ORIGINAL ARTICLE FOUND AT:http://firefightertoolbox.com/understanding-fire-flow-part-1/
In a previous article I wrote titled The Engine Company’s Primary Mission I stated that the engine company officer will have to make rapid and accurate decisions to quickly identify and initiate the appropriate course of action at the scene of a fire.
After deciding the “Mode of Attack” the officer will have to determine the needed “Fire Flow” to extinguish the fire. In a later article I went into detail about how to determine the necessary fire flow for the conditions you are confronted with, but after receiving some feedback it became clear that there is a lot of confusion and misunderstanding about what fire flow is and how important it is to fire suppression.
Many people believe that fire flow is the amount of water you will need to extinguish a given fire and although partially correct there is more to it than that. But before I go any deeper into defining fire flow we need to make sure that we have a thorough understanding of the theory of how to safely and efficiently extinguish a fire. We are taught in basic training that water extinguishes fire by absorbing heat. But in order to be effective water must be applied at a rate faster than the rate of heat being generated.
The goal is to cool the involved combustibles dropping them below the temperature at which they produce ignitable vapors and heat to support the fires growth. There is also an extinguishing affect when water is converted to steam which dilutes the oxygen supply. Although steam has no ability to cool or absorb heat, it can reduce the temperature of the flames by modifying the air which supplies the flames oxygen.
It has become very obvious to me from seeing the changes take place on the fire ground and from reading all of the research available that fires are hotter and more unpredictable than any time in the history of the fire service. The main reason for this problem is the heat release rate of modern fuels.
Heat release rate (HRR) is defined as the amount of time needed for a given fuel depending on its mass to produce enough energy (heat) to influence combustion.
Modern Fuels +
To fully understand the effects of HRR we have to understand the fuels found in the modern fire environment. Today’s fire environment is far more hostile then it was 15 years ago. The main reason for this is the endlessly expanding role of plastics. Plastics are derived from petrochemicals, (hydrocarbons) and can be found partially or wholly in every consumer product available in the market place today. Given the opportunity they burn very rapidly and produce a tremendous amount of heat, smoke and toxic gases.
Increased Fuel Loads =
The other problem is the sheer amount of fuels found in the average home. As society has become more affluent families have introduced increasing amounts of combustible material into their homes and apartments. As the amount of combustible materials has increased so too must the heat energy potential. Plastics release heat during combustion at 3 times the rate of traditional Class-A combustibles. Not only do they introduce more heat into the burning compartment but they do it three times faster which drastically increases the fires growth rate.
Increased Fire Flow Rates
Just as the heat release rate and fire growth potential have increased in modern fuels so must the fire flow increase. A fire flow rate between 95 and 125 GPM was a safe and efficient flow 15-20 years ago but not today. Due to the many challenges we face in our synthetic society a safe and efficient fire flow is now between 150 and 200 GPM. The increased HRR have drastically altered the time it takes for a fire to flashover. Fires were typically thought to reach flash over conditions 10 minutes after ignition today that time to flash over is less than 4 minutes. Not only must we apply water at a rate faster than the heat being generated but due to the unpredictable and rapid nature of the fires growth our application rate must have a forgiveness factor built into it as well.
Another critical fact that affects fire flow rate is the stage of burning the fire is at when the fire department arrives. As stated earlier fires are reaching flash over conditions much quicker.
As fate would have it most fire departments are arriving sooner in the incident thanks to modern computer aided dispatch systems. Firefighters will be making entry into the building at the same time the fire has reached flash over conditions which means that your fire flow rate at a minimum, must be capable of absorbing the maximum potential HRR to immediately halt the fires transition to flash over.
In our next issue we will discuss how the type of nozzle and stream affects fire flow.
According to NFPA, fire suppression can be accomplished by:
1. Cooling the gaseous combustion zone by flowing water into the overhead smoke layer and thermal column to disrupt the flow of heat and combustible gases.
2. Cooling the solid or liquid combustible by flowing water directly onto the burning solid materials to prevent the production of combustible vapors and gases.
3. Application rate and type of stream are the key factors which will ultimately determine the speed of extinguishment.The rate at which water is applied (GPM) is the definition of fire flow. And it is this application rate (GPM) that will have the greatest impact on how quickly we extinguish a fire. As stated above the type of stream will also play a significant role but without applying water at a rate faster than heat is being generated the type of stream will have very little impact on the speed of extinguishment.
In Part 2 of this series we will discuss how the challenges of the modern fire ground have created a need for higher fire flows then typically used in the past.
As stated in Part 1 of this series the application rate (Flow Rate) and the type of stream used are the two critical factors that determine the speed of extinguishment. So far we have focused solely on the application rate but if we are to have a clear and thorough understanding of fire flow then we must discuss fire streams. A fire stream has a specific shape mass and velocity based on the type of nozzle its rated flow and the pressure applied to the nozzle. There are two basic types of fire streams.
1) Broken stream produced by a fog nozzle
2) Solid streams produced by a smooth bore nozzle.
When selecting a flow rate and fire stream, firefighters must not only consider the stream’s heat-absorbing ability but also the ability of the stream to reach, penetrate, and cool.
One gallon of water can absorb only so much heat per second.
• If the heat-absorbing capability, or knockdown power, of the flow rate is greater than the heat produced by the fire, (HRR) and the stream has sufficient velocity and mass to reach the fire than the fire will be extinguished quickly.
