Dr. subhash technical campus

Centrifugal pumps

In this part of the lesson we will take a closer look at Centrifugal pumps

Centrifugal pumps - theory and characteristics

A centrifugal pump can be further defined as a machine which uses several energy transformations in order to increase the pressure of a liquid. The energy input into the pump is typically the fuel source energy used to power the driver.

Centrifugal pumps - theory and characteristics

Energy input Most commonly, this is electricity used to power an electric motor. Alternative forms of energy used to power the driver include high-

pressure steam used to drive a steam turbine. Fuel oil used to power a diesel engine. High-pressure hydraulic fluid used to power a hydraulic motor. Compressed air used to drive an air motor. Regardless of the driver type for a centrifugal pump, the input energy

is converted in the driver to a rotating mechanical energy, consisting of the driver output shaft, operating at a certain speed, and transmitting a certain torque, or horsepower.

Centrifugal pumps - theory and characteristics

Velocity and pressure levels Fluid flow

A particular feature of centrifugal pumps is that the power absorbed is a minimum at zero flow, and therefore can be started up against a closed valve.By increasing the size of the impeller, and/or the speed of pump rotation, we can achieve larger pumping rates.

The diagram illustrates that velocity and pressure levels vary as the fluid moves along the flow path in a centrifugal pump.

Centrifugal pumps - theory and characteristics

Consequently, no pumping action can occur unless this non-condensable gas is first eliminated, a process known as priming the pump.

Hence we need a fluid flow through the impeller in order to achieve a vacuum. Thus when these pumps are not primed, or loose suction during operation they will not self-prime themselves. In order to prime or re-prime these pumps we can use a priming system

If vapours of the liquid being pumped are present on the suction side of the pump, this results in Cavitation, which can cause loss of prime or even serious damage to the pump.

Centrifugal Pumps - Cavitation

When the pressure falls below the vapour pressure of the liquid at a given temperature, boiling occurs and small bubbles of vapour are formed. These bubbles will grow in the low-pressure area and implode when they are transported to an area of pressure above vapour pressure. The term given to this local vaporisation of the fluid is Cavitation.

Centrifugal Pumps - Cavitation

The collapsing of the bubbles is the area of Cavitation we are concerned with, as extremely high pressures are produced, which causes noise and erosion of the metal surface.

The area of pipeline ...

This cavitation effect...

To reduce cavitation ...

Click on an item to jump to it.

Centrifugal Pumps - Cavitation

When the pressure falls below the vapour pressure of the liquid at a given temperature, boiling occurs and small bubbles of vapour are formed. These bubbles will grow in the low-pressure area and implode when they are transported to an area of pressure above vapour pressure. The term given to this local vaporisation of the fluid is Cavitation.

Centrifugal Pumps - Cavitation

The collapsing of the bubbles is the area of Cavitation we are concerned with, as extremely high pressures are produced, which causes noise and erosion of the metal surface.

The area of pipeline where Cavitation mainly occurs is the pump suction, where the liquid is subjected to a rapid rise in velocity, and hence a fall in static pressure.

Centrifugal Pumps - Cavitation

When the pressure falls below the vapour pressure of the liquid at a given temperature, boiling occurs and small bubbles of vapour are formed. These bubbles will grow in the low-pressure area and implode when they are transported to an area of pressure above vapour pressure. The term given to this local vaporisation of the fluid is Cavitation.

The collapsing of the bubbles is the area of Cavitation we are concerned with, as extremely high pressures are produced, which causes noise and erosion of the metal surface.

Centrifugal Pumps - Cavitation

This Cavitation effect on the pump can cause damage on the casing and impeller.

During Cavitation, a liquid/vapour mixture of varying density is produced.

This results in fluctuations in pressure (caused by the liquid column being drawn in), and causes fluctuations in the discharge pressure, pump power absorbed (shown on the ammeter), and hence pump revolutions.

Centrifugal Pumps - Cavitation

When the pressure falls below the vapour pressure of the liquid at a given temperature, boiling occurs and small bubbles of vapour are formed. These bubbles will grow in the low-pressure area and implode when they are transported to an area of pressure above vapour pressure. The term given to this local vaporisation of the fluid is Cavitation.

Centrifugal pumps - Priming

Centrifugal pumps although suitable for most general marine duties,suffer in one very important respect; they are not self priming and require some means of removing air from the suction pipeline and filling it with the liquid.

Centrifugal pumps - Priming

Where the liquid to be pumped is at a higher level than the pump, opening an air release cock near the pump suction will enable the air to be forced out as the pipeline fills up under the action of gravity. This is often referred to as "flooding the pump".

Alternatively, an air-pumping unit can be provided to individual pumps or as a central priming system connected to several pumps.

The water ring or liquid ring primer can be arranged as an individual unit mounted on the pump and driven by it, or as a motor driven unit mounted separately and serving several pumps, known as a central priming system.

Centrifugal Pumps - Central Priming System

Centrifugal pumps-component parts

Centrifugal pumps-component parts

This is a vertical , single stage ,single entry , centrifugal pump for general marine use .

The mainframe and casing, together with a motor support bracket, house the pumping element assembly.

The volute casing is split in two halves along a vertical plane.

Since the suction and discharge nozzles are provided in the rear half of the casing, the rotating element can be taken out by removing only the front half casing without disturbing the rest of the pump.

Centrifugal pumps - component parts

The pumping element is made up of a top cover, a pump shaft, an impeller, a bearing bush and a sealing arrangement around the shaft.

The sealing arrangement may be a packed gland or a mechanical seal and the bearing lubrication system will vary according to the type of seal.

Replaceable wear rings are fitted in the casing around the top and bottom faces of the impeller.

The motor support bracket has two large apertures to provide access to the pumping element, and a coupling spacer is fitted between the motor and pump shaft to enable the removal of the pumping element without disturbing the motor or vice versa

Packing

Centrifugal Pumps - Double Entry Pumps