Gaseous systems: the importance of design, installation and maintenance

November 2001 Fire & Safety

With the advent of the many new but more costly fire suppressants to replace halon, there is understandable pressure to minimise the expense of supply, installation and system maintenance. Yet safety margins relating to the use of these new gases are now extremely stringent, so there is certainly no room for corners to be cut.

Skimping on the quality of system engineering is false economy in the extreme. In fact, a poorly designed and installed fire suppression system has the potential to cause more damage than a fire itself.

Consideration of the many complex issues involved in the design and installation of gaseous extinguishing systems needs to start at the earliest 'pre-design' stage. A system engineered by experts takes a host of different factors into account.

Firstly, there are the most obvious considerations relating to the type of fire risk that exists, and the precise area that is to be covered. A thorough review of the risk needs to be conducted in order to ascertain the 'geometry' of the hazard, to facilitate the appropriate positioning of the cylinders discharge nozzles and agent distribution. There is a need to undertake a full site survey.

The objective is to extinguish a fire so quickly that damage is minimised, this leading to the lowest possible levels of combustion and decomposition products. The system must also be designed so that a homogenous mix of air and extinguishing agent is achieved following the discharge cycle, ideally with a rich stream of suppressant being directed at or across the fire source during the discharge phase to accelerate the extinguishing process.

An in-depth knowledge of the air/agent mixing process is vital here. Some of the agents available on the marketplace will arrive at the discharge nozzle in liquid form, only flashing to a gas beyond the nozzle. This gives the benefit of reducing pipe diameters and consequently reducing the total labour and material cost, plus also produces quicker discharge times for the agent to be delivered, this leading to faster suppression times. However, it is imperative that the agent is not directed in the liquid phase onto valuable or sensitive equipment, unless so desired, as this may cause irreparable and expensive damage.

Nozzle positioning

Also, anyone coming into direct contact with the discharge stream close to the nozzle might conceivably receive frostbite burns. It is essential, therefore, that nozzles are installed so that their streams are directed away from exit paths where a person could potentially be struck.

Other considerations for nozzle positioning include the physical structure of the room that is being protected. It is important to identify obstructions that might impede the even distribution of extinguishant. For safety reasons, it is important to avoid placing discharge nozzles next to a fluorescent light fitting where the discharge may shatter the tube. Further consideration should be given as to where agent losses may occur. If this phenomenon can be designed-out of the system, this will assist in the overall efficiency of the extinguishing system.

The installation process

An intelligent approach to installations requires that the original design criteria for a system should always be double-checked up to and beyond commissioning. It can make a fundamental difference to the design of the system if anything has subsequently changed from the time of the design to the time of installation. Suppose, for example, that an enclosure, which was originally designated as an electrical equipment area, is now to be used as a computer/data processing area instead. The fire risk in a computer room may include magnetic tape material or perhaps the presence of stacked paper. For some agents this type of deep-seated risk may call for an increased design concentration, or flame extinguishment may not be possible.

Ideally, the design and installation teams should work in close harmony with one another, so that immediate decisions can be made when changes come to light. Good communication ensures that remedial action can be taken quickly to avoid compromising the effectiveness of the system and reducing unnecessary further visits to site and the costs associated with such visits.

For this reason, use of a company that can design, supply and install the fixed extinguishing system can save the end user a great deal of time and money. In fact, the ability to take concerted action in this way can be instrumental in ensuring that a project stays on schedule.

Of course, the physical quality of the installation itself is of equal importance. Any point of failure in the system at the time of discharge could be catastrophic, so the quality of the materials used and the installation procedure is equally critical. In order to ensure that the extinguishing agent is mixed properly with the air within the risk, it is essential that the pipe lengths and fitting locations are as designed and calculated, otherwise effective extinguishing may not take place. Where a concentric reducer is required to reduce the size of a pipe, use of an incorrectly positioned or wrong type of fitting/bush may create undesirable turbulence, which adversely affects the distribution of agent. This may starve a branch of agent and prevent fire suppression within a single void, compromising the whole system. These are all factors that could make or break the effectiveness of the system in the event of fire.

Working to a standard

For total peace of mind, it is best to engage the services of competent experts who can take full responsibility for the entire design, supply and installation process - preferably using a recognised standard. Such a standard now exists in the form of SABS14520 for clean agent gases. This applies to every part of a gaseous fire suppressant project from the initial design stages right through to installation and commissioning.

Install once; maintain for life

There is more to the maintenance of a gaseous fire extinguishing system than many end users ever imagine. The owner of the building in which the system is installed is ultimately responsible for it, and relevant standards dictate that electrical elements must be checked on a quarterly basis, with the mechanical element being maintained at least every six months.

A few areas that need vigilant inspection are:

* Is the quantity of agent and pressure in the cylinder still correct? The effects of a leak may not be immediately apparent when the ambient temperature is high. On the subject of leakage, the industry and end users alike, are all duty bound to limit or at least reduce any unnecessary emissions. Leakage will not only compromise the system's fire extinguishing capability but also contributes to unnecessary emissions and increases the cost of repair when the leak is finally identified.

* Have there been any structural changes since the system was installed? Alterations in room partitioning will affect the volume of the protected enclosures, making a difference to the overall gas requirements. This might compromise the system performance, or alternatively may lead to dangerous concentrations.

* Has the nature of the fire risk changed? As discussed earlier, change of use from, say, an electrical area to a computer processing room might necessitate fundamental changes to the system design.

* Have any of the discharge nozzles been obstructed in any way? Just a few boxes stacked in front of a nozzle could make all the difference to the effectiveness of the system.

* Regular routine maintenance carried out by a competent individual addresses all of the above.

Then there is the question of integrity testing - key issues that need to become common practise in SA. A gaseous suppression system can be in perfect working order mechanically, but if excessive enclosure leakage is present the system may be rendered ineffective should a fire occur. In the event of fire, it is critical that both the design concentration is achieved and the length of time is retained in the room remains at the originally specified level. This is a highly specialised area that demands expert attention. Let building owners beware!

For further details contact the FDIA on tel: (011) 397 1618.

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