FPASA BULLETIN FP 06
GASEOUS EXTINGUISHING SYSTEMS
Circumstances sometimes preclude the installation of fixed fire protection systems using
water as the extinguishing medium eg: where water sensitive storages and equipment
predominate or where its use could aggravate a fire’s intensive and spread.
It is therefore necessary to seek alternative systems that utilise the extinguishing
capabilities of carbon dioxide (CO2).
Definition
Gaseous extinguishing systems comprise a pre-determined quantity of extinguishing agent
connected to piping fitted with nozzles which discharge the agent directly onto the risk
(local application) or flood the entire protected volume (total flooding). Actuation may be
automatic and/or manual.
Agent used
CO2 is a colourless, odourless, inert gas which extinguishes fire by reducing the
concentrations of the oxygen in the air, or the gaseous phase of the fuel in the air, to a
point where combustion ceases (suffocation).
Electrically non-conductive gases.
Uses
CO2 is suitable for use on fires involving:
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flammable liquids and gases
electrical equipment eg: transformers, oil-filled switch-gear and circuit breakers
internal combustion engines using petrol and diesel
ordinary combustible materials, eg: records storage areas
data processing and other electronic equipment
This agent is ineffective on the fires involving:
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materials containing their own oxygen supply eg: nitro-cellulose, organic peroxides
and oxidising chemicals
reactive metals such as sodium, potassium, magnesium etc
metal hydrides
Methods of application
There are two principal methods of application.
First, total flooding, where the discharge nozzles are arranged to distribute evenly the
agent into a room to extinguish a fire anywhere within its volume. It is important to note
that the extinguishing concentration not only needs to be reached, but maintained until the
fire is extinguished. Surface fires are easily extinguished but where deep-seated fires may
occur, permeation, facilitated by time is necessary. Protected rooms or compartments
need to be relatively gas tight.
Where there are openings that cannot be closed
automatically, additional quantities of agent are used to compensate for leakage.
Total flooding systems are suitable for the protection of risks such as:
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computer suites
vaults
ovens and driers
transformer and switch rooms
flammable liquids stores
Secondly, local application, where nozzles are arranged to discharge the agent directly
onto or into the burning area. Duration of gas discharge varies – 30 seconds for CO2.
However when flammable liquids having ignition temperatures below their boiling points
are involved, the duration of discharge may be increased to as long as three minutes to
prevent re-ignition. Local application systems are suitable for the protection of:
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dip tanks
quench tanks
enclosed machine spaces
vapour vents
Agent quantities
The criteria for calculating the quantities of agent required include:
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the volume of the protected area
the nature of the hazard
whether the risk is enclosed or not
likely spread of fire
The design concentrations for flame extinguishment of CO2 for various fuels is shown in Table 1
Minimum design concentrations %
by volume
Fuel
CO2
Acetone
31
Benzene
37
Methane
30
Propane
36
Table 1
Personnel Hazards
When gaseous flooding systems are used to protect areas in which people work, the
effects of the discharge of gas must be considered.
These are:
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the high percentage of agent required to achieve fire extinction – particularly with
CO2 causes life safety hazards through oxygen deficiency.
other effects associated with the agent discharge include temporary reduced
visibility, excessive noise during discharge and low temperature – all of which can
adversely affect people’s ability to evacuate the area.
Where CO2 is used for total flooding in normally occupied areas, the system should be
fitted with lock-off devices to prevent agent discharge.
System components
With due adjustments for differences in the physical properties of the gas, the design of
gaseous extinguishing systems is similar, and related to protection requirements
Bulk gas storage
Two modes of storage are normally used:
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High pressure, where the agent is stored under pressure in a liquid state in
rechargeable cylinders at atmospheric temperature. The approximate pressure for
these containers is 5 860 kPa for CO2.
Low pressure storage. CO2 can only be stored at low pressure in temperature
regulated tanks. The liquid CO2 is kept at a temperature of about -18°C and
pressure of 2 000 kPa.
