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

STORAGE

TREATMENT

CONSUMER

DISTRIBUTION

Supply

Treatment

Storage

Consumer

1 psi = 2.31 ft of water

35-50 psi

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

hf = 10.44 * L * Q1.85

          C1.85 * d4.8655

 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 ProblemA 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 Hazen-Williams 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

hf = 10.44 * L * Q1.85

C1.85 * d4.8655

hf = 10.44 * 45 ft * (10 gpm)1.85

(140)1.85 * (3/4 in.)4.8655

hf = 14.44 ft

Head Loss in 3 in. Pipe

hf = 10.44 * L * Q1.85

C1.85 * d4.8655

hf = 10.44 * 120 ft * (10 gpm) 1.85

(140)1.85 * (3 in.) 4.8655

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