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What factors affect water flow?

by Manage My Life Last activity date:
April 26th, 2007

What factors affect water flow?

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Building Supplies
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The first thing we talk about is the pipe itself: how big is it, how smooth and how straight? It's obvious to most people that the bigger the pipe, the more water that you can move through it. What's not so obvious is how dramatic this is. For example, changing the pipe diameter from 1/2-inch to 3/4-inch makes a very large difference. Many would say that a 3/4 -inch pipe is 50 percent larger than a 1/2 -inch pipe, and you should be able to get 50 percent more water through it. Actually, a 3/4 -inch diameter pipe is

225 percent

of a 1/2-inch diameter pipe in cross-sectional area! If you're keen, you can do the math. The area of a circle is pi x r 2 . Pi is roughly 3.14. The radius of a 1/2 -inch diameter pipe is roughly 1/4-inch. The radius of a 3/4-inch diameter pipe is roughly 3/8 -inch.

In practice, the difference is even more dramatic than the cross-sectional area would suggest. If you consider 100 feet of 1/2 -inch diameter pipe, you will lose 10 psi of pressure running

3.5 gallons per minute

through the pipe. For 3/4-inch diameter pipe, you will lose 10 psi when you flow

9.4 gallons

per minute through it. The cross-sectional area of a 3/4 -inch pipe is 225 percent of a 1/2 -inch pipe, but the

flow

through a 3/4 -inch diameter pipe is 270 percent of the flow of a 1/2 -inch pipe with the same pressure loss!

We now have a pretty good sense of how important pipe diameter is. This should give you some appreciation to how little water flows through rusted galvanized steel pipes, for example. Not only does the surface become considerably rougher, which reduces flow (or increases pressure loss due to friction, whichever way you think of it) but it also reduces the pipe diameter. This dramatically reduces the ability of the pipe to move water. The profile or curve of a faucet fed by rusted galvanized steel piping, for example, becomes steeper with time. Homeowners become less and less happy with the rate of flow available at any given pressure.

Changing any piece of pipe along that 100-foot length will help. Since pressure is lost pushing water past every inch of pipe along the way, if we only changed half the pipe to 3/4-inch, we would still enjoy a benefit. For example, if we wanted to flow 6.5 gallons of water through 100 feet of 1/2 -inch diameter pipe, we might lose 30 psi.

If we started with 60 psi at the beginning of the pipe, we'd end up with 30 psi at the end of the pipe. If we put a gauge at the 50-foot mark, it would read approximately 45 psi because we'd lose 15 psi along the way. (Again, 6.5 gallons per minute would be flowing past any point along the pipe.)

Now, if we changed the first 50 feet of that pipe from 1/2 -inch to 3/4-inch, we would be much better off. Instead of losing 15 psi through that 50 feet, we'd lose about 3 psi. If we started with 60 psi, we'd still have 57 psi at the 50-foot mark. From that point on, we'd lose 15 psi again, yielding 42 psi at the tap, instead of the 30 psi we got earlier. We're better off than before, even though we didn't change all of the pipe.

A lot of people think you have to change the

upstream

(first) section of pipe to avoid a bottleneck problem. Let's try it the other way. If we changed the last 50 feet from 1/2 -inch diameter to 3/4 -inch, what would happen? The first 50 feet of pipe would look like it did originally. If we started with 60 psi and flowed the same amount of water, we would lose 15 psi. A gauge at the 50-foot mark would read 45 psi. Now, if we changed the last 50 feet of pipe to 3/4 -inch we'll only lose 3 psi through that part. The result at the tap is going to be the same flow (about 6.5 gpm) with 42 psi. As you can see, it doesn't make any difference whether we replace the upstream or downstream section of pipe!

People sometimes tell us that it doesn't make sense to change the piping inside the house until the pipe from the street to the house is replaced. We've just shown that that is not true. Increasing any piece of pipe in the entire system will improve the water supply. The more pipe we change, the better it gets.

We have been talking about straight pipes. What happens if you make the water change direction? Elbows, tees, etc., consume considerable energy (create pressure loss due to friction). Each elbow represents several equivalent feet of pipe length. Convoluted piping systems have lower water supply profiles.
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by Manage My Life |
April 26th, 2007
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