Irrigation pipe size affects the pressure available at the far end of a zone and the head the pump must supply. Compare candidate internal diameters at the design flow, then check pressure rating and transient conditions separately.
How to size an irrigation mainline
- Choose the operating case. Add the flows of the zones that will operate together and identify the flow through each pipe section.
- Compare actual internal bores. Enter the length, pipe material, fittings and valves for each candidate pipe.
- Calculate velocity and pressure losses. Account for friction, local losses and changes in elevation.
- Check the delivery pressure. Compare the pressure left at the outlet with the equipment requirement, then assess pipe pressure rating and surge separately.
The two-bore comparison shows the calculation. The 650 m supply-main report shows how to read a Tenselo result and its outstanding pressure checks. A pipe size alone does not establish a suitable pump: carry the required flow and head into the pump sizing guide.
Use the actual internal diameter
A nominal pipe size is not necessarily the waterway diameter. The bore depends on the material, dimensional standard and wall thickness. Obtain the internal diameter from the selected pipe's published dimensions before estimating losses.
- Flow through each section for the intended zone combination.
- Internal diameter, length, material and pipe condition.
- Water properties and temperature for the friction calculation.
- Fittings, valves and equipment losses, with their reference velocities.
- Elevation profile, source pressure and required delivery pressure.
A mainline carrying a constant flow is different from a drip lateral with water leaving along its length. The constant-flow example below must not be applied to an entire emitting lateral without accounting for the reducing flow.
Velocity, major loss and minor loss
With flow Q in m³/s and internal diameter D in metres, velocity v = 4Q / (πD²). Darcy–Weisbach major head loss is hf = f(L/D)v²/(2g). Fitting loss is hm = ΣK × v²/(2g) when all K values refer to that same velocity.
Here f is the dimensionless Darcy friction factor, not the Fanning friction factor. Determine it from the appropriate flow regime, Reynolds number and relative roughness for a real pipe. The EPA EPANET 2.2 manual, section 3.1 describes pipe head-loss methods and minor losses.
Worked example: 100 mm and 125 mm internal bores
Assume water at 10 L/s through a 200 m constant-flow pipe. For an arithmetic comparison only, use Darcy f = 0.025 for both bores, ΣK = 5, g = 9.81 m/s² and density = 1,000 kg/m³. A real design must calculate the friction factor for each candidate.
| Quantity | 100 mm bore | 125 mm bore |
|---|---|---|
| Velocity | 1.27 m/s | 0.81 m/s |
| Pipe friction loss | 4.13 m | 1.35 m |
| Fitting loss | 0.41 m | 0.17 m |
| Combined head loss | 4.54 m | 1.52 m |
| Equivalent pressure loss | 44.6 kPa | 14.9 kPa |
Under these assumptions the larger bore reduces combined loss by about 3.02 m of head. This comparison does not select a pipe material, nominal size or pressure class.
Finish the pressure check
Track elevation and losses along the complete route to check the pressure at the critical outlet. Check the maximum pressure as well: low points, pump shut-off conditions and pressure transients may govern the pipe or fitting rating. A low steady-state friction loss does not prove a system is protected from water hammer.
Compare installation cost, operating hours and pumping energy with maintenance and future demand. Keep the assumed flow and pipe dimensions beside the results so another person can reproduce the comparison.
Common pipe-sizing questions
What is the best irrigation pipe velocity?
There is no single value that selects every irrigation pipe. Use the project criteria, pressure losses, surge assessment and manufacturer limits. The velocities above are calculated results, not universal design limits.
Can I use Hazen–Williams instead?
It is a commonly used alternative for water systems, with its own coefficient and applicability assumptions. Do not substitute a Hazen–Williams coefficient for a Darcy friction factor or mix equations within one result.
Does doubling the flow double friction loss?
With a fixed friction factor and bore, the equations here make loss proportional to flow squared. In a real calculation the friction factor can also change with flow.
Read the 650 m supply-main example
The sizing-method example above uses assumed bores and a fixed friction factor. The report below comes from a separate recorded HYD-007 calculation: 34 L/s, 650 m of level supply main, water at 20°C, new pipe and a 10% equivalent-length allowance for fittings.
Inside the sample
Follow the flow through to the remaining pressure
The report records 34 L/s through 650 m of supply main, starting at 5.4 bar. The returned PE100 option has a 175.8 mm internal bore and a calculated end pressure of 4.80 bar.
Compare that end pressure with the actual requirement at the pivot inlet. The sample leaves that requirement unspecified, so it cannot confirm operation of a particular machine.
Read the supply-main report (PDF, 1 page)
The layout was prepared for demonstration from the application result. The calculator remains provisional; this is a supply-main example, not a complete pivot design or manufacturer approval.
- Check the actual bore. The returned generic PE100 option is DN200 SDR17 PN10 with a 175.8 mm internal diameter. Use the waterway diameter in the calculation, not 200 mm.
- Read velocity and friction together. The result shows 1.40 m/s and approximately 0.85 m friction loss per 100 m. Those values belong to this flow, pipe and calculation basis.
- Close the pressure budget. The calculated end pressure is 4.80 bar. The report supplies no required downstream pressure, so suitability remains unresolved until the pivot designer supplies that requirement.
A supply main carries the combined downstream demand. Flow reduces along a pivot span as sprinklers discharge. Use the machine and sprinkler-package workflow for those checks, and collect the site data with the fillable centre-pivot design brief (PDF, 2 pages).
Continue the design
Use this loss estimate in an irrigation pump duty calculation. For tools covering velocity, friction and pressure checks, see Tenselo's pipe hydraulics calculators. An emitting lateral also needs the separate checks described in the drip irrigation guide.