Nominal Pipe Size – (NPS)
Nominal Pipe Size (NPS) is a set of standard pipe sizes used for pressure piping in North America. The same pipe dimensions are used with different names in Europe.
Pipe size is specified with two non-dimensional numbers: a Nominal Pipe Size (NPS) and a schedule (SCH). The relationship of these numbers to the actual pipe dimensions is a bit strange. The NPS is very loosely related to the inside diameter in inches, but only for NPS 1/8 to NPS 12. For NPS 14 and larger, the NPS is equal to the outside diameter (OD) in inches. For a given NPS, the OD stays constant and the wall thickness increases with larger SCH. For a given SCH, the OD increases with increasing NPS while the wall thickness increases or stays constant. Pipe sizes are documented by a number of standards, including API 5L, ANSI/ASME B36.10M in the US, BS 1600 and BS EN 10255 in the United Kingdom and Europe, and ISO 65 internationally.
For NPS of 5 and larger, the DN is equal to the NPS multiplied by 25 (not 25.4).
The most commonly used schedules today are 40, 80, and 160. There is a commonly held belief that the schedule number is an indicator of the service pressure that the pipe can take. For example, the McGraw Hill Piping Handbook says the schedule number can be converted to pressure by dividing the schedule by 1000 and multiplying by the allowable stress of the material. However, this is not true Pressure rating actually goes down with increasing NPS and constant schedule.
The various standards for pipe schedule are not identical. Frequently some sizes—or even entire schedules—are present in some standards but not others. When different standards do overlap, they usually have the same dimensions. For this reason, the source of the schedules is not distinguished in the table below. Beyond NPS 8, however, there are differing version of schedules 5, 10, 40, and 80. One variation is the presence or absence of an ‘S’ suffix after the schedule number. Generally, ‘S’ indicates Stainless Steel Schedule.
Some specifications use pipe schedules called Standard Wall (STD), Extra Strong (XS), and Double Extra Strong (XXS), although these actually belong to an older system called Iron Pipe Size (IPS). The IPS number is the same as the NPS number.
STD is identical to SCH 40S, and 40S is identical to 40 for NPS 1/8 to NPS 10, inclusive. XS is identical to SCH 80S, and 80S is identical to 80 for NPS 1/8 to NPS 8, inclusive. Different definitions exist for XXS, but it is generally thicker than schedule 160.
8 to NPS 3-1/2
|Wall Thickness (inches)|
|SCH 5||SCH 10||SCH 30||SCH 40||SCH 80||SCH 120||SCH 160|
NPS 4 to NPS 8
|NPS||DN||OD (inches)||Wall Thickness (inches)|
|SCH 5||SCH 10||SCH 20||SCH 30||SCH 40||SCH 60||SCH 80||SCH 100||SCH 120||SCH 140||SCH 160|
NPS 10 to NPS 24
|Wall Thickness (inches)|
|SCH 5s||SCH 5||SCH 10s||SCH 10||SCH 20||SCH 30||SCH 40s||SCH 40|
NPS 10 to NPS 24 (Continue)
|Wall Thickness (inches)|
|SCH 60||SCH 80s||SCH 80s||SCH 80||SCH 100||SCH 120||SCH 140||SCH 160|
NPT (National Pipe Thread)
National Pipe Thread is a U.S. standard for tapered (NPT) or straight (NPS) threads used to join pipes and fittings. ANSI/ASME standard B1.20.1 covers threads of 60-degree form with flat crests and roots in sizes from ¹⁄16 inch to 24 inch Nominal Pipe Size. The taper rate for all NPT threads is ¹⁄16 (³⁄₄inch per foot) measured by the change of diameter (of the pipe thread) over distance. The taper divided by a center line yields an angle 1° 47′ 24″ or 1.7899° as measured from the center axis.
Commonly-used sizes are ¹⁄8, ¹⁄4, ³⁄8, ¹⁄2, ³⁄4, 1, 1 ¹⁄4, 1 ¹⁄2, and 2 inch, appearing on pipe and fittings by most U.S. suppliers. Smaller sizes than those listed are occasionally used for compressed air. Larger sizes are used less frequently because other methods of joining are more practical at 3 inches and above in most applications.
Nominal Pipe Size is loosely related to the inside diameter of schedule 40 pipe. Because of the pipe wall thickness, the actual diameter of the threads is larger than the NPS, considerably so for small NPS. Other schedules of pipe have different wall thickness but the OD (outer diameter) and thread profile remain the same, so the inside diameter of the pipe is therefore different from the nominal diameter.
Threaded pipes can provide an effective seal for pipes transporting liquids, gases, steam, and hydraulic fluid. These threads are now used in materials other than steel and brass, including PVC, nylon, bronze and cast iron.
The taper on NPT threads allows them to form a seal when torqued as the flanks of the threads compress against each other, as opposed to straight thread fittings or compression fittings in which the threads merely hold the pieces together and do not provide the seal. However a clearance remains between the crests and roots of the threads, resulting in a leakage around this spiral. This means that NPT fittings must be made leak free with the aid of thread seal tape or a thread sealant compound. (The use of tape or sealant will also help to limit corrosion on the threads, which can make future disassembly nearly impossible.)
There is also a semi-compatible variant called NPTF or Dryseal, designed to provide a more leak-free seal without the use of teflon tape or other sealant compound. NPTF threads are the same basic shape but with crest and root heights adjusted for an interference fit, eliminating the spiral leakage path.
Sometimes NPT threads are referred to as MPT (Male Pipe Thread), MNPT, or NPT(M) for male (external) threads and FPT (Female Pipe Thread), FNPT, or NPT(F) for female (internal) threads. An equivalent designation is MIP (Male iron pipe) and FIP (Female iron pipe).
Pipe thread sizes
|14 ” OD||14 “||8||0.12500|
|16 ” OD||16 “||8||0.12500|
|18 ” OD||18 “||8||0.12500|
|20 ” OD||20 “||8||0.12500|
|24 ” OD||24 “||8||0.12500|
A.R.I Flow Control Accessories (n.d.). NPS (Nominal Pipe Size). A.R.I Flow Control Accessories Ltd.