physics

Nominal Pipe Size

Nominal Pipe Size is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Nominal Pipe Size rather than just read about it. In short: Nominal Pipe Size (NPS) is a North American set of standard sizes for pipes used for high or low pressures and temperatures. "Nominal" refers to pipe in non-specific terms and identifies the diameter of the hole with a non-dimensional number (for example, "2-inch-nominal steel pipe" consists of many varieties of steel pipe with the only criterion being a 2.375-inch (60.3 mm) outside diameter). Specific pipe is ident…

Key takeaways

Nominal Pipe Size belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Nominal Pipe Size to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Nominal Pipe Size from memory before moving on to harder problems.

Reference excerpt

Nominal Pipe Size (NPS) is a North American set of standard sizes for pipes used for high or low pressures and temperatures. "Nominal" refers to pipe in non-specific terms and identifies the diameter of the hole with a non-dimensional number (for example, "2-inch-nominal steel pipe" consists of many varieties of steel pipe with the only criterion being a 2.375-inch (60.3 mm) outside diameter). Specific pipe is identified by pipe diameter and another non-dimensional number for wall thickness referred to as the Schedule (Sched. or Sch., for example, "2-inch-diameter pipe, Schedule 40"). NPS is often incorrectly called National Pipe Size, due to confusion with the American standard for pipe threads, "national pipe straight", which is also abbreviated as NPS. The European and international designation equivalent to NPS is DN (diamètre nominal/nominal diameter/Nennweite), in which sizes are measured in millimetres; see ISO 6708. The term NB, for nominal bore, is also frequently used interchangeably with DN. In March 1927 the American Standards Association authorized a committee to standardize the dimensions of wrought steel and wrought iron pipe and tubing. At that time only a small selection of wall thicknesses were in use: standard weight (STD), extra-strong (XS), and double extra-strong (XXS), based on the iron pipe size (IPS) system of the day. However, these three sizes did not fit all applications. Also, in 1939, it was hoped that the designations of STD, XS, and XXS would be phased out by schedule numbers; however, those original terms are still in common use today (although sometimes referred to as standard, extra-heavy (XH), and double extra-heavy (XXH), respectively). Since the original schedules were created, there have been many revisions and additions to the tables of pipe sizes based on industry use and on standards from API, ASTM, and others. Stainless steel pipes, which were coming into more common use in the mid 20th century, permitted the use of thinner pipe walls with much less risk of failure due to corrosion. By 1949 thinner schedules 5S and 10S, which were based on the pressure requirements modified to the nearest BWG number, had been created, and other "S" sizes followed later. Due to their thin walls, the smaller "S" sizes can not be threaded together according to ASME code, but must be fusion welded, brazed, roll grooved, or joined with press fittings.

Application Based on the NPS and schedule of a pipe, the pipe outside diameter (OD) and wall thickness can be obtained from reference tables such as those below, which are based on ASME standards B36.10M and B36.19M. For example, NPS 14 Sch 40 has an OD of 14 inches (360 mm) and a wall thickness of 0.437 inches (11.1 mm). However, the NPS and OD values are not always equal, which can create confusion.

For NPS 1⁄8 to 12, the NPS and OD values are different. For example, the OD of an NPS 12 pipe is actually 12.75 inches (324 mm). To find the actual OD for each NPS value, refer to the tables below. (Note that for tubing, the size indicates actual dimensions, not nominal.) For NPS 14 and up, the NPS and OD values are equal. In other words, an NPS 14 pipe is actually 14 inches (360 mm) OD. The reason for the discrepancy for NPS 1⁄8 to 12 inches is that these NPS values were originally set to give the same inside diameter (ID) based on wall thicknesses standard at the time. However, as the set of available wall thicknesses evolved, the ID changed and NPS became only indirectly related to ID and OD. For a given NPS, the OD stays fixed and the wall thickness increases with schedule. For a given schedule, the OD increases with NPS while the wall thickness stays constant or increases. Using equations and rules in ASME B31.3 Process Piping, it can be demonstrated that the pressure rating decreases as the NPS increases while the schedule remains constant. 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. XXS wall is thicker than schedule 160 from NPS 1⁄8 in to NPS 6 in inclusive, and schedule 160 is thicker than XXS wall for NPS 8 in and larger.

Blockage or ball test When a pipe is welded or bent the most common method to inspect blockages, misalignment, ovality, and weld bead dimensional conformity is to pass a round ball through the pipe coil or circuit. If the inner pipe dimension is to be measured then the weld bead should be subtracted, if welding is applicable. Typically, the clearance tolerance for the ball must not exceed 1 millimetre (0.039 in). Allowable ovality of any pipe is measured on the inside dimension of the pipe; normally 5% to 10% ovality can be accepted. If no other test is conducted to verify ovality, or blockages, this test must be seen as a standard requirement. A flow test can not be used in lieu of a blockage or ball test. See pipe dimensional table, Specification ASME B36.10M or B36.19M for pipe dimensions per schedule. Stainless steel pipe is most often available in standard weight sizes (noted by the S designation; for example, NPS Sch 10S). However stainless steel pipe can also be available in other schedules. Both polyvinyl chloride pipe (PVC) and chlorinated polyvinyl chloride pipe (CPVC) are made in NPS sizes.

NPS tables for selected sizes

NPS 1⁄8 to NPS 3+1⁄2 DN does not exactly correspond to a size in millimeters, because ISO 6708 defines it as being a dimensionless specification only indirectly related to a diameter. The ISO 6708 sizes provide a metric name for existing inch sizes, resulting in a 1:1 correlation between NPS and DN sizes. ISO 6708 does not include values for "DN 6" or "DN 8"; however, ASME B36.10M lists "DN 6" and "DN 8". Also, the European Standard EN 12 516-1 (Industrial valves – Shell design strength – Part 1: Tabulation method for steel valve shells) specifies the dimensions "DN 6" and "DN 8", respectively their equivalents NPS 1⁄8 and NPS 1⁄4.

Tolerance: The tolerance on pipe OD is +1⁄64 inch (0.016 in; 0.40 mm), −1⁄32 inch (0.031 in; 0.79 mm). As per ASME B36.10M -2018 Pipe wall thickness are rounded to nearest 0.01 mm (0.00039 in), while converting wall thickness from inch to millimetre.

NPS 4 to NPS 9

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Nominal Pipe Size

Start with the simplest possible case. Write down what Nominal Pipe Size claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Nominal Pipe Size before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Nominal Pipe Size ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Nominal Pipe Size

In research: Nominal Pipe Size appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Nominal Pipe Size in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Nominal Pipe Size is common in secondary-school and first-year university syllabi. It links to neighbouring topics Customary units of measurement in the United States, Mechanical standards, Piping, so understanding it makes those chapters shorter.
In everyday life: Look for Nominal Pipe Size outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Nominal Pipe Size in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Nominal Pipe Size means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Nominal Pipe Size out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Nominal Pipe Size in simple terms?

Why does Nominal Pipe Size matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Nominal Pipe Size?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Nominal Pipe Size.