Process safety

Chemical plants handle flammable, toxic and high-pressure material, and process safety is about stopping it getting out. A hazard is something that can cause harm (a tank of chlorine); risk combines how likely harm is with how bad it would be. Engineers reduce risk by making harm less likely, less severe, or both.

The best fix is inherently safer design: remove or shrink the hazard rather than guard it. Store less (minimize), use a less dangerous material (substitute), run at milder temperature and pressure (moderate), and make the design hard to get wrong (simplify). The Bhopal disaster in 1984 involved a large store of methyl isocyanate, a toxic intermediate that safer routes avoid storing at all.

A HAZOP (hazard and operability study) is a team walking through the P&ID line by line, applying guidewords to each design intent: No, More, Less, Reverse, Part of, As well as, Other than. "No flow" in the cooling water line: what causes it, what happens, what protects us? Each credible problem gets an action.

Protection comes in layers, so no single failure causes an accident: a good basic design and control system, alarms and operators, a safety instrumented system that trips the plant, relief valves and bursting discs that open before a vessel bursts, then containment (bunds, flares) and emergency response. Each layer is independent of the others.

A relief valve is sized for the worst credible case: a fire round the vessel, a blocked outlet with the pump still running, or a runaway reaction, which may produce gas and needs a much larger vent. Everything vented has to go somewhere safe, often to a flare that burns it.