Buildings and shelters — designed for the day everything goes wrong

Most buildings are designed for weather and people. Plant buildings are designed for the day the thing they sit next to fails.

Siting comes before design

API 752 governs the siting of permanent occupied buildings, and API 753 covers portable ones. The assessment weighs three things:

• Occupancy — how many people, for how long. A permanently staffed control room is a very different problem from an unmanned switchroom.
• Distance from credible release, fire and explosion sources.
• Hazard severity — what inventories are nearby and what they could do.

What plant buildings are for

Building	What is inside	What drives the design	Note
Control room	Operators, control system, alarms	Blast resistance and continuous habitability	Must stay usable during the event it is watching
Substation / switchroom	Switchgear, transformers, MCCs	Keeping ignition sources out of classified areas	Pressurised, with sealed cable entries and no process pipework through it
Analyser house	Process analysers and their sample systems	It contains process samples, so it is a hazard in itself	Ventilated and gas detected, often classified as a hazardous area
Laboratory	Sample testing, small chemical quantities	Ventilation, fume extraction and chemical storage	Sited away from process, with its own drainage
Workshop and stores	Maintenance work, spares	Access, lifting and laydown	Usually outside the hazardous area boundary entirely
Welfare and offices	People, for long periods	Occupancy — the more people, the further away	The building type that most often fails a siting assessment
Temporary buildings	Contractors during a turnaround	Siting assessment, exactly as for permanent ones	Portable units close to process have killed people. API 753 covers these.

Read the driver column. Very few of these are designed by the usual building criteria — what shapes them is blast, gas ingress, fire or the need to keep working when everything outside has stopped.

Note the last row. Temporary buildings brought in for a turnaround are assessed exactly like permanent ones. Portable units placed close to process units have killed people, and that is precisely why API 753 exists. “It’s only there for six weeks” is not a mitigation — a turnaround is when the plant is least stable and the site most crowded.

Three threats, three defences

Threat	What it does	Defence	Note
Blast overpressure	Collapses walls and roofs, and turns glass into projectiles	Distance first, then blast-resistant construction	Distance is always the cheaper defence — it is a layout decision
Flying glass	Causes most of the injuries in a survivable blast	Blast-rated glazing, laminated film, or simply no windows	A blast-resistant building with ordinary windows is not protected
Flammable gas ingress	A cloud drifts in and finds an ignition source inside	Pressurisation, gas detection on the intake, automatic HVAC shutdown	Intakes sited high and upwind of credible release points
Toxic gas ingress	Reaches the occupants directly, with no ignition needed	As above, plus escape sets and a defined shelter-in-place plan	H₂S is the usual concern, and it disables the sense of smell
Fire and radiant heat	Prevents escape and may render the building unusable	Fire rating, separation distance, protected escape routes	Two escape routes leading in different directions
Loss of services	The building stops functioning when it is needed most	Essential and UPS supplies, emergency lighting, backed-up comms	A control room without power is not a control room

The three threats are blast, fire and gas, and they call for different defences. A building can be blast resistant and still kill everyone inside by drawing toxic gas through its ventilation intake — which is why the assessment considers all three together rather than one at a time.

The control room

The building that must work during the event it is watching.

• Blast resistance sized to the overpressure from the siting assessment.
• Habitability — it has to stay usable, which means ventilation, lighting, welfare and communications all surviving.
• Essential and UPS power, because a control room without power is not a control room — see distribution.
• Internal layout that keeps operators away from external walls and glazing.
• Escape routes — two, leading in different directions.

Switchrooms and substations

A different problem: keeping ignition sources out of the hazardous area.

• Pressurisation with clean air, so flammable gas cannot enter. Loss of pressurisation is an alarmed condition.
• Sealed cable entries — every penetration is a path for gas, and this is a common finding.
• No process pipework through the room, ever. A leak inside a room full of arcing equipment is the scenario the whole arrangement exists to prevent.
• Fire detection and suppression, with careful thought about what suppression agent does to live switchgear.

This is where civil, electrical and hazardous area classification meet, and where Ex installation discipline shows up in the building fabric rather than the equipment.

Analyser houses

The awkward case: a small building that deliberately contains process samples. It is not a refuge from the hazard — it is a hazard in its own right.

So it is ventilated rather than sealed, gas detected, often classified as a hazardous area internally, and sited accordingly. It is one of the few buildings where you want air moving through freely rather than kept out.

What to take away

• Siting comes before design, and distance is the cheapest defence.
• Occupancy drives the assessment. The more people and the longer they stay, the further away.
• Glass causes most injuries in survivable blasts.
• A pressurised building distributes whatever reaches its intake. Detect at the intake and shut down.
• Switchrooms keep ignition sources out — pressurised, sealed, no process pipework.
• Analyser houses contain the hazard rather than excluding it, and are ventilated accordingly.
• Temporary buildings get the same assessment as permanent ones.