Pressurized Control Rooms: Where Should the Air Come From?
Drawn from an earlier revamp project on which DSIB principals held the risk and safety design role.
The Facility was installing a positive pressurization system for its control room — the emergency management centre for the relevant unit, required to shelter up to 25 persons during a toxic release event. Under the shelter-in-place strategy (API RP 752), the building must remain habitable and operationally functional throughout an emergency.
In normal operation it draws outside air continuously; in an emergency, the HVAC isolates and a dedicated instrumented process-air (PA) line takes over. The response time of that switchover — and the integrity of the PA line — are also relevant factors that directly constrain discussions on reliability of the proposed solution.
The core question was spatial: from exactly where on the building should outside air be taken? The control room is surrounded by VCM stacks, a waste-treatment plant, cold vents, and a PTA complex — each a potential toxic source. Neither PHA nor dispersion modelling studies existed for any surrounding units.
With no client specification or existing process hazard register, our principals first established a Terms of Reference with all parties — fixing scope, methodology and acceptance thresholds before any site work began, using the API RP 752 consequence-based procedure as the analytical background.
Understand the Site
A day and night shift hazard identification workshop with plant personnel revealed what no document could: prevailing wind behaviour as experienced on the ground in years, a chlorine migration pathway via underground drainage channels, seasonal fog patterns and all nearby emission source locations with their potential dispersion patterns.
Identify the Scenarios
The central output was a tiered scenario strategy: find a point that works for the dominant 80–90% of normal-operation conditions and delegate low-probability, high-severity releases to the active layer of gas detection, ESD logic and Process Air (PA) switchover.
Validate with Measurement
Dispersion behaviour — particularly the layered, descending plume from the VCM facility stacks arriving on the prevailing NNE-to-SSW wind — was a primary input to scenario classification. The assumptions were then validated through a three-day field measurement series at the proposed intake location, with measurement intervals timed across afternoon, early-morning and full-day windows to capture the full range of site conditions.
Swipe the figure sideways to reach the control room →
Decisions
01
No intake on walls facing the EO/EG process
The detection-to-response window — the time between gas detection and damper closure — means a release from nearby equipment at grade could reach the intake before the ESD can act. Walls with direct line of sight to the EO/EG process were excluded regardless of wind direction. This is a physical constraint, not a probabilistic one.
02
Do not raise the intake stack to avoid VCM emissions
An intuitive response to VCM stack emissions is to raise the intake above them. The dispersion analysis showed the opposite: the descending, stratifying plume reaches its highest concentration at an intermediate elevation as it approaches the control room. Raising the intake would place it directly into that zone. The intake was kept at rooftop level, below the descending plume profile.
03
The Cl₂ migration event is an emergency scenario — not a siting constraint
The documented chlorine exposure via underground drainage channels was real but episodic and non-atmospheric in nature. Designing the intake location to passively handle this scenario would force spatial compromises that degrade performance across far more frequent conditions. The correct response is PA activation via gas detection, supported by a migration route review and sourcing study of Cl₂.
04
Exclude the transformer area on lifecycle grounds
A geometrically viable position above the transformer substation was rejected not on hazard grounds but on operational ones: installation during construction and routine maintenance of detectors and dampers would create ongoing working-at-height exposure. Safety decisions must account for the full operational life of a system, not only the moment of design freeze.
05
Choose the rooftop position for intake
Alternative 1 — rooftop position offset from the transformer area — was confirmed as the clean air intake location following field measurement validation. All five parameters remained below OSHA limits throughout normal operating conditions.
Measurement Intervals
See Details
Air quality measurements should be repeated at the intervals specified in the field measurement report, and reassessed whenever operational changes occur at any surrounding facility.
SIF Integrity
See Details
A Functional Safety Assessment for the damper interlock and PA line activation sequences should be completed to confirm SIL compliance and validate the assumed detection-to-response time window — a critical assumption underpinning the entire shelter-in-place strategy.
Assumptions
See Details
All analytical assumptions must be formally re-validated once PHA documentation for the VCM, PTA, and waste treatment facilities revised.
Morning Fog
See Details
The morning fog layer reported by plant personnel warrants investigation. If it is process-related rather than atmospheric, it may become the dominant intake-siting scenario and could change the preferred elevation of the intake stack.
Cl₂ Migration
See Details
The Cl₂ migration pathway should be subject to a dedicated engineering review. Physical channel modification is likely a more robust mitigation than detection-based response alone.
Dependency of Active Layers
See Details
Gas detection coverage, alarm effectiveness and operational response procedures should be reviewed against the building siting evaluation criteria to confirm the active safety layer can reliably perform as per the assumptions.
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