Working mechanism of Pump
A centrifugal pump is one of the simplest pieces of equipment in any process plant; however, plant engineers consider it as the heart of their plant. A centrifugal pump indeed is one of the most critical equipments ensuring the continuity of any process. Its purpose is to transfer the desired liquid to the desired point in the desired quantity.
The pump delivers the flow by converting energy of a prime mover (an electric motor or turbine) first into velocity or kinetic energy and then into pressure energy of a fluid that is being pumped.
The energy changes occur by virtue of two main parts of the pump, the impeller and the volute or diffuser. The impeller is the rotating part that converts driver energy into the kinetic energy. The volute or diffuser is the stationary part that converts the kinetic energy into pressure energy.
Thus, by creating resistance to flow (pressure or head) the centrifugal pump delivers the required flow.
GENERATION OF CENTRIFUGAL FORCE
The process liquid enters the suction nozzle and then into eye (centre) of a revolving device known as an impeller. When the impeller rotates, it spins the liquid sitting in the cavities between the vanes outward and provides centrifugal acceleration. As the liquid leaves the eye of the impeller, a low-‐pressure area is created causing more liquid to flow towards the inlet. Because the impeller blades are curved, the fluid is pushed in a tangential and radial direction by the centrifugal force. This force acting inside the pump is the same as the one that keeps water inside a bucket that is rotating at the end of a string.
ENERGY IMPARTED BY THE IMPELLER
Once the liquid enters the eye of the impeller, it moves outward along the impeller. By virtue of its rotation, the impeller imparts kinetic energy to the liquid as it moves outward along the impeller.
The liquid accelerates and its kinetic energy increases and at the outer tip of the impeller the liquid possesses the highest velocity. The key idea is that the energy created by the centrifugal force is kinetic energy. The amount of energy given to the liquid is proportional to the velocity at the edge or vane tip of the impeller. The faster the impeller revolves or the bigger the impeller is, then the higher will be the velocity of the liquid at the vane tip and the greater the energy imparted to the liquid.
CONVERSION OF KINETIC ENERGY TO PRESSURE ENERGY (HEAD)
The rapidly moving liquid leaves the pump impeller and the liquid enters the diffusing element of the pump (the volute in the casing). Here an increase in the cross-‐sectional area of the flow passage occurs, causing the liquid to slow down. The deceleration of the liquid in the diffusing element converts the kinetic energy of the liquid to pressure energy. Therefore, the head (pressure in terms of height of liquid) developed is approximately equal to the velocity energy at the periphery of the impeller expressed by the following well-‐known formula:
After understanding the working principle of a centrifugal pump we now look at the basic mechanical design of a typical horizontal end-‐suction centrifugal pump.
Henceforth the term PUMP will always refer to CENTRIFUGAL PUMP.
The below figure depicts the operation of a typical end-‐suction back pullout horizontal centrifugal pump.