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On laying hose

So you think you want to be a fireman, eh kid?

The hose that runs from the fire hydrant to the fire truck is called supply line. Most supply line is 3 inches or more in diameter, and in Central Florida, it’s usually 5 inches. (Orlando uses 4 inch, but that is because they typically have fire hydrants that are close together).

First, a bit of engineering.

The reason for this is hydrodynamics and friction loss. The average water main pressure is about 65 psi. At 1,000 gallons per minute, a 3 inch hose loses 80 pounds of pressure every 100 feet of hose length due to friction between the moving water and the hose itself, while a 4 inch diameter hose loses 20 pounds of pressure, and a 5 inch hose loses only 8 pounds. That means, if you want longer hose lays with high flow, the larger the diameter of your supply line, the better.

There is a lot of math involved in being the driver of a fire engine. You need to be able to calculate your friction losses in your head, rapidly, and remember that the lives of the guys in the burning building depend on you getting it correct. When you are flowing 2,000 gallons per minute through half a dozen different hose lines at 2 in the morning at a burning strip mall isn’t the time to realize that you are math deficient.

5 inch supply line has what is called a “sexless coupling” meaning that there is no male or female end, the couplings are interchangeable (butbutbut WHAT ABOUT THE OTHER 872+ GENDERS?!?—M). This allows you to start laying from either the fire to the hydrant, called a reverse lay, or from the hydrant to the fire, called a forward lay. There are advantages and disadvantages to both, but we won’t talk about that in this post.

My fire truck carried 1200 feet of 5 inch diameter supply line. That means with standard hydrant pressure, I could get a bit more than 800 gallons per minute into my engine without having to put another fire engine at the hydrant to boost pressure.

A whole bunch more fascinating stuff regarding what-all you need to know but almost certainly don’t when it comes to how fires are fought nowadays is included in this must-read post from Divemedic. Even if you never cared anything about being a fireman when you grew up—I didn’t, I admit, nor about being a cowboy, although being an astronaut did sound pretty cool—this stuff is just too good to miss out on reading, and you shouldn’t. There’s a video too, just for additional incentive to go check it out.

Back when I was working at the H-D shop, my boss Goose wanted desperately to be a fireman, but after failing the dummy-drag test three times he finally had to give it up as a lost cause. Goose practiced and strength-trained for months and months—and being a former USMC F4 mechanic, you know he wasn’t lacking in either intelligence or iron-willed determination—but in the end he’s a small, slight fella and those damned dummies are damned heavy. In fact, I think the dummy actually outweighed him by about twenty-thirty pounds.

At any rate, from hearing Goose talk about it, I probably know more than the average bear about what it takes to be a fireman, but even so DM still covers things I never heard about before.

3 thoughts on “On laying hose

  1. There is always that one guy:
    “The average water main pressure is about 65 psi. At 1,000 gallons per minute, a 3 inch hose loses 80 pounds of pressure every 100 feet of hose length due to friction between the moving water and the hose itself…”

    65-80 = -15 psi at the end of the hose which is impossible unless there is no further flow, and the hose is held up vertically, with the supply end unattached, inducing a reverse flow.

    Flow and pressure should not get mixed up. Yes, there is a pressure loss due to length and the friction which will reduce the flow rate.

    1. By the way, I just recently had to run new water line to my home from the meter, just a hair short of 300ft. 3/4 line and for some reason, in spite of even higher friction than the 3′ fireman’s line, I still get water at my place which is also 40 foot higher than the street elevation.

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