Andrew Brown looks at the pros and cons of specifying gas cartridge and stored pressure
OVER THE past five years, the more traditional gas cartridge fire extinguishers have been increasingly
replaced by stored pressure equivalents. There are many arguments given by extinguisher companies and service agents as to
why this is the case but the decision to replace cartridge types with stored pressure vessels may not be as clear cut as it
seems and specifiers should consider a number of factors before selecting equipment.
Stored pressure extinguishers contain the extinguishing medium in the body of the extinguisher,
along with dry air or nitrogen at a pressure ranging from 5 to 15 bar. Discharge is initiated by either opening the valve
or by piercing a sealing disc. Upon discharge, the compressed gas expands to fill the extinguisher body and expel the extinguishing
In gas cartridge extinguishers, the extinguishing medium resides in the body of the extinguisher,
with a siphon tube and a gas pressure charge (ranging from 12 to 220g in capacity) usually fixed to the head-cap, which acts
as an actuating mechanism as well as the discharge control facility (see Figure 1).
In the case of dry powder units, gassing assemblies are also incorporated into the extinguisher
to fluidify the powder to ease its application. These gassing assemblies range from a single plastic tube surrounding the
cartridge to a separate tube with gas inlets and rubber shrouds at intervals. A bursting disc is also sometimes fitted in
the hose assembly to allow proper fluidification and pressure build up. Once the cartridge has been pierced and the powder
fluidified, the extinguisher behaves with the same characteristics as a stored pressure unit.
There are points for and against using both types of unit and the following issues should be considered:
Stored pressure units usually feature a tell-tale gauge or pressure disc on their operating assembly
or body to indicate the pressure held in the container. In the case of a gauge, there is a green-coloured zone where the needle
points if the container possesses the minimum pressure to force out the required amount of agent to fulfil the fire rating
of the unit (in the case of a new unit). If the needle is in the red zone, the unit has insufficient pressure to operate to
its full capability.
The gauge does not always give a reliable indication of the true pressure and, although gauge
testers are on the market, not all gauges have a test facility. The diaphragm or bourdon tube coil can become distorted (especially
if the unit has undergone pressure or hydraulic tests) and may give a fully operational reading when the unit has, in fact,
become totally depressurised. With some diaphragm gauges the internal mechanism can also develop problems caused by stress
on components over time. Surprisingly, these problems are not confined to cheaper models of gauges. In cases involving the
bourdon tube coil, the gauge can be tampered with again to give a false reading of pressure.
Sometimes thumb pressure discs, now rare, can never be depressed even when the head assembly is
totally removed from the extinguisher body.
The gas cartridge unit, however, has no pressure gauge, which means the cartridge must be weighed
for any carbon dioxide loss. If a loss greater than 10% of the total weight is detected, the cartridge should be replaced.
Content and lining inspection
Since stored pressure units are constantly pressurised, it is impossible to open them every year
to inspect either the contents for discoloration or the internal linings and threads for damage or corrosion without having
to repressurise the unit afterwards. As a result, internal inspections are usually delayed until a test discharge is carried
out. This can be from four to 20 years depending on the model and extinguishing agent. Even when the internal inspection takes
place, the neck aperture is sometimes too small to give an opportunity to inspect all of the internal lining thoroughly enough
to detect any problems. If a problem goes unnoticed for a considerable period of time, the pressure-holding integrity of the
steel cylinder could be adversely affected, possibly leading to disastrous or fatal results. This problem would not of course
apply to the same extent to stainless steel units or small aluminium bodies.
Cartridge extinguishers can be opened at every inspection and, as a result, problems can be detected
early and remedied quickly. If a problem arises in a stored pressure unit in the first, second or third year it could go unnoticed.
Speed of operation
Because the principle of stored pressure units is simple, their operation is usually marginally
quicker than cartridge examples as there is no gassing and liquefying or bursting disc procedure. If, however, the contents
of the extinguisher have settled and compacted to form cake [is this only in powder extinguishers?] in between servicing,
the gassing procedure can assist in reducing this to a limited extent by agitating the contents. If the unit is stored pressure
the result could be failure to operate.
It is also worth noting that the delays experienced would be negligible, especially in those units
conforming to the new standards, as they must begin discharge within ten seconds of initial operation (four seconds for BS
5423: Specification for portable fire extinguishers) before they can
be certificated to European and UK standards.
