Effects of Sprinkler System in Compartment Fires - Literature review Example

Effects of Sprinkler System in Compartment Fires The automatic Sprinkler System For decades of years, the automatic sprinkler system has been used as a very effective means of fire protection. Studying the effects of sprinkler systems in compartment fires may not suffice without understanding the mode of operation of the sprinkler system. The sprinkler head types within an automatic sprinkler are sensitive to temperature (Hoffmann, Galea & Markatos, 1989). They are made to react at certain set temperatures, but independently one from the other. When the predetermined temperature is attained, there is a mechanism that forces a valve to open and water flows out in a stream which spreads over a certain predetermined space. Through a pipe system specially designed for the purpose, the water flows to the sprinklers. The sprinkler heads are set at certain specific intervals (Hoffmann, Galea & Markatos, 1989). A prominent feature of the automatic sprinkler is the distributor or deflector plate which was designed in the 1950s. The stream of water is made to flow towards the plate from where it is spread out to form a spray of the shape of an umbrella under the sprinkler and no water droplets are sprayed upwards (Poon 2013). This characteristic of the sprinkler system releases a uniformly distributed spray of water under the sprinkler, regardless of whether it came from an upright sprinkler or one of a pendent type. Early designs of sprinklers heads, manufactured before the 1950s released a spherical spray pattern with almost 60 percent of water going downward and the remaining 40 percent rising upward to the ceiling of the building (Hoffmann, Galea & Markatos, 1989). When designing a sprinkler system, certain factors must be considered. First, the spray umbrella has to be covered. The quantity of water to be discharged and the pressure pushing the water as well as the size of the opening on the sprinkler are other important factors. It is also necessary to factor in the operating temperature as well as the speed at which the sprinkler will operate (Hoffman, Garlea & Markatos 1989, p. 299). Compartment Fires An understanding of the nature and behaviour of compartment fires can go a long way in helping to understand the effects of the sprinkler spray on these fires. Most compartment fires are characterized by a pattern of clear phases. However, the timescales and sizes of these phases can vary significantly from one fire to another. After the fire has been ignited, it goes through the first phase which is the period of growth (Zhang & Chow, 2014). In this stage, the fire develops and spreads out to the nearest fuel. With continued development of the fire, the temperature inside the fire cell will increase and all exposed surfaces absorb radiation heat coming from the fire, the surfaces of the compartment and in some cases a hot layer on the upper side. If the compartment temperature goes to 600°C, all the surfaces exposed to the fire can ignite spontaneously. This process is called flashover. After this flashover stage, the fire starts burning in a stage of full development. If the fire in this phase is not controlled, and there are vents, of broken windows or any other opening, the fire goes on burning until when it exhausts all the fuel then it starts the decay phase (Poon, 2013). In the initial fire stages, the phase of growth is critical when attempting to assess the rate of production of smoke, how high the smoke layer is and the conditions inside a compartment. If there are sprinklers, they tend to actuate at the time the phase is passing through the growth stage so that they start to bring down the rate of heat generation. This to a large extend reduces the possibility of have a flash over take place. As a result of the exothermic reactions taking place in fire types that raise the surrounding temperatures, there is an early stage where a stream of gas rises higher than the source of fire and into fresh air that has not been contaminated (Yea, et al., 2016). When the pressure in a compartment is constant, combustion products are heated and this reduces the density of the gases. The resultant difference in gas density between the air around and the gas stream makes the stream to move upwards. Buoyancy forces force less dense streams to rise when compared to their surroundings. The rising stream of gas is always turbulent and the rising gases and the flames with them form the fire plume. The fire plume has 3 major regions known as the continuous region that contains a luminous flame, the intermittent region that at other times has or may lack a luminous flame and buoyant part of the plume which carries only the burnt substances and entrained air (Poon, 2013). Smoke and Water Interactions Sprinkler systems provide a reliable means of fire control in buildings. They work best on small fires and when they are close to the fire, although deluge systems operate differently because many sprinklers are activated at once (Zhang & Chow, 2013). Sprinklers control the first stages of the growth of fire and release of heat and keep it within a single compartment. The sprinklers extinguish fires through the displacement of air from compartments by producing steam. A sprinkler spray also cools the room and the fuel, thus reducing the radiation of heat back to combustible materials (Poon, 2013). Direct contact of spray water with fuel hinders the release of combustible vapour through the cooling of the fuel. The sprinkler pre-wets the any combustible substance to prevent any more spread of the fire. According to Ganapathy (2002, p. 105), when smoke interacts with water droplets from spray activated sprinklers, the droplets take away buoyancy from the products of combustion and create air currents that overcome the outflow from vents thus transporting combustion products down to the floor. There are mixed opinions about the effects that automatic sprinkler sprays have on compartment fires. Some researchers argue that there is no need for controlling smoke in sprinkled compartments because sprinklers check the growth of fire, reducing its size and cutting down the production of smoke to harmless levels (Nell & Zenzen, 2007). Other researchers believe that sprinklers benefit smoke control systems by reducing the rates of airflow and differences in pressure so that smoke is effectively