• If the (HRR) is greater than the heat-absorbing capability of the flow rate, or the stream does not have the velocity and mass to reach the fire, the fire will not go out in a timely fashion to adequately protect interior operating nozzle teams and trapped occupants.
STREAM PENETRATION AND REACH
With increased HRR and rooms in residential structure fires (reaching flashover in less than 4 minutes there is little room for error, the attack crew must select a stream that has:
• The Reach
• Thermal Penetration
• And Droplet Size
To reach not only the burning fuel base but the primary radiant heat sources, which are:
• The Ceiling Gases and Smoke
• The Burning Ceiling
• The Burning Wall Materials Simultaneously!!!!!!!!!!!!!!
This stream, at a minimum, must be capable of, absorbing the maximum potential HRR at the ceiling. It also must have enough heat-absorbing capacity and mass so that a significant portion of the stream will not turn to steam.
• These un-evaporated droplets, if they are big enough will fall to the floor in droplet form.
• Suppressing the primary fuel source preventing un-ignited combustibles from flashing.
This series of pictures above is a good example of how quickly a fire can be extinguished when we use the appropriate stream and application rate.
Units arrived to find a 2 ½ story single family dwelling with heavy fire on the 1st floor rapidly extending to the 2nd floor. Due to the percentage of fire involvement and a report of people trapped the OIC decided to mount an exterior attack to rapidly knock down the fire so the truck crew could start their search. Crews stretched a 1 ¾ attack line with a fog nozzle with a fire flow rate of 150 GPM. They chose a straight stream so the stream would have enough reach, thermal penetration and droplet size to reach the burning fuel base and the primary radiant heat sources and made a quick knock and stopped the fire from extending to the 2nd floor.
In the next and final issue we will discuss the physical challenges presented by increased fire flow rates.
(Photos Courtesy: Bryan T. Smith)
In my last installment on the subject of fire flow we will be discussing the physical challenges created by the need for increased fire flow rates.
In order for an engine company to achieve its primary mission on the fire ground the firefighters who ride the engine will be expected to perform some hazardous and strenuous tasks. One of the more physically challenging tasks is flowing water from an attack line and the ability to control the hose line and its reaction forces.
The issue of increased nozzle reaction while fighting fires is a serious concern that can ultimately lead to some bad and dangerous outcomes. When firefighters are struggling to fight reaction forces they gate back the nozzle to make handling the nozzle easier. This is dangerous because although the nozzle will be easier to handle firefighters may not be flowing enough water to overcome the HRR of the burning fuels.
So the question is how to achieve adequate fire flows to safely fight the fire and not fight the nozzle. First we have to understand what nozzle reaction is and is not. Nozzle reaction is a perfect example of Newton’s third law of motion which states “For every action there is an equal and opposite reaction.”
Nozzle reaction is created by two components:
1.The volume of water leaving the nozzle.
2.The pressure at which the water leaves the nozzle.
In simple terms at equal nozzle pressure a higher volume of water (GPM) will have a higher reaction. And equally so at equal GPM flows (Volume of water) a greater nozzle pressure will produce a greater reaction. Captain David Fornell of the Danbury Connecticut FD and Paul Grimwood of the London Fire Brigade have conducted extensive research and testing into the topic of fire flow and nozzle reaction. Their work has established a base line for the ideal reaction pressures on nozzles based on crew size.
Crew Size vs. Reaction Forces
• 60 lbs of reaction force will require 1 firefighter behind the nozzle
• 75 lbs. of force will require 2 firefighters behind the nozzle
• 95 lbs. of force will require 3 firefighters behind the nozzle
The main culprit in reaction forces is nozzle pressure. If you can find a nozzle and hose combination that allows you to achieve higher fire flow rates at reduced nozzle pressure you can reduce nozzle reaction forces.
Standard Fog Nozzles
• A SFN flowing 150 GPM @ 100 psi NP has a nozzle reaction of 76
• A SFN flowing 200 GPM @ 100 psi NP has a nozzle reaction of 101
• A 7/8’s tip flowing 160 GPM @ 50 psi NP has a nozzle reaction of 57
• A 15/16’s tip flowing 180 GPM @ 50 psi NP has a nozzle reaction of 66
• A 1” tip flowing 210 GPM @ 50 psi NP has a nozzle reaction of 75
Low Pressure Fog Nozzles
• A LPF flowing 150 GPM @ 55 psi NP has a nozzle reaction of 56
• A LPF flowing 175 GPM @ 55 psi NP has a nozzle reaction of 66
• A LPF flowing 200 GPM @ 55 psi NP has a nozzle reaction of 75
In order to achieve higher fire flow rates and lower nozzle reaction forces the fire service will need to embrace some new and old technology. Standard pressure and flow fog nozzles do not offer many flow range options that address the issue of nozzle reaction. Smooth bore and low pressure fog nozzles do offer some very real solutions to higher flow rates and lower reaction forces.
Appropriate and safe water application rate plus the right nozzle with low reaction forces and a fire stream with the reach thermal penetration and droplet size to reach the burning fuels and the radiant heat sources is the definition of fire flow!
Be Safe and God bless!
About the Author
Bryan is a career captain with over 30 years in the fire service. He is a state and nationally certified fire service instructor and has been a part of developing and teaching numerous training programs for the fire service. He is also published in Firehouse magazine. Ask questions below or email btsmith@FirefighterToolbox.com