Flow, pressure and head

Flow is how much water passes in a set time: litres per hour (L/h) for a dripper, or cubic metres per hour (m³/h) for a pump or a pipe. One cubic metre is 1,000 litres.

Pressure is how hard the water pushes inside the pipe, measured in bar. Head is pressure given as a height of water, in metres: one bar is about 10 metres of head, which is why FAO describes class 6 PVC pipe as rated for 60 metres.

Total head is the height the pump must lift the water, plus the pressure the system needs at the far end, plus what is lost to friction in the pipes, all in metres. Every pipe, valve and fitting takes a share of it.

Sources: FAO (2007), Handbook on Pressurized Irrigation Techniques, chapter 4: System design; FAO (2001), Irrigation Manual Module 8: Sprinkler Irrigation Systems, on pipe classes.

The main: the buried pipe, of a pressure class above the most it will carry
Isolation valve: a hand valve to shut the line for repairs
Zone valve: opens and shuts this zone when the controller tells it to
Pressure regulator: lowers the pressure to what the drip line needs
Gauge: shows the pressure, so you can check it against the design
Drip line: lets the water out a few drops at a time beside the plants

From the main to the drip line: a hand valve and a zone valve in their box, then a regulator and a gauge.

PVC-U and PE pipe

FAO's handbook describes two plastic pipes for irrigation. PVC-U, also called uPVC or rigid PVC, is stiff and comes in straight lengths. PE, polyethylene, is flexible and black and comes in coils. Each is made in pressure classes: the PN number, in bar, is the highest pressure the pipe is made to carry all the time in ordinary use.

Compare	PVC-U	PE
Where it goes	Buried: it must lie underground, out of the sun and away from heat	It can lie on the surface: carbon black in the plastic protects it from sunlight
Sizes	50 to 225 mm, named by the outside diameter	12 to 110 mm, named by the outside diameter
Pressure classes	PN 4, 6, 10 and 16	PN 2, 4, 6, 10 and 16
How it comes	6 m lengths with a socket at one end	Coils of 50 to 400 m
Joints	Solvent cement on small sizes, rubber-ring sockets on larger ones; buried fittings need thrust blocks	Compression or insert fittings; barbed insert fittings only on small pipe at low pressure
Life	About 50 years buried	12 to 15 years on the surface
Usual job	Mains and sub-mains	Drip lines in soft PE; mains and sub-mains in hard PE

Source: FAO (2007), Handbook on Pressurized Irrigation Techniques, chapter 3: Irrigation equipment and jointing techniques.

Choosing the pressure class

Choose the class for the highest pressure the pipe will ever see, not the pressure while the system runs. A closed valve can hold the pump at its shut-off head, the most it can give, and valves shutting or the power failing send surges along the pipe. FAO's pumping manual says the rating must at least cover the working pressure plus the surges.

Heat weakens plastic pipe. A class holds at the temperature its maker rates it for, and FAO's table of service factors starts at 23 °C: at about 32 °C a PE pipe keeps about 81 percent of its rating, and PVC-U about 75 percent.

Worked example, for illustration only

Is PN 6 enough for a surface PE main?

Assumptions

A PE main lies on the surface, fed by a pump whose chart shows a shut-off head of 55 m.
On hot afternoons the water in the pipe reaches about 32 °C.
FAO's service factor for PE at 32.2 °C is 0.81, and one bar is about 10 m of head.

Working

PN 6 pipe: 6 bar × 0.81 = about 4.9 bar, or about 49 m of head. That is less than the 55 m the pump can reach against a closed valve.
PN 10 pipe: 10 bar × 0.81 = 8.1 bar, or about 81 m of head. That is above the 55 m, with about 26 m to spare for surges.
Hotter pipe: black pipe in full sun can get hotter than 32 °C, above all when the water stands still behind a closed valve. FAO's last factor for PE, at 37.8 °C, is 0.70: PN 10 still gives 10 bar × 0.70 = 7 bar, or about 70 m.

Result: In this example PN 6 is not enough and PN 10 is. The answer turns on the pump's shut-off head and the water's temperature, so check both before you choose a class; for pipe hotter than FAO's table goes, ask the maker, or bury or shade the main.

Reading the shut-off head on a pump's chart

Sources: FAO (2001), Irrigation Manual Module 5: Irrigation Pumping Plant, chapters 4, 9 and 10; FAO (2007), Handbook on Pressurized Irrigation Techniques, chapter 3: Irrigation equipment and jointing techniques.

