Pumps - An Introduction

What is a pump ? How are pumps classified ?

A Pump is a mechanical device which moves liquid from a lower level to a higher level.  The pump draws the liquid inside pressurizes it and discharges it through the outlet.  A pump is driven by a prime mover which is, generally, an electric motor.  IC engines and turbines can also be used as prime movers to drive the pump.

Pumps are usually classified into two broad categories

Rotodynamic pump and
Positive Displacement Pumps

Rotodynamic Pumps

In these pumps, a rotary device with blades, called the impeller drives the liquid.  The liquid gets kinetic energy in the process.  The kinetic energy is converted into pressure by means of the design of the pump.

The rotodynamic pumps can be divided into

Centrifugal pumps : Here, the impeller with blades drives the liquid radially outwards towards the casing.  The liquid gets pressurized as it exits the pump.

Axial Pumps:  In these pumps, the liquid is driven axially by the impeller.  The flow of the liquid is parallel to the axis of the impeller.

Positive Displacement Pumps

Positive Displacement pumps are another major category of pumps.  In positive displacement pumps, the liquid is drawn into a chamber, pressurized and expelled at the discharge side.

These pumps are in turn classified into two types

Reciprocating Pumps: In these pumps, a piston moves inside a cylinder.  The piston creates low pressure when it moves up.  This sucks the liquid inside.  Once inside, the piston moves down and pressurizes the liquid which is discharged through a port.  The handpump used to pump water is a reciprocating pump. Eg. Plunger Pump

Rotary Pump: In these types of pumps, two rotating gears or screws move inside a casing.  As the screws or the gears move, the liquid is progressively taken into the pump.  The cross section of the casing is reduced as the liquid moves.  This causes pressure at the discharge side.  Examples: Screw Pumps, Gear Pumps

Rotodynamic Pump

A rotodynamic pump is a pump in which the impeller imparts kinetic energy to the fluid. The term Rotodynamic is a broad one encompassing all pumps with rotary impellers.

Centrifugal pumps are a type of rotodynamic pumps.  The impeller of the centrifugal pump draws in water from the suction and pushes the water radially giving kinetic energy to the liquid.

Apart from centrifugal pumps, axial flow pumps in which the water flows radially, parallel to the axis of the shaft, are also called rotor dynamic pump.

Positive displacement pump ?

A positive displacement pump is a pump which draws a fixed amount of the liquid from the inlet and discharges it in the outlet at high pressure.

Positive displacement pumps have an expanding cavity in the inlet and a decreasing cavity near the inlet.  Positive displacement pumps have constant volume.  The pumps deliver a constant flow regardless of the discharge pressure.  The pressure depends on the speed of the pump.

Positive displacement pumps can be further classified into reciprocating pumps, rotary pumps, etc.

Positive displacement pumps should never be operated with the outlet closed. Since the pump works on a fixed volume of liquid,  the pump can get seriously damaged if it is accidentally operated with the outlet closed.

A special pressure relief valve is provided for protection against excess pressure.

Image courtesy: commons.wikimedia.com

A comparison of Centrifugal and Positive Displacement Pumps


Priming
Centrifugal pumps need to be primed separately.  The priming can be manual or through a separate priming arrangments

Positive displacement pumps are self priming as they develop a low pressure which can draw the fluid inside.

Flow Rate
Centrifugal Pumps have a flow rate which is dependent on the discharge pressure.  Positive Displacement pumps have a constant flow rate regardless of the pressure

Viscous Fluids
Centrifugal pumps cannot handle viscous fluids due to increased friction between the impeller and the liquid.  Positive displacement pumps can handle viscous fluids.

Efficiency
Centrifugal pumps have lower efficiency as the viscosity increases. Positive displacement pumps have high efficiency as the viscosity increases

Method of operation
Centrifugal pumps build pressure by imparting velocity to the liquid and then converting it into pressure.  Positive displacement pumps develop pressure by drawing a fixed amount of liquid and pressurizing it.

What are the different parts of the Centrifugal Pump?

The centrifugal pump consists of the following main parts.

The Impeller
The Impeller is the heart of the pump.  The impeller provides kinetic energy to the water entering the pump from the suction pipe.

The Volute
The volute refers to the tubular casing of the pump which increases in size as it approaches the discharge port.  The function of the volute is to convert the velocity of the water from the impeller into pressure.  It achieves this by a gradual increase in volume.

The Suction Pipe
The Suction pipe connects the sump to the pump inlet.

The Foot Valve
The foot valve is a non-return valve which is connected on the suction side.  The foot valve prevents the flow of water from the overhead tank which is at a higher level to the sump when the pump is not running.

The Strainer
The Strainer prevents the entry of debris into the pump

The Delivery Pipe
The Delivery Pipe serves to supply water to the tank from the discharge side of the pump.

The Delivery Valve
The delivery valve is a valve at the output of the pump in the delivery line.  The function of this valve is to control the output of the pump.  The delivery valve is closed when the pump is first started during the priming process.  It is then gradually opened.