Page 185 - Water Engineering Hydraulics, Distribution and Treatment

specified level, in order to avoid pressures and flows that
could otherwise have undesirable effects on the system. A
The effect of temperature changes is small if pipe joints per-
pressure or a hydraulic grade is used to control the operation
mit adequate movement. Steel pipe laid with rigid transverse
of the valve.
joints must either be allowed to expand at definite points or
be rigidly restrained by anchoring the line.
5.9.6 Pressure-Sustaining Valves
5.9.13 Anchorages
Pressure-sustaining valves maintain a specified pressure
upstream of the valve. Similar to the other regulating valves,
Anchorages are employed (a) to resist the tendency of pipes
they are often used to ensure that pressures in the system
to pull apart at bends and other points of unbalanced pressure
(upstream, in this case) will not drop to unacceptable lev-
when the resistance of their joints to longitudinal stresses is
els. A pressure or a hydraulic grade is used to control the 5.9.12 Expansion Joints 5.9 Appurtenances 163
exceeded, (b) to resist the tendency of pipes laid on steep
operation of a pressure-sustaining valve. gradients to pull apart when the resistance of their joints
to longitudinal stresses is inadequate, and (c) to restrain or
direct the expansion and contraction of rigidly joined pipes
5.9.7 Pressure Breaker Valves under the influence of temperature changes.
Anchorages take many forms as follows:
Pressure breaker valves create a specified head loss across
the valve and are often used to model components that cannot
1. For bends—both horizontal and vertical—concrete
be easily modeled using standard minor loss elements.
buttresses or kick blocks resisting the unbalanced
pressure by their weight, much as a gravity dam
resists the pressure of the water behind it, taking into
5.9.8 Flow Control Valves
consideration the resistance offered by the pipe joints
A flow control valve limits the flow rate through the valve themselves, by the friction of the pipe exterior, and
to a specified value in a specified direction. A flow rate is by the bearing value of the soil in which the block is
used to control the operation of a flow control valve. These buried
valves are commonly found in areas where a water district 2. Steel straps attached to heavy boulders or to bedrock
has contracted with another district or a private developer to
3. Lugs cast on pipes and fittings to hold tie rods that
limit the maximum demand to a value that will not adversely
prevent movement of the pipeline
affect the provider’s system.
4. Anchorages of mass concrete on steel pipe to keep
it from moving, or to force motion to take place at
expansion joints inserted for that purpose—the pipe
5.9.9 Throttle Control Valves
being well bonded to the anchors, for example, by
Throttle control valves simulate minor loss elements whose angle irons welded onto the pipe
head loss characteristics change over time. With a throttle 5. Gate chambers so designed of steel and concrete that
control valve, the minor loss K is adjusted based on some they hold the two ends of steel lines rigidly in place.
other system flow or head.
In the absence of expansion joints, steel pipe must be
anchored at each side of gates and meters in order to pre-
5.9.10 Manholes vent their destruction by pipe movement. In the absence of
anchors, flanged gates are sometimes bolted on one side to
Access manholes are spaced 1,000–2,000 ft (300–600 m)
the pipe—usually the upstream side—and on the other side
apart on large conduits. They are helpful during construc-
to a cast iron nipple connected to the pipe by means of a
tion and serve later for inspection and repairs. They are less
sleeve or expansion joint.
common on cast iron lines than on steel and concrete lines.
5.9.14 Other Appurtenances
5.9.11 Insulation Joints
Other appurtenances that may be necessary include the fol-
Insulation joints control electrolysis by introducing resis- lowing:
tance to the flow of stray electric currents along pipelines.
Modern insulation joints make use of rubber gaskets or rings 1. Surge tanks at the end of the line to reduce water
and of rubber-covered sections of pipe sufficiently long to hammer created by operation of a valve at the end of
introduce appreciable resistance. the line