How Pipe Fittings Work

Tapered (“Pipe”) Threads

There are three types of tapered threads commonly used in industrial applications.
     •        NPT/NPTF
     •        BSPT
     •        Metric Taper
All three thread styles noted above use the same basic metal-to-metal sealing design for achieving a seal. Although very similar, there are differences in the thread dimensions, pitch, and flank angle that do not allow interchangeability.

NPT / NPTF Threads

NPT threads, when assembled without a sealant, leave a spiral leak path at the crest-root junction. To seal pressurized fluid, NPT threads require a suitable sealant. NPTF threads (Dryseal), on the other hand, when assembled, do not leave the spiral leak path. This is because they have controlled truncation at the crest and root, ensuring metal-to-metal crest-root contact prior to, or just as the male-female thread flanks make contact. Upon further tightening, the thread crests are flattened out until the flanks also make metal-to-metal contact. Thus, theoretically at least, there is no passage left for the fluid to leak, provided all surfaces are flawless and dimensions exact. In reality, this is not the case and a sealant/lubricant is necessary to achieve a leak free joint, even with NPTF threads. The sealant/lubricant fills all imperfections in the surfaces affecting the seal and also provides lubrication to ease assembly and minimize galling.