Quantity of discharge
It has been suggested that a cartridge extinguisher will discharge proportionally less of the
contained agent than a stored pressure counterpart. But if the extinguisher is certificated to the relevant standards this
will not be the case, since the requirements are based on agent type and capacity relating to proportion of contents discharged,
not the method of operation. Table 1 compares extinguisher type and percentage to be discharged appertaining to BS 5423 (superseded
in part by BS EN 3: Portable fire extinguishers).
Table 1: Percentage content discharge for extinguisher types
Percentage content discharge
Water and foams
Powder (after continuous discharge)
It is also worth noting that the percentage of contents discharged will depend on the pressure
contained in an extinguisher body, relative to its size and how far away the siphon tube is from the internal edge of the
base of the extinguisher.
In cartridge units the lever type operation usually has two functions:
- to pierce the cartridge
- to begin and interrupt discharge
It could be said that there is more of a margin for mechanical failure due to more operations
being carried out; but today piercing pins, although only forced into the head-cap and not definitely fixed, are usually hollow.
As a result, if the pin is torn out of the head-cap and remains in the cartridge diaphragm, carbon dioxide will still be allowed
to escape into the extinguisher body.
Rating to weight ratio
Due to the extra mechanisms required for gas cartridge dry agent extinguishers, it may be said
that stored pressure units give a better rating to weight ratio meaning that a stored pressure unit will give the same (or
sometimes better) fire rating but will be lighter than its cartridge counterpart. The weight involved is not, however, a great
deal for an able-bodied person, and as lighter materials are increasingly used in cylinder and component manufacture, this
factor will become even less of an issue as the total weight of one charged unit must not exceed 23kg under any circumstances.
Extinguishers might be installed in areas with a high risk of physical damage, chemical or meteorological
attack. Even though measures can be taken to prevent this they can often be inadequate or, in cases of chemical or meteorological
attack (which can happen between regular maintenance checks), the effects can become evident very quickly. One example would
be an area where industrial processes are carried out. If a fork-lift truck or other sharp object makes contact with an extinguisher
body with any great force the unit could be punctured or the head could become badly damaged. In the case of the cartridge
unit only the contents will have to be cleared.
In the case of a stored pressure unit, there is, however, the possibility that the contents would
be expelled at force and pieces of the unit could become projectiles possibly injuring people or damaging property. The contents
would also be spread over a much larger area, possibly damaging or contaminating expensive materials.
Recharging and depressurising
As closures and threads on cartridge models do not have to be totally gas-tight until operation,
when units are refilled or refurbished, there are fewer difficulties experienced in getting the unit to contain and retain
pressure since it is contained in the cartridge until it is pierced (assuming weight loss does not occur). Even if leaking
does occur from the cartridge, the closures are sufficient to retain at least some pressure in the body. Stored pressure units
can lose pressure over a long period on site and this can sometimes go unnoticed until the unit is next serviced. When recharging
a stored pressure unit a compressed air container is needed as well as a range of charging adaptors. These items can be expensive
With cartridge units, a stock of spare cartridges can be retained on the premises if required,
especially if extinguishers are used regularly. Recharging is simple and quick if carried out properly and safely. In high-usage
areas, such as foundries, this would be a distinct advantage. Spare charges are also cost effective.
Ease of service
It is also worth noting that, from a servicing point of view, stored pressure units are much more
cost effective in terms of maintenance due to the minimum amount of work required.
It is evident that the automatic replacement of old cartridge extinguishers with stored pressure
cylinders is not as straightforward as it might at first seem. Many factors have to be taken into consideration and it would
be true to say that some factors, such as the nature of the working environment, would have more of a bearing on the decision
than others, such as the minimal rating to weight ratios assuming that the premises to be supplied are staffed by 100% able-bodied
So it could be argued that the higher the risk of damage to the unit, the more sensible it would
be to install a gas cartridge extinguisher as long as any damage is rectified immediately. But the decision must ultimately
be an informed one following careful consideration of all possible alternatives and avenues.
It is important to note that the problems described can only multiply if annual or more frequent
maintenance is not carried out to a high standard q
Andrew Brown MIFireE is a Watch Manager with the West Midlands Fire Service
References and bibliography:
1. Manual of Firemanship Book 3 Hand
Pumps, Extinguishers and Foam Equipment
2. Fire Extinguishing Trades Association,
Guide To Servicing Portable Fire Extinguishers
3. BS 5423: Specification for portable
fire extinguishers: 1987 (withdrawn)
4. BS EN 3: Portable fire extinguishers