Pressure points

Andrew Brown looks at the pros and cons of specifying gas cartridge and stored pressure extinguishers

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.

Extinguisher operation

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 medium.

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:

Pressure gauges

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

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.

Moving parts

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.

Adverse conditions

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 and bulky.

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.

Informed decisions

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 employees.

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