Water Pressure Assessment for Irrigation Systems

1. Determining Available Water Pressure

Before we can do our final check, we need to know the pressure we have to work with. This isn't as simple as just using the city's stated water pressure. The pressure available to your irrigation system is always less than the static pressure at the water main.

Let's define two key terms:

• Static Pressure: The water pressure in the pipes when no water is flowing. This is the maximum pressure in the system. You can measure it by attaching a pressure gauge to an outdoor tap and ensuring no other water is running.
• Dynamic (or Working) Pressure: The pressure in the pipes when water is flowing. This is the pressure that actually matters for our design. It's always lower than static pressure because of friction losses that occur in the city's service line and through the water meter before the water even reaches your irrigation system's main line.

The workbook we've used before has an excellent section on how to properly account for these initial losses to determine the true amount of water and pressure available for your system.

As you read, the key takeaway is that we must subtract the pressure losses from the service line and water meter from our initial static pressure. This gives us the Available Dynamic Pressure at the point of connection for our irrigation system.

Available Dynamic Pressure = (Static Pressure) - (Service Line Loss) - (Water Meter Loss)

2. The System Check: Required vs. Available Pressure

Now we have the two critical values needed for our final check:

1. Required Pressure: The Total Dynamic Head (TDH) of our most demanding zone (which you calculated in the last lesson).
2. Available Pressure: The Available Dynamic Pressure at the source.

The moment of truth is a simple subtraction:

• If the result is a positive number (ideally with a small safety margin of 5-10%), your system has enough pressure and the design is valid.
• If the result is negative, you have a pressure deficit. The system will not work as designed, and sprinklers will underperform.

The "System Operation Check" section in the Toro design workbook provides a perfect walkthrough of this entire verification process.

3. Worked Example: Bringing It All Together

Let's use the lawn zone from our previous lesson's example and see if it works.

Given:

• Required TDH: From the last lesson, we calculated this to be 42.8 meters (4.28 bar).
• Static Pressure: Measured at the property's connection point is 4.5 bar.
• System Components before Irrigation Zone:
  • 3/4" Water Meter
  • 20 meters of 1" pipe for the service line
• Zone Flow Rate: 35 GPM (approx. 8 m³/h) - this is a high flow for a residential system, chosen to illustrate the point.

Step 1: Calculate Available Dynamic Pressure

First, find the pressure loss in the water meter and service line. We can use friction loss charts for this. For simplicity, let's assume we've looked up these values:

• Pressure loss through a 3/4" meter at this high flow rate is significant, let's say 0.6 bar.
• Friction loss in 20m of 1" service line is about 0.2 bar.

Total pressure loss before the irrigation system = 0.6 bar + 0.2 bar = 0.8 bar.

Now, calculate the Available Dynamic Pressure:
$\text{Available Dynamic Pressure}=4.5\text{ bar (Static)}-0.8\text{ bar (Losses)}=3.7\text{ bar}$

Step 2: Perform the System Check

$\text{Pressure Surplus/Deficit}=3.7\text{ bar (Available)}-4.28\text{ bar (Required)}=-0.58\text{ bar}$

Conclusion: The design fails. We have a pressure deficit of 0.58 bar (or 5.8 meters of head). The sprinklers will not receive their required operating pressure.

4. What To Do When The Design Fails

When faced with a pressure deficit, you have two primary options.

Option A: Reduce the Total Dynamic Head (TDH)

The largest component of TDH that you can control is pipe friction loss. By increasing the diameter of your main and lateral lines, you can significantly reduce friction and lower the overall TDH.

This video demonstrates the powerful effect of changing pipe size on the pressure requirements of a system.

In our example, we would need to recalculate our pipe friction losses using larger pipe sizes to see if we can reduce the TDH by at least 0.58 bar.

Option B: Add a Booster Pump

If modifying the pipe sizes is not practical or not sufficient, the solution is to add a booster pump. A booster pump's job is to increase the pressure from the source to meet the system's requirements.

The required pressure boost is simply the deficit you calculated:
$\text{Required Boost}=\text{Required TDH}-\text{Available Dynamic Pressure}$
In our example, we need a pump that can add at least 0.58 bar (5.8 m of head) at our zone's flow rate.

This article provides an excellent overview of booster pumps and how to size them.

Once you know the required flow and head, you select a pump using its pump curve, which is a graph showing the pump's performance. You look for a pump that can provide the required flow rate (on the horizontal axis) at the required head (on the vertical axis).

Conclusion

Congratulations! You have now completed the entire hydraulic design and verification process. You can now not only calculate the pressure a system needs but also determine if the available water source can provide it.

Key Takeaways:

• The Available Dynamic Pressure is the static pressure minus losses from the service line and water meter.
• A system is viable if the Available Dynamic Pressure is greater than the Required TDH of the most demanding zone.
• If there is a pressure deficit, your two main solutions are:
  1. Reduce TDH by increasing pipe sizes.
  2. Add a booster pump to increase the available pressure.
• A booster pump is selected using a pump curve to match the required flow and head (pressure).

This lesson was the final piece of the technical design puzzle. In our next module, you will begin the Capstone Project, where you will apply everything you've learned—from site analysis and hydrozoning to full hydraulic calculations and system checks—to design a complete irrigation system for a 600 m² villa.