Sizes and threads that do not mix

Metric plastic pipe is named by its outside diameter. The inside is smaller, and depends on the material and the class: in FAO's table, a 63 mm PVC-U pipe of PN 6 is 59.2 mm inside, but a 63 mm hard PE pipe of PN 6 is 55.8 mm inside, because PE needs a thicker wall. Newer PE grades of the same class differ again, so take the inside diameter from the maker's sheet. Joints differ too: PVC-U is glued with solvent cement or pushed into rubber-ring sockets, while PE takes compression or insert fittings.

Threads are named in inches after old steel pipe sizes, not after what you can measure. FAO gives 1 inch for a 25 mm steel pipe, yet a 1 inch thread measures about 33 mm across. When you order an adaptor, name both ends: the pipe's outside diameter, and the thread's size and standard.

There are two main thread standards. BSP threads have a 55 degree thread angle and NPT, the American standard, 60 degrees. At ½ and ¾ inch the two are so close in design that they are hard to tell apart, so ask for the standard by name on every threaded part. BSP also comes as taper, sealed on the thread with tape or paste, and parallel, sealed with a washer or an O-ring.

Sources: FAO (2007), Handbook on Pressurized Irrigation Techniques, chapter 3: Irrigation equipment and jointing techniques; Swagelok (2025), Thread and End Connection Identification Guide, MS-13-77, pages 13 and 21 to 28.

How much pressure friction takes

Water loses pressure as it rubs along a pipe. The loss grows fast as the flow rises, and falls fast as the pipe gets wider. Makers publish friction tables, usually as metres of head lost per 100 m of pipe, and FAO recommends using them.

Keep the water slow as well. FAO's handbook says plastic pipe makers recommend no more than 1.5 m/s, and treats 1.7 m/s as the most under normal conditions. The table gives FAO's largest recommended flow for some common pipes.

Pipe	Inside diameter	Largest recommended flow
Soft PE 16 mm, PN 4	12.4 mm	0.75 m³/h
Soft PE 20 mm, PN 4	16.4 mm	1.3 m³/h
Soft PE 25 mm, PN 4	20.6 mm	2.0 m³/h
Soft PE 32 mm, PN 4	27.2 mm	3.5 m³/h
Hard PE 50 mm, PN 6	44.2 mm	9 m³/h
Hard PE 63 mm, PN 6	55.8 mm	15 m³/h
Hard PE 75 mm, PN 6	66.4 mm	21 m³/h
PVC-U 63 mm, PN 6	59.2 mm	17 m³/h
PVC-U 90 mm, PN 6	84.6 mm	34 m³/h
PVC-U 110 mm, PN 6	103.6 mm	51 m³/h

Worked example, for illustration only

Two pipe sizes for the same zone

Assumptions

A zone takes 2.5 m³/h through 100 m of pipe on level ground.
The choice is soft PE 32 mm, 27.2 mm inside, or hard PE 50 mm, 44.2 mm inside, from FAO's table.
Friction comes from the Hazen-Williams formula in FAO's manual, with C = 140 for plastic pipe. Fittings are left out.

Working

32 mm pipe: about 6.8 m of head is lost over the 100 m, about 0.7 bar, and the water moves at about 1.2 m/s.
50 mm pipe: about 0.6 m is lost, under 0.1 bar, and the water moves at about 0.45 m/s.
25 mm pipe, 20.6 mm inside, would lose about 26 m, with the water at about 2.1 m/s: well past FAO's largest recommended flow for that size.

Result: In this example the 32 mm pipe takes about ten times the pressure the 50 mm pipe does, every time the zone runs, and the pump must make it up. A regulator further on cannot give it back, because a regulator only lowers pressure.

Sources: FAO (2007), Handbook on Pressurized Irrigation Techniques, chapter 3: Irrigation equipment and jointing techniques; FAO (2007), Handbook on Pressurized Irrigation Techniques, chapter 4: System design; FAO (2002), Irrigation Manual Module 9: Localized Irrigation, Equation 23.

Valves, regulators and gauges

Each valve on a line has one job. The table follows the water from the pump to the drip line. The store links below go to a hand valve, a zone valve and a filter regulator we design with and supply.

