Te c h n i c a l
Newsletter
Special Issue
IN THIS ISSUE:
Hydraulics
Published by Rain Bird Corporation, Turf Products
Winter 2001
Hydraulics – The basics behind
strong irrigation design
Efficient, low-maintenance irrigation designs are based on a solid foundation of hydraulics.
Understanding hydraulics, or the study of fluid behavior at rest and in motion, is essential for
creating cost-effective systems that avoid overdesign while minimizing the potential for damage.
Hydraulics begins with an understanding of water. Water responds to gravity or seeks its own
lowest level and takes the shape of the container it is in. It is relatively incompressible. One
gallon of water weighs 8.3 pounds.
Water pressure is the force created by the weight of
water above the measurement point.
One gallon of
water weighs 8.3
pounds and water
has a specific
weight per cubic
foot of 62.4
pounds. One cubic
inch of water
weighs 0.036 lbs.
The formula for water pressure is:
P
=
PRESSURE
pounds per square inch
f o r c e = F (lbs.)
a r e a = A (in2)
Water pressure is measured in psi (pounds per
square inch). Water elevation is described in feet
of head. One foot of water elevation is equal to
.433 psi. One psi equals 2.31 feet of water elevation.
A container 1 inch
square and filled
with water to a
height of one footthe pressure (psi)
would equal:
When the area is constant, the force of water depends on
its elevation. The pressure of one square-inch of water is
the same, whether it’s held within a narrow container or
in one square-inch at the base of a one-foot deep lake.
Static and Dynamic Pressure
Irrigation hydraulics examine two types
of pressure measurements:
P = F = 0.036lb/ in 3 x 12in 3 = 0.433lb
A
1in x 1in
1in 2
• Static (hydrostatic) pressure – measures water at rest or water that
is experiencing no friction or pressure loss due to movement.
• Dynamic (hydrodynamic or working) pressure – measures water in motion
or water that is experiencing pressure loss due to friction as it moves
through pipes, fittings, and valves.
figure 1
continued on next page
C
continued from front
figure 2
10'
Falcon Rotor
10 GPM each
4.33
PSI
loss
10'
4.33
PSI
loss
A
1 1/2" SCH 40
PVC Pipe
49.68
PSI
20
GP
M
100-PEB valve
Inlet 60 PSI Static
49.68 PSI
43.21 PSI
37.03 PSI
43.21
PSI
10
GP
M
B
Inlet of Sprinkler A
Inlet of Sprinkler B
Inlet of Sprinkler C
37.03
PSI
1" SCH 40
PVC Pipe
4.72 PSI loss
2.14 PSI loss
50'
50'
30 GPM
20 GPM
10 GPM
Once water begins moving, pressure is lost
(friction loss) as the water flows through
pipes and by fittings, valves and other
components – all of which offer resistance.
This dynamic or working pressure varies
throughout the system. The amount of
water flowing through system and the
physical size of the path affect
friction loss. (see figure 2: In this
example, we also lose pressure
Size
due to the elevation gain
O.D.
I.D.
(.433 x 10' =4.33 psi loss).)
Wall Thk
Friction Loss in Hydraulics
Friction loss increases as the
flow or speed of water (water
velocity) through the system
increases. The flow rate is
measured in gallons per minute
(gpm or m3/hr or l/sec). The
industry standard maximum
velocity for irrigation piping is
five feet per second (fps or
1,5 m/sec). Any velocities above
this can cause water hammer or
damaging surges. This standard
is one key consideration when
sizing irrigation pipes.
While it’s possible to mathematically calculate friction loss using
a formula, it’s easiest to use pipe
pressure loss/velocity charts
from the manufacturers to
measure and calculate friction
loss and velocity. (See figure 3)
Before beginning the design, it is important
to know critical information about the site’s
water source. The designer must determine
the flow (in gpm or m3/hr or l/sec) available
for irrigation as well as the working pressure
in psi at the POC.
Find the chart for the type of piping you are
using. Charts show pressure losses for various
sizes of pipe and a wide range of flows.
Shaded areas show velocities that exceed
industry standards and should be avoided.
