In this step you will make some preliminary selections of the equipment such as sprinkler heads, valves and more. When selecting your sprinkler equipment we need to also find out how much pressure loss each item creates. The amount of pressure loss may require that you reconsider one product over another. Keep reading and it will become clearer.
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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 consumes a little bit of that water pressure. A bit like a car uses more fuel for each mile it goes. 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 each component of the irrigation system. To start we will make some 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 has a page for each of the items that will tell you everything you need to know. I will explain to you all the pros and cons of the various product types available. For the more complex choices like backflow preventers, I will lead you through a series of simple questions that will guide you toward the best solution for your specific irrigation system. Then you will pick your actual equipment and enter the associated pressure loss value into the Pressure Loss Table on your Design Data Form.
Hopefully you picked up a copy of the Design Data Form earlier in the tutorial. If not, please go back to the appropriate page for your water supply source below so you can figure out the correct values for available GPM, PSI and get the form:
City Slicker Water. Water supply from a water company, utility, or district.
Country Bumpkin Water. You use a pump to get water from a well, stream, pond, etc.
Backwoods Water. Gravity flow water system from a tank or spring. No pump.
You’ll refer back to these values several times throughout the design process and you may need to change them a few times, so use a pencil so you can erase and rewrite values! 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 an item on the table doesn’t apply (for example, you don’t have a water meter) just enter n/a and a pressure loss value of 0 for that item.
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OK, here’s a typical Pressure Loss Table.
Pressure Loss Table
|Item (links jump to a page with details on each item)||Brand & Model||PSI Loss|
|Flow Sensor (optional)|
|Master Valve (optional)|
|Filter (optional but strongly suggested)|
|House Mainline 1 (pipe to house)|
|House Mainline 2 (pipe through house or to irrigation connection)|
|Valves (remote control valve, zone valve)|
|Elevation change (change in feet x .433 = PSI)|
|Sprinkler Heads or Drip Emitters|
|Lateral (“branch”) Pipes (20% of sprinkler head PSI Loss maximum)|
|Total Pressure Loss (add together the values above.)|
Don’t panic! The next few pages of the tutorial goes through each item on this table and helps you to figure out the pressure losses to enter on each line of the table. If you’ve used the tutorial before and already know a lot of this, the links in the table above will allow you to jump ahead in the tutorial to the page where the details on that item are located. For most people you will want to just keep reading the pages in order.
Pressure Regulators & Pressure Reducers
Before we continue on we need to address pressure regulators/reducers. A pressure reducer is another name used for pressure regulators. I’m going to use the name “pressure regulator” to avoid confusion. A pressure regulator is a special valve that reduces the water pressure to a set level and keeps it at that level. Some homes have these pressure regulators installed on the water supply, which can impact the values used in the Pressure Loss Table. If you have a municipal water supply, you already learned a little about pressure regulators on the City Slicker Water page of the tutorial. Back on that page you should have discovered if you have a pressure regulator and, if you do, you also decided if you would tap into your water supply before or after that pressure regulator.
If you have a pressure regulator on your house 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.
Installing a Pressure Regulator
If you are planning to install a new pressure regulator be aware that there are two types sold. The one you want to use on your house will be made of bronze or brass, have a pressure adjustment screw so you can set the downstream pressure you want, and generally are pretty expensive. If you need one for a sprinkler system I suggest it also be this more expensive type. There are also cheaper models that use a different principle to work, these cheaper ones are often used on drip irrigation systems. They are not adjustable, and not nearly as accurate and will often allow a damaging pressure surge to pass through them. They typically are barrel-shaped and constructed of plastic. They will not have a adjustment screw or knob on them.
Before you decide to take a shortcut and install a pressure regulator right before the valves so you can ignore pressure loss in the mainline, 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. Also remember that maximum water velocities also still apply to the mainline pipes, so you will still have to do the size calculations for the mainline.
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 may allow damaging pressure surges to pass through it. To restate this another way, the pressure regulator must reduce the pressure by 15 PSI or more for it to work accurately and reliably.
This article is part of the Sprinkler Design Tutorial Series
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