What is the difference between NPS, NPTF and NPT Fittings ?
- NPS = national straight pipe
- NPT = National Pipe Thread
- NPTF = National Pipe Thread Fuel
- Both NPT and NPS have the same thread angle, shape, and pitch (threads per inch). However, NPT threads are tapered and NPS threads are straight (parallel).
- Both threads have an included angle of 60 ° and have flat peaks and valleys. Tapered pipe thread is commonly found on pipe ends, nipples, and fittings (ex: couplings, elbows, tees, etc.). Steep-angle threads are critical for a tight joint without leaking.
- Thread sealant or tape is usually required to complete the seal. Straight pipe threads need a gasket or an O-ring to create a seal. While the NPT and NPS threads will mate, they do not seal properly with each other.
NPT Fittings vs. NPTF Fittings
- Requirements for NPT (National Pipe Thread) are detailed in ANSI B1.20.1
- Requirements for NPTF (National Pipe Thread Fuel) are detailed in ANSI B1.20.3
Both NPT and NPTF have the same threads per inch, pitch diameters, and taper per inch. The differences come in the major and minor diameters, the root and the crest of the threads.
The Fittings of NPTF thread crests are within the parameters of NPT requirements but have a smaller range than NPT. The roots of the NPTF thread are different from the roots of the NPT thread.
NPTF thread roots are designed to interfere with the crest of the mating thread with the intention of creating a mechanical seal through the deformation of the thread form in the assembly.
NPTF Fitting threads have two identified classes: Class 1 and Class 2. The roots of the NPT threads are designed to allow space with the corresponding thread ridges in the assembly.