Process Equipment Design and Mechanical Design
Core concepts
This domain tests the sizing and mechanical integrity of vessels, heat exchangers, columns and reactors. Know shell-and-tube exchanger layout (TEMA types, baffles, tube count, pitch), heat transfer area and pressure drop estimation, and fouling allowances. For vessels, understand design pressure and temperature, allowable stress, weld joint efficiency, and the ASME-style thin-wall formulas for shells and heads (torispherical, ellipsoidal, hemispherical). Cover nozzles, reinforcement, flanges, gaskets, supports, and the effect of wind and seismic loads. Distinguish internal vs external pressure, and know corrosion allowance. Column internals - trays (sieve, valve, bubble cap), packing (random vs structured), HETP, flooding and weeping - recur. Mechanical topics include material selection, brittle fracture, fatigue, and hydrostatic testing.
Traps
Thin-wall formulas are invalid for thick vessels (low D/t); check applicability. Use the correct joint efficiency and corrosion allowance. Do not confuse design pressure with operating pressure. For heads, the crown and knuckle radii matter; torispherical heads are weaker than ellipsoidal. In exchangers, tube-side vs shell-side assignments change pressure drop and cleaning considerations.
Worked rule of thumb
Thin cylindrical shell under internal pressure: t = P·D/(2·S·E − 1.2P) + corrosion allowance, where S is allowable stress, E joint efficiency, D internal diameter. Always round up to the next standard plate thickness. For a quick check, a 2 m diameter vessel at 10 bar with S = 140 MPa, E = 0.85 gives t ≈ 8.4 mm plus allowance.
Exam strategy
State the applicable code formula, list all symbols with units, and add corrosion allowance explicitly.