controlled. Another argument made by a number of researchers is that sprinklers make the condition of smoke worse. Interaction of Sprinkler Spray with a Smoke Layer When a sprinkler is actuated at a time when the smoke layer has a very small depth, then the effects of the interaction of the sprinkled spray and the smoke layer may not require consideration. In case there is a smoke layer, the effects of interaction of the sprinkler spray and smoke layer become significant and are supposed to be considered (Zhang & Chow, 2013). As the droplets of water released by the sprinkler spray go through the layer of smoke, the temperature of the smoke layer falls. As a result, the smoke layer loses a portion of its buoyancy and this makes it to fall down to lower levels (Cooper, 1995). There is an effect as well on the water droplets from the sprinkler spay because the droplets are subjected to an air drag effect which depends on the diameter of the droplets and the resultant effect is that the stratified smoke layer is pulled downward. It is important to consider the evaporation of the sprinkler spray droplets as they interact with a layer of smoke (Zhou, 2014). Droplets moving through the smoke layer absorb heat and upon reaching boiling point they start evaporating. The effect of this is a reduction in the diameter of the droplets and leading to a reduction in the air drag effect. Cooper (1995) described the relationship between smoke and sprinkler spray. When the sprinkler is in the lower smoke layer, at the surface or below, as it may be in a fire in a shopping mall that has large unconfined ceilings. The interaction between the smoke layer and sprinkler spray is very little. The head of the sprinkler is in the upper layer and this is what the spray interacts with (Cooper, 1995). When the sprinkler has actuated the spray is able to entrain gases from the upper layer which has a higher temperature, fall down and cools, releasing humidity to the upper layer gases as a result of droplet evaporation. A develops which contains downward flowing gases and the jet becomes a fixed spray cone enveloping the fire. Within the layer beneath, if the buoyant forces flowing upwards are weak and they cannot drive the jet upwards to the upper layer, then it remains within the lower layer. If the buoyant forces flowing upwards are enough to force the jet towards the layer above, then a huge amount of lower gases settle there. The effect is a compromise in the stability of the smoke layer and this result in thorough mixing with air in the lower parts which is always cooler. This effect is called smoke logging (Cooper, 1995). In fire disasters, the mode of operation of the sprinkler and the attributes of the combustible materials play a critical role in influencing fire safety. Yea, et al., (2016) performed a study to using CFD simulation software called FDS to investigate the various sprinkler system modes of activation in various fire scenarios in big commercial buildings. The researchers discovered that during the first stages the sprinkler system smothered the fire and stopped it from developing. This happened through the inhibition of the pyrolysis of combustible substances. The effect of the sprinkler system significantly reduces the heat transfer process; thereby allowing for a longer time of evacuation in the building (Yea, et al. 2016). There are two types of fire protection for buildings which include the passive and active types (Poon, 2013). For structures, there are codes that demand the provision of passive protection. The active protection afforded by sprinklers is needed in buildings that have a bigger fire hazard such as high rise buildings (Qu & Wu, 2016). Using fire tests and statistics, studies have demonstrated the ability of sprinkler systems to contain the growth of fires. As opposed to passive protection, active fire protection systems like sprinklers need to have motion and response for them to work. Compared to passive protection, the reliability of active protection systems to function whenever there is the need is thought to be lower. Poon (2013) performed a study using the risk assessment approach. He sought to discover by what means people can rely on sprinklers to give protection to structures so that they can achieve the goal of fire safety in their buildings. Preliminary assessments showed that fire rated compartment barriers and sprinklers displayed similar performance in their ability to contain fires. However, sprinklers performed a bit better. Generally, the sprinkler spray provides the ability to curb fire before is develops fully (Poon 2013). The reduction of the potential development of fire has other benefits including the reduction of potential damage and making it possible for people to access to fire intervention and rescue. There is a potential of lowering the fire resistance power of the barriers inside a compartment without a compromise on safety, especially in scenarios where sprinklers exist (Poon, 2013). To compare the effects that sprinklers have on fire, several tests featuring un-sprinklered and sprinklered scenarios on the same fire compartment were used. In Harbin University in 2004, a study of flash over office fire tests were performed. The set up was made using a chair, a table with papers and books placed on top, a cupboard and a computer box inside the fire room. Without the use of sprinklers, the rate of heat generation at the peak was approximately 1.8 MW at 800 s. When sprinklers were used, the head activation was done when the rate of heat generation hat hit 1 MW, but it rose to 1.5 MW. The fire intensity based on the amount of energy generated fell significantly to approximately 17 percent of the fire that was not sprinkled. Another study involving full scale experiments on fire in empty dormitories was done by the National Institute of Standards and Technology (NIST). The experiments were aimed at studying the effect of sprinklers in the room where the fire started, with the door opened and later with the door closed (Ganapathy, 2002). It was observed that the sprinklers got activated within two minutes and smothered the fire before reaching to flashover. The fire temperatures remained under 100 degrees Celsius. With no sprinklers in use, the fire went on for 10 minutes with maximum temperatures of between 800 and 850 degree Celsius. A number of full scale fire experiments on residential premises were