threads together. No gasket is used. “NPT” and “NPSH” threads are similar in diameter and threads per inch. (Credit: Red Head Brass, LLC)

Why aren’t tapered pipe threads (NPT) available in swivels?

They typical coupling consists of three main components; male, band, swivel. The swivel’s job is to bring together the male and the band, squeezing a gasket between them, sealing the water path. With straight threads, like NH and NPSH, the swivel turns on the male as far as necessary to make this seal. However, NPT threaded components seal by jamming the mating threads as they are tightened. No gasket is used. The turns required to tighten may vary. If NPT threads are used in a coupling, the end of the male may not stop at the correct place in the swivel. The male can be tight into the swivel but, even with a gasket, it may not seal against the band. This would allow water to leak out of the joint. Thus, NPT threads are not available in swivels. However, a coupling set with a NPT male and a NPSH swivel can work. NPSH thread is the straight version of NPT thread. A NPT male will screw into a gasketed NPSH swivel and seal against the band, provided the end of the male is cut square. (Credit: Red Head Brass, LLC)