Pressure Vessels, Tanks, and Boilers
Which code governs
ASME Section I covers power boilers, Section IV heating boilers, Section VIII Divisions 1 and 2 pressure vessels, Section IX welding and Section V nondestructive examination. In Canada, CSA B51 requires a Canadian Registration Number for each boiler or pressure vessel before installation and sets rules for boiler external piping and testing.
Shell and head thickness
Hoop stress governs a cylindrical shell. For Section VIII Division 1, with inside radius R: t = P·R ÷ (S·E − 0.6P), plus corrosion allowance. For a 2:1 ellipsoidal head: t = P·D ÷ (2S·E − 0.2P). Section I uses its own thickness form and a 1.5 × hydrotest rather than Division 1's stress-ratio test.
Worked example: shell thickness
Inside radius 900 mm, design pressure 1.5 MPa, allowable stress 138 MPa, joint efficiency 0.85, corrosion allowance 3 mm. t = (1.5 × 900) ÷ (138 × 0.85 − 0.6 × 1.5) = 1350 ÷ (117.3 − 0.9) = 1350 ÷ 116.4 = 11.6 mm. Add the corrosion allowance: 14.6 mm. With mill under-tolerance and rounding, specify 16 mm plate. R must be the inside radius; substituting the outside radius inflates the result.
Details that carry marks
- Nozzle reinforcement by area replacement: metal removed is replaced by excess thickness in shell, nozzle or pad within prescribed limits.
- Post-weld heat treatment is triggered by thickness, material and service - check the code table before scheduling.
- Tubes are rolled and beaded or welded; tube-to-tubesheet joints are inspected for leak tightness.
- Manways, handholes and openings need reinforcement and correct gasket and stud selection.
Traps
- Mixing Section I and Section VIII formulas and test pressures.
- Forgetting corrosion allowance, or applying it to the wrong surface.
- Hydrotesting stainless with chlorinated water; chloride is typically limited to about 50 ppm, then drained and dried.
- Testing at a metal temperature near the minimum design metal temperature, inviting brittle fracture.