Page 21 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

and pressure is added to the liquid (again Bernoulli’s Principle). The
liquid leaves the pump at discharge pressure, prepared to overcome the
resistance in the system.
The flow from a centrihgal pump is mostly governed by the speed of
the driver and the height of the impeller blades. The pressure or head
that the pump can generate is mostly governed by the speed of the
motor and the diameter of the impeller. Other factors play a lesser role
in the pump’s flow and pressure, like the number, pitch, and thickness
of the impeller blades, the internal clearances, and the presence and
condition of the wear bands.
In simple terms, we could say that PD pumps perform work by
manipulating the available space inside the pump. Centrihgal pumps
perform work by manipulating the velocity of the fluid as it moves
through the pump. There is more on this in Chapter 6.

Pressure measurement
Force (F) is equal to Pressure (P) multiplied by the Area (A):

F=P xA.

F
Pressure is equal to the Force divided by the Area: P = -
A
If we apply pressure to the surface of a liquid, the pressure is
transmitted uniformly in all directions across the surface and even
through the liquid to the walls and bottom of the vessel containing the
liquid (Pascal’s Law). This is expressed as pounds per square inch
(lbs/in2, or psi), or kilograms per square centimeter (k/cm2).

Atmospheric pressure (ATM)

Atmospheric pressure (ATM) is the force exerted by the weight of the
atmosphere on a unit of area. ATM = 14.7 psia at sea level. As
elevation rises above sea level, the atmospheric pressure is less.

Absolute pressure (psia)
__
Absolute pressure is the pressure measured from a zero pressure
reference. Absolute pressure is 14.7 psia at sea level. Compound
pressure gauges record absolute pressure.

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