Process Design, Engineering Controls and Change Management
Inherent safety and engineering controls
Design should follow the inherent safety hierarchy: minimise (reduce inventory), substitute (less hazardous material), moderate (dilute, refrigerate, lower pressure) and simplify. Engineering controls - relief systems, interlocks, safety instrumented systems, containment, detection and deluge - sit above procedural controls in the hierarchy of control.
Candidates must understand safety instrumented function concepts: safety integrity level (SIL), proof testing, and the difference between a control function and a protective function. Relief and venting design, flare systems, and the need for safe disposal of relieved material are frequently examined.
Management of change
MOC is a formal process ensuring changes are assessed, approved, implemented and communicated before operation. It covers plant, process, materials, software, organisation and procedures. Key steps: request, hazard review proportionate to risk, approval by authorised persons, update of documents and drawings, training, and close-out. Temporary changes require the same rigour plus an expiry date. Emergency changes must be retrospectively reviewed. Pre-startup safety review confirms construction and installation meet design intent before hazardous material is introduced.
Traps in questions
- Treating a like-for-like replacement as exempt when the specification has changed.
- Forgetting organisational and software changes.
- Confusing MOC with a permit-to-work - they are complementary, not interchangeable.
- Missing the pre-startup safety review on new or modified plant.
Rule of thumb
If any of material, quantity, pressure, temperature, flow path, control logic or organisation changes, run the MOC process.