Hose Diameter Decisions

Hose diameter — particularly which is better — is a popular topic in the fire service. Is 1.75 inches better than 1.5 inches? Is 2 inches better than 1.75 inches? Is 5 inches better than 4 inches? The answer depends on what you need, so here's some information you might find useful.

Size of the hose: What size a hose is labeled may or may not be the "true size" of the hose. For example, a few years ago, I took the time to measure the diameter of all the 1.75-inch hoses I could get my hands on. I found that inside hose diameters on such hoses ranged from 1.75 inches up to 1.94 inches — almost 2 inches.

There is a significant difference in cross-section area between a 1.75-inch hose and a 2-inch hose — more than 3/4 of a square inch. It doesn't sound like much, but for the same flow rate, say 150 gallons per minute, the 2-inch hose has 17 pounds per square inch less friction loss per 100 feet of hose. Stated another way, for the same friction loss (35 pounds per square inch), you get 150 gallons per minute from the 1.75 inches, and 210 gallons per minute from the 2 inches. Big difference.

When you compare a true 1.75-inch hose to the 1.5 inches, the results are similar: the cross-section area is 3/5 of a square inch bigger, and at 150 gallons per minute, the friction loss is 19 pounds per square inch less per 100 feet. For the same friction loss, you get 186 gallons per minute from the 1.75 inches versus 150 gallons per minute for the 1.5 inches.

So what's the moral of this story: No one can say that one size is always better than another. You need to make choices based on your response district and associated factors.

I'll discuss supply lines next week.

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