There are some common fire service debates that just seem to be staples of our craft. In recent light a new one has arisen, traditional helmets versus European. While that one seems to be gaining attention, that won’t be what we discuss today. There are statements out there that say, “What goes around comes around.” and “Everything comes full circle.” This is true. If you talk to anyone that has been in their line of work for any amount of time, they’ll tell you common practices come and go in their trade, but very few things arise that are “new”.
In that case, low pressure nozzles and smoothbores are nothing new, but it seems that the fire service is making its way back towards a low-pressure high-volume service as I see more and more departments going to smoothbore or fixed gallonage nozzles that have higher flows and less nozzle reaction. While departments do their research, many members often turn to their favorite social media platform to ask fellow peers’ which nozzle they should go with. I think the most common one comes from the 1.75” hand line and the debate between a 7/8” or a 15/16” smoothbore tip size. In the following dialogue, I want to discuss these two tip sizes a little more in depth and give my opinion on which one you should go with.
The first point I want to make in regards to smoothbore hand lines is their operating pressures. This can be a huge factor in making your decision, along with a few other things. A smoothbore handline’s optimum tip pressure is 50 PSI. This means at 50 PSI, your tip size of choice should give you good reach, penetration, a desired nozzle reaction, and good stream qualities. However, 50 PSI is not the only tip pressure available to you when it comes to smoothbores. A hand line smoothbore nozzle has an operating range of 40-60 PSI. This means you can under pump or over pump the nozzle of your choice, but as you go in either direction, you get more or less flow and higher or lower nozzle reactions. The fact that you can over or under pump them means you have options as a person or group on deciding which nozzle works best for you.
The chart above is for reference. I posted it awhile back but thought I would put it out there for a little more of the masses. This chart shows the ranges of the two most common 1.75” smoothbore tip sizes. When we evaluate the chart, we see the flows and nozzle reaction of both tip sizes at various tip pressures and how the nozzle reaction and flow ranges change at each pressure. This is where the decisions have to be made, so let’s create a scenario or two. Let’s say you had a target flow of 175 GPM because you have 175@50 fixed gallonage nozzles on your apparatus and were looking to purchase a smoothbore nozzle to put in service and give your firefighters their option as to which they used. There might be someone who says a 7/8” isn’t for you because it flows 160 GPM. This is true, but it is also capable of flowing 175 GPM with a nozzle pressure of 60 PSI as you can see from the chart.
When you pump the 7/8” to 60 PSI, the nozzle reaction rises to 71 pounds of reaction force. This is where the options come in. If you look at the 15/16” side of the chart, and the target flow of 175 GPM, you see it’s under pumped to a tip pressure of 45 PSI and you only have a nozzle reaction of 61 pounds. They are both flowing 175 GPMs, but there’s a difference of 10 pounds in nozzle reaction, which is huge in the grand scheme of things. The same could be said if you had a 150@50 fixed and were looking to add a smoothbore. A 15/16” might not be best because the lowest it flows in the operating range of 40-60 PSI is 165 GPM, but to keep a 150 GPM target flow you could under pump the 7/8” to 45 PSI NP and get 152 GPM. That brings the nozzle reaction to 53 pounds and that also happens to be the same nozzle reaction a 150@50 fixed has, meaning you have two lines that are the same from a hydraulics and nozzle reaction standpoint. You can get 165 GPM out of both, over pump the 7/8” to 55 NP and get 168 with 65 pounds of NR or under pump the 15/16” to 40 NP and get 165 GPM with 54 NR. There are options.
I want to point out that there are advantages and disadvantages to under and over pumping nozzles. The advantage of over pumping is higher flows, but the disadvantages are higher nozzle reactions, poor steam qualities at times, and more back pressure making the hose line more stiff and harder to maneuver. The advantage to under pumping is less nozzle reaction. The disadvantages are less flow and less back pressure in the line. If you have less back pressure in the line that could lead to having more kinks, lower stream velocities, and less reach to name a few depending on your hose. Your hose brand and model are a factor in all of this as well, you can’t just take a nozzle and just throw it on any hose. It’s a complete system.
I told you I was going to recommend which nozzle is best for you. My recommendation is to perform a nozzle evaluation with the right equipment and with your hose and apparatus. This means using flow meters and a pitot gauge to confirm the tip pressures and flows. When you use these nozzles in the various ranges above, you will see how your hose reacts at different pressures. I mentioned above that hose is part of the full attack package, but if new hose isn’t on the horizon, you need to try both nozzles within these ranges and see what works best with your current hose. There might be more or less nozzle whip, more or less kinking, and so on. So, in the end, no one is deciding this for you, you are deciding for yourself with your hands on the equipment so your firefighters can feel the difference and performance of each. I could ask a few questions and give an overall recommendation, but only you can decide which nozzle is best for you.
We have always been told “You can’t have your cake and eat it too.” I think with a full understanding of these nozzles and their operating range through testing, you might can, as I gave a couple examples of above. In the meantime, open your nozzles pointing high, keep the bales all the way open, sweep the floor on the advance, and hit all four corners of the room!
© 2023 Created by fireeng. Powered by
You need to be a member of Fire Engineering Training Community to add comments!
Join Fire Engineering Training Community