Its purpose
A preliminary hazard analysis asks “what in this could kill someone?” before detailed engineering begins. Its output is not a detailed risk table but a hazard list that steers the design.
What is looked at
- the hazardous properties and quantities of the substances to be used,
- energy sources: pressure, temperature, electricity, height, motion,
- siting: distance to surroundings, neighbours and housing,
- events that have happened at similar plants,
- points not yet decided in the design where later change would be expensive.
Why timing decides everything
The cheapest way to remove a hazard is to design it out — choose a less hazardous substance, hold a smaller inventory, site the hazardous unit away from occupied areas. All of these are cheap early on; once the plant exists the same outcome can only be bought with added layers of protection, and at a higher price.
Relation to later analyses
The PHA sets the agenda for the detailed work that follows: which nodes go to HAZOP, which scenarios get a layer of protection analysis, which barriers are tracked in a bowtie. It does not replace any of them.
A common mistake
Dismissing the PHA because “there is no detail yet”. Major hazards are visible without detail; by the time the detail arrives it is too late to change them.