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Pressure Loss Adjustments

 

Now it is time to make sure the sprinklers will have enough pressure to operate properly.

Adjust Your Pressure Loss Data:

Pull out your Design Data Form.  One of the top 3 sections should be completed (City Slicker, Country Bumpkins, or Backwoods Water) as well as the Pressure Loss Table at the bottom of the form.  (If you haven’t completed the Design Data Form you need to go back to the beginning of this tutorial and work forward to this page.)

Look for the line called “Design Pressure” on your Design Data Form (it will be in the appropriate section for your water supply source, ie; City Slicker, Country Bumpkins, or Backwoods Water.)  Now look at the “TOTAL PRESSURE LOSS” on the last line of your Pressure Loss Table.  The  TOTAL PRESSURE LOSS value must be less than or equal to the “Design Pressure” you entered on your form.   If it is, congratulations, you have enough water pressure to operate your system using the equipment you have or selected.  If so, you’re done with this page, continue to the next page of the tutorial.

If the Total Pressure Loss from the table above is greater than your Design Pressure, then you must decrease the Total Pressure Loss or your irrigation system will not work. Don’t panic, it generally isn’t hard to do.  Here are a few methods you can use to lower the Total Pressure Loss.  Each method has advantages and disadvantages.  I’ve ordered the list so that generally the easiest and/or less expensive ways are higher on this list:

Increase the mainline size. This would lower the pressure loss in the mainline. Of course, for any existing mainlines already installed, this is NOT an easy method!  This is often the method I often use on my larger irrigation systems.  It costs more to use a larger pipe, but often it works out to be the least expensive method in the long run.  Increasing the pipe size also has other advantages, by reducing turbulence and the velocity of the water in the pipe it can decrease maintenance costs and increase the life of the sprinkler system.  It also doesn’t cause any negative “ripple impacts” on the rest of the design.  Unfortunately small sprinkler systems often have short mainlines with almost no pressure loss, so this method is not very effective for them.

Lower the sprinkler head operating pressure. This is an easy and fast way to lower your Total Pressure Loss, however it comes at a price. When you lower the operating pressure of the sprinkler head, you decrease the distance the sprinklers can be placed apart. Thus you will likely need more sprinkler heads which will increase costs.  But keep in mind that most people don’t use as many heads as they should anyway, so if you are typical, adding more sprinklers may save your behind in the end.  Be sure you don’t lower the pressure too far, remember Stryker’s Rule for rotor heads and rotary nozzles, the spacing in feet can’t exceed the pressure in PSI.  (Ie; 30 PSI = 30 feet between heads maximum.)  If you lower the sprinkler head operating pressure there is also a ripple effect, you will probably also need to change the lateral pressure loss value to keep it less than 20% of the sprinkler head operating pressure.

Reduce the pressure loss in the laterals. This means the lateral pipes will be larger and cost more, but often using larger pipe is the best way to reduce the pressure loss.  There’s no ripple effect and using larger lateral pipes will help your sprinkler system perform better and last longer (due to less water turbulence and velocity in the pipes.)

Increase the size of the backflow preventer or valves.  In most cases this will not be very helpful, but sometimes it makes a difference. Also, sometimes different brands of backflow preventers and valves will have lower pressure losses for the same flow rate.  Be sure to double check to be sure the larger valve or backflow preventer is still within the manufacturer’s recommended flow range!

Lower the Initial Design Flow.  As you remember we have been using a value called “Initial Design Flow” up until now, and I mentioned that we might need to lower that number. This is where you may need to lower it! Lowering the Initial Design Flow will reduce the pressure loss values for the water meter, backflow preventer, mainline, and valves. Start by lowering it by 10% and entering the new lower value on your Design Data Form. There is a big ripple effect for this method!  You need to go back to the Pressure Loss Table and recalculate all of the pressure losses using the new Design Flow.  Ughhh…  If that still didn’t reduce the pressure loss enough, try a new Design Flow that is another 10% lower. For example, if your original design flow was 20 GPM, try lowering it to 18 GPM, or even 16 GPM.

cautionIMPORTANT NOTE: If you have a pump (you used the Country Bumpkin Method to measure your water) you should only lower the Design Flow as a last resort! Try everything else above first.  Lowering the Design Flow can cause your pump to cycle on and off. With pumps it is important to keep the pump from cycling, which creates pump wear.  Pump cycling also hurts the efficiency of your sprinkler system due to the pressure fluctuations it causes in the sprinkler system.

Add a booster pump to increase the water pressure. This is obviously a very expensive option for a homeowner and should be avoided if possible.  Booster pumps are more commonly used on parks, schools, and golf courses.  But in some situations this is the only option that will work, especially if you have a Design Pressure under 40 PSI.

