Irrigation pipe size and pressure loss calculator
Sizing an irrigation pipe starts from the flow it must carry. We pick the smallest pipe in the TS EN 12201 tables that keeps the water under 1.5 m/s, then work out the pressure loss with the Hazen-Williams formula. Add the height difference and you get the pressure the pipe needs at its start. The next size up sits beside it, so you see what a bigger pipe saves at the pump.
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A worked example
A 25-decare soilless strawberry greenhouse in Serik. Each valve zone draws 36 tonnes an hour. A 400 m PE100 10 bar main runs from the filter to the farthest valve; the valve sits 8 m above the filter and needs 1.5 bar at its inlet.
- Smallest inside diameter 92.1 mm for 1.5 m/s
- Suggested pipe Ø110 × 6.6 mm, 96.8 mm inside; water speed 1.36 m/s
- Pressure loss 8.6 m with 10% for fittings, which is 0.84 bar
- Total with the height difference 8.6 + 8 = 16.6 m, which is 1.62 bar
- Pressure needed at the start 1.5 + 1.62 = 3.12 bar
- One size up, Ø125 1.05 m/s, 0.45 bar lost, 2.73 bar at the start. The pump pushes about 4 m less.
[The example’s numbers await Perge’s check.]
How it is worked out
First the speed limit gives the smallest inside diameter: D = √(4 × Q ÷ (π × v)), with Q the flow in m³ a second and v the top speed. Then the first standard pipe wider than that is picked. A pipe’s inside diameter is its outside diameter less two walls. For the same outside size, a higher pressure class has a thicker wall and a narrower bore, so a 16 bar pipe loses more than a 10 bar one.
The pressure loss comes from the Hazen-Williams formula: hf = 10.67 × L × Q^1.852 ÷ (C^1.852 × D^4.87). We take C = 140 for PE and 150 for PVC. We add 10% to the friction for bends, tees and valves. If the end of the pipe is higher than its start, the height difference is added too; 10.2 m of water makes 1 bar.
The loss falls with about the fifth power of the inside diameter. So one size up often halves it. On long lines a bigger pipe saves on the power the pump uses each year; the tool shows the two side by side.
Questions
How do I work out the size of an irrigation pipe?
Take the flow through the pipe and a top water speed. Inside diameter = √(4 × flow ÷ (π × speed)). 36 tonnes an hour at 1.5 m/s needs at least 92 mm inside, which is a Ø110 pipe in 10 bar PE100. Then check the pressure loss for the pipe’s length.
How fast should water flow in an irrigation pipe?
In mains and sub-mains, under 1.5 m/s is the usual aim. The faster the water, the faster the pressure loss grows, and the harder the water hits when a valve shuts. Very slow water means the pipe is bigger than it needs to be.
How much pressure does a pipe lose per 100 m?
It depends on the flow, the size and the pipe type. 36 tonnes an hour loses about 0.2 bar per 100 m in a Ø110 PE100 10 bar pipe. The same water loses about 0.56 bar in a Ø90 pipe and about 0.11 bar in a Ø125.
Which pressure class of PE100 pipe should I use?
Pick it for the highest pressure in the line, with room for water hammer. On flat land, if the pump gives no more than 4 bar, 6 bar pipe can do. Irrigation mains mostly use 10 bar. High-pressure pump outlets and steep land need 16 bar.
How does a height difference change the pressure?
Every 10.2 m of height takes 1 bar. If the pipe runs uphill, add it to the pressure needed at the start. If it runs downhill you gain pressure, but the pressure at the low end rises, so pick the pipe class for that.