Room & Contents Fires Fire-fighting Flowrates for Gaseous & Fuel-phase Suppression

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1 Room & Contents Fires Fire-fighting Flowrates for Gaseous & Fuel-phase Suppression Paul GRIMWOOD Compartment, enclosure or room fires present themselves in two distinct burning modes and 3D water-fog techniques, using spray patterns to pulse or burst finely divided water droplets into gaseous-phase combustion, along with direct applications of solid stream jets, to tackle the fuel-phase of combustion, effectively require different water flow-rates to achieve their individual objectives. However, in practical terms there is generally no real conflict, as we will discover. With approximately 112,000 structural fires annually, the average room & contents fire accounts for around 85% of the UK firefighter s structural workload. The average room (compartment) fire is generally confined to an area between 12-50m2 with area fire involvement in excess of 100m2 accounting for only a very small percentage of incidents. It is noteworthy that 25% of serious working fires actually become worse, following UK fire service arrival, before control and suppression is achieved. This is due, in the main, to a forced entry by firefighters that allows additional air to reach the fire via the approach route they have created for an interior fire attack. It is also due, in part, to a distinct lack of flow-rate on the primary attack hose-line, which is traditionally a first-aid 19mm high-pressure hose-reel where reliance is made of secondary support hose-lines of larger diameter being laid in promptly. This concept of playing catch up against the potential for any sudden escalation in fire development is fraught with dangers and recent experiences might suggest that firefighters entering an average room of 75m3 (for example), existing in a state of only partial fire involvement, should be equipped with a minimum flow-rate capable of dealing safely and effectively with any rapid fire development. The UK s firefighters experience an event; termed abnormal rapid fire development around 600 times every year and this alone should prompt a tactical approach that ensures a capable flow-rate from the point of initial entry. Further still, Compartment Fire Behavior Training (CFBT) in the UK has generally failed to establish a two-tier strategy for interior fire attack and there now exists a grey area where the transition between interior uses of pulsing water-fog and direct attack jet streams lies. I have always encouraged that this transition should occur where gaseous/fuel-phase fire involvement beyond 70m2 exists when direct attack, via high-flow jet streams, becomes the most effective means of suppression. The only time this might change is where the area of fire involvement is broken down into several manageable compartments (rooms) or perhaps where a stair-shaft is involved as such fires are generally well suited to the convecting water-vapour created by pulsed water droplets. However, the overall rate of heat (energy) release should also be taken into account when risk assessing compartment fires and deciding on the method of attack. Paul GRIMWOOD 1 PaulG@fire2000.com

2 The fire-ground formula for tactical flow-rate is A (m2) x 4 = LPM but ensuring a minimum flow-rate of 200lpm as a starting point. Therefore 50m2 (or below) of fire involvement requires 200lpm according to the formula, or 200m2 of fire would require 800lpm to achieve fire suppression effectively during the growth or steadystate stages of fire development. However, there are certain other factors that might affect that formula. How hot can 'average' room fires get? The average room fire can produce ceiling temperatures as high as degrees C and room fires are often quantified in their rate of heat (HRR), or energy release, in kw (or MW). In general, the greater the heat release rate, the greater the temperature. However, not all energy released is actually heat as it may not have sufficient air (oxygen) to burn inside the room. As serious room fires are confined and searching for much needed oxygen they generally burn in a state of ventilationcontrol. That is, much of the energy release is unable to combust within the room itself but will turn to heat-release as it gets near to areas of higher ventilation, ie; doorways and windows. As the fire gases leave the window there is a plentiful supply of oxygen and they may combust with some great ferocity. Therefore, when discussing HRR in relation to a compartment fire it is important to establish if it refers to energy release or heat release as much of the energy release will convert to flaming as it leaves the compartment. Doorway (side view of room) Fig.1 - A side view of a modelled design fire demonstrates that heat release rate is never stable and that fire gases may burn hotter at various locations in a room, depending on the positioning and status of ventilation points, ie; windows and doorways. Therefore a burning gas layer may burn at HRR's of 1 to 3 to 7 to 9 to 11KW and possibly more, at locations where the ventilation is good. The practical implications of this are that firefighters may need even greater cooling capability in the firefighting stream at the entry doorway than inside the room itself. The practical implications of this are that firefighters may need even greater cooling capability in the firefighting stream at the entry doorway than inside the room itself. Many times I have heard firefighters say that they just could not get in the doorway as the heat was so severe. However, subsequent investigation demonstrated that they were equipped with an adequate flow-rate to handle the fire load of the compartment. This suggests that the energy release was dynamic at the ventilation points and entry doorway, demonstrating high amounts of heat release at these locations in particular. Paul GRIMWOOD 2 PaulG@fire2000.com

