Product Manual A

Prepared by: AAD Rev. #:0
Approved by: TED 1 Date:10/18/01
1200510
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 valve. 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.
Slip is the greatest factor affecting efficiency of a displacement pump, and this factor is greatly influenced
by the condition of and wear on the working parts.
DEFINITIONS
HEAD OF WATER -- vertical depth of water measured in feet or in pressure per unit or area. In
hydraulics, head always represents pressure and it is expressed interchangeably in feet of water or
pounds per square inch and sometimes in inches of depth of mercury.
STATIC HEAD -- the pressure that is exerted by a stationary column of water of a given height or depth.
TOTAL HEAD OR TOTAL DYNAMIC HEAD -- the maximum height above the source of supply to
which the pump would elevate the water plus all the resistance to flow in the pipe or hose line.
DISCHARGE HEAD -- the pressure measured at the discharge outlet of a pump.
SUCTION HEAD -- the positive pressure measured at the suction entrance of a pump (when pumping
from an elevated tank or hydrant).
VELOCITY HEAD -- the equivalent pressure represented by fluid in motion as measured by means of a
Pitot Gage.
STATIC LIFT -- the vertical height of the center of the pump above the source of supply (when pump
from draft).
TOTAL SUCTION LIFT -- the static lift plus the friction in suction line plus entrance losses.
NET PUMP PRESSURE -- the total dynamic head of the pump.
EFFECTIVE NOZZLE PRESSURE -- the pump discharge pressure minus hose friction plus or minus the
difference in elevation above or below pump.