Calculate your pipe resistance!

When we push water through a pipe, it creates resistance. The more water we try to push through the same pipe per hour, the more resistance occurs. If we also increase the length of the pipe, the resistance increases even further!

Being able to calculate this resistance is extremely useful. It allows us to understand the expected backpressure in existing setups, helping us find the right pump. For new installations, it helps determine which pipe diameter is suitable in combination with a specific pump.

An example:

You received an irrigation reel from your neighbor, but you don't have a pump yet. The reel has a sprinkler with a technical table attached. Based on this table, you've concluded that you need a flow of approximately 30 m3/hour at a pressure of 4 bar (at the sprinkler nozzle!).

The reel contains 200 meters of HDPE pipe with an outer diameter of 75mm. To get from the water source to the reel, there is 100 meters of HDPE transport pipe with an outer diameter of 90mm.

You can now use the calculator to determine the pipe resistance. Please note: you must subtract the wall thickness from the outer diameter to get the correct internal diameter!

* This calculator defaults to new HDPE or PVC piping. Resistance may vary with different or older materials.

Entering the data:

Regarding the reel, we enter:
– Capacity = 30 m3/hour
– Internal diameter = 66mm (75mm outer with a 4.5mm wall thickness)
– Pipe length = 200 meters

Result = 16.87 meters

For the transport pipe, we enter:
– Capacity = 30 m3/hour
– Internal diameter = 79mm (90mm outer with a 5.5mm wall thickness)
– Pipe length = 100 meters

Result = 3.52 meters

Adding both together, we have a total resistance of 20.39 meters.

'And what do I do with this information?'

Great question! Now that we know the pipe resistance and a few other factors, we can calculate the required duty point of the pump and find the right model!

What else do we know?

A. We only calculated pipe resistance, not the fittings (elbows, valves, etc.). We add 5% to the pipe resistance as a safety margin and round up. If there are many fittings, we recommend mapping them specifically. 20.39 x 1.05 = +/- 21.5 meters.
B.  The water level is at 5 meters depth and your land slopes up, placing the reel 5 meters higher than the source. So: 5m source depth + 5m elevation = 10 meters, which must be added. 21.5 + 10.0 = 31.5 meters.
C.  The nozzle requires 4 bar for optimal reach and spray patterns. 1 bar is roughly 10 meters, so we need an additional 40 meters of pressure for the nozzle. 40 + 31.5 meters = 71.5 meters.

Conclusion:

We need a pump with a duty point (nominal) of roughly 30 m3/hour at (minimum) 71.5 meters (7.15 bar).

Slightly more pressure is never an issue, but we advise against 'over-specifying' as it won't necessarily result in a better system and eventually just wastes electricity (and money).

The above is a rough but very quick way to gain insight into the required duty point for your situation. This calculator is a tool; always use your common sense and contact us if in doubt!