Pressure Loss (Landscape Sprinkler Design Tutorial)

Jess Stryker's Landscape Tutorial Series
Jess Stryker's
Landscape Sprinkler Design Tutorial
Step #2

Pressure Loss Adjustments

Now you should have all the lines on your Pressure Loss Table filled in. Here are links back to each of the descriptions of what goes on each line just in case:

Pressure Loss Table

_____ PSI - Water Meter Click here
_____ PSI - Backflow Preventer Click here
_____ PSI - Filter (optional) Usually 2 PSI - check for loss value with the manufacturer.
_____ PSI - Mainline Click here
_____ PSI - Valves Click here
_____ PSI - Elevation change (change in feet x .433 = PSI) Click here
_____ PSI - Sprinkler Heads or Drip Emitters Click here
_____ PSI - Lateral Pipes (maximum is 20% of sprinkler head PSI above) Click here

_____ PSI - Total Pressure Loss (add together the values above)

Fill in a preliminary value for each item on the Pressure Loss Table.

Adjust Your Pressure Loss Data:

The Total Pressure Loss on the last line of your Pressure Loss Table must be less than or equal to the "Design Pressure" that you noted earlier on your Design Data Form. If it is, then you're done with this page, continue to the next page.

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. Here are a few things you can do to lower the Total Pressure Loss:

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. Now go back to the Pressure Loss Table and recalculate all the losses using the new Design Flow. 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.

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 they can be placed apart. Thus you will likely need more sprinkler heads. Remember that rotor heads don't work very well if you use less than 30 PSI. You will probably also need to change the lateral pressure loss to keep it less than 20% of the sprinkler head operating pressure.

IMPORTANT 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 first.

Increase the mainline size. This would lower the pressure loss in the mainline. Of course, if the mainline is already installed, it is a lot of work to replace it.

Increase the size of the backflow preventer or valve. 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 your new flow is not under the manufacturer's minimum flow rate!

Reduce the pressure loss in the laterals. This means the pipes will be larger and cost more, but sometimes using larger pipe is the cheapest way to reduce the pressure loss.

Finally, in some cases you may have to add a booster pump to increase the water pressure. This is obviously a very expensive option for a homeowner. Booster pumps are more commonly used on parks, schools, and golf courses where the typical sprinkler head needs more than 50 PSI to operate!

Example:

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 - the sprinkler system won't work.

Warning: don't use the pressure losses above 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 5 PSI!

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. The easiest way to do that 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 choice. When we lower the sprinkler head PSI we also have to lower the PSI loss for the laterals. 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!

Warning: don't use the sample pressure losses above 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 5 PSI!

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!

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