Page 315 - Water Engineering Hydraulics, Distribution and Treatment

293
Problems/Questions
EXAMPLE 9.2 REQUIRED PRESSURE FOR A PRESSURE–LEAKAGE TEST
After the pipe is full of water for a pressure–leakage test, it should sit idle for at least 24 hours before the test start. Then the pressure
is brought up by a booster pump to a required pressure level at least 50% higher than the normal expected operating pressure, or
2
150 psi (1034 kPa or 10.5 kg/cm ), whichever is larger, and maintained for at least 4 hours. The normal expected operating pressure
of the water pipe to be tested is 115 psi (798.1 kPa). At what pressure should the pipe be tested?
Solution 1 (US Customary System):
The test pressure = (normal operating pressure) × 1.5 = 115 psi × 1.5 = 172.5 psi, which is greater than 150 psi. Therefore, the
selected test pressure is 172.5 psi.
Solution 2 (SI System):
The test pressure = (normal operating pressure) × 1.5 = 798.1 kPa × 1.5 = 1,197 kPa, which is greater than 1,034 kPa. Therefore,
the selected test pressure is 1,197 kPa.
EXAMPLE 9.3 PRESSURE–LEAKAGE TEST
A 24-hour pressure–leakage test is performed on 1,200 ft (365.76 m) of 12-in. (30.48-cm)-diameter ductile iron pipe with a test
pressure of 150 psi (1,034 kPa). The ductile iron pipes are 18 ft (5.4864 m) long each. According to AWWA, at the test pressure
2
of 150 psi (1041 kPa or 10.545 kg/cm ), the allowable leakage is 23.3 gpd/mi-in. (21.58 Lpd/km-cm) of ductile iron pipes for both
3
mechanical and push-on joints. During the 24-hour test period, 22 gallons (83.27 L = 0.08327 m ) of water were added to maintain
the 150 psi (1034 kPa) pressure. Did the pipe pass the pressure–leakage test?
Solution 1 (US Customary System):
L AD = Q ∕Ld = 22 gpd∕[1,200 ft (mi/5,280 ft) × 12 in.] = 8.07 gpd/mi-in.
D
Since L AD = actual leakage of ductile iron pipe = 8.07 gpd/mi-in., which was less than the allowable leakage rate of 23.3 gpd/
mi-in., the pipe passed the pressure–leak test.
Solution 2 (SI System):
L AD = Q ∕Ld = 83.27 Lpd∕[0.36576 km × 30.48 cm] = 7.469 Lpd/km-cm.
D
Since L AD = actual leakage of ductile iron pipe = 7.469 Lpd/km-cm, which was less than the allowable leakage rate of
21.58 Lpd/km-cm, the pipe passed the pressure–leak test.

PROBLEMS/QUESTIONS 9.7 Protection against sanitary deficiencies from cross-
connections is an important engineering consideration in water and
9.1 What is the definition of a cross-connection?
sewer system design. Visit the website www.10statesstandards.com
9.2 Why do we have cross-connections? to learn about the latest government regulations regarding the sep-
9.3 How many types of cross-connections are there? aration distance of drinking water mains crossing sewers.
9.4 What is the difference between backpressure and back- 9.8 How frequently should backflow preventers be tested to
siphonage? ensure their proper functioning? Who is qualified to perform back-
flow preventer testing? Consult with your local government for the
9.5 What types of devices can be used to correct cross-
local rules and regulations.
connections?
9.9 In addition to the many cross-connections that may exist
9.6 Successful control of cross-connection hazards depends not
on the premises of a water system’s customers, the water sys-
only on inspection for cross-connections by the water system and
tem itself may own or control cross-connections. These potential
by water users, but also on an enforceable cross-connection con-
cross-connections should be subject to the same scrutiny as those
trol program. If a community subscribes to a modern plumbing
that are privately owned. Examples of cross-connections that can
code, such as the National Plumbing Code, its provisions will gov-
pose a risk to water quality and public health can be found in
ern backflow and cross-connections. However, the water system
must obtain authority to conduct a community inspection program water treatment plants, pumping stations, or the distribution sys-
through an ordinance or other means and carry out a comprehensive tem. During a sanitary survey for a water treatment system, what
are the potential cross-connections that should be identified by an
program. Please outline the major components of a water system’s inspector?
cross-connection control program.