This storage method is particularly convenient where bulk quantities of CO2 are
required for the protection of large compartment.
The configuration of storage containers should facilitate inspection, testing and
maintenance operations. When siting storage containers, consideration should be given to
the likelihood of exposures to fire or explosion, mechanical damage or corrosion due to the
environment.
Actuation devices
Automatic systems rely on detection devices capable of detecting either heat, smoke or
flames. When the detectors sense a fire condition the gas release valves are actuated,
releasing gas into the discharge system and subsequently into the protected area.
Manual systems rely on human activation. Manual controls are strategically positioned
within or close to the protected area and can be immediately operated on discovery of a
fire. Manual operation is used in fully occupied areas, particularly when systems are
isolated temporarily from the automatic mode. Both manual and automatic systems should
incorporate an emergency manual operating device normally situated in the vicinity of the
agent release valve.
Control devices
These include agent releasing valves, discharge controls, system lock-off devices and
other trip mechanisms and switches designed to:
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operate door closing devices
close openings in ventilation ducts
isolate power supplies to air conditioning and other equipment.
Distribution system
Well supported piping capable of handling the pressures likely to be encountered are used
to convey the agent from the storage facility to the discharge nozzles. The nozzles are
secured in positions which should not be altered without consultation with the installers.
Where clogging may occur, nozzles can be fitted with frangible discs or blow-out caps.
Alarms
Audible and visual alarms are designed to indicate that the:
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system has operated and requires recharging
automatic system has operated and personnel response is required
discharge is to commence and the areas should be evacuated
supervised devices and equipment have failed
Safety requirements
In order to substantially reduce the hazards to personnel, the following provisions should
be made in addition to alarms and lock-off devices:
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adequate escape routes and exit doors
emergency lighting and signs to aid evacuation
instruction and warning notices
facilities for prompt ventilation of protected areas
emergency plans and regular evacuation drills
provision of self-contained breathing apparatus where people may become trapped.
Testing and maintenance
As is the case with all fire control installations, gaseous systems require regular inspection
and maintenance. Only personnel who have received thorough training should perform
these tasks as accidental discharge may occur. In 1961, seven men died and 42 others
were hospitalised following the accidental discharge of a CO2 system in the engine room of
a vessel under repair in Rotterdam.
Acceptance test
Prior to commissioning, the system needs to be tested to show that it has been correctly
installed and is functional. Tests should include:
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pressure testing of system
a visual examination to ensure system is complete as per specification including
signs, notices and instruction manuals
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a discharge test, where practical, to confirm coverage of risk where local application
is being used and to check agent concentrations where total flooding systems are
concerned.
The future users of the system should be present during these tests.
Regular inspections
It is recommended that regular inspections such as those listed in the table below be
carried out.
Test
Thorough inspection and Test for proper operation to ensure
system is fully operational
Visual checks to be carried out between annual tests
following an acceptable check list or schedule
The mass of all high pressure cylinders to be taken and date
of last hydrostatic pressure test noted
Readings from low pressure CO2 containers. Liquid level
gauges to be noted. Replace/refill if 10 % of content has
been lost.
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Weekly
Intervals
Monthly 6 Monthly
Annually
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A discharge test should be carried out where regular tests show this is necessary
Cylinders require hydrostatic pressure testing at regular intervals as required by the
Occupational Health Safety Act Vessels under Pressure regulation
An inspection report showing results of all tests and recommendations should be
kept on the premises
References:
National Fire Codes 12, 12A, National Fire Protection Association, Quincy, Mass USA
Fire Suppression and Detection Systems, J L Bryan
Manual of Firemanship, Book 9, HMSO, London, England
Published by
Fire Protection Association of Southern Africa
(Incorporated Association not for Gain)
(Reg. No. 73/00022/08)
P O Box 15467
1472 Impala Park