carried out in a project undertaken by Fire Code Reform Centre (FCRC) in Melbourne Australia. The aim of the study was to study flashover fires. One of the tests had a sprinkler controlled fire in which the temperature achieved a peak of 250 degrees Celsius, though for a short time. On average, the temperatures did not rise beyond 100 degrees Celsius. Where the sprinkler was not used, the temperatures stood at a high of 1000 degrees Celsius. For 20 minutes, the temperatures were 500 degrees Celsius and above. The conclusion made was that the sprinkler system inhibits the growth of fire to high temperatures and disastrous proportions (Nell & Zenzen, 2007). Through various tests, sprinklers have been found to be the most workable solution to building fires. The water spray produces a cooling effect and they experience a drag force causing the stratified smoke layer to lose its stability. Zhang & Chow (2013) studied the cooling effect of sprinkle system water sprays. They also studied the stability of the smoke layer as well as the mass flow rate. In the simulation they applied the Fire Dynamics Simulator (FDS) code which they based on the large eddy simulation idea. Through the experiment, the researchers found that the decrease in temperature came very close to being linear with the sprinkler system’s working pressure. The spray strongly impacts on the motion of smoke within the compartment (Zhang & Chow 2013). Since it produces a cooling effect the activation of the sprinkler results in a decrease in temperature. The reduction in temperature was found to have an almost linear relationship with the working pressure. The water spray produced a drag force that caused the smoke to descend upon the activation of the sprinkler. Since the head of the sprinkler was put at a long distance from the vent, there is no big change in the inflow and outflow (Zhang, & Chow 2013). The interaction of the smoke layer and the sprinkler warrant much investigation because the instability of the smoke layer caused by water droplets from the spray and its eventual drop to the ground is a risk to ventilation and evacuation of people in the event of a fire. The issue of the stability of smoke when it encounters a water spray has been investigated by researchers in the past such as Bun (Yea et al, 2016). They assumed that the smoker layer was a constant thickness. They sprinkler spray was assumed to be drops with spherical shapes, with equal diameters calculated using the sprinkler pressure. A physical measurement called the “drag to buoyancy ratio” for the whole of the smoke layer was adopted as the criteria for its stability. Morgan and Baines worked on the model further to develop it and include the convective exchange of heat between the fire smoke and the sprinkler spray. They also included a distribution function of water droplets from the sprinkler. In another model developed by Cooper, it was assumed that the smoke layer below the nozzle of the sprinkler was dragged down by the drag force of the water droplets and forced upwards by its own buoyancy (Cooper, 2005). Since the water spray produces a drag force and cools the smoke, the system of ventilation would be ineffective. The spray droplets produce a drag force which causes the mass flow pattern to change. Several numerical simulations about the spray and fire smoke exist. However, the available simulations are based on the interaction between the spray and the plume. Rarely would studies be made on smoke stability. A significant part of the sprinkler spray protection process is where the fire plume interacts with the water spray (Zhou, 2014). So as to provide appropriate data for developing and validating a LES-based model of fire protection a number of experiments were carried out to investigate the way hot air plumes interact with water sprays via combined gas liquid speed and droplet size measurements. The researchers used Laser-based particle image velocimetry (PIV) to obtain the spatially resolved velocity data. They also used a shadow Imaging System for measuring the water droplet size and volume flux. They selected hot air plumes to interact with a spray of water. They then measured the velocity field of the ceiling floor and the hot air plume with and without the water spray. They also measured the volume flux and the size of the droplets of the water spray with and without the hot air plume. The results of the experiment revealed that the evaporation effect caused by the existence of hot air on the water droplets was instrumental in changing the structure of interaction and the pattern of the ceiling layer (Poon, 2013). It is clear from the above review of literature that studies on the effects of sprinkler systems in compartment fires are not in short supply. Many studies have been accomplished on this subject with almost similar results. As shown in this paper, all the studies arrive at the conclusion that because of the water spray’s cooling effect and drag force it produces, the smoke layer loses its stability and this sets in motion the process of weakening the fire and consequently it is smothered and extinguished (Zhang & Chow, 2013). Bibliography Cooper, L.Y. 1995. The interaction of an isolated sprinkler sprat and a two-layer compartment fire environment. Phenomena and model simulations, Volume 25, Issue 2, pp.89-107. Ganapathy, R. 2002. The Economics of Fire Protection. Taylor & Francis. Hoffmann, N., Galea, E.R & Markatos, N.C. 1989. Mathematical modelling of fire sprinkler systems. Appl. Math. Modelling, vol. 13.pp. 298-305. Nell S.G. & Zenzen M. 2007. M31 Distribution System Requirements for Fire Protection. American Water Works Association. Poon L. 2013. Assessing the reliance of sprinklers for active protection of structures. Procedia Engineering 62, 618 – 628. Qu, X. & Wu, H. 2016. Study of Grid Ceiling on Parametric Optimization Design of Automatic Sprinkler System. Procedia Engineering 135, 439 – 444. Yea, X., Maa, J., Shena, Y. & Linc, L. 2016. Suppression Effect of Sprinkler System on Fire Spread in Large Commercial Buildings. Procedia Engineering 135, 455 – 462. Zhang, C. & Chow, W. 2014. Numerical studies on the interaction of sprinkler and smoke layer, Procedia Engineering 62, 453 – 462. Zhou, X. (2014). Characterization of interactions between hot air plumes and water sprays for sprinkler protection. Proceedings of the Combustion Institute, 35(3), pp. 2723–2729. Read More

