National Pipe Thread

NPT pipe threades or American Standard Taper Pipe Threads are used frequently in the oil and gas industry.  The threads have a taper to not only provide a method of mechanical joining of two fittings or pipes but also to provide a leak proof joint.  In order to accomplish this, the threads become shallower the farther they are from the end of the pipe or fitting.  If you were to take the cross section of a NPT threaded end, you would see that the bottoms of the threads aren't on a cylinder, but a cone.  This is the taper which is 1/16 inch per inch, which is equivalent to 3/4 inch per foot.

Because of the taper, a pipe can only screw into a fitting a certain distance before it jams.  This is much different than threading a nut on a bolt. ASME B1.20.1 specifies the effective thread distance.  It also specifies the engagement distance which is the distance the pipe can be screwed in by hand, without much effort.  Engineers and piping designers need to take this into account when they are creating drawings.  Field workers need to know how many turns it takes to make up the fitting by hand or with a wrench.

 

National Pipe Thread Dimensions
Nominal size	Actual OD	Threads per inch	Length of engagement hand tight	Hand tight turns	Wrench makeup turns	Length of effective thread
1/16	0.3125	27	0.160	-	-	0.2611
1/8	0.405	27	0.180	4.50	2.50	0.260
1/4	0.540	18	0.200	4	3	0.401
3/8	0.675	18	0.240	4.50	3	0.408
1/2	0.840	14	0.320	4.50	3	0.534
3/4	1.050	14	0.340	4.50	3	0.546
1	1.315	11.5	0.400	4.50	3.25	0.682
1-1/4	1.660	11.5	0.420	4.50	3.25	0.707
1-1/2	1.900	11.5	0.420	4.50	3.25	0.724
2	2.375	11.5	0.436	5	3	0.756
2-1/2	2.875	8	0.682	5.50	3	1.136
3	3.500	8	0.766	6	3	1.2000
3-1/2	4.000	8	0.821	6.50	3.50	1.2500
4	4.500	8	0.844	6.75	3.50	1.3000
5	5.563	8	0.937	7.50	3.75	1.4063

 

standards

• ASME
  • ASME B1.20.1 - Pipe Threads, General Purpose, Inch