SUBJECT CODE NAME OF SUBJECT TOPIC :::::: 151903 FLUID POWER ENGINEERING CENTRIFUGAL PUMP Parul...

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TOPIC

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151903FLUID POWER ENGINEERING

CENTRIFUGAL PUMP

Parul Institute of Engineering & Technology

Centrifugal pumps

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

FLUID POWER ENGINEERING CENTRIFUGAL PUMP

Centrifugal pumps - theory and characteristics

Centrifugal pump duties are usually for the movement of large volumes of liquid at low pressures, although higher pressures can be achieved with multi-staging.

Energy input

Energy transformations inside the Dump

Velocity and pressure levels Fluid flow



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.



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.



The remaining energy transformations take place inside the pump itself. The rotating pump shaft is attached to the pump impeller, which rotates in a volute housing.

The impeller imposes a centrifugal force, which imparts kinetic energy to the fluid. Kinetic energy is a function of mass and velocity (K.E = $Sv7.Raising a liquids velocity increases its kinetic energy.



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.


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 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 ...



When the pressure falls below the vapour pressure of the liquid at a given temperature, boiling occurs and small bubbles of vapour are formed.



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 - 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.


Centrifugal Pumps - Central Priming System


Centrifugal Pumps - Central Priming System

Where several pumps require a priming aid, for example a cargo pumping system or a number of engine room pumps, a central priming system is often used

This reduces the number of priming pumps, saving spares, maintenance and cost.

With this system a central priming unit consisting of two or more liquid ring primer pumps is arranged to pull a vacuum on a central tank.The tank has connections to float chambers in each of the suction lines for the system pumps isolated by either manually operated or solenoid operated valves.


Centrifugal pumps - losses

When assessing the amount of power needed to operate a centrifugal pump you must always take into account the various losses.


Centrifugal pumps - losses A characteristic curve for a centrifugal pump is obtained by operating the pump at rated speed with the suction open and the discharge valve shut. The discharge valve is then opened in stages to obtain differentdischarge rates and total head corresponding to them. The data can then be represented graphically as a curve.


Centrifugal pumps - losses Losses can be caused by:

Capacity reduction Excessive vibration Failure to deliver caused by loss of suction may be due to Failure to deliver Insufficient supply head Air leakage at suction pipe (e.g. valve open to empty bilge, etc) Loss of priming facility or leaking shaft gland


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SUBJECT CODE NAME OF SUBJECT TOPIC :::::: 151903 FLUID POWER ENGINEERING CENTRIFUGAL PUMP Parul...

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