Reactor and Digester Equipment Design
What this domain tests
This domain covers mechanical design of batch and continuous digesters: shell thickness, nozzle reinforcement, thermal fatigue, corrosion allowance, and the process cycles that load the equipment. Design pressure, design temperature and liquor chemistry must be stated together.
Core rules and concepts
- Continuous digesters have impregnation, heating, cooking and washing zones. Pressure is set by cooking temperature and vapour pressure; the shell sees hot caustic liquor and cyclic heating.
- Batch digesters see repeated pressurization, relief and blow. Cycling drives fatigue at openings and supports.
- Use ASME BPVC Section VIII Div 1 (or Div 2 where specified) for shell thickness; add corrosion allowance to the calculated thickness, not to the pressure.
- Nozzle reinforcement, weld joint efficiency and support loads often govern, not the plain shell.
- Dead zones and poor circulation create concentration cells and localized corrosion.
Worked example
Cylindrical shell, internal design pressure P = 1.2 MPa, inside radius R = 2,000 mm, allowable stress S = 138 MPa, joint efficiency E = 0.85, corrosion allowance CA = 3 mm. Div 1 thin-wall formula: t = PR/(SE - 0.6P) = (1.2 x 2,000)/(138 x 0.85 - 0.6 x 1.2) = 2,400/(117.3 - 0.72) = 20.6 mm. Add CA: 20.6 + 3 = 23.6 mm, so specify 24 mm plate. Check nozzle reinforcement and hydrotest separately.
Traps that cost marks
Using outside radius in the Div 1 formula; omitting joint efficiency; adding corrosion allowance to the pressure term; ignoring cyclic thermal stress at the chip chute and liquor nozzles; assuming a thicker shell fixes nozzle or support problems.