The 2 1/2-inch hose offers enormous punch for the engine company and should be considered the standard diameter for fire attack in buildings with standpipes for several reasons.
One reason is the potential loss of pressure as we go higher and higher. If we have limited pressure to supply our lines, we need to maximize what is available. The only way to do this is with lower operating pressures on our nozzles and with a larger diameter line. Problems involving pressure reducing vales (PRV's) or pressure regulating valves can also limit the flow of water and should be considered ahead of time.
Also, there is "reflex time." The reflex time is basically the time from when the call is received to when water is actually flowing on the fire. To get a line into operation on an upper floor (say the 15th floor) can be very time consuming, maybe as long as 20 minutes. This, coupled with the rate of heat release in today's residential occupancies is severe and if you throw in a windswept fire, then you have the need for a flow rate far greater than is required for the typical 2 1/2-story wood-frame dwelling.
The 1 3/4-inch smoothbore nozzle flows 325 gallons per minute (gpm) at 50 psi nozzle pressure, which translates into a powerful punch. The nozzle reaction from this much water flowing does present a danger if it isn't controlled properly and trained with ahead of time.
We will not debate the smoothbore nozzle verse fog nozzle debate here. Many good departments put out fires with both types of nozzles on a day-to-day basis and if applied properly, the fire goes out.
I would like to take this time to emphasize why I opt for the smoothbore nozzle for a high-rise pack and for standpipe operations. The answer has nothing to do with the issues of water application typical of each school of the smoothbore verse fog nozzle debate. The problem with standpipe operations is the potential for debris in the building's pipes, risers, and infrastructure.
Debris causing a nozzle to clog 15 floors up could be catastrophic. It is no easy task to go get another nozzle or to go to the fire attack stairwell or the floor below (areas of refuge) to clean the debris from the nozzle. To limit the chance of clogging and obstructions, a smoothbore orifice allows most debris to pass without the firefighter on the nozzle ever being aware of it. The lower operating pressure also has advantages but this can also be achieved with a low-pressure fog nozzle. Nevertheless, only the smoothbore nozzle will pass significant amounts of debris with ease.
Past building fires in residential high rises in the recent past that my organization responded to very dramatically showed that the stream that was discharged initially was a dirty black or brown stream that exited initially from the nozzle when it was opened. There is no telling just how much rust, scale, or other debris came out of those nozzles initially.
So as we see, the 2 1/2-inch hose and smoothbore nozzle have great advantages for fires in commercial or residential occupancies where the hook up is made from a standpipe system.
Both residential occupancies and commercial occupancies demand that you have the maximum punch available. The fire may have a huge headway and even though it is a residential building, the longer time to get water on the fire may require a massive punch, hence the reason for the 2 1/2-inch and 1 1/4-inch tip combination.
Commercial occupancies, with their larger square footage areas and higher fire loads obviously demand the 2 1/2-inch line and 1 1/4-inch tip and most certainly demand a backup line of similar punch. Can your department muster such an attack? Even more importantly, can your department maintain the momentum of the attack as the air supply in their breathing apparatus is rapidly depleted?
200 Feet Of Hose As A Minimum
The amount of hose in the three element system also reveals that we have a total of 200 feet of 2 1/2-inch hose. We hook up on the floor below the fire because it is considered an area of refuge or safety. It is an area where we can prepare and then deploy in a safe manner. This length allows us to use a length of hose to get from the floor below up to the fire floor followed by an additional 150 feet to get to the fire.