I’ll be brutally honest with you here. This is going to be incredibly boring, but necessary if you want to really understand pumps, so hang in there!

In the USA the pressure output of pumps is measured as “feet of head”, which is normally shortened down to the term “feet head” and abbreviated as ft.hd.. If you need metric measurements you’ll want to make reference to the Conversion Formulas where you’ll find the necessary information for converting to your favorite measurement system!

Feet of head is really pretty easy, it is simply height of elevation. As everyone knows, water is pretty heavy. (Try carrying a 5 gallon jug of water up a flight or two of stairs!) That weight of the water is what creates water pressure! Think of a tall column of water. The”water pressure” at the bottom of that column is simply the total weight of all the water in the column above the point where you are measuring it. In fact, at any point in the column the water pressure is equal to the weight of the water above that point. So as you move up toward the top of the column the water pressure decreases. Inversely, just like in the ocean or a swimming pool, the deeper you go, the greater the water pressure! That greater pressure is what makes your ears hurt if you dive down to the bottom of a deep swimming pool!

In the USA pressure is normally expressed as “pounds per square inch” (PSI). Notice the weight connection?

It’s pounds per square inch, the weight of the water! Well, for pumps we simplify that even more by measuring the pressure (or weight) as feet of water depth! Now that’s really simple! The water pressure in feet head is just the depth of the water in feet above the point at which the pressure is measured.

Example: Let’s say you have a swimming pool that is 8 feet deep. At the very bottom of the pool the water pressure will be equal to 8 feet of head. Pretty simple! If you want to know the pressure in PSI you can convert it by multiplying feet head times 0.433. So the pressure in PSI would be 8 ft. hd. x 0.433 = 3.46 PSI. If you swam under water at a depth of 5 feet below the surface then the water pressure on your body would be 5 feet head or 2.17 PSI. The Titanic rests on the sea floor at a depth of 12,600 feet below the surface. Therefore the pressure on the hull of the Titanic is 12,600 feet of head or a bone crushing 5,456 PSI! Consider that the plastic pipe in your sprinkler system will burst at somewhere around 300 PSI of pressure!

Ok, now the difficult part. Since water is essentially a non-compressible liquid it exhibits the unique trait of transferring pressure horizontally when in a confined space. What this means is that water in a pipe exhibits the same pressure as it would if the pipe were perfectly vertical, even if the pipe isn’t. The best way to demonstrate this is with a picture.

Tower of (Water) Power

In the picture above the water pressure in the water tank at the top of the water level is 0 feet, or 0 PSI. This is because there is no water above it to create pressure. (Yes, I know there would be a small amount of water pressure due to the air pressure above the water, but let’s try not to confuse things. This is hard enough to understand! So we’re going to say that the water pressure at the water surface in the tank is 0 feet head. Ok?)

The ground level is 40 feet below the water level in the tank. Therefore the water pressure at ground level is 40 feet of head, or about 17 PSI. So far, pretty straight forward.

Now the hard to understand part. The water enters the house at a level 100 feet below the water level in the tank. So the static water pressure at the house is 100 feet of head, or about 43.3 PSI. Note that I said “static”pressure. So now you’re likely wondering how this could be? The water level is not just 100 feet above the house there is also easily 180 feet of pipe between the tank and the house! The answer is that distance does not matter when the water is static (not moving) in the pipes. Because the water is a non-compressible liquid it transfers the pressure horizontally along the pipe route for pretty much any distance without any lose of pressure! If we measured the pressure with the water flowing the pressure would be termed “dynamic pressure”. With the water in a dynamic state (flowing in the pipe) the water would loose pressure due to friction on the sides of the pipe and we would get a lower pressure reading at the house. But static pressure means no flow, no friction, and no pressure loss! Read that last sentence again! Think about it for a second, go back look at the picture again if you need to. It makes sense if you think about it. My professor in college spent a week drilling this concept into us and a lot of people in the class never did understand it! So if you still don’t “get it” don’t feel bad and don’t get discouraged! Just continue on with the next paragraph.

In most cases we measure water pressure in the static state when designing irrigation systems (or any other water piping system for that matter). Then we use calculations to figure out the friction loss that will occur in the pipes and subtract it from the static pressure to arrive at the dynamic pressure. Why not just turn the water on and measure the dynamic pressure with the water flowing? It would seem that then we would not have to prepare a separate calculation for friction loss, right? Well, that is correct, however dynamic pressure is very difficult to measure. You have to get the flow just right and then hold the flow at that level for a minute or two while the pressure stabilizes. This is a real pain in the rear to do and not nearly as easy as it sounds! Plus, what if the pipe isn’t installed yet? Then you can’t measure the dynamic pressure at all. So, the result is that we almost always will work with static pressures. It’s just easier, and who wants to do it the hard way?

Now go back and look at that picture above again. As the water flows to the house the water level in the tank will go down. So the elevation of the top of the water in the tank will not be as high above the house. When the tank is almost empty the difference might be only 95 feet. So the water pressure would also be lower. This happens all the time and is normal! If the elevation varies, then so will the water pressure. I know I keep saying the same things over and over in different ways, but I’m trying to drive home some important but hard to understand principles! My apologies if you got it the first time through and are getting bored!

Still confused? Don’t worry about it, just follow through the procedures that follow and you’ll be alright even if you don’t fully understand why you’re doing some of these things! Just remember that when I use the term”feet head” I’m talking about water pressure and whenever you measure water pressure with a gauge you need to turn off the water.

Source: http://www.irrigationtutorials.com

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