Jess Stryker's
Landscape Sprinkler Design Tutorial
Step #2
Lateral Pressure Loss
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Jess Stryker's Lateral Pressure Loss |
Types of Commonly used Irrigation Pipe
Two types of pipe are commonly used for sprinkler systems laterals, polyvinyl chloride (PVC) and Polyethylene (Poly). Both are types of plastic. PVC is usually white or gray color and semi-rigid. Polyethylene is usually black and is flexible.
- PVC is the type most commonly used in warm winter climates. PVC pipe is rated by two different systems, the first is the "class" system (Cl) the other is the "schedule" system (SCH). It is not possible to say that one is always better than the other. Schedule pipe is rated by the pipe's wall thickness, while class pipe is rated by the pipe's operating pressure. All PVC pipe of the same size will have the same outside diameter, regardless of which type or rating they are, so the same fittings will fit all of them. Most PVC pipe is connected together using "PVC fittings" which are glued in place. The fittings are typically rated as SCH 40 (standard white PVC fittings), some are available as SCH 80 (stronger and normally gray color). Sometimes PVC pipe has threaded ends just like steel pipe. PVC pipe and steel pipe have the same outside diameters, and threaded steel pipe fittings will fit on threaded PVC pipe.
- Poly pipe is commonly used in areas with very cold winters, and where special situations occur such as very rocky soil. Poly pipe doesn't break as easily if water freezes in it, but also doesn't hold up as well under higher pressures. Poly pipe is more forgiving in rocky soils (big rocks, like granite boulders) as it is less likely to crack if it is installed against the side of a large rock. Poly is rated by a system of "SDR" ratios. The lower the SDR number, the stronger the pipe is. All poly pipe of the same size will have the same INSIDE diameter. This is because poly pipe uses "insert fittings" also sometimes called "barbed fittings". Insert fittings shove into the pipe and have barbs to hold them in place. Do not rely on the barbs! Use a stainless steel clamp to hold the fitting in place. (Exception: sprinkler risers using special "twist on" barb fittings don't need clamps. See the installation tutorial.)
Use CL 200 for your laterals!
I recommend that the other non-mainline pipes be CL 200 PVC and buried at least 10" deep (12" deep is the industry standard for commercial irrigation systems). Many homeowners use the CL 125 PVC pipe because it is cheap, but it breaks easily and they often regret using it later. If you can't find CL 200 PVC then use SCH 40. In some areas where winters are long and cold you may wish to use polyethylene pipe which holds up better when frozen. Polyethylene is also often used in areas where the soil is very rocky.Hard frost areas:
In areas with hard frost pipes should be installed below the frost line or you should have drain valves installed at all low points to allow them to be drained in the winter. You can also have the pipes winterized by blowing the water out of them with air for the winter season. Even if you plan to have them blown out I still recommend installing a drain valve at the lowest point of each mainline. I've written a separate tutorial on ways to winterize your sprinkler system that you should read through if you're in a cold winter area.
Decide which type(s) of pipe best fits your needs and make a note of it on your Design Data Form.
Laterals:
"Lateral pipe" is the name given to the pipe sections between the control valves and the sprinkler heads or drip emitters. If you visit your local warehouse hardware store you may find that they use the name "branch" pipe for laterals. We will determine the actual sizes of these pipes later using the pressure loss value that we establish here. So this is an important value!
For most residential sprinkler systems on City size lots a lateral pressure loss value of 4 PSI will work great. It gives a nice balance between cost savings and performance and is a safe figure to use with any type of sprinkler. I would start with 4 PSI for this value. If you find later that you need to raise it, come back and read the rest of this page before you do. If you decide you want to use a lower value you can do so without problems.
You can skip to down "pencil" logo near the bottom of this page if you are going to use 4 PSI for your lateral pressure loss value.
At this point in the design we need to make somewhat of an "educated guess" for the lateral pressure loss. However there is a guideline for maximum allowable pressure loss, so we can use that as a starting point. This rule is that the lateral pressure loss may never be greater than 20% of the sprinkler head or drip emitter operating pressure. The sprinkler or emitter operating pressure is established for us by the manufacturer, and you should have entered it in your Pressure Loss Table already.
Sprinkler Head Pressure x 0.20 = Maximum Lateral Pressure Loss
Example:
Say the sprinkler heads we want to use have an operating pressure of 30 PSI. Then the lateral loss may not be more than 6 PSI (30 x 0.20 = 6 PSI). Therefore we make an educated guess that a lateral pressure loss of 6 PSI will work.
No doubt some are wondering why the lateral pressure loss is limited to 20% of the sprinkler or emitter operating pressure. This is an industry standard for limiting the variation in performance between the sprinkler heads or emitters controlled by the same valve. We know that the first sprinkler after the control valve will most likely (but not always, see last paragraph below) have more water pressure than the furthest sprinkler from the control valve. After all, the water has to pass through a lot more pipe and fittings to reach that last sprinkler, so a lot of energy is going to be lost getting there! Since both sprinkler and emitter performance is directly related to water pressure it is necessary to limit the pressure difference between the first head and the last. Otherwise the first head might flood the area around it with water before the last head even got the area around it wet. I've seen poorly designed sprinkler systems where the grass is dark green by the valve, and gets yellower and yellower as you move toward the last sprinkler! This is even more critical with drip systems. I remember a Eucalyptus tree farm I visited a few years back, where they had planted long rows of trees for firewood production. The first trees (next to the valve) in each row were twice the size of the trees at the other end of the row! All because the first trees were getting much more water.
Now I guess I need to explain why the last sprinkler head on a line might not have the lowest operating pressure. There's only one situation I can think of where this might happen, and that is where the sprinkler system is installed down the side of a steep hill. If the valve and first head were at the top of the hill and the last head was at the bottom, then the added pressure that results from the elevation change (gravity adds energy) might be great enough to cancel out all of the pressure loss in the pipes! More on elevation changes and pressure loss next!
Sprinkler Risers:
Sprinkler risers are the short pipes that connect the sprinkler heads to the laterals. They are usually flexible to allow the sprinkler head to move without breaking the lateral pipe. That's a nice feature to have when you drive over a sprinkler with the car or hit a sprinkler with the mower. To simplify things a default pressure loss value for the sprinkler risers is built into this Tutorial. Hurray! Finally, something you don't need to worry about! So as long as you use this tutorial for your design and a reasonably standard riser for your sprinkler heads, you don't need to worry about the pressure losses. A standard riser would be a PVC swing joint using pipe and fittings the same size as the sprinkler inlet, any of the poly pipe risers commonly know as "Funny Pipe", "swing pipe" or equal (keep the riser pipe less than 18" long), or Cobra Connectors (keep them less than 12" long). Don't worry about those riser names for now, more information on risers will come later in the Installation Tutorial. In fact, now would be a good time not to worry about risers at all! I only mention them here because people get to this point and decide I have made a grievous mistake by leaving them out.
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