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

http://www.lawntechlandscaping.com

Here’s the basic procedure to follow if you’re selecting a pump for a new irrigation system.

1.         Decide on the type of pump that best fits your needs, end-suction centrifugal, submersible, turbine, jet pump, etc. Go back to the first page if you don’t have a clue what I’m talking about.

2.         Estimate your flow (GPM) and pressure (feet of head) requirements. The remainder of this page will explain and demonstrate how to do this.

3.         Research the available pump models and select a preliminary pump model that meets the requirements you established above.

4.         Create a first draft irrigation design. The irrigation should be designed for the flow and pressure the pump will produce.

5.         Once you have a first draft of your irrigation you may be able to fine tune your pump selection based on that design. Would a different pump lower your irrigation costs or better fit your irrigation system design? Return to the pump selection process and re-evaluate your pump selection. Make your final pump selection.

6.         Return once again to your irrigation design. Can it be fine tuned to better match your final pump selection? Make any necessary adjustments.

Although this method requires considerable effort it will give you an excellent balance between pump and irrigation system, leaving you with a very efficient irrigation system! You’re going to save you money for years to come.

For a Existing Irrigation System:

If you just want to increase the water pressure in an existing system that uses a municipal water supply, go down to the next section on booster pumps.

If the irrigation system currently has a pump on it, you just use a new replacement pump rated for the same flow and pressure as the previous one. Warnings:

•           Do not select a replacement pump based on horsepower alone. Two pumps can have the same exact horsepower and produce radically different flows and pressures. See Pressure vs. Flow on the first page of this tutorial.

•           Keep in mind that if your old pump is over 5 years old it is likely worn, and as pumps begin to wear out their performance decreases. So while you may be thinking you need a bigger pump because the irrigation isn’t working as good as it should, this may not be true. When you install a new pump you may find it performs much better than the old worn out one did, even though they are both exactly identical. Many people buy a bigger pump when they really don’t need one. You may want to discuss this issue with your pump sales consultant.

Here’s the basic procedure to follow if you’re selecting a pump for an existing irrigation system that did not previously use a pump. I’m assuming you are switching the water supply for the irrigation from a municipal water system to another source such as a canal, pond, or creek.

1.         Decide on the type of pump that best fits your needs, end-suction centrifugal, submersible, turbine, jet pump, etc. Go back to the first page if you don’t have a clue what I’m talking about.

2.         Estimate your flow (GPM) and pressure (feet of head) requirements. If you have the original irrigation design the plans should tell you this information. If not, you will need to figure out the irrigation system demand. Calculating the flow demand is pretty easy, you just look to see which nozzles are installed in the sprinklers, look up the flow requirement for that sprinkler and nozzle (use the sprinkler manufacturer’s website), then add up the flow requirements of all the sprinklers that operate at the same time. Determining the pressure requirements is often much more difficult. If the irrigation system is operational install a pressure gauge on the pipe as close as possible to where the pump is going to be installed and measure the pressure with the system turned on. You may need to tap into the pipe to install the pressure gauge. If what I just said doesn’t make sense, or you can’t take a pressure reading on the existing system, the best suggestion I can give for this is to read through the Sprinkler System Design Tutorial to learn how pressure requirements for a sprinkler system are calculated. You should be able to then reverse engineer your existing sprinkler system to figure out the GPM and PSI it requires to operate. You may have to dig up some pipes in places to determine what size they are. Be warned that reverse engineering is not going to be easy to do and will take some time.

3.         You will need to add some new pipe and other equipment to the existing irrigation system to accommodate the new pump. This includes the pump intake pipe or manifold, any screens and filters, pump control valves if used (ie; anti-cycle valves,) and a new mainline pipe from the pump to the point you tie it into the old irrigation system. Each one of these new items causes some water pressure loss, so you need to add pressure to the pump pressure requirement for each of them. For valves and filters the manufacturer’s can tell you how much water pressure loss they cause. For the pipes the pressure loss can be calculated using the pressure loss calculators on the Landscape Irrigation Formulas page of this website.

4.         Research the available pump models and select a pump model that meets the minimum pressure and flow requirements of your irrigation system.

Booster Pumps

As you remember from the first of this tutorial (back when you were still awake), booster pumps are used to increase the water pressure. Therefore the required booster pump pressure is simply the desired pressure minus the existing pressure. Just remember that for most pump brands the pressure must be expressed in feet of head, not PSI!

PSI x 2.31 = feet head

Feet head x 0.433 = PSI

Example: The existing pressure in the water company mainline you will use to supply water for your sprinkler system is 35 PSI static. Static pressure means the water pressure when measured with all water flows shut off; no faucets running, ice maker is off, no sprinklers on, nobody taking a shower (don’t turn off the water if someone is in the shower!!!), etc. To measure static water pressure just get a pressure gauge at the hardware store and attach it to a water outlet someplace reasonably close to the irrigation system. Make sure all other water outlets are turned off, then turn on the water to the gauge only. The gauge will show the static water pressure.

Where was I? Oh yeah, you have 35 PSI existing pressure. But let’s say your irrigation system needs 50 PSI to operate correctly. So you decide to add a booster pump to create more pressure. The pressure increase needed is 50 – 35 = 15 PSI. So you need a booster pump that produces 15 PSI of pressure at whatever flow rate the irrigation system requires. But wait, for most pumps the pressure needs to be expressed in feet head, not PSI! So convert PSI to feet head. 15 PSI * 2.31 = 35 feet head (round the result up to the next whole number.) That wasn’t difficult at all!

There are many companies that build and sell pre-packaged booster pump systems. These pump packages come with everything you need pre-assembled and ready to go. Typical assemblies include the pump, electrical controls, any needed control valves, a frame to hold everything and an enclosure to protect it. All you do is install it on a concrete pad, connect the pipes, and connect it to the power source. For most people this is the best way to buy a booster pump.

Source: http://www.irrigationtutorials.com

http://www.lawntechlandscaping.com