Technical·6 min read
CO₂ Local Application (Object Protection): Designing to Protect a Single Device in a Large Space
The device protection only CO₂ can do (local application): the engineering of protecting a single machine/equipment without enclosing a whole space. ISO 6183 / NFPA 12's two methods — rate-by-area (flat surfaces/tanks) and rate-by-volume (three-dimensional machines), the assumed enclosure (+0.6 m, min 1.2 m), discharge rate by open/closed state 16→4 kg/min/m³, percentage enclosure, the vapour compensation factor 1.4 for the effective liquid portion.
A-Pro Engineering
Local application (object/device protection) is the signature use that sets CO₂ apart from every other gaseous agent: instead of flooding an entire space with gas, you protect a single device or object at fire risk directly. In very large, open or unclosable spaces, this is in practice the only solution for protecting one machine — and only CO₂ can do it. This article covers local application design under ISO 6183 / NFPA 12. For cases where the whole volume must be protected, see our CO₂ Total Flooding (Volume Protection) article; for general agent selection, see Gas Suppression: Design and Agent Selection.
What is local application? Why only CO₂?
Total flooding fills a closed volume with gas to a given concentration; local application applies enough CO₂ directly to the target object and achieves extinguishment on/around the object. The space need not be enclosed.
The reason only CO₂ can do this is physical: as high-pressure liquid CO₂ leaves the nozzle, part of it flashes to solid “snow” and gas; this mixture falls onto the object, both smothering (oxygen displacement) and cooling it, leaving no residue. Halocarbon (FM200/NOVEC) and inert gases (IG-541/IG-01) work only in a closed volume; dispersing rapidly in open air, they cannot form a suppressing atmosphere over a device. That is why local application = CO₂.
Typical uses: wave-solder and paint/coating lines, quench and oil tanks, printing/rotary presses, turbine oil units, test rigs, open fuel/oil processes — all sharing the fact that enclosing the space is impractical/uneconomical while a specific device carries a high fire risk.
Two methods: rate-by-area and rate-by-volume
ISO 6183 (§7.5) and NFPA 12 define two calculation methods for local application. The method is decided by the object’s geometry.
| Method | For | Example |
|---|---|---|
| Rate-by-area (surface-based) | Flat surfaces and low objects | Quench/oil tank surface, flat bath |
| Rate-by-volume (volume-based) | Three-dimensional, irregular objects | Production machine, wave solder, press |
Rate-by-area (surface-based method)
Used for a horizontal flammable surface (e.g. the liquid surface of an oil/quench tank) or a low object. The basic logic:
- The nozzle is placed above the surface to be protected; each nozzle protects a given area (a coverage area taken as a square). The edge length is l = √(coverage area).
- The nozzle rate and count are selected from the approval listing based on the nozzle’s position and projection distance.
- When the nozzle is angled (not perpendicular to the surface), ISO 6183 applies the Table 5 aiming factors: if the nozzle angle is 45–60° the aiming point is taken at 1/4 of the surface, and at 90° (directly overhead) at 1/2 (the centre).
- For a deep flammable liquid the liquid surface must sit at least 150 mm of freeboard below the tank rim; otherwise the discharge can splash the liquid and spread the fire.
Rate-by-volume (volume-based method)
Used for three-dimensional, irregular objects (production machines, presses, wave-solder lines) and is the most common local-application method.
1) Assumed enclosure
An imaginary box is defined around the protected object: +0.6 m in every direction from the object’s outer limits (2 ft ≈ 0.61 m in NFPA 12). Rules:
- Each side of the enclosure must be at least 1.2 m.
- No volume is deducted from within the enclosure — the entire interior including the object itself counts.
- The enclosure volume gives the base volume the discharge rate is multiplied by.
2) Percentage enclosure and discharge rate
The base rate depends on how much the assumed enclosure is surrounded by real building surfaces:
- A fully open enclosure uses 16 kg/min/m³, an almost fully closed one 4 kg/min/m³.
- The intermediate value: rate = 4 + {(1 − enclosure ratio) × (16 − 4)} kg/min/m³
- Percentage enclosure = closed edges/surfaces ÷ total edges/surfaces × 100. (The more real walls/structure around the enclosure, the less gas escapes and the lower the rate.)
3) Vapour compensation factor (1.4) and discharge time
In local application only the liquid portion of CO₂ reaches the object and is effective; the portion that vaporises during discharge does not contribute. To compensate this loss the stored quantity is increased by a vapour compensation factor (typically 1.4 in HP):
Stored CO₂ = calculated rate × 1.4 × discharge time
NFPA 12 also gives a simplified ×0.7 multiplier by combining a 0.5-minute discharge with the 1.4 factor. Per ISO 6183 Table 6, the discharge time in local application is at least 30 seconds (HP min 30 s; LP vapour ≤30 s + liquid ≥30 s, combined ≈40 s per VdS).
When is it combined with total flooding?
Some facilities have both a volume risk and a point risk in one space; then a combined solution is built: total flooding for the volume and local application for the critical device (e.g. an exhaust duct protected by total flooding while an added nozzle/local application handles the deep-seated load beneath it). The gas quantities of the two applications are calculated separately and the total storage sized accordingly. For the details of volume protection see our CO₂ Total Flooding article.
System equipment and life safety
The basic logic of manometers/pressure switches, damper automation and the correct discharge sequence is common to all gas systems; for details see Gas Suppression: Design and Agent Selection. The critical CO₂-specific point is life safety: even when local application is done in an open space, the discharged CO₂ can reduce ambient oxygen to a dangerous level. Therefore a time delay, pre-discharge audible-visual alarm, door abort/lock-off and post-discharge ventilation/purge are mandatory in local application too.
