Hydraulic calculation is the foundation of a reliable irrigation system. Without it, you risk having zones with insufficient pressure where nozzles can't reach their design radius.
Why do you need hydraulic calculation?
Water loses pressure through friction as it moves through pipes. The longer the pipe, the smaller the diameter, or the higher the flow — the greater the losses. If pressure at the farthest sprinkler drops below minimum, it won't work properly.
The Hazen-Williams formula
The primary formula for calculating pressure loss in plastic pipes:
hf = 10.67 × L × Q^1.852 / (C^1.852 × d^4.87)
Where: hf = head loss (m), L = pipe length (m), Q = flow rate (m³/s), C = roughness coefficient (150 for PE pipe), d = internal diameter (m).
Step 1: Determine source parameters
- Static pressure — pressure with no flow (gauge on closed valve)
- Dynamic pressure — pressure at working flow rate
- Maximum flow rate — from bucket method (working = 75% of maximum)
Step 2: Calculate flow velocity
Water velocity in pipes should not exceed 1.5 m/s (5 ft/s). Formula: V = Q / (π × r²). Here's a quick reference:
| Pipe diameter | Max flow at 1.5 m/s |
|---|---|
| 25 mm / 1" (ID 21 mm) | 0.52 m³/h = 2.3 GPM |
| 32 mm / 1.25" (ID 27 mm) | 0.86 m³/h = 3.8 GPM |
| 40 mm / 1.5" (ID 35 mm) | 1.44 m³/h = 6.3 GPM |
Step 3: Check each zone
For each zone, sum up all nozzle flow rates, calculate pressure loss from source to the farthest sprinkler, and verify that residual pressure is sufficient for the nozzle to operate.
Practical example
Source: 3 bar static pressure, 18 L/min. PE pipe 32 mm, length 25 m. Zone: 4 × MP2000 (180°) = 4 × 2.8 = 11.2 L/min.
Losses: hf ≈ 0.3 bar. Remaining: 3 - 0.3 = 2.7 bar — sufficient for MP Rotator (min 1.7 bar / 25 PSI).
Our planner performs hydraulic calculations automatically using the Hazen-Williams formula for each zone.