Process Design Safety: 23-Chem-A5 Chemical Plant Design and Economics, May 2015, Q6

Question 6: Process Design Safety (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Process-design safety follows the checklist structure in Sinnott & Towler Ch. 9 (and the classic Kletz "inherently safer design" philosophy): for each equipment class, identify the hazards and design them out or provide protective systems. At least six factors per item follow, framed for a Canadian plant governed by the provincial OH&S regulations, CSA B51/ASME BPVC for pressure equipment, and the National Fire Code.

(a) Materials

Flammability — flash point, flammable/explosive limits (LEL/UEL), autoignition temperature.
Toxicity — exposure limits (TLV/OEL), acute vs chronic effects, routes of exposure.
Chemical stability — shock/heat-sensitive, spontaneously combustible, or peroxide-forming substances.
Reactivity and incompatibility — violent reaction with water, air, or other process chemicals.
Corrosivity — attack on materials of construction; compatibility of gaskets/seals.
Dust-explosion potential for powders; safe substitution of a less hazardous material where possible (inherent safety).

(b) Reactors

Exothermic heat of reaction and adequacy of cooling / heat-removal capacity (runaway prevention).
Thermal-runaway and decomposition scenarios; consequences of loss of cooling or agitation.
Reaction kinetics and the effect of mischarging, wrong ratios, or catalyst error.
Temperature and pressure control and independent high-temperature/high-pressure trips.
Emergency measures — reaction inhibition/quench, dump/blowdown, adequate relief (vent sizing per DIERS).
Formation of hazardous by-products or side reactions; provision for safe venting of gaseous products.

(c) Pressure Systems

Design pressure and temperature with margin above the maximum operating condition; code compliance (ASME BPVC / CSA B51 registration).
Pressure-relief provision — relief valves/bursting discs correctly sized for the worst-case (fire, blocked outlet, thermal expansion).
Safe discharge routing of relief to flare, scrubber or safe location.
Overpressure and vacuum protection (external pressure/collapse).
Materials at the design temperature (brittle-fracture at low temperature, creep at high temperature).
Inspection, testing and certification — hydrotest, in-service inspection, isolation for maintenance.

(d) Control Systems

Fail-safe action of valves and final elements on loss of power, air or signal.
Independent, redundant protective instrumentation (trips/interlocks) separate from control (SIS separate from BPCS).
Alarms with adequate operator response time; prioritisation to avoid alarm flooding.
Safety instrumented functions sized to the required SIL; trip testing and bypass management.
Consequences of common-mode failure, instrument drift, and sensor location/reliability.
Emergency shutdown (ESD) logic, manual trip access, and safe start-up/shutdown sequencing.

(e) Storages

Minimising inventory of hazardous materials (inherent safety — "what you don't store can't leak").
Secondary containment / bunding sized for the largest vessel plus rainfall; spill control and drainage.
Segregation and separation distances between incompatible materials and from ignition sources.
Overfill protection — independent high-level alarms and trips; vent and vacuum relief.
Blanketing/inerting of flammable-liquid tanks; vapour-loss and emission control.
Fire protection, grounding/bonding, and safe loading/unloading arrangements.

(f) General

Plant layout — spacing, separation of hazardous units, and siting relative to the site boundary and occupied buildings.
Means of escape, access for emergency services, and safe operator/maintenance access.
Ventilation and classification of hazardous (electrical) areas; suitable electrical equipment.
Drainage, effluent and emissions control; environmental protection (CEPA / provincial permits).
Provision for maintenance — isolation, purging, lock-out/tag-out, and safe entry to confined spaces.
Noise, static electricity, and protection against natural hazards (wind, seismic, freezing).

(g) Fire Protection

Elimination/control of ignition sources and area electrical classification.
Fire detection and alarm (heat, smoke, flame, gas detection).
Fixed protection — fire water ring main, hydrants, monitors, sprinkler/deluge and foam systems.
Passive protection — fireproofing of structural steel and vessel supports; fire walls.
Emergency isolation and depressurising/blowdown to reduce inventory feeding a fire.
Access for firefighting, escape routes, and adequate fire-water supply/containment of contaminated run-off.

Concept & Theory

Topic. Systematic process-design safety by equipment-class checklist and inherently safer design.

Key relations. Hazard-control hierarchy: eliminate → minimise → substitute (inherent safety) → engineered protection (relief, trips, containment) → procedural/PPE. Layers of protection (LOPA) and hazard studies (HAZOP) formalise the checklist for each item.

Why this works. Grouping the review by equipment class ensures no category of hazard is overlooked and forces the designer first to remove hazards at source (Kletz: "what you don't have can't leak") and only then to add protective systems. It maps directly to the design deliverables — material data sheets, reactor safeguards, relief sizing, SIS design, tank bunding and the fire plan.

Common pitfall. Treating the checklist as add-on protection rather than as design intent — bolting on relief valves and alarms instead of reducing inventory, moderating conditions, or substituting safer chemistry. Another is stopping at fewer than the required factors per item, or conflating control (regulation) with protection (independent trips), which the question separates.

Source. Sinnott & Towler, Chemical Engineering Design Ch. 9 (safety and loss prevention, safety checklists); T. Kletz, Process Plants: A Handbook for Inherently Safer Design; CSA Z767 / provincial OH&S process-safety-management practice.

Question 6 of 6: Process Design Safety