Examples:

Lets assume that you previously determined that your system has a “Design Pressure” of 45 PSI and a “Initial Design Flow” of 20 GPM. Using that information you might start out with the following pressure losses:

2.2 PSI – Water Meter
0 PSI – Backflow Preventer (none, we’re using anti-siphon valves)
2 PSI – Mainline (23 feet of 1″ SCH 40 PVC mainline)
5 PSI – Valves (using 1″ anti-siphon type)
4 PSI – Elevation change (about 9 feet)
30 PSI – Sprinkler Heads or Drip Emitters (using spray heads)
6 PSI – Laterals (20% of 30 PSI)
_________________________________
49.2 PSI – Total Pressure Loss
More than 45 PSI so the sprinkler system won’t work.

Warning: don’t use the pressure losses shown here for your design. Get the actual losses for the equipment you plan to use. For example, many brands of anti-siphon valves have much higher losses than the 5 PSI in this example!

As you can see, the Total Pressure Loss is greater than the “design pressure” of 45 PSI, so the irrigation system will not work. The pressure loss must be lowered. We could increase the mainline size, but with only 2 PSI of loos already, lowering it to 1 PSI would not be much help.  It seems the easiest way in this case is to lower the sprinkler head pressure to 25 PSI. This means the sprinklers will need to be a little closer together, but we have little other good choices. When we lower the sprinkler head PSI we also have to lower the PSI loss for the laterals (to keep the loss in them below 20% of the sprinkler head pressure.) The total will now be 43.2 PSI which is less than 45 PSI:

2.2 PSI – Water Meter
0 PSI – Backflow Preventer (none, we’re using anti-siphon valves)
2 PSI – Mainline (18 feet of 1″ mainline)
5 PSI – Valves (using 1″ anti-siphon type)
4 PSI – Elevation change (about 9 feet)
25 PSI – Sprinkler Heads (using spray heads, lowered to 25 PSI)
5 PSI- Laterals (lowered to 20% of 25 PSI)
____________________________
43.2 PSI – Total Pressure Loss
Less than 45 PSI so now the sprinkler system will work!

Another option we could have tried would be to leave the sprinklers at 30 PSI and lower the laterals to 1 PSI. The result would have been 44.2 PSI of Total Pressure Loss which would work. The downside is that we might have some pretty large lateral pipes (maybe even as large as 1 1/2″ diameter!). Usually lateral losses under 2 PSI don’t work out well, but again, sometimes this is the best choice. The only way to know for sure is to try both ways!

Once your Total Pressure Loss is less than or equal to your Design Pressure you are ready to move on to the next page.  We’re finally ready to start figuring out where sprinklers will be placed.  You’ve learned a lot to get to this point!  You have reason to feel good about your accomplishment.

Curious how long it would take a professional designer to get to this point in a design?   Reading the previous 10 pages of the tutorial was all about learning basic hydraulics and the parts of an irrigation system, which pros already know.  The time it takes a pro to look up pressure loss values, fill out the Pressure Loss Table, and adjust the pressures is about 10-20 minutes.


High Water Pressure

Sometimes your water pressure may be too high.  If the Design Pressure is more than 15 PSI higher than the Total Pressure Loss in your Pressure Loss Table then you should consider reducing the pressure.  Too much pressure can make the sprinklers work inefficiently and/or damage the sprinkler system.  There are two ways you can do this.

The first way to reduce the water pressure for the sprinkler system is by installing a pressure regulator or reducer on the whole sprinkler system.  Typically this is installed near the point your irrigation system mainline taps into the water supply.  On residential systems, typically if there is too much pressure for the sprinkler system, then there is also too much for the typical household appliances like washing machines, so the pressure regulator is put on then entire house’s water supply.

The second way to reduce the water pressure for the sprinklers is to use special pressure reducing automatic zone control valves for the sprinkler system. These are solenoid valves that have a pressure regulating module added to them.   This allows you to set a reduced pressure level individually in each different watering zone.  The use of pressure regulating zone valves has the disadvantage of sometimes being more expensive (the more zone valves there are, the more likely this option will be more expensive.)  A second disadvantage is that it does not reduce the pressure in the mainlines, so they may be more susceptible to water pressure damage.  There is an advantage to using pressure regulating zone valves if you have sprinklers that require different water pressure levels.  Each zone can be set to a different pressure, so spray heads could have lower pressure to prevent misting, while rotors valves are set to a higher pressure to achieve a greater radius.

Pressure regulators are discussed more in-depth at the bottom of the introductory page about Selecting Your Sprinkler Equipment.



This article is part of the Sprinkler Irrigation Design Tutorial
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