Part	What it does	Where it goes
Non-return valve	Lets water flow one way only, so it cannot run back through the pump when it stops	Straight after the pump, at its outlet, next to the gate valve
Isolation valve	Shuts a line or a zone by hand: a ball valve with a quarter turn, a gate valve only fully open or fully shut	Wherever part of the system may need shutting off for repair
Zone valve	An electric valve that opens and shuts one zone when the controller tells it to; a handle opens it by hand	At the start of each zone, usually in a valve box
Air valve	Lets trapped air out, and lets air in when the pipes drain, so that neither trapped air nor a vacuum damages them	At high points, after the pump, at both ends of long lines; vacuum breakers on drip lines laid on the soil
Pressure regulator	Lowers the pressure to a set level, but only when more than that reaches it	At the start of a drip zone
Pressure gauge	Shows the pressure, so you can check the system against its design	At key points, where it is easy to read and look after

Cepex PVC ball valve Hunter PGV zone valve Hunter filter regulator

Sources: FAO (2007), Handbook on Pressurized Irrigation Techniques, chapter 3: Irrigation equipment and jointing techniques; FAO (2007), Handbook on Pressurized Irrigation Techniques, chapter 3, gauges and air valves; FAO (2007), Handbook on Pressurized Irrigation Techniques, chapter 3, emitters and automation; FAO (2001), Irrigation Manual Module 5: Irrigation Pumping Plant, chapters 4, 9 and 10.

Pressure regulators for drip

Drip lines work in a narrow band of pressure. FAO notes that water enters drippers at about 1.0 bar, and its design rule keeps the difference in pressure between any two drippers in a zone to no more than 20 percent, so that their flows differ by no more than 10 percent.

A regulator holds the pressure after it at its setting, so a drip zone near the pump gets the same pressure as one far away. It can only lower pressure: if less than its setting reaches it, less goes out. Take its setting from the drip line maker's sheet and its size from the zone's flow, and fit a filter ahead of it.

Pressure-compensating drippers keep a steady flow once the pressure is above their working level, and their sheet gives the range they work in.

Choosing a drip filter Drip tape or dripline

Sources: FAO (2007), Handbook on Pressurized Irrigation Techniques, chapter 3: Irrigation equipment and jointing techniques; FAO (2007), Handbook on Pressurized Irrigation Techniques, chapter 3, emitters and automation; FAO (2007), Handbook on Pressurized Irrigation Techniques, chapter 4: System design.

What to measure

The flow each pipe will carry: that of the largest zone or block on it
The highest pressure the pipe will see: the pump's shut-off head, or the supply's pressure with everything shut
The length of each run, and how far the ground rises or falls along it
The pressure the drip lines or sprinklers need, from their sheets
Whether the pipe will lie in the sun or be buried, and where it crosses a road or track
The size and thread of everything you must connect to: the pump outlet, the filter, any valve already there

What to compare on the spec sheet

Pipe: the material, outside diameter, PN class, the standard it is made to, the temperature the class holds at, and the friction table
Fittings: made for your pipe's material and outside diameter, and rated at or above the pipe's class
Threads: the size and the standard, BSP or NPT, and whether taper or parallel
Zone valves: the size, the pressures and flows they work at, and the controller voltage they need
Regulators: the pressure they give out, the flows they handle, and the most pressure they can take in
Air valves: the class they are rated for; FAO's handbook says they are made for at least PN 10
Warranty: what each part's warranty covers, for how long, and who holds spare parts

Sources: FAO (2007), Handbook on Pressurized Irrigation Techniques, chapter 3, gauges and air valves; FAO (2007), Handbook on Pressurized Irrigation Techniques, chapter 3, emitters and automation.

When to ask for site-specific sizing

The main is long, the ground rises or falls along it, or it crosses a road
The pump's shut-off head is close to the class of the pipe you plan
Several zones of different sizes share one main
New pipe must join old pipe of unknown size, class or thread
Valves shut fast on a long main, or the pump can stop suddenly when the power fails

Check before ordering

Every pipe and fitting is the same material and outside diameter, or the adaptor between them is named by both ends
Every part is rated at or above the class the main needs, after allowing for heat
Every thread is the same size and standard, and taper or parallel
No pipe carries more than the largest recommended flow for its size
Each zone valve suits the controller and the zone's flow
Each drip zone has a regulator set for its drip line, with a filter ahead of it
There are air valves at the high points and at the ends of long lines, and a non-return valve at the pump
There are spare fittings and a repair coupling for each pipe size

Trademarks belong to their owners. ASFA IRRIGATION NG LIMITED is an independent company; no partnership is implied.

Valves and automation Water and filtration in the store Drip irrigation

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Choosing pipes, valves and pressure regulators.