Fire Safety Engineering Problems: Case Studies

The fire safety engineering has been finding increased use in the recent time as with the rapid globalization and interconnectivity. FDIs have been flowing into the infra market and which has put the focus on how to improve the fire safety particularly in engineering applications. This review basically focusses on innovative solutions to fire safety problem in those cases where prescriptive building regulations are costly and there are some restrictions in building design. It basically presents case studies where the use of fire science and fire safety engineering to address various aspects of building design where sprinklers systems are involved are present. The first case is basically a case where an alternative fire suppression system is used in place of sprinklers. The second case discusses about an airport where how the rationalization of the sprinkler system helped in saving money. The third case is about how spread of fire from floor to floor can be prevented in balconies by using partial sprinkler system.

Case Study 1

This basically studies an alternative to sprinkler system in a hall

The exhibition hall is 280 m long, varies in height from 14 to 24m and is 70m wide. For this the hall was divided into 4 separate halls. Now the problems regarding Sprinkler Efficiency in the halls is

Limited impact which the sprinklers offer due to high height of the roof. Now one of the reasons for installing a sprinkler system is keep fire small and manageable, in case it is not extinguished. In case of office with relatively low ceiling sprinklers do make a significant impact. By calculation it can be shown that in a sprinkler protected office with a 3m ceiling height, sprinklers would activate when the fire intensity is approximately 2.5 MW, and a properly designed and installed sprinkler system would ensure that the fire did not grow any further. In an unsprinklered office a fire could reach 10 MW quite easily, depending on the ventilation factor and fuel load. A reduction of fire intensity from 10 MW to 2.5 MW is quite significant.

In the Hall, however, sprinklers are not likely to produce such a meaningful reduction in fire size. Hall, however, sprinklers are not likely to produce such a meaningful reduction in fire size. Calculations using fast-response sprinklers with a Response Time Index (RTI) of 50 and a sprinkler activation temperature of 68°C, show that the lowest sprinklers, namely those at 14.5 m above the floor, would activate when an ultra-fast fire is at an intensity of approximately 4.4 MW. For the sprinklers at 20.5 m high, the fire size at sprinkler activation is approximately 10 MW. Significantly, in approximately 50% of the building the ceiling is above 20.5 m and the resultant fire size at sprinkler activation would be greater than 10 MW. Where the highest sprinklers (24.5m) were located the shape of the roof in this building makes predictions on sprinkler activation times unreliable.

Now talking about reduced effectiveness of sprinkler water spray

With fires of intensities around 10 MW the interaction of sprinkler water droplets with the fire plume would reduce the suppression and fire control effectiveness of the sprinklers due to the upward velocity of the hot gases in the plume and the evaporation of the water due to the heat. In addition, the drop size distribution of a sprinkler spray is such that not all droplets would be capable of penetrating the fire plume due to the upward velocity of the hot gases and evaporation.

The Alternative

On the assumption that the fire growth rate could be ultra-fast, the 10 MW heat output would be reached in approximately 4 min from the time that the fire reached the so-called established burning stage of around 10 kW). On this basis it would not be sufficient to rely on Fire Brigade intervention alone. Although Fire Brigade response time might be in the vicinity of 4 min, additional time would be required to locate the seat of the fire, and set to up hoses for firefighting. Attack on the fire is also not likely to commence until evacuation is complete.

In terms of water quantity and effectiveness of firefighting, hydrants would theoretically be adequate to control a 10 MW fire. If a system providing water equivalent to that from a hydrant or hydrants could be designed to get water onto a fire in less than 4 mins then the design objectives (10 MW max) could be met.

The Method

In considering the approach used in some large aircraft hangars where automatic foam monitors are used to apply a foam solution over the floor in the event of an aircraft fuel spillage, it was decided that one could control a 10 MW fire in the Exhibition Halls by a similar method, namely fixed monitors, but without the foam.

A major benefit of such a monitor system would be its ability to apply water to the seat of the fire from the side of the plume, rather than from above the plume as is the case with sprinklers. This would obviate the difficulty of having to penetrate the plume for its full height, and the fire suppression effectiveness would be substantially enhanced

The functionalities of the monitor installed

In order to ensure total coverage of the floor area, bearing in mind high obstructions that could be part of an exhibit, four monitors were installed in each Hall. The monitors were mounted at an elevated position at a height of approximately 4 m above the Exhibition Hall floor. The nozzle size was selected by the mechanical consultants on the basis of the quantity of water that should be delivered to provide a density at least equal to that of a sprinkler system, together with the maximum distance that the water had to be projected. The maximum effective distance reached by each monitor was 40 m, using four monitors per hall. Being attached to the Hydrant system the water supplies were required by the authorities to allow the hydrants to operate simultaneously with two monitors. In operation, each monitor would be preset to oscillate through a certain arc and at a certain elevation when activated. Activation could be either remotely from the Control Room or manually by an operator at the monitor. The operators of the monitors would be either trained staff or Fire Brigade personnel.

In conclusion, the fire safety engineering solution to the fire suppression problem in this building was not only effective but achieved significant cost savings.

Case Study 2

This case involves the use of the Sprinkler system at airport

It basically involves fire-safety engineering methods used in Sprinkler system which has estimated cost of ten million dollars. The engineering approach was motivated with cost savings in mind, but at the same time fire safety objectives as required.

