Two words, two jobs

Pipe conveys; tube is measured. The two words describe not two metals — same mill, same chemistry, often the same machine — but two buying languages with different tolerances, and orders get mispriced when they are mixed. A pipe order is written as a nominal size with a wall series attached, because pipe has to mate with fittings, flanges and thread gauges standardised long before the order. A tube order is written as an outside diameter against a wall in millimetres, because tube has to fit a bundle layout, a clip or a compression fitting that cares about the actual dimension. The pipe category on this site runs from A106 pressure pipe to square sections, and the distinction is visible inside it: the products sold as pipe carry schedule tables, and the products sold as tube carry outside-diameter lists.

The nominal size is a name, not a measurement

Nominal size survives because it keeps the outside diameter fixed while the wall moves. The A53 page sells 1/2 to 36 inch NB with walls from 3 to 12 mm, and notes that sizes up to 26 inch OD are available on a limited basis — a sentence that only makes sense once NB and OD are understood as different things. A one-inch nominal pipe does not measure 25.4 mm across the outside; the OD is fixed by the schedule system so that a fitting for one wall fits another, and the wall grows inward. Metric practice does the same: the API 5L listing runs DN6 to DN1200, which is 4 to 1219 mm of outside diameter across 1 to 60 mm of wall. Stainless blurs the boundary openly: the 304/304L page offers 3 to 2200 mm outside diameter and 0.08 to 170 mm of wall under the word pipe, and the A106 programme cold-draws down to 15.0 mm OD — at which point pipe and tube describe the same object with different paperwork.

Wall thickness is sold in three languages

The first language is the schedule. The S31803 duplex listing spans SCH5 through SCH160 and XXS, the seamless line pipe page runs SCH 10 to 160 with STD, XS and XXS beside them, and the P91 page adds Schedule 20 to XXS with heavier walls on request — a wall named by a code whose millimetre value changes with every diameter. The second is the millimetre, and it is the language of tube: the A213 boiler tube listing states 12.7 to 114.3 mm outside diameter with 0.8 to 15 mm of wall, and the square and rectangular sections run 10x10 to 300x300 mm and 10x20 to 120x180 mm at 0.5 to 75 mm. The third is the gauge, inherited from sheet practice; it survives because a gauge number maps to a thickness band rather than a single value. The three languages can name the same wall but cannot be converted by arithmetic alone; a schedule, a gauge and a millimetre figure agree only where the standard says so.

Tolerance is where the standards part company

The tolerances tell the two words apart more clearly than the names do. A106 allows ±0.40 mm on outside diameter up to 48.3 mm and ±1 per cent above 60.3 mm, with the wall at ±12.5 per cent — generous, because pipe is welded into a system that absorbs the variation. The API 5L table is tighter at the small end, +0.41 and −0.80 mm below 60.3 mm and ±0.75 per cent of diameter above it, with the wall at +15 and −12.5 per cent. The stainless pipe page quotes ±1 per cent outright. Then the same catalogue quotes cold-drawn tube at ±0.1 mm and cold-rolled at ±0.05 mm on the P91 and A53 listings — tolerances a hot-rolled schedule pipe never carries. The ERW versus seamless page adds the process angle: wall tolerance is generally tighter for seamless, which matters when the design sits close to the minimum. Ask for tube and receive schedule pipe, and the bundle may still not close.

The test programme follows the word

Testing is written for the duty the word implies. A106 hydrostatically tests every length, or applies a nondestructive electric test over the full body as the alternative, and expects the pipe to bend cold through 90 degrees and to be capable of meeting the flattening test. A53 performs tension, bend and flattening tests and issues EN 10204 3.1B mill certificates. Tube raises the stakes because the wall is thinner and the duty is thermal: the A213 listing puts eddy current testing, hydrostatic testing or ultrasonic examination on every tube, with single and double random lengths to a maximum of 25,000 mm. The 347H page adds flattening, flaring, visual and chemical tests to the same list, and the duplex programme names third-party inspection by TUV, SGS, BV, ABS and LR alongside the stainless pipe range it serves. The lesson is not that one is better; it is that the certificate should name the tests the word on the order implies.

Length, ends and what the fitting expects

Length and end preparation finish the separation. Pipe travels in random lengths — A53 between 3 and 18 m, API 5L line pipe fixed at 5.8, 6, 11.8 or 12 m as SRL or DRL — or in the long custom lengths the P91 page lists at up to 16,000 mm, the duplex listing at 12,000 mm and the large diameter thick wall listing cuts to order. Ends follow the joint: plain, beveled, threaded and grooved are all standard offers, protected by plastic caps, because pipe is welded, screwed or coupled on site; the 2205 listing even spells the codes out — BOE, POE, TBE, BLE and plain or beveled. The A106 page states the expectation directly — suitable for bending, flanging and similar forming, and for welding — and the black or hot-dipped galvanized finishes on the galvanized pipe line are ordered the same way the joint is. Tube, by contrast, is usually bent to a radius and joined by fittings that grip the outside diameter, which is why its tolerance and surface finish — annealed and pickled, bright, polished — are part of the dimensional argument, not an appearance choice.

What to write so the order survives translation

Six lines keep the words straight. Write the standard and the word together — A106, A213, A790 — instead of assuming the seller will infer them. Give the size in the language of that standard: nominal size with schedule for pipe, outside diameter against wall in millimetres for tube. State the tolerance class, because ±12.5 per cent of wall and ±0.1 mm of diameter are different products at the same price band. Name the delivery condition and finish, from annealed and pickled to bright. Name the test — hydrostatic, eddy current, ultrasonic, flattening — and the certificate. Finally, say what joins the product: welded, threaded and coupled, or a compression fitting, and let the processing service quote the cutting, welding and surface treatment on the same sheet. An order written this way returns the product that was meant, whether the drawing called it pipe or tube.