Environmental Engineering Reference
In-Depth Information
Systems may require multiple pumps, placed either in parallel or in series. Par-
allel pumps are effective at increasing system discharge in low-friction, low-loss
systems. Series pumps typically provide only modest increases in flow rate but
significantly increase the head for high-friction or high-static lift systems.
When trying to match pumps to piping systems with variable head-discharge
requirements, variable-speed pumps should be considered for installations when
a wide range of flow conditions is required. The variable-speed drive can oper-
ate over a significant range of motor speeds. Variable-speed drives offer several
advantages. Single-speed drives start the motor abruptly, subjecting the rotat-
ing element to high torque and electrical current surges several times the full
load current, along with possible transient and vibration problems. In contrast,
variable-speed drives provide a soft-start, gradually ramping the motor to oper-
ating speed.
One of the primary justifications for using a variable-speed drive is the cost
savings resulting from reduced power demands and reduced maintenance of the
motor, pump, and discharge control valve. With constant-speed pumps, flow
reduction in a given system can only be achieved by throttling the control valve.
This is somewhat analogous to operating an automobile at full throttle and trying
to control speed by applying the brake. The excess pressure drop across the
valve to reduce the flow rate results in wasted energy and creates the possibility
of cavitation.
Additional details regarding series and parallel pump selection and perfor-
mance, pump cavitation, pump operation, and suction piping considerations are
provided by Tullis (1989, 1993).
7 OTHER PIPELINE OPERATION CONSIDERATIONS
There are many details that need consideration in pipeline design. In addition to
the items already discussed, a few additional design considerations are briefly
discussed.
7.1 External Loads
There are situations where the external load is the controlling factor in deter-
mining the risk of pipe collapse. The magnitude of the external load and the
resistance of the pipe to collapse depend on numerous factors, including inter-
nal pressure, pipe diameter, pipe material and wall thickness, pipe deformation
(deviation from a circular cross section), trench width, depth of cover, specific
weight of the soil, degree of soil saturation, type of backfill material, method
used to backfill, degree of compaction, and live loads. The cumulative effect of
all these sources of external loading requires considerable study and analysis,
beyond the scope of this chapter. There are no simple guidelines for evaluating
external pipe loads; the reader is referred to (Watkins and Anderson 2000) and
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