With continued development of the fire, the temperature inside the fire cell will increase and all exposed surfaces absorb radiation heat coming from the fire, the surfaces of the compartment and in some cases a hot layer on the upper side. If the compartment temperature goes to 600°C, all the surfaces exposed to the fire can ignite spontaneously. This process is called flashover. After this flashover stage, the fire starts burning in a stage of full development. If the fire in this phase is not controlled, and there are vents, of broken windows or any other opening, the fire goes on burning until when it exhausts all the fuel then it starts the decay phase (Poon, 2013).

In the initial fire stages, the phase of growth is critical when attempting to assess the rate of production of smoke, how high the smoke layer is and the conditions inside a compartment. If there are sprinklers, they tend to actuate at the time the phase is passing through the growth stage so that they start to bring down the rate of heat generation. This to a large extend reduces the possibility of have a flash over take place. As a result of the exothermic reactions taking place in fire types that raise the surrounding temperatures, there is an early stage where a stream of gas rises higher than the source of fire and into fresh air that has not been contaminated (Yea, et al., 2016). When the pressure in a compartment is constant, combustion products are heated and this reduces the density of the gases.

The resultant difference in gas density between the air around and the gas stream makes the stream to move upwards. Buoyancy forces force less dense streams to rise when compared to their surroundings. The rising stream of gas is always turbulent and the rising gases and the flames with them form the fire plume. The fire plume has 3 major regions known as the continuous region that contains a luminous flame, the intermittent region that at other times has or may lack a luminous flame and buoyant part of the plume which carries only the burnt substances and entrained air (Poon, 2013).

Smoke and Water Interactions Sprinkler systems provide a reliable means of fire control in buildings. They work best on small fires and when they are close to the fire, although deluge systems operate differently because many sprinklers are activated at once (Zhang & Chow, 2013). Sprinklers control the first stages of the growth of fire and release of heat and keep it within a single compartment. The sprinklers extinguish fires through the displacement of air from compartments by producing steam.

A sprinkler spray also cools the room and the fuel, thus reducing the radiation of heat back to combustible materials (Poon, 2013). Direct contact of spray water with fuel hinders the release of combustible vapour through the cooling of the fuel. The sprinkler pre-wets the any combustible substance to prevent any more spread of the fire. According to Ganapathy (2002, p. 105), when smoke interacts with water droplets from spray activated sprinklers, the droplets take away buoyancy from the products of combustion and create air currents that overcome the outflow from vents thus transporting combustion products down to the floor.

There are mixed opinions about the effects that automatic sprinkler sprays have on compartment fires. Some researchers argue that there is no need for controlling smoke in sprinkled compartments because sprinklers check the growth of fire, reducing its size and cutting down the production of smoke to harmless levels (Nell & Zenzen, 2007). Other researchers believe that sprinklers benefit smoke control systems by reducing the rates of airflow and differences in pressure so that smoke is effectively controlled.

Another argument made by a number of researchers is that sprinklers make the condition of smoke worse. Interaction of Sprinkler Spray with a Smoke Layer When a sprinkler is actuated at a time when the smoke layer has a very small depth, then the effects of the interaction of the sprinkled spray and the smoke layer may not require consideration.

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