NPT & NPTF 区别

NPT ：美制一般用途圆锥管螺纹（密封）
NPTF：美制干密封圆锥管螺纹
NPT = National Pipe Thread
NPTF = National Pipe Thread Fine

使用时前者可以在螺纹副内填加密封介质，而后者则完全依靠螺纹自身形成密封，螺纹间无任何密
封介质。所以前者属于经济型螺纹，产品设计中常用，而干密封管螺纹则规定了较为严格的公差，
属精密型螺纹，仅用在特殊场合。它们两者的牙型角、斜度等指标都是相同的，关键是牙顶和牙底
的削平高度不一样（螺纹牙高不一样。NPT可能有过盈也可能有间隙，但NPTF没有间隙；NPT内外
螺纹均为圆锥，而NPTF外螺纹是圆锥，内螺纹则有圆锥、圆柱两种；NPT有右旋也有左旋，NPTF
只有右旋一种旋向。）
http://www.ring-plug-thread-gages.com/ti-NPT-vs-NPTF.htm
NPT vs NPTF

THREAD DESIGN
The requirements for NPT (National Pipe Thread) are spelled out in ANSI B1.20.1
The requirements for NPTF (National Pipe Thread Fuel) are spelled out 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 crest of the threads.

The NPTF thread crests fall with in the parameters of the NPT requirements, but have a smaller range than
the NPT.

The NPTF thread roots are different than the NPT thread roots. NPTF thread roots are designed to interfere
with the crest of the mating thread with the intention of creating a mechanical seal through thread form
deformation at assembly. NPTF threads have two classes identified: Class 1 and Class 2. NPT thread roots
are designed to allow clearance with the mating thread crests on assembly.
http://www.cutting-tool-supply.com/TechTips/Tapping/NPTvsNPTF/NPTVsNPTF.htm
NPT Vs. NPTF Taper Pipe Threads
The two most common taper pipe threads used in the United States are NPT and NPTF. Applications range
from electrical conduits and hand railings to high-pressure pipe lines that carry gas or caustic fluids. NPT
threads are for mechanical or low-pressure air or fluid applications and require the use of sealing
compounds like Teflon tape, to provide the seal. When the application is more critical, and the sealing
compound may fail due to high heat or pressure, NPTF Dryseal threads are used. This mechanical seal is
produced by the mating and slight crushing of the threads when a wrench is applied to tighten the fittings.

Visually, both threads appear to be identical. Both have a ¾” taper over one foot of length. Both have the
same pitch diameter at the top of the hole of internal threads or end of the pipe on external threads, and
both have the same thread lengths or depths. However, there is a subtle difference in the thread form that
differentiates the two. The major and minor diameters of both threads differ slightly. With NPT threads,
after a wrench is applied, slight spaces at the major and minor diameters may exist that would allow the
assembly to leak and therefore a sealing compound is used to fill any gaps. On the other hand, NPTF
threads are designed to ensure that sufficient crushing of the entire thread form will take place to produce
a mechanical seal.

How does the difference in thread forms effect the tooling used to produce NPT and NPTF threads? Taps
are available for both NPT and NPTF threads having the appropriate form to produce each type of thread.
Since NPT threaded parts require sealing compounds, it is acceptable to use an NPTF tap for NPT
applications. However, NPT taps cannot be used for NPTF applications, as it will likely produce a thread that
will leak. The same is true of external threads. In most cases the tap drill is the same for both forms.

The most significant difference in the two threads is the inspection required. Since sealing compounds will
be used for NPT threads, only a single plug with a step, known as an L1 plug (internal thread) or a single
thin L1 ring (external) are required to check size. However, since the taper and the position of major and
minor diameters are so critical to the sealing of NPTF threads, the additional threads in the assembly
known as L2 and L3, and the major and minor diameters are inspected with either special plug or ring gages.