A designer sits down with a line list for a flue-gas scrubber recirculation line and fills in the design inputs one by one: fluid, pressure 7.0 MPa, temperature 200 C, material - Hastelloy C276, nominal size DN100. Then comes the row that says "wall thickness," and this is where C276 stops behaving like an ordinary pipe calculation. On a carbon steel line, the pressure term dominates and corrosion allowance is an add-on. On this line, the pressure term will barely register, and a completely different field on the input sheet will decide which schedule you buy.
Product Forms
Bar & Rod
Plate & Sheet
Coil & Strip
Pipe & Tube
Fitting: Flange, Tee, Elbow, Reducer etc.
Forging: Ring, Shaft, Circle, Block etc.
The Inputs, and Where Each One Comes From
The B31.3 pressure-design term for straight pipe:
t = P x D / [2 x (S x E x W + P x Y)]
| Symbol | Value used | Source |
| P (design pressure) | 7.0 MPa | Line list |
| D (outside diameter) | 114.3 mm (DN100 / NPS 4) | Dimensional standard |
| S (allowable stress) | About 160 MPa at 200 C (about 23.3 ksi at room temperature, nearly flat to 450 C) | B31.3 Table A-1 for N10276 - verify against the current code edition for contract use |
| E (weld joint quality factor) | 1.0 seamless (B622); 0.85 typical for welded (B619) | Table A-1A/B |
| W (weld joint strength reduction) | 1.0 at this temperature | B31.3 Table 302.3.5 |
| Y (coefficient) | 0.4 (austenitic / duplex class, at temperature) | Table 304.1.1 |
| c (corrosion allowance + tolerances) | See below | Project corrosion engineer |
Before the arithmetic, one C276-specific input deserves its own paragraph: the corrosion allowance. The general corrosion rate of C276 in most scrubber and acid service is very low - fractions of 0.1 mm per year - so projects typically carry a corrosion allowance of 1 to 3 mm, sometimes less, where a stainless line would carry 3 to 6 mm. That single input will move the answer more than everything else in the formula.
The Calculation, Term by Term
| Step | Arithmetic | Result |
| Pressure term | t = 7.0 x 114.3 / [2 x (160 x 1 x 1 + 7.0 x 0.4)] = 800.1 / 325.6 | t = 2.46 mm |
| Add corrosion allowance (CA = 3 mm case) | 2.46 + 3.0 | 5.46 mm required |
| Add mill undertolerance (-12.5% on wall) | 5.46 / 0.875 | 6.24 mm minimum nominal |
| Select schedule | SCH 40S = 6.02 (fails); SCH 80S = 8.56 | SCH 80S |
| Repeat with CA = 1 mm (low corrosion rate) | (2.46 + 1.0) / 0.875 = 3.95 mm | SCH 40S (6.02) passes |
Read the two bottom rows against each other. The pressure term contributed 2.46 mm. The corrosion allowance swung the schedule. On the CA = 3 mm line you buy SCH 80S; on the CA = 1 mm line you buy SCH 40S - a difference of roughly 35% in weight per metre - and the pressure inside the pipe never changed.
This is the CA reversal of C276: because the allowable stress of the alloy is high and its corrosion rate is low, the wall is decided by the corrosion allowance figure, not by the process pressure. For a buyer, it means the CA number on the enquiry is the most commercially loaded field on the whole spec sheet - argue it with corrosion data, not habit.
The Temperature Non-Event, and Why It Matters
| Material | Allowable stress, room temperature to 400-450 C | Practical consequence |
| 316L (S31603) | Noticeable decline | Wall must thicken as service warms |
| Duplex 2205 (S32205) | Declines, plus a service ceiling around 250-300 C | Temperature costs wall twice over |
| C276 (N10276) | Almost flat - room-temperature S holds nearly to 450 C | The pressure term barely grows with temperature |
Most corrosion-resistant alloys pay for heat with wall: allowable stress decays, the pressure term expands, the schedule climbs. The C276 curve is one of the flattest in the code table - a consequence of the Ni-Mo-Cr solid-solution matrix, with no precipitate system to soften and no duplex phase balance to protect.
At 200 C the temperature term did nothing. If your line list has a row "design temperature 350 C" and the material cell says 316L-or-equal, that is precisely the application where the flat C276 curve earns part of its premium back in wall thickness - before anyone counts the corrosion rate.
Seamless or Welded: The E-Factor Decision
For B622 seamless pipe, E = 1.0 and the arithmetic above stands as written. For B619 welded C276 pipe, the weld joint factor pulls the allowable stress down (E typically 0.85), inflating the pressure term by roughly 18% - on the CA = 1 mm case that pushes the minimum nominal from 3.95 mm to about 4.5 mm, still inside SCH 40S.
The practical rule: at small bore and high pressure the E-factor pushes you to seamless; on large-bore, corrosion-governed walls (where CA dominates anyway), the welded penalty vanishes inside the allowance - which is why heavy-wall FGD ducting and large scrubber headers are routinely B619 welded product.
What to Put on the Enquiry
- Design pressure and temperature, so the supplier can check the schedule against the same B31.3 table, not a stock list.
- CA as an explicit number - it is the commercially loaded field.
- Seamless B622 or welded B619, stated, because the E-factor moves the schedule.
- Schedule selected, plus the statement that wall undertolerance (-12.5%) has been accounted for - this prevents the classic dispute where a mill ships at minimum wall against a schedule chosen without the tolerance step.