Velocity fps
psi loss
1
2
3
4
5
6
7
8
9
10
11
12
14
16
18
20
22
24
26
28
30
figure 3
1.05
2.11
3.16
4.22
5.27
6.33
7.38
8.44
9.49
10.55
11.60
12.65
14.76
16.87
18.98
21.09
0.43
1.55
3.28
5.60
8.46
11.86
15.77
20.20
25.12
30.54
36.43
42.80
56.94
72.92
90.69
110.23
Velocity fps
Calculating Water Meter Capacity
and Working Pressure
To determine the maximum system
working flow in gpm or m3/hr, follow these
three rules. Once you’ve completed all
three calculations, use the most restrictive
value (or the lowest flow of the three) as
your available flow:
1 The pressure loss through the water
meter should not exceed 10% of the
minimum static pressure available in the
1"
1.315
1.049
0.133
3/4"
1.050
0.824
0.113
Flow GPM
• service line length (from water
source to POC)
• average high/low pressure (Obtain this
from the purveyor. Remember to use the
lowest psi figure for summer daylight
hours or the worst case condition.)
Begin at the Source –
Determining the Water Supply
1/2"
0.840
0.622
0.109
• service line size and type (for instance,
whether it’s 3/4-inch K copper, or
3/4-inch sch. 40 galv., etc.)
• working water pressure (water running)
- if available when running a flow test
at the site.
1.85 PSI loss
100'
Static pressure is the starting point for any
hydraulic irrigation design. The designer
must know the static water pressure at the
point of connection (POC) or water source.
This can be obtained from a pressure reading or from the water purveyor. (see figure 1)
• water meter size
• static water pressure (no water running)
11/4" SCH 40 PVC Pipe
30 GPM
5.60 PSI
The designer should know:
psi Loss
Velocity fps
0.60
0.11
0.37
1.20
0.39
0.74
1.80
0.84
1.11
2.40
1.42
1.48
3.00
2.15
1.85
3.60
3.02
2.22
4.20
4.01
2.59
4.80
5.14
2.96
5.40
6.39
3.33
6.00
7.77
3.70
6.60
9.27
4.07
7.21
10.89
4.44
8.41
14.48
5.19
9.61
18.55
5.93
10.81
23.07
6.67
12.01
28.04
7.41
13.21
33.45
8.15
14.42
39.30
8.89
15.62
45.58
9.64
16.82
52.28
10.38
18.02
59.41
11.12
PVC SCHEDULE 40 PIPE / PSI LOSS PER 100 FEET OF PIPE
1 1/4"
1.660
1.380
0.140
1 1/2"
1.900
1.610
0.145
psi Loss
Velocity fps
psi Loss
Velocity fps
psi Loss
0.03
0.12
0.26
0.44
0.66
0.93
1.24
1.59
1.97
2.40
2.86
3.36
4.47
5.73
7.13
8.66
10.33
12.14
14.08
16.15
18.35
0.21
0.42
0.64
0.85
1.07
1.28
1.49
1.71
1.92
2.14
2.35
2.57
2.99
3.42
3.85
4.28
4.71
5.14
5.57
5.99
6.42
0.01
0.03
0.07
0.12
0.18
0.25
0.33
0.42
0.52
0.63
0.75
0.89
1.18
1.51
1.88
2.28
2.72
3.20
3.17
4.25
4.83
0.15
0.31
0.47
0.62
0.78
0.94
1.10
1.25
1.41
1.57
1.73
1.88
2.20
2.51
2.83
3.14
3.46
3.77
4.09
4.40
4.72
0.00
0.02
0.03
0.05
0.08
0.12
0.15
0.20
0.25
0.30
0.36
0.42
0.56
0.71
0.89
1.08
1.29
1.51
1.75
2.01
2.28
city water main.
PRESSURE LOSS THROUGH WATER METERS
This keeps heavy pressure loss from
occurring early in the system. Use the
manufacturer’s Water Meter Loss charts
to determine this. (See figure 4)
Pressure Loss: psi
Flow GPM
2 The maximum flow through the meter
for irrigation should not exceed 75% of
the maximum safe flow of the meter
(AWWA standard).
Again, the manufacturer’s charts will show
meter size and gpm flow rate.
3 The velocity of flow through the service
line should not exceed 5 fps or 1,5 m/s.
Using the proper pipe friction loss chart,
find the column and velocity that does
not exceed 5 fps.
By using the lowest flow of these three
values, you have the maximum system
working capacity.
Sizing The Pipes
Pipes should be sized so that the necessary
flow and pressure reach each sprinkler, while
allowing water to travel through the
piping at a safe velocity. Determining
the smallest size pipe to accomplish this
helps keep the job economical. Use the
manufacturer’s pipe friction loss/velocity
charts to select appropriate pipe sizes.
