Pipes & Tubes

What you are actually specifying

A pipe is defined by four things: the grade, the outside diameter, the wall thickness and the manufacturing route. Get any one of them wrong and the material is useless to you, however good the mill was.

Outside diameter is fixed by NPS. This surprises people new to piping: a 4" pipe has an outside diameter of 114.3 mm whether it is Sch 10S or Sch 160. Increasing the schedule thickens the wall inwards and reduces the bore. That is what makes the schedule system work — fittings, flanges and supports built for a given NPS fit any schedule of that size.

Wall thickness comes from the schedule. Carbon and alloy pipe follows ASME B36.10M (Sch 40, 80, 160, XXS). Stainless follows ASME B36.19M, whose S schedules — 5S, 10S, 40S, 80S — exist because stainless usually needs no corrosion allowance and can therefore be run thinner for the same duty. Note that 40S and Sch 40 are identical up to NPS 10 and diverge above it, and 80S and Sch 80 diverge above NPS 8. If a drawing says only “Sch 40” on a stainless line above 10", ask which standard is meant before you order.

Choosing the grade

Grade selection follows the service, not the budget. In rough order of increasing capability:

• 304 / 304L — general purpose austenitic. Fine for water, steam, food and mild chemical duty. No molybdenum, so it pits in chlorides.
• 316 / 316L — adds around 2% molybdenum and with it genuine pitting resistance. The default process grade. The L means low carbon, which prevents chromium carbide precipitation at the grain boundaries during welding.
• 321 / 347 — stabilised with titanium and niobium respectively. Specify these when the line will sit for long periods in the 425–815 °C sensitisation range, where even a low-carbon grade will eventually sensitise.
• 310S — high chromium and nickel for heat resistance to around 1050 °C. A furnace and heat-treatment grade, not a chloride grade.
• 904L — high nickel, molybdenum and copper. Its territory is sulphuric acid and other reducing environments where 316L is consumed.
• Duplex S31803 / S32205 — roughly double the yield strength of 316L, with a PREN around 35. Higher strength often means a thinner wall and a lighter line. Watch the upper temperature limit, usually taken as about 300 °C.
• Super Duplex S32750 / S32760 — PREN above 40. Seawater, brine and reverse osmosis reject.
• Nickel alloys — Inconel 625 for very wide corrosion coverage, Hastelloy C276 for reducing and mixed acids, Monel 400 for seawater and hydrofluoric acid, Incoloy 825 for phosphoric and sulphuric service.

If you are choosing between two grades on cost, calculate on installed cost and expected life rather than on price per metre. A duplex line that runs at a reduced wall thickness is frequently cheaper installed than the 316L line it replaces.

Seamless, ERW or welded

Seamless pipe has no longitudinal weld and is the usual choice for highly cyclic service, sour service, and wherever the specification simply forbids a seam. Welded pipe — ERW, HFW or EFW — is formed from strip and welded along the seam, giving tighter dimensional tolerance, better surface finish and a lower price, and it reaches thin walls that seamless cannot practically achieve. In a properly made stainless seam no filler is added, so the weld is the same alloy as the pipe. Both routes are supplied under ASTM A312; the larger fusion-welded sizes fall under ASTM A358, where the class number tells you whether the weld was radiographed.

What to tell us when you enquire

Send the grade and standard, the NPS and schedule (or the OD and wall in millimetres), the length and quantity, the end preparation, and the certification level. If the service is sour, add NACE MR0175 / ISO 15156. If it is hygienic, tell us the required internal finish and whether the bore must be ID scarfed. If it falls under Indian boiler regulations, say IBR at enquiry stage — retro-fitting IBR certification to material already made is not possible.