Pumps – Classification and General Concepts

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.

These 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: Here, the liquid is driven axially by the impeller. The flow of the liquid is parallel to the axis of the impeller.

Positive Displacement Pumps

Here, the liquid is drawn into a chamber, pressurized and expelled at the discharge side.

These are, in turn classified, into two types:

Reciprocating Pumps: Here, 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 lift water is a reciprocating pump. Eg. Plunger Pump

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

Capacity

The Capacity of the pump refers to the discharge rate of the pump at a specific head.

It is generally expressed in cubic metres per hour, second or minute. Sometimes, it is also represented in litres per second or hour.

Specific Speed

The Specific Speed is a method of describing a pump by parameters such as the speed, head and flow rate. The Specific Speed helps in deciding the type of pump for a specific application. It also helps in comparing different types against a single requirement.

The Specific Speed is the speed at which a geometrically similar pump would have to run to deliver a flow rate of 1 liter of fluid at a head of 1 meter.

A single stage centrifugal pump consists of an impeller and a volute. The liquid is drawn inside at the centre of the impeller and discharged radially.

Multistage centrifugal pumps have more than one impeller and volute. A three stage multistage pump will have three impellers and three volutes. The output of the first impeller will be fed to the input of the second impeller and so on.

The multistage pump can generate more pressure than a single stage pump.

For a given output pressure, the multistage pump will have impellers of smaller diameter. The efficiency of a multistage pump will also be greater than a single stage pump.

It is possible to retrofit a single stage pump assemble with a multistage assembly with minimal rework in the piping.

In certain multistage pumps, there are provisions to by pass an impeller. This may be useful for applications such as in firefighting where the pressure of the liquid will be different at different situations (based on the height of the building). In such cases, the last impeller may be bypassed and the output can be drawn before the last impeller.

Multistage pumps will have more vibration than single stage pumps.