3 A computer model might estimate that a compartment fire load, fully involved in fire, would provide a 16MW HRR (for example). However, a compartment fire may only burn with an efficiency factor of 50 percent and the actual amounts of HRR occurring inside the compartment will not be as high as this. How are water requirements for effective fire suppression calculated? The needed flow-rate to effectively and safely suppress any particular fire can be matched against estimated fire load energy densities, which are measured in MJ/m2 and are around 500 MJ/m2 for residences and 800MJ/m2 for modern open plan offices. The needed flow-rate can also be based on empirical research of real fires or laboratory experiments. Cliff Barnett developed the FIRESYS model that generates data based on a fire occurring in a compartment with an applied fire load, depending on occupancy type. It takes into account various factors in the fire s growth and development stages to estimate energy release. The required flow-rate for effective fire suppression is then provided based on 50% for interior firefighting and 50% based on exterior exposure. The National Fire Academy (NFA) formula is perhaps the most well researched and popular method used by firefighters to estimate needed flow-rates on the fire-ground. However, how well do you understand the limitations of the NFA formula? NFA's Field Formula provides an excess of needed water flow-rate as it is designed with two important factors in mind - It accounts for TWO primary firefighting streams. It accounts for structures that are going to be opened up by firefighters to ventilate dangerous combustion products from within. This will probably increase the rate of burning and therefore needed fire flows are also increased to suppress the fire effectively. It is stated (Burns & Phelps) that the NFA field formula is also size limited, in that it may not be 100% reliable for fires where the flow-rate is in excess of 1,000gpm (3,780lpm) or where the fire area exceeds 3,000 ft2 (280m2). The author s (Grimwood) approach to estimating needed firefighting flow-rate has evolved from empirical research and offers a simple rule-of-thumb formula for calculating flow requirements. Based on the need for a single attack line operating from an interior position in compartments up to 600m2 the formula recommends a minimum and safe flow-rate from the primary attack line is A x 4 = LPM Where A = Area in m2 (based on 2.3m ceilings); and 4 is a factor that is variable as follows 4 = average fire load between MJ/m2 (ie., residential to open plan office) or 6 = fire spread to involve structural elements or is wind assisted creating forced draughts, PPV etc; 2 = moderate to light fire load. Paul GRIMWOOD 3 PaulG@fire2000.com

4 What is the minimum safe flow-rate for a room fire? Although there are several factors that go to making up an effective interior firefighting stream, including nozzle pressure, stream velocity, flow-rate, nozzle reaction, application pattern, technique and design, the minimum flow-rate required for any stream is around 200lpm. This is because a primary attack hose-line must be capable of dealing with any potential for rapid fire development in a compartment. In a worse case scenario, in the average room fire, based on the above calculations the flow of 200lpm is a minimum (using either high or low-pressure streams)! What is the difference between critical and safe (tactical) flow-rates? Critical flow-rates are those that are working right on the limits. They are bordering on being unsafe and yet they may well suppress the fire with success if the conditions are right. The Tactical Flow-rate is that which is calculated with an inbuilt safety margin for error. It is perhaps twice that of the CFR but it takes account of the unpredictability of a compartment fires progress through the stages of growth and development towards steady state burning. It also accounts for the dynamic movements of energy release that may be encountered inside compartment fires, as discussed above. What happens where the critical flow-rate is not met? The fire will not be effectively suppressed until the fire load has begun to burn itself out and the fire is progressing into the decay stages of development. This should be avoided at all costs and firefighters should be adequately equipped to deal with compartment fires during their growth and steady state stages before structural elements become involved and structure collapse becomes an additional hazard. How do efficiency factors affect flow-rate? Firefighting streams are never 100% efficient. It is estimated that solid streams are about 50% efficient and fog patterns are about 75% efficient in their ability to control and suppress fires. Therefore, only 250lpm will be having any effect on a fire when using a 500lpm solid stream. Do modern day 'room & contents' fires burn hotter than 30 years ago? There are many articles that suggest modern furnishings and room contents burn hotter than thirty years ago, as there are probably an equal number of articles that state the opposite view that a compartment fire can only burn to a maximum heat output due to burning efficiency factors etc. The truth is, the fire gases produced in modern fires are greater in fuel content than thirty years back and when a sufficient amount of air is encountered, then the combustion will be more intense. Even a small wind entering a compartment may assist this process or extensive venting tactics may serve to provide those additional amounts of air. Some modern plastics also may provide their own o2 supply in the process of burning! Yes fires can and do burn hotter but Paul GRIMWOOD 4 PaulG@fire2000.com

5 they generally require more air than can be provided by a single open window on a still day. Needed flow-rates for various Tactical Applications Solid stream (Direct) fire attack CFR (2 lpm/m2); TFR (4lpm/m2) Water-fog (3D) fire attack CFR (3.25 lpm/m2); TFR (8.13 lpm/m2) Water-fog (Indirect) CFR (0.3 lpm/m2); TFR (2.5 lpm/m2) Tactical Considerations In all cases a secondary support hose-line of equal flow, or greater than that of the primary line already in operation, should be laid in support as soon as is practical. Interestingly, the NFA calculation discussed above, accounts for two lines or twice the flow-rate when compared to the Tactical Flow-rate discussed by the author. As an example, 100m2 (1,076ft2) of fire involvement will require 400lpm (tactical flow-rate); or 330lpm (IOWA); or 1,372lpm (NFA 2 x hose-lines); or 480lpm twice the flow-rate) using a solid stream attack. Remember pressure gets water to the fire but it s FLOW that puts the fire out! Paul GRIMWOOD 5 PaulG@fire2000.com