Page 182 - Water Engineering Hydraulics, Distribution and Treatment

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Chapter 5
Water Hydraulics, Transmission, and Appurtenances
5.8.3 Durability
section to be tested. The pipe is then filled with water and
Experience with all but coated ductile iron pipe (length of life
placed under pressure, the water needed to maintain the pres-
100 years) has been too short and changes in water treatment
sure being measured by an ordinary household meter. Where
have been too many to give us reliable values on the length
there is no water, air may be substituted. Losses are assumed
of life of different pipe materials. The corrosiveness of the
to vary with the square root of the pressure, as in orifices.
water, the quality of the material, and the type and thickness
The allowable leakage of bell-and-spigot cast iron pipe
of protective coating all influence the useful life of the various
that has been carefully laid and well tested during construc-
types of water pipes. External corrosion (soil corrosion) is
tion is often set at
important, along with internal corrosion. Pipes laid in acid
nd p
√
soils, seawater, and cinder fills may need special protection.
Q =
(US customary units)
(5.43a)
1,850
where Q is the leakage, gal/h; n is the number of joints in the
5.8.4 Transportation and placing a temporary header or plug at the end of the
length of line tested; d is the nominal pipe diameter, in.; and
When pipelines must be built in rugged and inaccessible
p is the average pressure during test, psig. A mile (1.6 km)
locations, their size and weight become important. Ductile
of 24 in. (600 mm) cast iron pipe laid in 12 ft (3.6 m) lengths
iron pipe is heavy in the larger sizes; steel pipe relatively
and tested under a pressure of 64 psig (444 kPa), for example,
much lighter. The normal laying length of cast iron pipe is
can be expected to show a leakage of
12 ft (3.7 m). Lengths of 16.4, 18, and 20 ft (5, 5.5, and 6 m)
√
are also available in different types of bell-and-spigot pipe. Q = (5,280∕12) × 24 × 64∕1,850 = 46 gal∕h (174 L∕h).
The length of steel pipe is 20–30 ft (6–9 m). Both prestressed
Considering that the pipe has a carrying capacity of
and cast reinforced concrete pipe are generally fabricated in
250,000 gal/h (946,000 L/h) at a velocity of 3 ft/s (0.90 m/s),
the vicinity of the pipeline. The sections are 12 and 16 ft
the expected leakage from joints (46 × 24 gal/day)∕24 in./
(3.6 and 4.9 m) long and very heavy in the larger sizes. A
mi = 46 gal/day/in./mi, which is relatively small.
diameter smaller than 24 in. (600 mm) is unusual.
The equivalent metric leakage equation using the SI units
is as follows:
5.8.5 Safety √
nd p
Breaks in cast iron pipes can occur suddenly and are often Q = 32,500 (SI units) (5.43b)
quite destructive. By contrast, steel and reinforced con-
where Q is the leakage, L/h; n is the number of joints in the
crete pipes fail slowly, chiefly by corrosion. However, steel
length of line tested; d is the nominal pipe diameter, mm;
pipelines may collapse under vacuum while they are being
and p is the average pressure during test, kPa. For the same
drained. With proper operating procedures, this is a rare
example introduced above, the expected leakage calculated
occurrence. Fiberglass pipe fails suddenly, much like cast
using SI units is
iron pipe.
[ ]
√
Q = (1.6 × 1,000∕3.6) × 600 444 ∕32,500 = 173 L∕h.
5.8.6 Maintenance
The expected leakage from joints is equal to
Pipelines of all sizes and kinds must be watched for leakage (173 × 24 L∕day) ∕600 mm∕1.6km = 4.32 L/day/mm/km.
or loss of pressure—outward signs of failure. There is little
choice between materials in this respect. Repairs to precast 5.9 APPURTENANCES
concrete pipe are perhaps the most difficult, but they are
rarely required. Cast iron and small welded-steel pipes can To isolate and drain pipeline sections for test, inspection,
be cleaned by scraping machines and lined in place with cleaning, and repairs, a number of appurtenances, or aux-
cement to restore their capacity. New lines and repaired lines iliaries, are generally installed in the line (Figs. 5.25 and
should be disinfected before they are put into service. 5.26).
5.8.7 Leakage 5.9.1 Gate Valves
All pipelines should be tested for tightness as they are con- Gate valves are usually placed at major summits of pressure
structed. Observed leakage is often expressed in gal/day/in. conduits because (a) summits identify the sections of line
diameter (nominal)/mile of pipe using the US customary that can be drained by gravity and (b) pressures are least at
units, or in L/day/mm diameter (nominal)/km of pipe using these points, making for cheaper valves and easier operation.
the SI units. The test pressure must naturally be stated. To For the sake of economy, valves smaller in diameter than the
conduct a leakage test, the line is isolated by closing gates conduit itself are generally installed together with necessary