Critical Flow Rates for Fire Attack

Critical Flow Rates

for Compartment Fire Attack

Shan Raffel EngTech MIFireEInstitution of Fire Engineers, Australia

Shan Raffel EngTech MIFireE

Simple Answers

The influence of Swedish Water-fog techniques (more recently termed 3D Fire Fighting Techniques) and CFBT (Compartment Fire Behaviour Training) has led to safer and more efficient firefighting. While the author is a strong advocate of these techniques, there is an emerging belief by some firefighters that “less water always means safer and more effective firefighting”.


Matching the Actions to the Situation

Correct in some situationsSmall to medium intact compartment fires in the early stages of developmentBalanced CFBT programs emphasise the advantages as well as the imitations of 3D techniques Need to exercise caution in the marginal areas. MORE water does not always mean safer firefighting


Advantages

3D Firefighting Tactics can dramatically reduce the volume of waterImprove conditions – visibility – thermal layersLess stressLess Water DamageScene preservation


Limitations

As a rough guide, the authors of 3D Fire Fighting use an area - up to 70m2 for (normal modern fire loading) as the practical limit for the application of pulsed 3D techniques

In some circumstances may be extended by increasing the water

flow and staging of hose-lines. Beyond this threshold, more traditional high flow hose-line capabilities may be required.


Canadian Research Council (2002)

The 3D water fog technique is not designed to replace the direct fire attack but rather to complement existing forms of fire attack in an effort to increase the safety and effectiveness of fire fighting teams. Compared to the traditional straight-stream attack, the 3D water fog technique has advantages in controlling steadily growing fires where the space can still be entered, but where the seat of the fire cannot be attacked directly. It has also been used for offensive attack to control flashover. REVIEW OF THREE DIMENSIONAL WATER FOG TECHNIQUES FOR FIREFIGHTING


Safety Margin

3D attack is not suitable for every fire could find themselves in situation where they are outgunnedThere is a need to be aware that the “right situation” cannot be accurately assessed on every occasionSound understanding of fire behaviour essential


Avoiding Tunnel Vision

Don’t Underestimate the required flow

Taking a hose-line with sufficient flow to handle the situation in case the extent of development is greater that anticipated

Lay out higher flow support line

“hope for the best, plan for the worst”.


“Fire Flow Rate Calculations”

How are they calculatedPractical Fireground modelsThe need for fire officers to exercise caution


So How are Flow Rates Calculated?

Complex formula used to calculate needed fire flow

Construction planning

Pre-fire planning


Common Basis

Matching theoretical cooling properties of water against known rates of heat release (MW) of the fuelsIn the real world it is almost impossible to deliver that water perfectly over the fuel surface. An efficiency factor to take this into account. Data from real fire incidents and compared it with the results of researchVariables such as the fuel (type, loading, spacing), construction type, stage of fire development, occupancy etc also need to be considered. We also need to consider response time, crew sizes, equipment available etc. Type of attack (offensive or defensive) and type of nozzles, size of the hoseline and nozzle pressure.


Needed Fire Flow

NFFi = (Ci)(Oi)(1+(X+P)i)whereNFFi = the needed fire flow in gallons per

minute (gpm)Ci = a factor related to the type of construction

Oi = a factor related to the type of occupancyX = a factor related to the exposure buildingsP = a factor related to the communication

between buildings


National Fire Academy

NFF = A ÷ 3

Where:

NFF is the Needed Fire Flow in GPMA is the area in ft2

Equates to about 13.5 lpm/m2


Iowa State Formula

NFF = V/100

Where:

NFF is the Needed Fire Flow in GPM

V= the volume of the room in cubic feet


Comparision for 100 m2

NFA1076 ÷ 3 = approx 358 GPM (1355 lpm)

Iowa State FormulaNFF = (1076 x8) ÷ 100= approx 86 GPM (325 lpm)


NFA

The NFA formula is based on the amount of water required to cool the burning fuel surface area (a 2 dimensional view) to below the temperature at which it can continue to supply the pyroylised fuel. The figure also takes into account the traditional North American approach which is to “vent early and often” (Brunacini).


Iowa State

The Iowa State formula works on the basis that when water is applied into an intact compartment that the oxygen level is dramatically reduced and when combined with the surface cooling effect, suppression can be achieved with much lower flow rates.


