B) American National Standard Pipe Thread Form
C) Specification for General Purposes Taper Pipe Threads, NPT
D) Specifications for Internal Straight Threads in Pipe Couplings, NPSC
E) Specifications for Railing Joint Taper Pipe Threads NPTR
F) Specifications for Straight Pipe Threads for Mechanical Joints, NPSM, NPSL, NPSH
This American National Standard covers dimensions and gaging of pipe threads for general purpose applications.
The types of pipe threads included in this Standard are designated by specifying in sequence the nominal pipe size *, number of threads per inch and the thread series symbol as follows:
3/8 - 18 NPT
1/8 - 27 NPSC
1/2 - 14 NPTR
1/8 - 27 NPSM
1/8 - 27 NPSL
1 - 11.5 NPSH
For left hand threads add LH to the designation, otherwise right hand threads will be understood. For example:
3/8 - 18 NPT - LH
Each of these letters in the symbols has a definite significance as follows:
N = National (American) Standard
P = Pipe
T = Taper
C = Coupling
S = Straight
M = Mechanical
L = Locknut
H = Hose Coupling
R = Railing Fittings
or Plated Threads.
The threaded product specifications covered in this Standard do not include an allowance for coatings or plating.
Where pressure-tight joints are required, it is intended that taper pipe threads conforming to this Standard be made up wrench-tight with a sealant. To prevent galling on certain piping materials such as stainless steels, the sealant usually contains a lubricant.
A gaging method and tolerances are prescribed in this Standard to effect a functional inspection of the handtight L 1 engagement threads. However, conformance to this Standard requires that all basic design dimensions be met (within applicable tolerances) including extension of the thread elements to provide for wrench-tight makeup. Therefore, additional methods of gaging may be employed to evaluate conformance to the basic design dimensions. When additional methods of gaging are employed, they shall be agreed upon by the supplier and the purchaser.
* Where it is necessary to use decimal notation for the size designation (as when inserting such in a computer or electronic accounting machine) the decimal equivalent of nominal pipe size may be substituted for fractional pipe sizes.
The form of thread profile specified in this Standard shall be known as the American National Standard Pipe Thread Form. The relations as specified herein, for form of thread and general notation are shown in Fig.1.
The angle between the sides of the thread is 60 deg when measured in an axial plane. The line bisecting this angle is perpendicular to the axis.
Truncation and Thread Height
The height of the sharp V thread, H, is
H = 0.866025p = 0.866025/n
p = pitch of thread n = threads per inch
The basic maximum depth of the truncated thread, h (see Fig.1), is based on factors entering into the manufacture of cutting tools and the making of tight joints.
h = 0.800p = 0.800/n
The crest and root of pipe threads are truncated a minimum of 0.033p. The maximum depth of truncation for the crest and root of these pipe threads will be found in Table 1. The crests and roots of the external and internal threads may be truncated either parallel to the pitch line or parallel to the axis.
The sketch in Table 2, giving a sectional view of this Standard thread form, represents the truncated thread form by a straight line. However, whet, closely examined, the crests and roots of commercially manufactured pipe threads appear slightly rounded. When crests and roots of threading tools or chasers lie within the limits shown in Table 1, the pipe threads of products produced by such means are acceptable on the basis of in-process control.
Taper Pipe Threads
Threads made in accordance with these specifications consist of an external taper and an internal taper thread, to form the normal type of joint having general application on pipe and fittings. Fig.2.
NPT taper pipe threads are intended to be made up wrench-tight and with a sealant whenever a pressure-tight joint is required.
is affected by out-of-roundness which is possible between the
wrench-tight mated parts in final assembly. This will vary depending
on the method for producing the thread in conjunction with the
elasticity and/or ductility of the mating parts and the resultant
conformance at final assembly.
Thread Designation and Notation.
American National Standard Taper Pipe Threads are designated in accordance with A.2.1 as follows: 3/8 - 18 NPT
Standard notation applicable to American National Standard Taper Pipe Threads is shown in Fig.3.
