Spray heads or rotors: how many per zone on your tap

Background

“My sprinklers won’t pop up” and “how many can I put on one zone?” are questions that have kept coming back on Italian forums for years [71][72]. In one documented case a user measures 3.6 bar with no water flowing, about 2 bar with one sprinkler open and less than 1 bar with two [71]. Static pressure is not enough to go on: what counts is how much is left when the water is flowing.

How to use it

1. Measure the static pressure, that is with no water flowing, using a pressure gauge on the tap, as the Rain Bird design guide says [68].
2. Do the bucket test: the seconds needed to fill a bucket of known volume, with the tap fully open [69].
3. Enter the pipe up to the first sprinkler: length and internal diameter. For PN6 low-density polyethylene, an Italian manufacturer’s catalogue gives an internal diameter of 16.6 mm for 20 mm pipe, 20.6 mm for 25 mm pipe and 26.4 mm for 32 mm pipe [70]: check the internal diameter of your own pipe on its data sheet.

The table shows, for each nozzle or sprinkler, how many a zone can run at the recommended pressure and at the minimum declared by the manufacturer.

An example

A tap giving 3 bar static pressure and 40 l/min in the bucket test, 10 m of PE 25 PN6 pipe:

• Rain Bird HE-VAN-12, 360°: 4 per zone at the minimum pressure of 1 bar, about 28 l/min, with the last one at about 1.25 bar. With fewer sprinklers the pressure would exceed the maximum in the table, 2.1 bar.
• Rain Bird R-VAN18, 360°: 3 per zone at the minimum of 2.1 bar. At the recommended 3.1 bar, none: the tap does not reach 3.1 bar even with no water flowing.
• Claber Colibrì 360°: 6 per zone within the table, which starts at 1.5 bar.

Open the example in the calculator and try changing the pipe from 25 to 32 mm, that is 26.4 mm internal.

Spray heads, rotary nozzles and rotors do not mix at random

The precipitation rate, that is how many millimetres of water fall in an hour, varies a lot between types. It is calculated from flow and spacing, as in the Rain Bird guide [68]:

• HE-VAN spray heads declare 40–45 mm/h;
• R-VAN rotary nozzles 16 mm/h;
• the 3504 series rotors in the example give about 5–7 mm/h.

On the same zone, in one hour a spray head wets many times more than a rotor. According to the FAO, a clay soil absorbs 2–8 mm/h and a medium-textured soil 8–16 [74].

How much water a system needs

Rain Bird gives about 30 l/min (8 US gallons per minute) as the minimum flow for designing a sprinkler system [73]. With less water, a dripline is often more suitable: see how long it can be.

In the pipe, Rain Bird gives 1.5 m/s as the general maximum velocity [68]: the calculator flags it when you exceed it.

How we do the sums

• Tap: from the two measurements we work out how much pressure is left at each flow. We assume losses proportional to the flow raised to the power 1.75, the exponent for turbulent friction in smooth pipes [65][67]. This is our own model, not a measurement.
• Pipe: losses with Darcy-Weisbach [1][2][3], calculated section by section between one sprinkler and the next [65]. Each sprinkler works at its own pressure, read from the manufacturer’s table. With identical outlets the result matches Christiansen’s F factor [66].
• Sprinklers: all identical, in a row, head-to-head at the declared radius.
• Excluded: fittings, valves, filters and water meter, so the real pressure will be somewhat lower. No extrapolation beyond the tables.

More details in where our data comes from.