Comparision

Iowa State formula is limited to small to medium sized intact compartmentsNFA formula more applicable to well developed and ventilated compartments.Applying the NFA formula to a 100 m2 intact compartment would require using two 64mm hand-lines capable of delivering approximately 650 lpm eachPotential for excessive water damage and disruption to the thermal balanceVery difficult to manoeuvre


Iowa State

Iowa formula would allow for the hose-line layout could consist of a 51mm (or 38mm) low pressure delivery or two high pressure deliveries. These are much lighter, easier to manoeuvre and less likely to cause excessive water damage and disruption to the thermal balance


Critical Flow-rate (CFR)

The Critical Flow-rate (CFR) has been defined as the: minimum amount of water-flow needed to fully suppress a fire at a given level of involvement’ (ie; during growth or decay stages of development). The actual CFR for compartment fires of a given size (m2), existing in different stages of fire development, may be widely variable. (Grimwood)2 lpm/m2


Swedish Research

Särdqvist et.al. CFR does not give the most efficient extinguishing effect. Increasing the flow rate above the critical value would actually result in a decrease in the total volume of water requiredOptimal rate results the lowest total volume of water. Increase above optimal rate will result increased volumes of water being applied the fire.


Tactical Flow Rate

The optimum flow rate is calculated under known and controlled conditionGrimwood recommends that we need to add a safety marginTactical Flow Rate


English Research

Grimwoods 1989 research of data from 100 fires serious working fires in London resulted in an estimated flow rate between 200-400 lpm was generally successful in extinguishing developing compartment fires up to 100m2 in area.


Grimwood Formula

A x 4 = lpm

Where A = area of fire involvement in m2

Based on average office fire loads

Higher fuel loads or involvement of structural elements - should be increased by 50%

A x 6=lpm


Swedish Research

Svensson and Sardvquist conducted large scale extinguishing testsFloor area of approximately 108 m2 (14.0 x 7.7) and a ceiling height of 6.3 mComparisons 25mm high-pressure (25 bar nozzle pressure) attack lines and 51 mm low-pressure (7 bar nozzle pressure) lines at flow rates of 115, 226 and 340 lpm. Six test burns with the similar fire loads. Attacks commenced after the temperature peakedSame two highly trained professional firefighters


Results

At 115 lpm neither the high-pressure or low-pressure attack lines were able to gain control of the test fires in the specified time frames. Flow rates of 226 and 340 were able to control the fire successfully. Data showed that the 25 mm high-pressure hose systems (at 226 and 340 lpm) reduced the compartment gas temperature faster and to a lower level than the low pressure lines. Fuel surface cooling effect of the 25 mm high-pressure hose-line at a flow of 226 lpm was as effective as the low-pressure hose-line at a flow of 340 lpm.


Equipment and Techniques

Demonstrated the significant effect of different hose-line systemsTechniques used were Swedish style and involved alternating between gas cooling (450 upward spray pattern) and the controlled application of water to the fuel surfaceAir controlThis contrasts with the traditional US approach which involves early ventilation and solid stream jets.


Applying the Grimwood Formula

High ceiling and could be expected to carry a high fire load it will be adjusted up by 50%.

TFT = A x 6 = 650 lpm.

It should be noted that this formula is used as a guide for the initial attack line. The time taken to layout secondary lines can vary enormously and thus is not considered as forming part of the TFR formula. It is recommended that the initial attack lines be supported by secondary support lines of equal or greater flow. Strategic placement of these lines will allow for greater total flow if required.


Rough Guides

The Grimwood and Iowa State formula provide a rough guide for initial attack lines in situations where the structure is still intact and the floor area is less than 600 m2. When the structure is well ventilated or relatively large, flow rates approaching the NFA formula may be more applicable


Margin for Error

When choosing the initial size and type of hose-lines required for a given task it is essential to allow a margin for the unexpected. It must also be remembered that even when dealing with a relatively small fire that large quantities of un-burnt fuel could have accumulated in uninvolved parts of the structure. Ignition of these accumulated gases can lead to rapid and often unexpected fire spread


In Search of a Simple Answer

It can be seen that the theoretical calculation of the flow rates required to extinguish a compartment fire is complex and many variables need to be considered. Fireground formulas need to be simple and easy to calculate. It must be remembered that their simplicity also limits their accuracy and thus that can only be a rough guide at best.


No Substitute for Training

The corner stone of accurate size-up is a sound practical understanding of fire behaviour

The safest and most effective method of developing this underpinning knowledge is through balanced realistic live fire training and the analysis of case studies


Conclusion

There are no simple or easy answers for complex problems.

Train as if you life depends upon it…..because it does!

Documents

Critical Flow Rates for Fire Attack