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Pressure Loss & Selecting Your Sprinkler Equipment
Collect Pressure Loss Data:
When selecting your sprinkler equipment we need to consider pressure loss in the irrigation system.
Like all other mechanical systems an irrigation system consumes energy when it operates. The irrigation system uses energy in the form of water pressure which, as we noted earlier, we will be measuring in PSI (pounds per square inch). Each component in the irrigation system that the water passes through uses up a little bit of that water pressure. If we run out of water pressure before the water makes it through the system, then the irrigation system will not work. So we need to calculate how much pressure will be lost in the irrigation system. This is done by means of educated guesses, which are then confirmed and adjusted by using a trial and error process. Don't worry, it's easy to do...
Below is a Pressure Loss Table that lists items that you MAY need to factor into your pressure loss calculations. Some of the items may not be necessary in some situations. The tutorial is going to take you item by item through the list below. As we come to each piece of equipment, such as sprinkler, valves, etc. I will explain to you all the pros and cons of the various product types available and we will figure out which one, if any, will work best for you. Then you will pick your actual equipment and enter the associated pressure loss value into the Pressure Loss Table on your Design Data Form.
On your Design Data Form there is a copy of the Pressure Loss Table below. As we look at each item on the next few pages you will pull out the Pressure Loss Table and enter a value for the item. If an item on the table doesn't apply (for example, you don't have a water meter) just enter 0 for that item. You'll refer back to these values several times throughout the design process and you may need to change them a few times, so, once again, use a pencil! If you have bad handwriting skills like me, you may wish to write a bit neater than normal so you can read it later! There's nothing worse than having to go back and recalculate your data because you can't read your own handwriting. Believe me, I've had to do it way too many times! If you didn't make a copy of the Design Data Form you will need to copy down the Pressure Loss Table below:
Pressure Loss Table
|_____ PSI||Water Meter|
|_____ PSI||Backflow Preventer|
|_____ PSI||Mainline ("main", "pressure pipe")|
|_____ PSI||Elevation change (change in feet x .433 = PSI)|
|_____ PSI||Sprinkler Heads or Drip Emitters|
|_____ PSI||Lateral ("branch") Pipes (maximum is 20% of sprinkler head PSI above)|
|_____ PSI||Total Pressure Loss (add together the values above.|
Don't panic if that you don't know what the items in the table are. We'll learn what each of these items is and start filling in these values on the next few pages. Or you can jump ahead to the description by clicking on the link on the item in the table.
A quick note concerning Pressure Regulators and Pressure Reducers. A pressure reducer is another name used for pressure regulators. It is not commonly used in the industry, so I'm going to use the preferred name "pressure regulator". If you have a pressure regulator then you get to take a little shortcut. On your pressure loss table you get to ignore the pressure loss for everything upstream of the pressure regulator. But before you decide to really take a shortcut and install the pressure regulator right before the valves, consider that the higher pressure may not be good for those upstream components. Generally I try to avoid pressures over 100 PSI in any portion of my sprinkler systems. I strongly recommend that you do likewise. When placing a pressure regulator on an irrigation system I normally install it right after my main irrigation system shut-off valve at the place where I tapped into the water supply. Thus I have: connection to water supply --> emergency shut-off valve --> pressure regulator --> irrigation system. Don't forget the pressure setting of a pressure regulator must always be at least 15 PSI lower than the incoming pressure. So if the incoming pressure is 80 PSI the pressure regulator must be set at 65 PSI or less. Otherwise the pressure regulator will not work accurately and will allow damaging pressure surges to pass through.
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Text and Images by Jess Stryker unless noted. Copyright © Jess Stryker, 1997-2011. All rights reserved.