Problems & Solutions
Published: 01 January, 2007
Question
Recently, a rail road tank car containing a flammable liquid ignited while on a railroad siding (side track) at an industrial facility. Several firefighters asked - which type of master stream nozzle/tip would be most appropriate for fighting this type of fire?
Answer:
Let us first review the two common types of master stream (over 400 g.p.m. - gallons per minute) nozzles commonly found in fire departments and fire brigades. The solid stream tip is a smooth bore nozzle that delivers a solid stream of water. On master streams this nozzle usually is supplied with 80 p.s.i. (pounds per square inch) at the nozzle discharge orifice with the flow varying based on the orifice size.
Most of these tips fall in the range of 600-1,000 g.p.m. and are usually stacked to allow the operator to switch tip size based on flow needs/availability. Solid stream tips are excellent for penetration through thermal columns and have an increased range compared to other options. If you need to deliver a large quantity of water over a distance or need to make sure it doesn’t vaporise as it passes through heated air currents, this is the nozzle of choice.
The combination nozzle, also referred to as a fog nozzle is designed to break the stream of water entering the nozzle into many small droplets of water. The nozzle allows the operator to adjust the stream delivered on the outbound side of the nozzle into a wide fog pattern or down to a narrow straight stream.
However, even in the straight stream format the water is still delivered to its target in droplet form. This type of nozzle and the associated stream it produces is excellent for rapid heat absorption from heated air currents (convection) and for vapor dispersion / absorption.
In some cases where an exposure might be damaged from the impact of a solid stream, a fog nozzle set on the straight stream position may be desirable to minimise damage to the surface of the exposure, including the breaking of windows.
With these two nozzle types in mind we must then determine what the strategic goal is for the application of the master stream. If the objective is to cool the tank, then large quantities of water must make it through the thermal column created by the fuel fire and actually strike the tank surface itself to absorb the heat. The law of latent heat transfer and the effect of conduction will allow the cold water running across the tank surface to absorb heat that has been radiated back onto the tank surface from the fire. As the water runs off the tank, now carrying heat away from the tank surface, a cooling effect will occur.
The nozzle required to achieve this objective with maximum efficiency would be the solid tip and the flow factor should be as large as possible based on the water supply. Remember cooling is based on application rate, not just the flow rate of a nozzle.
If the objective is to disperse the vapours being released from the tank car due to the fire situation then you need a stream that is comprised of small droplets of water.
The combination / fog nozzle tip will provide this effect for you. As the many droplets of water enter the heated air space around the tank car fire they will either bond with the vapours being released or at least cool those vapors resulting in cooler materials falling to the ground.
This effect limits the spread and may in some cases cool the vapours below their ignition temperature (depending on the fuel that is involved). Remember that if this stream has to pass through high heat environments that some of the water from the stream will vaporize before reaching the target. Penetration is not one of the benefits of this stream, so know what type of distance and impact you need to make with the stream before selecting the combination nozzle.
One last consideration on this topic. When attempting to suppress a tank car fire we have to remember that we also have the option of using various types of foam concentrates to enhance the suppression capabilities of water. It is important to select the correct type of foam for the job at hand, to have the foam eductors matched to your nozzle tip flows, and to be properly trained in the application of foam master streams as you can waste foam very quickly if you are not well trained on how to apply these types of streams. Be safe!
KEVIN MELLOTT
