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Richard Gatley: ‘Pressure-relief vents offer a means of ensuring sufficient venting area and close as the room pressure subsides, thus preventing post-discharge dilution. |
The demise of halon will necessitate an increased role for air-pressure-relief vents within gas fire suppression systems if safety standards are to be maintained, writes Richard Gatley.
Early gas fire-suppression systems using halon gases involved the introduction of relatively low volumes of gas to extinguish fires. Concentrations of around 5% were sufficient to achieve the desired reactive effect to extinguish fire. The effect on room pressure of these systems was generally negligible, as rooms are never likely to be completely leakproof and therefore unlikely to result in room pressures in excess of 500 Pa, which most structures would be able to withstand.
Oxygen starvation
As from the end of 2003, when the halon phase-out is completed, these gases will be eliminated from the fire suppression gas arsenal. Indeed, a number of alternative gases and mixtures are being widely used. Many of these agents extinguish fires by reducing the oxygen concentration from the normal 20% to about 12%, effectively asphyxiating a fire. This typically requires the addition of about 40% of the volume of the room of inert gas. If we were to make the assumption that the room was leakproof, the room pressure would increase to around 40 kPa — a significant increase, and one that would certainly result in structural damage to the room.
Clearly there will be various levels of leakage depending on the particular application. The presence of room leakage requires us to consider not only the volume of gas but also the rate at which it is introduced. The rate at which air will escape from the room is directly dependent on room pressure and leakage area.
Integrity tests
All structures have a maximum allowable internal pressure, the most common being 500 Pa. So, as many gas fire-suppression systems have a peak discharge rate within 2 s and are almost fully discharged within 10 s, it is imperative that proper consideration is given to ensuring that there is sufficient leakage area to prevent damage to the building through over-pressurisation.
Room integrity tests are often carried out to establish the natural leakage of the room and to establish the required additional venting area necessary to prevent over-pressurisation.
However, such information should be regarded with a degree of caution, as alterations in the natural leakage can occur over the life of the room.
For example, if the room is constructed of porous blockwork with just a light coating of emulsion paint, then as subsequent coats of paint are applied, the porosity (and hence the natural leakage area) will decrease. Over-pressurisation on discharge therefore becomes a distinct possibility. Some gas fire-suppression engineers now choose to provide sufficient venting area through air-pressure-relief vents, irrespective of the results of the room integrity tests.
It must also be recognised that once the gas has been discharged, its concentration must be maintained for a period to ensure that the fire does not rekindle. At this stage of the process it is necessary to prevent gas dilution in the room by fresh air entering through the leakage paths. It is a reasonable assertion that a room with sufficient natural leakage area to prevent over-pressurisation will not be effective at preventing post-discharge dilution.
Ideal design
Pressure-relief vents offer a means of ensuring sufficient venting area and close as the room pressure subsides, thus preventing post-discharge dilution.
The ideal design of a pressure-relief vent is one that will remain closed at pressures less than 100 Pa and open fully as the pressure differential increases above 100 Pa. Units should also close as the pressure subsides, and well before the room pressure drops to ambient level.
For protecting rooms within rooms, it is important to remember that it is not the absolute pressure within the room that is important, but the differential pressure generated across the walls, ceilings and floors. Careful consideration needs to be given to how the air, vented from a room during a gas discharge, is handled or dissipated. If a pressure-relief vent is fitted to an outside wall in conjunction with an external weather louvre then, assuming they are of matched sizes, the size must be chosen on the basis of which of the two elements provides the least free venting area.
Fire suppression systems are installed with the hope that they will never be required to operate. However, when fires occur in systems without proper pressure venting, structural damage will occur — with potential fatal injuries, thereby presenting significant risks to the gas fire-suppression industry as a whole.
Apreco Ltd, The Bruff Business Centre, Suckley, Worcs. WR6 5DR.
info@apreco.co.uk
