DARLEY ZSP, Nstallation, Operation, Maintenance, Repair And Troubleshooting Instructions

Page: 31 / 155

Prepared by: EAP Rev. #:1
Approved by: MCR
2
Date: 1/29/07
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 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.
Slip is the greatest factor affecting efficiency of a displacement pump, and this factor is greatly
influenced by the condition of and wears on the working parts.
IF FURTHER INFORMATION IS NEEDED, CALL W.S. DARLEY & CO. AT
CHIPPEWA FALLS, WI. AT 800-634-7812 or 715-726-2650