HAZOP

A hazard and operability study reviews the process design, node by node, once the piping and instrument diagrams exist and before they are issued for construction. The plant is split into nodes: feed, digester, gas line, scrubber, upgrading, compression, boiler, hydrogen storage, and fuel storage where the product is a liquid fuel. For each node the team applies guide words — no, more, less, reverse, other than — to flow, pressure, temperature, level, and composition, and asks what happens.

The room needs the drawings, the cause-and-effect list, and someone who knows how that process behaves. A guide word with no cause and no consequence is not an entry. A credible deviation is written with its causes, how far it can go, the safeguard that already exists, and the action if that safeguard is missing or untested. Oxygen in biogas, a blocked gas line, reverse flow into a digester, loss of boiler feed water, and a hydrogen compressor started against a closed discharge are the kind of deviations these plants produce. The study does not invent a new process. It tests the one on the drawing.

The register comes back to design. A missing trip becomes an instrument and a line in the automation matrix. A missing relief path or a wrong valve becomes a change on the detailed design drawing. Operability is recorded as well: a unit that cannot be started, isolated, or drained will be lined up some other way if the drawing does not allow a safe one. HAZOP is not the site safety assessment of layout and access, and it is not the job-by-job risk assessment used on shift. The three are read together before commissioning.