Pipe Fitting Wall Schedule Selection: SCH 10 to SCH 160 Explained
Purchase orders for buttweld pipe fittings frequently specify "SCH 40" or "SCH 80" without explaining why. Here is what the schedule means, how pressure rating is calculated, and how to specify correctly for your design pressure.
What "Schedule" Means
Pipe schedule is a nominal designation for wall thickness. ASME B36.10M (carbon and alloy steel) and ASME B36.19M (stainless steel) define the schedule system. For a given nominal pipe size (NPS), the outside diameter (OD) is fixed — only the wall thickness changes with schedule. Common schedules: SCH 5S, 10S, 20, 40, 80, 120, 160, and XXH (extra-extra heavy). The higher the schedule number, the thicker the wall and the higher the pressure rating.
Stainless Steel "S" Schedules
Stainless steel fittings use "S" schedules (5S, 10S, 40S, 80S) per ASME B36.19M. SCH 40S has the same wall thickness as SCH 40 for sizes up to NPS 10; above NPS 10 they diverge. Always verify the actual wall thickness in millimetres against B36.19M for the specific NPS — the schedule number alone is not sufficient for pressure calculation.
Pressure Design Formula
The minimum required wall thickness for a buttweld fitting is calculated using the modified Barlow formula (ASME B31.3, paragraph 304.1.2):
tm = (P × D) / (2 × (S × E + P × Y))
Where: P = design pressure; D = outside diameter; S = allowable stress for the material at design temperature (from ASME Section II Table 1A); E = weld quality factor (1.0 for seamless fittings); Y = temperature-dependent coefficient (0.4 for T <900°F).
The specified schedule wall must exceed this calculated minimum by the mill tolerance (typically 12.5% underthickness allowance per ASTM) plus any corrosion allowance specified by the process engineer.
EN 10253 Type B and Schedule Matching
Under EN 10253-2 Type B, the fitting bore is precision-machined to match the inside diameter of a specific pipe per EN 10216-2 or EN 10217-2. This means the Type B fitting wall thickness is inherently tied to the pipe schedule — you cannot simply specify "Type B" without also specifying the pipe OD series and wall series. In practice, European projects typically specify the pipe standard (e.g. EN 10216-5 for stainless steel seamless pipe) and the fitting schedule follows from the pipe specification automatically.
Common Schedule Specifications by Industry
| Industry / Service | Typical Schedule | Notes |
|---|---|---|
| Process plant — general | SCH 40 / 40S | ASME B31.3 standard |
| Pharmaceutical / food (CIP) | SCH 5S / 10S | Thin wall, lighter weight, electropolished bore |
| High-pressure process | SCH 80 or 160 | Design pressure >40 bar at elevated temperature |
| Power generation (CrMo steam) | Heavier — calculated | Creep allowance drives wall — always calculate, do not use schedule tables alone |
| Offshore subsea (duplex/super duplex) | Per project spec | High yield strength allows thinner walls vs CS/SS at same pressure |
| Cryogenic LNG | SCH 40S / 80S | ASME B31.3, cryogenic insulation jacket clears NPS OD |
What to State on the Purchase Order
A complete fitting specification on the PO: NPS size (e.g. NPS 4), schedule (e.g. SCH 40S), fitting type and angle (e.g. 90° LR Elbow), material grade (e.g. ASTM A403 WP316L), product standard (e.g. ASME B16.9 or EN 10253-2 Type B), end finish (bevelled per ASME B16.25), and certificate type (EN 10204 3.1). Incomplete schedules — particularly for high-pressure or cryogenic service — are the most common cause of fitment issues on site.