Centrifugal pumps acceleration

According to Bernoulli s Law, when velocity goes up, pressure goes down. This was explained in Chapter 1. A centrifugal pump works by acceleration and imparting velocity to the liquid in the eye of the impeller. Under the right conditions, the liquid can boil or vaporize in the eye of the impeller. When this happens we say that the pump is suffering from vaporization cavitation. 

A centrifugal pump is used to pump a liquid in steady turbulent flow through a smooth pipe from one tank to another. Develop an expression for the system total head A/t in terms of the static heads on the discharge and suction sides zd and zs respectively, the gas pressures above the tanks on the discharge and suction sides Pd and Ps respectively, the liquid density p, the liquid dynamic viscosity p, the gravitational acceleration g, the total equivalent lengths on... 

Diameter of centrifugal pump or length of stroke of reciprocating pump Earth s gravitational acceleration... 

Some process equipment such as reciprocating pumps, compressors, and conveyors must be started while fully loaded other drives such as centrifugal pumps, fans, and agitators have light loads at starting. Many applications require considerably more torque to start and accelerate the load than to run it the reverse is also true. Starting torque, maximum or breakdown torque, inertia of the load, and the accelerating time must be considered. 

A centrifugal pump is the ultimate in simplicity. A basic representation is shown in Figure 11-2. Liquid flows to the suction nozzle and into the impeller eye (center). The impeller usually rotates at 3,600 rpm. The liquid undergoes centrifugal acceleration. 

Cavitation in a centrifugal pump, which has a significant, negative effect on performance, is the most common failure mode. Cavitation not only degrades a pump s performance, but also greatly accelerates the wear rate of its internal components. 

This is an example of Bernoulli s equation in action. A steam vacuum ejector (jet) works in the same way. Centrifugal pumps and centrifugal compressors also work by converting velocity to pressure. Steam turbines convert the steam s pressure and enthalpy to velocity, and then the high velocity steam is converted into work, or electricity. The pressure drop we measure across a flow orifice plate is caused by the increase of the kinetic energy of the flowing fluid as it rushes (or accelerates) through the hole in the orifice plate. 

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