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Elevation Pressure Loss
Hills, Valleys, & Slopes:
Irrigation Suggestion: "Consider the slope or you'll look like a dope!" Elevation changes can add or subtract water pressure from your irrigation system. That seriously changes how well the system works. Each foot of elevation change is equal to 0.433 PSI of water pressure. Think of a vertical column of water. At the bottom of the column the weight of all the water above is felt, this weight creates pressure. Have you ever swam down to the bottom of a deep swimming pool and felt your ears pop or hurt? That's caused by the water pressure pressing against your eardrum. In the USA we measure water pressure in pounds per square inch (PSI). That's the weight in pounds of the water on a one-square-inch surface area.
You can skip down past this section if you wish. Look for the next horizontal line. This section is for those who need to know "why?" 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 (which is a confined space) exhibits the same pressure as it would if the pipe were perfectly vertical, even if the pipe isn't. This isn't an easy principle to understand, so be patient and re-read as needed. The best way to demonstrate this is with a picture.
In the picture above the water pressure in the water tank at the top of the water surface level is 0 feet of head, or you could also say there is 0 PSI. This is because there is no water above it to create pressure. Head is another word that indicates pressure, it is mostly used when measuring pressure created by the depth of water. So 10 feet deep water will create 10 feet of head at the 10' deep level. So 10 feet of depth = 10 feet of head. Ok? (Yes, Mr. Physics Professor, I know there would be a small amount of additional 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 there is 0 feet of head at the water surface.)
Looking again at the picture above, we see that 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. Again 40 feet of depth = 40 feet of head. Now lets convert that to pressure measured in PSI. As noted earlier, 1 foot of elevation change creates 0.433 PSI of water pressure. So in this case 40 feet of head is going to be about 17 PSI. (40 ft head x 0.433 psi/ft = 17.3 PSI.) Again, the formula is "feet of head x 0.433 = PSI." So far, pretty straight forward. Read again if you're confused.
Static Water Pressure
Now the hard to understand part. In the drawing above, 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, using the formulas in the previous paragraph. Note that I said this is the "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 the length of a pipe does not matter when the water is static in the pipes. Static means the water is not flowing, it is not moving, it is standing still. This is very important! Because the water is a non-compressible liquid it transfers the pressure horizontally along the pipe route for pretty much any distance without any loss of pressure! Now on the other hand, if we measured the pressure with the water flowing, then 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 there is no flow, so there is 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. Our professor spent a week drilling this concept into us back in college 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 accept it on faith (I wouldn't lie to you) and continue on with the next paragraph.
In most cases we use static water pressure values when designing irrigation systems (or any other water piping system for that matter). Then we use calculations and charts to estimate the friction loss that will occur in the pipes when the sprinkler system is operating, 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 extremely difficult to measure accurately! 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, in the case of most sprinkler systems the pipe isn't installed yet! You can't measure the dynamic pressure if the pipe isn't installed! So, the result is that we almost always will work by using static water pressure and then use calculations to determine the dynamic pressure. Its just way easier to do, and who wants to do it the hard way?
Now go back and look at that picture at the top of this page of the tank and house 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 since the water depth is less, the water pressure would also be lower. This happens all the time and is normal! If the top of the water elevation varies, then the water pressure will also vary. So if the water level will vary at your water source, the pressure will also vary. 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 all right even if you don't fully understand why you're doing some of these things! Just remember that whenever you measure water pressure with a gauge you need to turn off all the water outlets so the water is static, that is, not flowing.
Time to wake up!
Hills, Valleys, & Slopes Continued...
In a nutshell: Just remember every foot of elevation change causes a 0.433 PSI change in water pressure. If your pipe is going downhill add 0.433 PSI of pressure per vertical foot you go down. If the pipe is going uphill subtract 0.433 PSI for every vertical foot you go up. The word "vertical" is critical. If the pipe goes up a slope the vertical distance is how high the slope would be if the pipe were perpendicular, straight up. Do not use the length of the pipe, use the change in elevation! If you don't want to accept my word for it then you're going to have to go back and read all that boring Hydraulics 101 stuff above!
Because elevation changes effect the water pressure we must take this into account when determining pressure loss. If the area to be irrigated is lower than the water source we will gain pressure, so we may be able to gain some beneficial added pressure to our system. Care must be taken though. We can only add pressure if ALL the irrigation system is lower. If portions of it are not lower, or are higher than the water source, then those portions aren't going to be getting that extra pressure. It is safest to just not add pressure for elevation changes unless you're really sure.
On the other hand if portions of the irrigation system are higher than the water source you will always need to subtract out the pressure loss created by the elevation gain. Pressure gained can be easily disposed of, pressure lost however, is very difficult to replace. So, for every foot of elevation gain (higher) in the irrigation system, you should subtract 0.433 PSI from the design pressure.
The far corner of the irrigation system is 9 feet higher than the water source. 9 feet * 0.433 PSI = 4 PSI loss (loss because it is higher).
One corner of the irrigation system is 20 feet lower than the water source. Another corner is 12 feet higher than the water source. 12 feet * 0.433 PSI = 5 PSI loss. The 20 feet lower corner isn't considered as it will not lower our water pressure (it will add to it).
A final example:
The water source is on a hill. The highest part of the irrigation system is 50 feet lower than the water source. The lowest part of the irrigation system is 60 feet lower than the water source. In this case you can add pressure because the ENTIRE irrigation system is lower. But the pressure added can only be the difference between the water source and the highest part of the irrigation system. 50 feet x 0.433 PSI = 22 PSI pressure GAIN. So you would subtract this amount from the total system pressure required. In other words you would enter a negative number in your Pressure Loss Table for Elevation Pressure Loss.
Too Much of a Good Thing:
What if one corner of the irrigation system is a lot lower than the other? While unusual, it is possible to have too much pressure! With too much pressure the sprinkler heads might not work as well, or they might even blow apart! For spray type sprinklers 40 PSI at the sprinkler head is the most pressure you want. For rotors it varies, but most small systems shouldn't have more than 70 PSI at the rotor sprinkler head. If you have too much pressure you will need to reduce it. Reduce the pressure by installing a spring-loaded check valve on the pipe leading to the low sprinkler heads. The spring-loaded check valve absorbs the extra pressure. Check valves are available with varying amounts of pressure loss through them. Some are adjustable. You should probably get these at a full line irrigation store where you can get help in selecting the proper check valve to use. You can also install a pressure regulator on the pipe to reduce the pressure. Regulators are much more accurate, but cost a lot more than the spring-loaded check valves!
Enter the pressure loss or gain in PSI on the "Elevation Change" line of the Pressure Loss Table. Feet x .433 = PSI
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Text and Images by Jess Stryker unless noted. Copyright © Jess Stryker, 1997-2011. All rights reserved.