Nothing causes more friction in a pipe enquiry than the three numbers every buyer quotes and few people define: the nominal size, the schedule and the grade. Two purchasers can order "2 inch pipe" and receive tubes with different bores, different walls and different pressure ratings, and both deliveries can be exactly to standard. Pipe dimension language is a set of conventions, each with a history, and the conventions are easy to read once somebody has explained them.

Nominal bore is neither the bore nor the outside diameter

NPS, nominal pipe size, names a family of dimensions rather than a measurement. The pressure table on the A53 carbon steel pipe page lists the pairs: NPS 1/2 has a 0.84 in outside diameter, NPS 2 is 2.375 in, NPS 6 is 6.625 in and NPS 12 is 12.75 in. From NPS 14 upward the convention flips and OD equals the nominal number. The same gap exists in metric: the API 5L pipe page quotes DN6 to DN1200, 1/8" to 48", which is 4 to 1,219 mm of actual OD. So the bore of a 2 inch pipe is smaller than 2 inches by exactly two wall thicknesses, and it changes every time the schedule changes. On a drawing, NB fixes nothing; OD and wall fix everything. Order by OD and wall, and use NB only as shorthand.

Schedule: one label, many walls

Schedule is a wall series, not a wall. SCH 40 on NPS 1/2 is a few millimetres of steel; SCH 40 on NPS 8 is more than twice that. The label indexes a set of walls that thicken with size so that pressure capacity climbs roughly in step across the range. The A53 page lists SCH 40, SCH 80, SCH 160, XS and XXS with an overall wall band of 3-12 mm. Stainless runs wider still: the large diameter thick wall stainless pipe range covers walls of 1-150 mm across SCH 10 to XXS in ODs from 6 to 2,500 mm. XS and XXS are older than schedules, short for standard weight and extra strong, and they survive on drawings because mills keep producing them. When a design calculation gives a required wall, quote that wall with its tolerance class, not a schedule alone.

Tolerances: where the money hides

Nominal wall is a midpoint, and the tolerance clause decides what can actually arrive. On A106 seamless pressure pipe, wall tolerance is ±12.5%. API 5L is asymmetric: +15.0% and -12.5% on wall, so a nominal 10 mm wall can conform at 8.75 mm. Outside diameter is controlled separately: A106 cold drawn tube is ±0.40 mm up to 48.3 mm and ±1% above, while API 5L allows +0.41/-0.80 mm below 60.3 mm OD and ±0.75% above. The A53 specification adds process-dependent tightness, ±0.1 mm on cold drawn and ±0.05 mm on cold rolled work. The honest caveat is printed on the 304/304L stainless pipe page itself: seamless tube has uneven wall thickness and lower surface brightness, and the thinner the wall, the higher the processing cost, which is why heavy wall is the economical direction. If a calculation needs a minimum wall, order nominal wall plus the maximum negative tolerance, or the design margin quietly disappears at the mill.

The grade rides on the same geometry

Grade changes the metal, never the dimensions, which is why two pipes of identical OD and wall can differ by 40% in strength. A53 comes in Grade A at 48,000 psi tensile and 30,000 psi yield, or Grade B at 60,000/35,000. A106 steps to three grades, A, B and C at 48,000, 60,000 and 70,000 psi tensile with carbon ceilings of 0.25, 0.30 and 0.35%. In line pipe the X number is the yield in ksi: X42 is 42,000 psi yield, X70 is 70,000, and PSL2 tightens phosphorus to 0.025 and sulphur to 0.015 while capping the yield-to-tensile ratio. Temperature moves the choice to alloy: A335 P22 carries 1.90-2.60% chromium and 0.87-1.13% molybdenum for boiler service, and A335 P91 reaches 415 MPa tensile with 205 MPa yield. In corrosive duty the 2205 duplex pipe page shows 550 MPa yield and 800 MPa tensile, and 2507 is limited to below 316 °C but out-performs it in chloride pitting. Same geometry, different metal: the medium and the temperature pick the grade. Within austenitic stainless the same OD and wall bands run right across the family, from the 310S pipe for furnace service to 317LMN tube where added molybdenum is needed against pitting. Boiler orders add the stabilised grades, A213/SA312 Grade 347 and its high-carbon 347H variant. Where chlorides rule out austenitics altogether, S31803 and S32205 are simply the two UNS names behind a 2205 callout, and they arrive on the same schedules.

Ends and lengths

The last two fields of a pipe line are ends and length. Ends are plain, beveled or threaded on A53; the 2205 page spells out the full abbreviation set, BOE, POE, TBE, BLE, PBE and the rest, all of which reduce to three intentions: plain for cut-to-length spooling, beveled for welding, threaded for screwed assemblies. Lengths are more varied than buyers expect. A53 is supplied black or hot-dip galvanized from 3 to 18 m. A106 defaults to 6 m or specified lengths. API 5L offers fixed 5.8, 6, 11.8 and 12 m plus SRL and DRL, single random and double random length, roughly 6-7 m and 11-12 m. The P91 page lists single random, double random and cut length as separate supply modes, and the big stainless range offers 4,000, 5,800, 6,000 and 12,000 mm. The 5.8 and 11.8 m lengths exist because a 20 ft container has 5,898 mm of internal length; a 6 m stick does not ship in one. Not every section is round, either: the square stainless pipe range runs 10x10 to 300x300 mm and is measured across flats, wall, then length.

What the mill actually tests

A106 requires a hydrostatic test on every length, or a full-body nondestructive electric test in its place, plus a cold bend through 90° around a mandrel and flattening capability. A335 adds transverse or longitudinal tension, hardness and, above NPS 25 with a diameter-to-wall ratio of 7 or less, a bend test instead of flattening. The stainless range runs squash, extended and water pressure tests, and the A53 supply page confirms mill certificates to EN 10204 3.1B. Sour service is a separate conversation: NACE MR0175, TM0177, TM0284, HIC and SSC testing are all offered on request and must be written into the order, because they change the melt practice, not just the paperwork. On arrival, measure OD with a pi tape, wall with a caliper or ultrasonic gauge at four points around the circumference, and read the certificate against the heat number stamped on the pipe, not against the packing list.

A pipe line that reads grade, OD and wall, schedule as shorthand, end preparation, length type and certificate level is unambiguous in any language, whether it points at A53 supply or at the wider steel pipe and stainless pipe ranges. Anything less leaves the mill, or the forwarder, to fill the gaps, and neither of them knows what the calculation needed.