Designation of Plated Threads.
The product specifications of this Standard do not include an allowance for plating. If plating is desired, it may be necessary to modify the threads since the same final gaging requirements must be satisfied for plated and unplated parts. This may be emphasized by adding the words AFTER PLATING to the designation. For manufacturing purposes, notes for plated taper pipe threads may specify the gage limits (turns or threads engagement) before plating followed by the words BEFORE PLATING. These should be followed by the standard gage limits (turns or threads engagement) after plating and the words AFTER PLATING.
3.1.3 Form of Thread.
The form of the thread for American National Standard Taper Pipe Threads is that specified in section B.
3.1.4 Taper of Thread. The taper of the thread is I in 16 or 0.75 in./ft measured on the diameter and along the axis.
3.1.5 Diameter of Thread. The basic pitch diameters of the taper thread are determined by the following formulas' based on the outside diameter of the pipe and the pitch of the thread:
E0 = D - (0.05D + 1. 1) 1 In
= D - (0.05D + 1.1) p
E I = E0 + 0.062 5L
D = outside diameter of pipe
E0 = pitch diameter of thread at end of pipe or small end of external thread
E1 = pitch diameter of thread at the gaging notch or large end of internal thread
L I = normal engagement by hand between external and internal threads
n = threads per inch
2 For the 1/8-27 and 1/4-18 sizes, E1 approx.= D - (0.05D +0.8 27) P.
Length of Thread.
The basic length of the effective external taper thread L2, is determined by the following formula based on the outside diameter of the pipe and the pitch of the thread:
L2 (0.80D + 6.8) 1 In
(0.80D + 6.8) p
D = outside
diameter of pipe n = threads per inch
This formula determines directly the length of effective thread which includes two usable threads slightly incomplete at the crest.
3.1.7 Engagement Between External and Internal Taper Threads.
The normal length of engagement between external and internal taper threads when screwed together handtight is shown in col. 6, Table 2. This length is controlled by the construction and use of the gages. It is recognized that in special applications, such as flanges for high pressure work, longer thread engagement is used, in which case the pitch diameter (dimension E1 , Table 2) is maintained and the pitch diameter E0 at the end of the pipe is proportionately smaller.
The basic dimensions of taper pipe threads, derived from the above specifications, are given in Table 2 .All dimensions are given in inches unless otherwise specified.
Manufacturing Tolerance on Product.
The maximum allowable deviation in the commercial product is one turn large or small from gages made to the basic dimensions.
Tolerances on Thread Elements. The permissible deviations in thread elements are given in Table 3. This table is a guide for establishing limits of the thread elements of taps, dies, and thread chasers. Conformance to these limits may be required on product threads, in which case specifications shall require control and checking of thread elements.
On pipe fittings and valves (not steel or high grade alloys used in critical services) for steam pressures 300 lb and below, it is intended that plug and ring gage practices as established in this Standard be used in conjunction with tooling control of thread elements, e.g., taps and dies, to provide satisfactory control of functional size. Therefore, no tolerances on thread elements have been established for this class. For service conditions, where more exact checks are required, procedures have been developed by industry to supplement the standard plug and ring gage method of gaging
Straight Pipe Threads in Pipe Couplings
Threads in pipe couplings made in accordance with these specifications are straight (parallel) threads of the same thread form as the American National Standard Taper Pipe Thread specified in B.1. They are used to form pressure-tight joints when assembled with an American National Standard external taper pipe thread and made up wrench-tight with lubricant or sealant.
The American National Standard Coupling Straight Pipe Threads are designated in accordance with A.2.1 as follows:
1/8 - 27 NPSC
Dimensions and Limits of Size.
The dimensions and pitch diameter limits of size are specified in Table 4. Die pitch diameter limits of size correspond to one and one-half turns large or small of the standard taper pipe thread. The major and minor diameters vary with the pitch diameter, as the American National Standard Pipe Thread form is maintained within the truncation tolerances shown in Table 1.
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