Materials, Corrosion and Equipment Design
What the exam tests
Material selection, corrosion mechanisms and allowances, pressure-vessel and piping design codes (ASME VIII, B31.3), brittle fracture, welding and inspection.
Core rules
- Thin-wall shell thickness: t = PR/(SE − 0.6P) + CA, with R the inside radius and E the joint efficiency.
- Corrosion allowance is added to calculated thickness and applied to every wetted surface, including trays and nozzle necks.
- Galvanic corrosion needs dissimilar metals plus an electrolyte; the more active metal becomes the anode and corrodes.
- Chlorides drive pitting and crevice attack on stainless steel; chloride stress-corrosion cracking of 304/316 is a risk above roughly 60 °C - duplex grades or stress relief may be needed.
- Sour service follows NACE MR0175/ISO 15156, with hardness limits and cracking-resistant steels.
- Minimum design metal temperature and impact testing control brittle fracture; post-weld heat treatment relieves residual stress and lowers SCC risk.
Worked example
A vessel has P = 2.0 MPa, R = 1,000 mm, S = 138 MPa, E = 1.0 and CA = 3 mm. t = (2.0 × 1,000)/(138 × 1.0 − 0.6 × 2.0) + 3 = 2,000/136.8 + 3 = 14.6 + 3 = 17.6 mm, so specify 18 mm plate. With E = 0.85 (spot radiography), t = 2,000/(117.3 − 1.2) + 3 = 17.2 + 3 = 20.2 mm - joint efficiency changes plate thickness and cost directly.
Traps
- Substituting outside diameter for R in the ASME shell formula.
- Omitting the corrosion allowance, or applying it to only one side.
- Specifying 316L for hot chloride service where SCC still occurs.
- Confusing erosion-corrosion with stress-corrosion cracking.
- Overlooking low-temperature brittle fracture on a letdown line cooled by Joule–Thomson expansion.