Page 314 - Water Engineering Hydraulics, Distribution and Treatment

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Chapter 9
Cross-Connection Control
and this shutoff valve cannot be operated from the ground
km-cm). Gpd/mi-in. stands for “gallons per day per
mile of pipe per inch of diameter,” while Lpd/km-cm
surface because it is buried and there is no valve box. New
stands for “liters per day per kilometer of pipe per
pipes should not be filled with water at a rate greater than 1
centimeter of diameter.”
foot (0.3048 m) of pipe length per second, because excessive
velocities could cause pipe movements and leaks. For an
existing water main with water flow, all air must also be
cially available is 18 ft (5.4864 m). At the test pres-
removed through corporation stops and hydrants before the
2
sure of 150 psi (1041 kPa or 10.545 kg/cm ), the
pressure and leakage testing starts.
allowable leakage is 23.3 gpd/mi-in. (21.58 Lpd/km-
cm) for both mechanical and push-on joints.
3. Plastic pipe: The pipe length is variable; there-
9.7.2 Pressure and Leakage Tests
fore, the allowable leakage is decided differently
from other types of pipes. At the test pressure of
After the pipe is full of water, it should sit idle for at least 2. Ductile iron pipe: The longest pipe length commer-
2
24 hours before the pressure and leakage tests start. Then 150 psi (1041 kPa or 10.545 kg/cm ), the allow-
the pressure is brought up by a booster pump to a required able leakage is 1.45 gal (5.4883 L) per hour for each
pressure level at least 50% higher than the normal expected 100 joints for a 6 inch (152.4 mm) plastic pipe. At the
2
operating pressure, or 150 psi (1034 kPa or 10.5 kg/cm ), same required test pressure, the allowable leakage is
whichever is larger, and maintained for at least 4 hours with 1.88 gal (7.1158 L) per hour for each 100 joints for
or without refilling the pipe with water. Any loss of water an 8 inch (203.2 mm) plastic pipe.
pressure would indicate leakage. In case the required pressure
level cannot be maintained for at least 4 hours, the leakage is Equation (9.7) can be used for the calculation of the actual
serious and the pressure test fails. No further leakage test is leakage of asbestos-cement and ductile iron potable water
needed. If any joints do show serious leakage, they must be pipes:
checked, adjusted, or repaired.
L = Q ∕Ld (9.7)
If the pressure test is successful, meaning that the AD D
required pressure level can be maintained for at least 4 hours,
where L AD = actual leakage of either asbestos-cement or
the leakage test is simultaneously conducted by measuring
ductile iron pipes, gpd/mi-in. or Lpd/km-cm; Q = leak flow
D
the amount of water needed to refill the pipe. If no refill
rate, gpd or Lpd; L = pipe length, mi or km; and d = pipe
water is needed during the 4-hour (or longer) successful
diameter, in. or cm.
pressure test, there is no pipe leakage. AWWA has speci-
Equation (9.8) can be used for the determination of the
fications for allowable small leakage, which are introduced
actual leakage of potable water plastic pipes:
in below:
L = Q ∕N (9.8)
H
P
1. Asbestos-cement pipe: The longest pipe length com-
mercially available is 13 ft (3.9624 m). At the test where L = actual leakage of plastic pipes, gph/100 joints
P
2
pressure of 150 psi (1041 kPa or 10.545 kg/cm ), or Lph/100 joints; Q = leak flow rate, gph or Lph; and
H
the allowable leakage is 30 gpd/mi-in. (27.78 Lpd/ N = number of 100 joints.
EXAMPLE 9.1 PRESSURE AND LEAKAGE TESTS PREPARATION
A 520-ft (158.5-m)-long section of a 12-in. (30.48-cm or 0.3048-m)-diameter water main is being filled with water for a pressure–
leakage test. If the water main is filled at a flow rate of less than 1 foot of pipe length per second (1 ft/s or 0.3048 m/s), what should
be the maximum flow meter reading for filling the water main?
Solution 1 (US Customary System):
2
Flow rate Q = Av = [0.785 × (1 ft) ] × (1 ft/s)
3
3
3
= 0.785 ft /s = (0.785 ft /s) (7.48 gal/ ft ) (60 s/min)
= 352 gpm.
3
For preparation of the pressure–leakage test, do not allow the flow meter’s reading to exceed 0.785 ft /s or 352 gpm.
Solution 2 (SI System):
3
2
Flow rate Q = Av = [0.785 × (0.3048 m) ] × (0.3048 m/s) = 0.0222 m /s = 1,334 L/min.
3
For preparation of the pressure–leakage test, do not allow the flow meter’s reading to exceed 0.0222 m /s or 1,334 L/min.