ASME B31.3 Section 304.1.2 gives one wall thickness equation:
t = (P × D) / [2(SEW + PY)]
The equation is identical for carbon steel, austenitic stainless, and duplex stainless. What changes is the material data fed into it. For UNS S32205 (Duplex 2205), three values differ from the values most engineers default to (316L or carbon steel):
|
Symbol |
Meaning |
S32205 Value (B31.3 Table A-1M) |
Common Mistake |
|
S |
Basic allowable stress at design temperature |
205 MPa at ≤38°C, drops to 182 MPa at 300°C |
Engineers often use the 316L value (138 MPa) or 304L value (115 MPa), which over-thickens the wall |
|
E |
Longitudinal joint efficiency |
1.00 (seamless) |
Correctly applied |
|
W |
Weld strength reduction factor |
1.00 for duplex stainless per B31.3 Table 302.3.5(e) |
Engineers often apply 0.85 (austenitic RT) and over-thicken the wall |
|
Y |
Temperature coefficient |
0.4 for ferritic and duplex stainless |
Engineers often apply 0.7 (austenitic 300-series) and over-thicken the wall |
For the same service conditions (8″ NPS, 15 bar, 50°C), feeding the wrong S, W, and Y can shift the calculated wall thickness by 15–30% - pure material data error, not a real engineering difference.
S32205 Material Properties That Drive the Calculation
|
Property |
Value |
Source |
|
UNS designation |
S32205 (also S31803, lower strength) |
ASTM A790 |
|
Density |
7,805 kg/m³ |
ASTM A790 |
|
Tensile strength, min |
655 MPa |
ASTM A790 |
|
Yield strength (0.2% offset), min |
485 MPa |
ASTM A790 |
|
Elongation, min |
25% |
ASTM A790 |
|
PREN |
35 (S32205), 32 (S31803) |
%Cr + 3.3×%Mo + 16×%N |
|
Max service temperature (B31.3 design) |
300°C (σ-phase concern above 250°C) |
ASTM A923 |
|
Min design temperature |
-50°C (impact tested per ASTM A370) |
ASME B31.3 |
Worked Example: 8″ NPS Seawater Injection Line
Service conditions (typical Saudi Aramco offshore):
Fluid: filtered seawater, Cl⁻ ≈ 19,000 ppm
Design pressure P: 15 bar (1.5 MPa)
Design temperature T: 50°C
Pipe: 8″ NPS, OD 219.1 mm
Material: UNS S32205, ASTM A790 seamless
Step 1 - Pull material data at 50°C:
S = 205 MPa (Table A-1M, ≤38°C; no reduction at 50°C)
E = 1.00 (seamless)
W = 1.00 (duplex stainless, Table 302.3.5(e))
Y = 0.4 (ferritic/duplex, Table 304.1.1)
Step 2 - Calculate minimum required wall thickness:
t = (1.5 × 219.1) / [2(205 × 1.00 × 1.00 + 1.5 × 0.4)] t = 328.65 / [2(205 + 0.6)] t = 328.65 / 411.2 t = 0.80 mm
This is the pressure-design minimum. The Schedule must be selected after adding corrosion allowance and mill tolerance.
Corrosion Allowance: The Bigger Driver Than Pressure in Seawater
|
Service Environment |
Recommended ca |
Source |
|
Filtered seawater, T < 60°C, no cathodic protection |
3.0 mm |
NORSOK M-506, Saudi Aramco 32-SAMSS-005 |
|
Wet phosphoric acid (30–54% P₂O₅, T < 80°C) |
2.0 mm |
Field data, ~0.1 mm/yr uniform corrosion |
|
NACE MR0175 sour service, H₂S ≤ 0.05 bar |
0 mm |
No SSC risk within MR0175 limits for 2205 |
|
Neutral oil/gas, non-corrosive |
1.0 mm |
Mechanical damage allowance only |
|
Hydrochloric acid, any concentration |
N/A - do not use 2205 |
Use Hastelloy C276 or C22 |
For seawater service: ca = 3.0 mm.
