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WATER HAMMER -- a series of shock waves produced in a pipeline or pump by
a sudden change in water velocity. A sudden change in flow velocity can
result from rapid closure of valves. A pressure wave is set up which travels
back and forth in the water column at extremely high speed producing rapid
vibrations that may be violent and destructive if the water column is long.
THE MAXIMUM THEORETICAL LIFT of a pump is 34 feet, which is the
pressure of the atmosphere at sea level. The maximum practical total lift at
sea level is 20 to 25 feet (depending on the type and condition of the pump)
and this decreases with drops in barometric pressure.
OPERATING CHARACTERISTICS OF PUMPS
CENTRIFUGAL PUMPS: A centrifugal pump develops pressure by centrifugal
force of the liquid rotating in the impeller wheel. The pressure developed
depends upon the peripheral speed of the impeller (increasing as the square
of the speed) and it remains fairly constant over a wide range of capacities up
to the maximum output of the pump, if speed remains constant.
If the discharge outlet of a centrifugal pump is entirely shut off, with speed kept
constant, there is a small rise in pressure, the water churns in the pump
casing and the power drops to a low value. If the discharge is opened wide,
with little resistance to flow the pressure drops while the capacity and power
both increase to their maximum.
A centrifugal pump is an extremely simple mechanism mechanically, but rather
complex hydraulically; in that many factors enter into the design of the
impeller and water ways which will affect the pump's efficiency.
DISPLACEMENT PUMPS: Rotary and piston pumps are termed "Positive
Displacement" pumps because each revolution displaces or discharge
(theoretically) an exact amount of liquid, regardless of the resistance. The
capacity is, therefore, proportional to the number of revolutions of the pump
per minute and independent of the discharge pressure except as it is reduced
by "slip" (leakage past the pistons or rotors). For a given speed the power is
directly proportional to the head. If the discharge is completely shut off, the
pressure, power, and torque climb indefinitely until the drive power is stalled
or breakage occurs.
Prepared by: RJG
Approved by: TED
Revised by: TED
Rev. A
Date: 03/04/10
Rev. Date: 30June2015
1200627.doc
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