Pipeline friction effects overcome by diameter

While there has been major investment in effluent ponds and storage tanks, what are the mechanics of getting ‘harvested’ effluent out onto paddocks to unlock its full potential?

There is a common misconception that to pump effluent over any reasonable distance from a storage area you will need a pump with high flow rates and high pressure. 

Anyone who has sweetened the missus up by creating a water feature in the garden will know that the flow over the waterfall needs lots of water to create the desired effect – it’s advisable to use larger diameter hose.

The physics of the matter is quite simply friction. When pumping with a 150mm delivery hose, the pressure drop resulting from frictional losses will be only about 25% of that in a system using a 100mm hose.

Looking at a typical 100mm set-up with a flow rate of 150m³/hr pumped over 1000m, the likely pressure drop is about 12.6 bar, and the effluent will travel at 4.5m/sec. 

Upsizing to 150mm hose with a corresponding set-up will see a system pressure drop of only 4.8 bar, and the effluent travels in the pipe at a speed of 3.04m/sec.

So it stands to reason that if you can reduce your overall system requirements for very high pressures by fitting bigger diameter hoses, there is huge potential saving fuel for the pumping tractor and a corresponding reduction in wear and tear on the pump and the pipeline, while delivering the required volumes of effluent during the working day.

We can look at the mechanics in more detail by comparing four typical set-ups, all trying to deliver 170m³/hr:

System 1

600m of 150mm supply hose; pressure loss 1.52 bar
400m of 125mm drag hose; pressure loss 2.41 bar

Total pressure required 3.93 bar

System 2

600m of 150mm supply hose; pressure loss 1.52 bar
400m of 100mm drag hose; pressure loss 6.34 bar

Total pressure required 7.86 bar

System 3

600m of 125mm supply hose; pressure loss 3.62 bar
400m of 100mm drag hose; pressure loss 6.34 bar

Total pressure required 9.96 bar

System 4

1000m of 100mm supply hose; pressure loss 15.86 bar

Total pressure required 15.86 bar

Note that in examples 1 and 2 the output requirement of 170m³/hr was achieved with a tractor running at 1450 engine rpm, and the pressure requirement in example 4 is not achievable even with the best products in the marketplace.

So while it’s sensible to do some homework and find the best pump and spreader system for your particular operation, don’t overlook the need for a pipe that can deliver the volumes with low pressure drops. 

A 150mm pipe might show a reduced effluent speed in the pipe, but that pipe has a 50% greater carrying capacity and is the easy way to get high daily delivery rates and reduced fuel/power costs.