Procuring stainless steel pipe involves navigating pipe sizing systems, wall thickness schedules, material grades, and end-finish specifications. This guide covers the essential parameters that buyers and engineers need to specify correctly to get the right product for fluid transport, structural, and pressure-containing applications.
Stainless steel pipe is specified by nominal pipe size (NPS) in inch-based markets or by nominal diameter (DN) in metric markets. NPS is a nominal designation that approximately equals the pipe's inside diameter for sizes up to NPS 12, but for NPS 14 and above, NPS equals the exact outside diameter.
Key reference: For NPS 1 through NPS 12, the outside diameter is fixed and significantly larger than the nominal size. Example: NPS 2 pipe has an OD of 60.3 mm (2.375 inches), not 50.8 mm (2.0 inches). This convention originates from early pipe manufacturing and is standardized across ASTM, ASME, and ISO systems.
Quick reference of common OD sizes:
| NPS | OD (mm / inches) | |---|---| | 1/2" | 21.3 mm / 0.840" | | 3/4" | 26.7 mm / 1.050" | | 1" | 33.4 mm / 1.315" | | 1-1/2" | 48.3 mm / 1.900" | | 2" | 60.3 mm / 2.375" | | 3" | 88.9 mm / 3.500" | | 4" | 114.3 mm / 4.500" | | 6" | 168.3 mm / 6.625" | | 8" | 219.1 mm / 8.625" | | 10" | 273.0 mm / 10.750" | | 12" | 323.8 mm / 12.750" |
Pipe wall thickness is specified by schedule number (SCH) under ASME B36.19 (stainless steel pipe) and ASME B36.10 (carbon steel pipe, also referenced for stainless). The schedule number is an approximate relationship to internal pressure rating: SCH = 1,000 × P/S, where P is internal working pressure and S is allowable stress.
Common stainless steel pipe schedules:
| NPS | SCH 5S (mm) | SCH 10S (mm) | SCH 40S (mm) | SCH 80S (mm) | |---|---|---|---|---| | 1/2" | 1.65 | 2.11 | 2.77 | 3.73 | | 1" | 1.65 | 2.77 | 3.38 | 4.55 | | 2" | 1.65 | 2.77 | 3.91 | 5.54 | | 3" | 2.11 | 3.05 | 5.49 | 7.62 | | 4" | 2.11 | 3.05 | 6.02 | 8.56 | | 6" | 2.77 | 3.40 | 7.11 | 10.97 | | 8" | 2.77 | 3.76 | 8.18 | 12.70 | | 10" | 3.40 | 4.19 | 9.27 | 15.09 | | 12" | 3.96 | 4.57 | 10.31 | 17.48 |
The "S" suffix (e.g., SCH 40S) designates stainless steel schedules, which differ from carbon steel schedules at some sizes. SCH 10S is the most frequently specified wall thickness for process piping in chemical, food, and pharmaceutical plants where internal pressures are moderate. SCH 40S is standard for higher-pressure service. SCH 80S serves high-pressure and severe-service applications.
304/304L (UNS S30400/S30403): The default grade for general-purpose process piping, water lines, food processing, and architectural applications. 304L (low carbon, ≤0.03% C) is preferred for welded pipe to avoid sensitization at the weld zone.
316/316L (UNS S31600/S31603): Specified where chlorides, acids, or marine atmospheres demand molybdenum-enhanced corrosion resistance. Chemical plants, pharmaceutical manufacturing, coastal water lines, and offshore platforms use 316L pipe. The cost premium over 304 is typically 20–40%.
2205 Duplex (UNS S32205/S31803): Selected for high-pressure, high-chloride, or stress-corrosion-cracking environments where 316L would fail. The dual-phase microstructure provides twice the yield strength of 316L, enabling thinner pipe walls for the same pressure rating.
904L (UNS N08904): High-nickel, high-molybdenum austenitic alloy for severe acid service, particularly sulfuric and phosphoric acid at elevated temperatures. Used in chemical processing and pollution control equipment.
Standard pipe lengths are SRL (single random length, 5–7 m) and DRL (double random length, 10–13 m). The actual delivered lengths vary within these ranges unless "exact length" is specified. For projects requiring precise cut lengths, confirm with the supplier whether fixed-length cutting is available and at what surcharge.
Stainless steel pipe weight per meter:
Weight (kg/m) = (OD − WT) × WT × 0.02491
Where OD = outside diameter (mm), WT = wall thickness (mm), and 0.02491 is the factor for austenitic stainless steel (density ≈ 7.93 g/cm³). For duplex stainless (≈7.80 g/cm³), use 0.02452.
Example: NPS 4 SCH 40S (OD 114.3 mm, WT 6.02 mm): (114.3 − 6.02) × 6.02 × 0.02491 = 16.2 kg/m.
When requesting a stainless steel pipe quotation, specify: material grade (304L, 316L, etc.), NPS or OD, wall thickness or schedule, pipe standard (ASTM A312 for welded/seamless, A358 for EFW, A790 for duplex), end finish, length requirement, and certification level (EN 10204 3.1 minimum, 3.2 for classed applications). Include any supplementary requirements such as PMI (positive material identification), intergranular corrosion testing, or NDE.
Zunkai Stainless Steel stocks 304/304L and 316/316L welded pipe from 1/2" to 12" NPS in schedules 10S and 40S. Seamless pipe and duplex grades are available on mill order. Contact the sales team with your pipe specification for stock availability and current pricing.
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