(See figure 3)
Before sizing the pipe, the designer
must determine the type of pipe and
length of the circuit.
Sizing the sprinkler lateral lines should
be done in reverse, starting with the
pipe segment that supplies water to
the sprinkler farthest from the valve
(See figure 5). Then, work backwards
or toward the valve, using the same
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
22
24
26
28
30
32
34
36
38
40
figure 4
5
⁄8"
0.2
0.3
0.4
0.6
0.9
1.3
1.8
2.3
3.0
3.7
4.4
5.1
6.1
7.2
8.3
9.4
10.7
12.0
13.4
15.0
3
⁄4"
0.1
0.2
0.3
0.5
0.6
0.7
0.8
1.0
1.3
1.6
1.9
2.2
2.6
3.1
3.6
4.1
4.6
5.2
5.8
6.5
7.9
9.5
11.2
13.0
15.0
(nominal size)
1"
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.4
1.6
1.8
2.0
2.2
2.8
3.4
4.0
4.6
5.3
6.0
6.9
7.8
8.7
9.6
1
1 ⁄ 2"
2"
In addition to sizing pipes, the designer
must also size valves and backflow
preventers using these guidelines:
0.4
0.5
0.6
0.7
0.8
1.0
1.2
1.4
1.6
1.8
2.1
2.4
2.7
3.0
3.3
• Valve pressure loss should not exceed
10% of the static pressure in the
irrigation mainline.
• Valves should be same size or no more
than one size smaller than the largest
lateral pipe they service.
• Valves should not be larger than largest
lateral pipe they each service.
0.8
0.9
1.0
1.2
1.3
Sprinkler
Water Pressure Loss Through Water Meter
charts from the manufacturer are set up much
like pipe flow loss chart. In this case, find the
pressure loss that is closest to, but does not
exceed 10%.
procedure to select pipe sizes that can
supply the necessary gpm to all down-
Add up the system pressure/friction
losses and add the pressure required at
the sprinklers on this valve. This is the
Circuit #1
1 3/4"
2"
1 1/4"
Sprinkler
Circuit #2
3/ "
4
Sprinkler
1 1/4"
Remote
Control
Valve
1 1/4"
Calculating System Pressure
Requirements
Now is the time – while the system is
still on paper – to make necessary
adjustments. To determine if the entire
system will work, select the worst case
valve circuit. This is the one with the
largest flow, with sprinklers requiring
the highest pressure, the one furthest
from the water source and at the
highest elevation.
Sprinkler
figure 5
Remember to add the total gpm flow
rate of all sprinklers in each zone that
the pipe supplies. This sizing procedure
follows the critical circuit length –
the longest path in the circuit that the
water must travel from the valve.
By using a split-length lateral circuit
design, you create mirror image
sections that share common pipe
sizes and can be more economical.
(See figure 6, back page)
Main
Remote
Control
Valve
stream sprinklers. The manufacturer’s
catalog shows the needed gpm flow rate
for various sprinklers.
Sprinkler
3/ "
4
Sprinkler
3/ "
4
Sprinkler
Sprinkler
answer is negative, it’s time to review
the design to see where to make
adjustments (to pipe sizes, valves
and sprinklers) to lower the overall
pressure loss.
total system pressure requirement.
Subtract this total system pressure from
the static pressure available at the water
source. If the number is positive, the
system should work as designed. If the
By thoroughly following the basic principles of hydraulics, designing cost-efficient, effective irrigation systems should
become easier and more dependable
both in theory and in the field.
figure 6
1/ "
2
3/ "
4
3/ "
4
1/ "
2
4
8
8
4
Main
4 GPM/head
Remote Control Valve
2 GPM/Head
6 GPM/head
10 GPM/head
1/ "
2
1/ "
2
3/ "
4
3/ "
4
2
4
6
3/ "
4
1"
1 1/4"
1"
3/ "
4
1/ "
2
8
16
12
8
4
1 1/4"
1 1/2"
2"
1 1/2"
1 1/4"
3/ "
4
12
18
24
32
24
16
8
1"
1 1/4"
1 1/2"
2"
2 1/2"
2"
1 1/2"
1"
10
20
30
40
48
36
24
12
6
8 GPM/head
3/ "
4
1 1/4"
1 1/4"
3/ "
4
8
16
16
8
4 GPM/head
8 GPM/head
12 GPM/head