Schedule Selection with Mill Tolerance
ASTM A790 seamless pipe has ±12.5% mill tolerance on wall thickness. To ensure the thinnest point meets design:
t_nominal ≥ (t + ca) / 0.875 t_nominal ≥ (0.80 + 3.00) / 0.875 t_nominal ≥ 4.34 mm
|
Schedule (8″ NPS) |
Nominal Wall (mm) |
Min Wall at –12.5% |
Meets 4.34 mm? |
|
5S |
3.76 |
3.29 |
No |
|
10S |
4.19 |
3.67 |
No |
|
40S |
8.18 |
7.16 |
Yes - selected |
|
80S |
12.70 |
11.11 |
Yes (over-design) |
Result: Sch 40S for 8″ S32205 seawater injection at 15 bar / 50°C / 3.0 mm ca.
If design pressure rises to 30 bar (deepwater injection), t = 1.59 mm, t_nominal ≥ 5.25 mm - Sch 40S still works. In this pressure range, corrosion allowance drives the Schedule, not pressure.
When to Move Beyond 2205
|
Service |
2205 OK? |
Recommended Material |
|
Seawater, T < 60°C |
Yes |
- |
|
Seawater, T > 60°C, with sand |
Marginal |
2507 (S32750, PREN ≥ 40) + cathodic protection |
|
Sour service, H₂S > 0.05 bar, T > 90°C |
No |
25Cr super duplex or nickel alloy |
|
HCl, any concentration |
No |
Hastelloy C276 |
|
H₂SO₄ > 15%, T > 50°C |
No |
Hastelloy C22 |
The wall thickness calculation only matters once material selection is correct. Wrong alloy, no Schedule compensates.
S31803 vs S32205 - Which One Are You Actually Getting?
Modern procurement defaults to S32205, but legacy P&IDs may still specify S31803. The allowable stress differs:
|
Grade |
Yield Strength (min) |
S at 50°C (B31.3) |
Wall Thickness Impact |
|
S31803 |
450 MPa |
189 MPa |
+8% wall vs S32205 |
|
S32205 |
485 MPa |
205 MPa |
Baseline |
For 8″ / 15 bar / Sch 40S service, both grades end up at the same Schedule. For thin-wall high-pressure service (e.g., 6″ Sch 10S at 40 bar), the difference can move the Schedule one step. Always confirm the UNS on the MTC before running the calculation.
FAQ
Q1: What is the basic allowable stress for S32205 at 200°C?
A: 190 MPa per ASME B31.3 Table A-1M (down from 205 MPa at ≤38°C). At 250°C, S = 186 MPa; at 300°C, S = 182 MPa. Above 300°C, σ-phase precipitation risk typically excludes 2205 from B31.3 design.
Q2: Why is W = 1.00 for duplex stainless?
A: ASME B31.3 Table 302.3.5(e) lists duplex stainless steels alongside ferritic stainless with W = 1.00. This reflects that qualified welds in duplex have full base-metal strength when properly solution-annealed and pickled. It is an advantage over 300-series austenitic, where W ranges from 0.85 to 1.00 depending on radiographic examination.
Q3: Is 3.0 mm corrosion allowance excessive for seawater?
A: It is the standard value for unlined, uncoated 2205 in seawater injection service. NORSOK M-506 and Saudi Aramco 32-SAMSS-005 both use 3.0 mm for this service. Operators with cathodic protection plus online corrosion monitoring sometimes reduce to 2.0 mm - but this requires documented CP system reliability and ongoing monitoring budget.
Q4: Can I use Sch 10S with online corrosion monitoring instead of Sch 40S?
A: The 4.34 mm nominal threshold excludes Sch 10S (4.19 mm). Online monitoring does not change the mill tolerance requirement. If the design basis allows reduced reliability and operator-side inspection cost, some operators accept Sch 10S - but this is not the default for major Middle East operators.
Q5: What is the maximum design temperature for 2205 in B31.3?
A: 300°C, limited by σ-phase precipitation in the 250–300°C range. ASTM A923 Method C detects deleterious intermetallic phases. For sustained operation above 250°C, consider 2507 (S32750) or nickel alloys.
Need Free quote and sample?
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