A schedule number does not answer the pressure question

Pipe enquiries often arrive with a schedule and a grade and nothing else, as though the pair settled the pressure question. It does not. A schedule is a wall-thickness series indexed to nominal size, and its number means nothing until it is read against a diameter and a grade. The A53 pipe page is one of the few places on this site that publishes the arithmetic instead of the vocabulary, with a pressure-rating table giving maximum allowable pressure in psi against NPS, outside diameter and schedule. At NPS 6 the listed values run from 2,071 psi to 6,390 psi; at NPS 12, from 961 psi to 5,042 psi. Two buyers can order six inch pipe in the same grade and be a factor of three apart on what it holds, and neither is wrong.

Where the pressure number comes from

The calculation is not mysterious. Hoop stress in a thin-walled cylinder is pressure times diameter divided by twice the wall, so the wall required is pressure times outside diameter divided by twice the allowable stress. Everything difficult sits in that last term. Allowable stress is not yield; it is a fraction of yield set by the design code, and the fraction accounts for the consequences of yielding rather than for the yield point itself. The A106 page supplies the inputs, with 48,000, 60,000 and 70,000 psi minimum tensile for Grades A, B and C and 30,000, 35,000 and 40,000 psi minimum yield. A designer who divides by yield rather than by allowable stress produces a wall thinner than any code permits; one who ignores the seam or casting factor produces a wall thicker than the quotation expects. Neither error surfaces until someone repeats the calculation.

Temperature moves the allowable stress, not the wall

A wall sized cold and installed hot is a different pipe, and this is the step most order sheets skip. The P91 page carries a table worth copying: 0.2 per cent proof stress falls from 410 MPa at 100 degrees C to 380 at 200, 360 at 300, 340 at 400, 300 at 500 and 215 MPa at 600. That is a loss of 48 per cent against the number a cold calculation would have used, and the wall does not grow to compensate. The same page fixes the room-temperature floor at 585 MPa tensile, 415 MPa yield and 250 HB maximum, and offers Schedule 20 through XXS, heavier on request. Lower down the temperature scale, the P22 pipe covers 19.05 to 114.3 mm outside diameter at 2.0 to 14 mm wall, while the boiler tube range listed as T2 through T12 runs 12.7 to 114.3 mm at 0.8 to 15 mm, where the same argument is usually made inside a boiler rather than along a line. Above that band the stabilised austenitics take the duty. 347H holds columbium between eight times the carbon content and 1.10 per cent precisely so the carbon stays out of solution at temperature, and its mechanical floor is 515 MPa tensile, 205 MPa yield and 35 per cent elongation. The 310S pipe programme runs 0.5 to 75 mm of wall, and the 310S alloy page is where the oxidation side of the same limit is set out.

The mill tolerance eats the wall that was calculated

Then the pipe arrives thinner than the drawing. The A106 tolerance block permits minus 12.5 per cent on wall thickness for cold-drawn stock, and that is not a defect, it is the specification. A 7.1 mm wall may be supplied at 6.2 mm and still be fully compliant. Outside diameter carries its own band, plus or minus 0.40 mm up to 48.3 mm and plus or minus one per cent above 60.3 mm. The seamless API 5L page states its band as plus 0.41 and minus 0.80 mm, asymmetric because the process is. Wall thickness therefore has to be ordered above the calculated figure by at least the tolerance, with the corrosion allowance sitting on top of that rather than inside it. A buyer who writes 7.1 mm minimum and accepts a certificate reading 7.1 nominal has bought 6.2 mm of pressure boundary.

Corrosion allowance is a second wall

In carbon service the allowance is frequently the larger of the two numbers. A line losing half a millimetre a year across a twenty-year design life needs 10 mm of sacrificial wall before the pressure calculation begins, which is why API 5L line pipe is stocked from 1 mm to 60 mm across SCH 5 to XXS and from DN6 to DN1200. That range exists because one bore serves both a 40 bar gas line and a low-pressure slurry line. Where the allowance cannot be carried, the alloy changes instead of the wall. The 317LMN tube adds molybdenum and nitrogen for pitting and crevice resistance on a low-carbon base, the S31803 duplex pipe spans 6 to 1174 mm outside diameter at 1 to 80 mm wall, and the 2205 pipe page quotes 550 MPa yield against 800 MPa tensile, which buys wall back by raising the allowable stress rather than by thickening the pipe. From there the ladder is continuous: S32205 covers 0.3 to 50 mm of wall, 2507 and S32760 raise pitting resistance again for chlorides the 300 series cannot hold, and 2205 plate sits alongside them where the pressure boundary is a vessel rather than a run of pipe. Alloy is cheaper than 10 mm of sacrificial carbon wall only when the tonnage amortises the difference.

Seam or no seam is also a pressure decision

Seamless is not automatically correct and welded is not automatically cheap. The ERW comparison sets out where the seam becomes the limiting feature under cyclic or high-pressure duty, and the A53 page is explicit that one grade arrives as Type S, Type E or Type F with different chemistry ceilings for each, which is not a distinction the schedule shows. What the standards do guarantee is that the pressure boundary gets tested. The A106 page requires every length to pass a hydrostatic test without leakage through the wall, or alternatively to be examined over its full body by a nondestructive electric test, and the P22 page adds a nondestructive examination on every pipe as well. Testing confirms the wall that was made. It does not replace the wall that was specified.

Six lines for the enquiry

Give design pressure and design temperature together, because either alone is unanswerable. Give outside diameter rather than nominal bore, since large diameter thick wall pipe runs 6 to 2500 mm and 304 pipe covers 0.08 to 170 mm of wall, so eight inch no longer identifies a product. State the allowable stress basis, whether it is the code table or your own. State the corrosion allowance and whether the ordered wall already includes it. State the mill tolerance you will accept, then order above it. Name the test and the document in the same paragraph, because a 12.5 per cent tolerance and a 3.1 certificate describe the same delivery; add welding, cutting and custom processing as separate lines, since none of them is included in the price of a bare pipe.