Clean Agent Suppression System Design Guide

A gaseous system uses the enclosure as a container. The whole design reduces to one question: can the required concentration be held in this enclosure for the required time?

Step 1 — Decide whether it is suitable at all

A gaseous system is not suitable for every risk. It is not suitable where:

• The material produces its own oxygen (lithium-ion thermal runaway, for instance).
• The enclosure is permanently open or leaks.
• There is a risk of deep-seated fire within a stack.
• The reignition source cannot be removed.

Gas knocks down flame but does not cool. If the reignition source persists, the fire returns once the agent disperses.

Step 2 — Choose the agent

Group	Mechanism	Leading issue
Chemical clean agents	Heat absorption and chemical action	Small cylinder volume, short discharge time
Inert gases (argon, nitrogen, blends)	Oxygen dilution	Many cylinders, longer discharge, high pressure
CO2	Oxygen dilution and cooling	Severe restrictions in occupied spaces

Four criteria decide: life safety (whether people are present), the space needed for the cylinder room, discharge time, and environmental or regulatory constraints.

Step 3 — Calculate concentration and quantity

The design concentration is read from the relevant standard and the agent's approval document, derived by adding a safety factor to the extinguishing concentration. The quantity depends on volume, with these corrections:

• Temperature. The design temperature in the enclosure affects agent density.
• Altitude. A correction factor applies at height.
• Net volume. Large solid equipment inside may be deducted, but conservatively.
• Connected volumes. Raised floor and ceiling voids are included where they are not separate enclosures.

Step 4 — Verify the discharge time

Standards set an upper limit on discharge time. It is short for chemical agents and longer for inert gases, where staged discharge may be used. A short discharge produces a sudden pressure rise in the enclosure — which leads straight into the next step.

Step 5 — Prove the hold time

After discharge the concentration must be retained for a defined period — the time needed to prevent reignition and allow intervention. Tightness is measured with a door fan test and the hold time calculated.

Leakage paths: cable and pipe penetrations, door undercuts and perimeters, ventilation ducts, raised floor edges, ceiling junctions, cable tray penetrations. If the test fails, the fix is to seal the leaks, not to add more agent.

Step 6 — Establish life safety

1. Audible and visual warning before discharge, with a defined delay.
2. Manual release and abort controls in defined positions.
3. Doors opening in the escape direction and openable freely from inside.
4. A post-discharge ventilation scenario; the enclosure must be purged under control.
5. Warning signage and staff training.
6. Oxygen level and exposure assessment for inert gases and CO₂.

Step 7 — Design the pipe network and nozzles

Pipe calculation differs from a sprinkler calculation because of two-phase flow, and is generally done with the manufacturer's approved software. Check: nozzle balance (each nozzle delivering its predicted share), pipe pressure class, the cylinder manifold, and pipe hangers and supports — discharge produces high reaction forces.

Step 8 — Set up detection

Detection that releases suppression is generally built on two-device confirmation (cross-zoning) to prevent false discharge. Aspirating detection is preferred in data centres. Where a pre-action system is present, its logic must be kept separate from the gas.

Step 9 — Commissioning and periodic testing

• Door fan test and hold time verification.
• Release chain test: detection to warning to delay to shutdown commands to the discharge valve (simulated without releasing agent).
• Free movement of the pressure relief damper.
• Cylinder weight or pressure checks, recorded.
• Ventilation and damper closure testing.
• Repeating the door fan test after any refurbishment.

The six most common mistakes

1. The door fan test never done.
2. A pressure relief damper missing or shut.
3. The raised floor void not included in the volume.
4. Gas and pre-action released from the same detection logic.
5. No defined post-discharge ventilation scenario.
6. Tightness not re-verified after refurbishment.

Frequently Asked Questions

Is a gaseous system suitable for every risk?

No. It is unsuitable where the material produces its own oxygen (lithium-ion runaway), where the enclosure is permanently open or leaks, and where deep-seated fire is a risk.

How is hold time verified?

With a door fan test. Enclosure tightness is measured and the hold time calculated. If the test fails, the fix is to seal the leaks, not to add agent.

Why is a pressure relief damper needed?

The overpressure produced during discharge damages walls and doors and can lift a raised floor if no damper is fitted or it is undersized.

Is the raised floor void part of the volume?

Yes, unless it is sealed off as a separate enclosure. This is one of the most common calculation errors.

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