The study addressed the fire hazard associated with easily defined and separate areas typical of airports, such as Arrivals and Departures Concourses, Departures Hall, Baggage Claim Hall and Baggage Handling Areas, and because the layout, usage and fire loading in different compartments varied, a variety of methods were employed in the analysis.

The departures

This area had the seats which contained foam rubber filling it was necessary to establish the likely worst case fire scenario as accurately as possible. For this it was decided bum a selection of the seats in a furniture calorimeter in order to obtain data on their burning characteristics.

An analysis of corresponding sprinkler response times resulted in an increase in sprinkler spacing across the corridor, from the 4 m maximum allowed by the code, to 5.2 m. This arrangement would not give rise to any loss of sprinkler effectiveness because it was considered that those areas where the water application density was below standard would be at the pedestrian aisle between the rows of seats where the fire load was negligible.

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Departures hall

The floor areas are very large (around 10,000 m2 ), the average fire load was very low, and more importantly, those parts having a significant fire load, namely check-in counters, groups of seats or carousels full of baggage, were separated by unfurnished floor areas. for an assumed worst case fire, namely, an ultra-fast fire in a check-in counter full of baggage, sprinklers at the Extra Light Hazard spacing of 21 m2 per sprinkler, would operate when the fire had reached an intensity of approximately 4 MW.

It was concluded that the sprinkler system would not be required to handle a fire involving more than one check-in counter, and hence the recommendation to use Extra Light Hazard instead of Ordinary Hazard in this area was justified.

Canopy outside departures hall

In the context this canopy (140 m long) required two rows of sprinkler heads. A likely fire scenario was a car on fire at the kerb of the roadway. Being beyond the edge of the canopy there would be no convected flow of hot gases to activate the sprinklers. It was shown that a single row of sprinklers would provide adequate protection for the building in the assumed fire scenario.

Radiant Heat calculations showed that in the event of a car fire at the kerb of the roadway, the windows of the building closest to the car could be subjected to a heat flux of up to 13.5 kW/m2. A single row of sprinklers below the awning ceiling was considered to provide sufficient protection in the assumed fire scenario.

Ceiling space above departures hall

This large compartment contained a timber floor and other combustibles such as electrical wiring. The concrete slab above consisted of inverted troughs which were 2.5 m wide and arranged in groups of five. The Sprinkler Code required sprinklers to be installed in the narrow troughs but it was evident that the effectiveness of these sprinklers was questionable on two counts. Firstly, if the sprinklers we-re installed at the optimum distance from the slab (around 100- mm) the water spray would be baffled by the sides of the troughs. Secondly, heat from a fire immediately below a narrow trough would travel very quickly along the trough and activate an excessive number of sprinkler head.

Assuming that there were no sprinklers in the narrow troughs, the behavior of hot gases from a fire below these troughs was examined. It was found that a 10m length of the narrow trough would be filled with hot gases in less than 5 s. Therefore, if baffles were placed at 10m intervals along the trough, the hot gases would overflow from the narrow trough and into the adjoining larger troughs where the sprinklers would be activated with a minimal delay.

This resulted in the deletion of the 400 sprinklers from the narrow troughs. This in turn saved a lot of money for the airport.

Case Study 3

This involved how to prevent fire from spreading from one floor to other in a multi storey apartment.

The problem associated with enclosed balconies in a multi-storey building is related to the fire spread objectives in particular, the upward spread of fire from one storey to those storeys above the fire affected storey. Where balconies are not enclosed flames issuing from a fire would be deflected horizontally by the balcony above and this would prevent spread of fire to the floor above. In this case the balcony above would act as a spandrel. Where balconies were enclosed, the spandrel effect was effectively removed because the enclosed balcony becomes a room capable of reaching flashover conditions in a fire.

In an effort to overcome this problem it was proposed to install sprinklers in the enclosed balconies.

Because sprinklers would be installed only in the enclosed balconies and not in the apartments two fire scenarios were addressed, firstly a fire originating in the enclosed balcony and secondly, a fire originating in the adjoining apartment.

Fire Scenario I

On the basis that the sprinklers installed in the enclosed balconies It may be assumed that a fire occurring in an enclosed balcony would be controlled by the sprinklers and flashover is unlikely. In these circumstances fire spread to the upper level would not occur.

Fire Scenario 2

For fire scenario 2 a fire is assumed to occur in an apartment adjoining an enclosed balcony. Because the apartment is not sprinkler protected it was assumed that the fire would reach flashover proportions and break the windows to the enclosed balcony and then the external windows of the enclosed balcony.

The Effect of Balcony Sprinklers

In a flashover fire as the flames and hot gases flowed out of a window the gases would be cooled by mixing with outside air. At this temperature radiant heat from the flames issuing from the window below would be sufficient to cause breakage to the windows above, resulting in a potential for spread of fire to the level above the fire Where sprinklers are installed in the balcony the hot gases from the room of fire origin will be cooled by the sprinkIer water spray before they issue from the window.

A calculation showed that in a typical situation radiant heat received by a window from flames at 600 ° C issuing from a window below would be 22 kW/m2 , while with gases at 350 ° C in the same situation the window would receive less than 6 kW/m2 Standard windows can safely withstand a radiant heat intensity ofup to 10 kW/m2

It was concluded therefore that the provision of sprinklers in an enclosed balcony would reduce the temperature of fire gases flowing through the balcony to such an extent that there would be radiant heat hazard to the windows in the floor above