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Tapered Threads -NPT, NPTF

- Jan 09, 2018 -


Tapered threaded pipe is commonly used in systems that handle liquids, gasses, steam and hydraulic fluids.  Qiancheng fittings and adapters are manufactured primarily for use in hydraulic systems.  The most common fitting in the United States and Canada is the National Pipe Taper thread or NPT.

The seal for an NPT thread is a mechanical seal. The seal is formed as the male and female threads are tightened together forming a dry seal.  The thread angle on an NPT and NPTF thread is 60⁰ that have flattened peaks and valleys which is referred to as a Sellers form.  Threads per inch, pitch and the peaks and valleys are different from those found on BSPT.

NPT threads are designed to work in conjunction with a lubricant or PTFE tape, commonly referred to as Teflon tape.  The use of lubricant or PTFE tape will help prevent thread galling during application.

The National Pipe Taper Fuel (NPTF) standard is designed to provide a better seal than the NPT.  The NPTF thread is commonly referred to as a dry seal thread which is designed to work optimally without the use of lubricants or PTFE tape.  The differences in thread are with the major and minor diameters as well as the root and crest of the threads. NPTF and NPT threads call for a similar crest though the NPTF crests have a smaller range.

NPTF threads are designed to crush the thread producing a mechanical seal.  As the fittings is seated, the male thread cuts into the female threads forming the seal.  For this reason NPFT fittings are not reusable.  Though the same function does not happen with NPT fittings, it is not recommended that NPT fittings be reused.  Overtightening is common, particularly when a fitting is reused, which can cause galling or increase the likelihood of leakage.

Application is the main factor used to determine the use of NPTF or NPT.  When the application is critical and a sealing compound may fail due hear or high pressure NPTF threads are typically recommended.  A dry seal thread is likely to perform a better seal than the NPT.

It is not recommended that NPT and NPTF threads be used interchangeably as this will compromise the seal of the threads creating the potential for leakage.

NPT pipe is covered by the ANSI B1.20.1 standard and NPTF is covered by ANSIB1.20.3.

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