Understanding Fire Flow – Part 4

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

Smooth Bore Nozzles

• 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!