Water Management
Water is an important natural resource
that requires proper management.
Appropriate flow rate, pressure, and
water quality are necessary for
effective use.
Water System
WATER SUPPLY
TREATMENT
STORAGE
DISTRIBUTION
CONSUMER
Supply
Treatment
Storage
Consumer
35-50 psi
1 psi = 2.31 ft of water
Terminology
Static pressure (head):
No water flowing through the system
Head loss:
Pressure loss due to friction
Actual pressure:
Static pressure – head loss (all expressed in
psi)
Hazen-Williams Formula
1.85
hf = 10.44 * L * Q
1.85
4.8655
C
*d
Where:
hf = head loss due to friction (ft)
L = length of pipe (ft)
Q = flow rate of water through
the pipe (gpm)
C = Hazen-Williams constant
d = diameter of the pipe (in.)
Water Supply Problem
A business’s water supply connection at the
street is at elevation 1,257 ft and has a
pressure of 55 psi. The first floor fixture
elevation is 1,285 ft. Between the connection
and a first floor fixture, there are 120 ft of 3 in.
copper pipe, 25 ft of ¾ in. copper pipe, and five
(5) ¾ in. - 90o elbows. Discharge through the
fixture is 10 gpm. The equivalent length for a
single ¾ in., 90o elbow is 4 ft. The HazenWilliams constant for copper pipe is 140.
Water Supply Problem
What is the pressure loss due to elevation
change between the street connection
and the first floor fixture?
1st floor elevation – street elevation
1,285 ft – 1,257 ft = 28 ft
Convert to psi:
28 ft ÷ 2.31 ft/psi = 12.12 psi
Total Equivalent Length of ¾ in. Pipe
= Pipe length + fitting equivalent length
= 25 ft + (5 elbows * 4 ft/elbow)
= 45.00 ft
Head Loss in ¾ in. Pipe
Friction head loss using Hazen Williams
equation
1.85
hf = 10.44 * L * Q
1.85
4.8655
C
*d
1.85
hf = 10.44 * 45 ft * (10 gpm)
1.85
4.8655
(140)
* (3/4 in.)
hf = 14.44 ft
Head Loss in 3 in. Pipe
1.85
hf = 10.44 * L * Q
1.85
4.8655
C
*d
1.85
hf = 10.44 * 120 ft * (10 gpm)
1.85
4.8655
(140)
* (3 in.)
hf = 0.05 ft
Total Head Loss for ¾ in. & 3 in. Pipe
Total Head Loss (due to friction):
= Head loss, ¾ in. pipe + head loss, 3
in. pipe
= 14.44 ft + 0.05 ft
= 14.49 ft
Convert to psi:
14.49 ft ÷ 2.31 ft/psi = 6.27 psi
Water Supply Problem
What is the pressure in units of psi at the
first floor fixture?
Actual pressure = street pressure – static
difference – head loss
= 55 psi – 12.12 psi – 6.27 psi
= 36.61 psi