Question 3 of 6: Inherently Safer Design (ISD) and Inherently Safer Predesign (ISPD)

Question 3: Inherently Safer Design (ISD) and Inherently Safer Predesign (ISPD) (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.

Inherently safer design (ISD), pioneered by Trevor Kletz, seeks to remove or reduce hazards at source by the choice of chemistry, materials and process conditions, rather than by adding protective (“add-on”) safety systems that can fail. A plant that contains little hazardous material at mild conditions cannot cause a major incident even if its controls fail — the hazard has been designed out rather than merely controlled.

(a) Five ISD principles for preliminary design

1. Minimize (intensification). Use the smallest possible inventory of hazardous material — small, intensified reactors and heat exchangers, in-line reactors instead of large stirred vessels, and little or no intermediate storage of hazardous intermediates. “What you don't have can't leak.”
2. Substitute. Replace a hazardous material or reaction route with a less hazardous one — a less toxic or less flammable solvent, a safer catalyst, a non-phosgene route, or water-based rather than solvent-based chemistry.
3. Moderate (attenuate). Use hazardous materials under the least hazardous conditions or in the least hazardous form — lower temperature and pressure, dilution, refrigeration of volatile toxics, or handling a material as a dilute aqueous solution rather than an anhydrous gas.
4. Simplify. Design out unnecessary complexity so there are fewer opportunities for error and fewer components to fail — equipment that is tolerant of mis-operation, that cannot be assembled incorrectly, and that avoids elaborate control and interlock systems whose failure creates new hazards.
5. Limit effects (error tolerance). Where a hazard cannot be eliminated, arrange the design so the consequences of a failure are limited by the equipment and layout itself — e.g. change reactor geometry or siting, use spacing and segregation, or design vessels for the maximum credible pressure — rather than relying on added trips to prevent the event.

(b) Four steps of Inherently Safer Predesign (ISPD)

Inherently safer predesign applies the principles above during the early, conceptual stage — where design freedom is greatest and changes are cheapest — through a structured four-step loop:

1. Identify the hazards. Systematically catalogue the hazardous materials and energies in the proposed chemistry and process (toxicity, flammability, reactivity, stored pressure/heat), gathering the property and reactivity data needed to judge them. You cannot design out a hazard you have not recognised.
2. Generate inherently safer alternatives. For each hazard, brainstorm design options by applying the ISD principles — alternative reaction routes and solvents (substitute), reduced inventories and intensified equipment (minimize), milder conditions (moderate), and simpler configurations (simplify).
3. Evaluate and compare the alternatives. Assess each option against inherent-safety measures (e.g. inventory, reaction severity, consequence/inherent-safety indices) together with its effect on operability, technical feasibility and cost, so that the safety gain is weighed against any penalties.
4. Select and document the preferred option. Choose the alternative that best reduces the inherent hazard while remaining practical, record the basis of the decision, and carry it into detailed design — revisiting the loop as the design matures and new information appears.