Design and application considerations
- Method choice: flat surface/tank → rate-by-area; three-dimensional machine → rate-by-volume.
- Assumed enclosure: +0.6 m in every direction from the object, min side 1.2 m, no volume deducted.
- Percentage enclosure: the rate is set correctly between 16 (fully open) and 4 (almost closed) kg/min/m³.
- Vapour compensation: storage = rate × 1.4 × time (or the ×0.7 simplification for 0.5 min).
- Discharge time: at least 30 s in local application (HP); LP vapour ≤30 s + liquid ≥30 s.
- Freeboard: at least 150 mm for a deep flammable liquid.
- Aiming factor: in rate-by-area, Table 5 by nozzle angle (1/4 … 1/2 centre).
- Life safety: delay, alarm, lock-off and purge are mandatory even in an open space.
- Periodic maintenance: nozzle aim/blockage and cylinder/tank weighing are checked regularly; see our periodic maintenance of fire systems article.
Summary
CO₂ local application is the only way to protect just the device at risk without enclosing a whole space, and among gaseous agents it is possible only with CO₂. The backbone of the design is choosing the right method by the object’s geometry (rate-by-area or rate-by-volume); in the volume-based method, building the assumed enclosure (+0.6 m), setting the rate (16→4 kg/min/m³) by percentage enclosure, and sizing storage with the vapour compensation factor (1.4). Life-safety measures are mandatory even in an open space. If the whole volume is at risk, turn to the CO₂ Total Flooding solution; for the agent comparison, see Gas Suppression: Design and Agent Selection. For an object/device-based CO₂ design at your facility, contact us for a free site survey within Ankara.
This content is for information only. Binding calculations and project design specific to your facility require project-based work with a fire engineer under TS ISO 6183 / NFPA 12 and the relevant nozzle approval listing.
© 2026 A-Pro Mühendislik. This is original technical content produced by A-Pro Engineering; all rights reserved. Copying, reproducing or republishing it in whole or in part without attribution and permission is prohibited.
Frequently asked questions
What is local application (object protection) and why is it done only with CO₂?+
Local application means protecting a single device/object at fire risk (a machine, quench tank, oil unit) directly, instead of filling an entire space with gas — the only solution in unclosable, very large or open spaces. CO₂ is the only agent that can do this on its own: as it discharges, part of it turns to solid 'snow' and gas that both smothers and cools the object, leaving no residue. Halocarbon and inert gases work only in a closed volume (total flooding); they disperse in open air and cannot protect a device. If the whole volume must be protected, see our CO₂ Total Flooding article.
What is the difference between rate-by-area and rate-by-volume in CO₂ local application?+
ISO 6183 / NFPA 12 define two methods. Rate-by-area (surface-based) is for flat surfaces and low objects (e.g. the surface of a quench/oil tank): the nozzle protects a given area based on its position and projection distance; for a deep flammable liquid at least 150 mm of freeboard is left. Rate-by-volume (volume-based) is for three-dimensional, irregular objects (e.g. a production machine): an assumed enclosure is built around the object and the gas rate is calculated from that volume. The object's geometry decides the method.
How is the assumed enclosure built in rate-by-volume?+
An imaginary box is defined around the protected object: +0.6 m in every direction from the object's outer limits in ISO 6183 (2 ft ≈ 0.61 m in NFPA 12). Each side of this assumed enclosure must be at least 1.2 m, and no volume is deducted from it (including the object itself). The enclosure volume gives the base volume that the discharge rate is multiplied by.
How is the rate-by-volume discharge rate calculated?+
The base rate depends on how much the assumed enclosure is surrounded by real building surfaces. If the enclosure is fully open the rate is 16 kg/min/m³; if almost fully closed, 4 kg/min/m³; the intermediate value is rate = 4 + {(1 − enclosure ratio) × (16 − 4)}. Percentage enclosure = closed edges/surfaces ÷ total ÷ 100. This rate is multiplied by the enclosure volume for the total rate in kg/min, then by the discharge time for the required CO₂ mass.
Why is the vapour compensation factor (1.4) needed?+
In local application only the liquid portion of CO₂ reaches the object and is effective in extinguishing; the portion that vaporises during discharge does not contribute. To compensate this loss the stored CO₂ quantity is increased by a vapour compensation factor — typically 1.4 in HP systems. In practice: stored quantity = calculated rate × 1.4 × discharge time. NFPA 12 also gives a simplified ×0.7 multiplier by combining a 0.5-minute discharge with the 1.4 factor.
How long is the CO₂ local application discharge time?+
Per ISO 6183 Table 6, the effective (liquid) CO₂ in local application is discharged for at least 30 seconds. In HP systems a minimum of 30 seconds applies; in LP systems the pre-liquid vapour flow is at most 30 seconds plus a liquid discharge of at least 30 seconds (combined at most ≈40 seconds per VdS). For deep flammable-liquid fires, time and freeboard are considered together to prevent re-ignition.
Does local application protect only the device or the whole space?+
It protects only the target device/object and its immediate surroundings, not the whole volume. That is why it is ideal for protecting a single machine in a very large or unclosable space — instead of flooding a whole hall with gas you protect only the machine at risk, sharply reducing the gas quantity and cost. But if multiple devices or the whole volume are at risk, CO₂ total flooding should be considered.
In which facilities is local application used?+
Point risks in open or very large spaces: wave-solder and paint/coating lines, quench and oil tanks, printing and rotary presses, turbine oil units, test rigs, open fuel/oil processes. The common feature is that enclosing the space is impractical/uneconomical while a specific device carries a high fire risk.
Related systems
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