Parul Institute of Technology Subject Code : 151903 Name Of Subject : Fluid Power Engineering Name...

Parul Institute of Technology

Subject Code : 151903

Name Of Subject : Fluid Power Engineering

Name of Topic : Axial Flow Pump & Centrifugal Pump

Axial-flow Pumps

Introduction An axial-flow pump uses a screw propeller to axially accelerate the liquid. The outlet passages and guide vanes are arranged to convert the velocity increase of the liquid into a pressure as distinct from the centrifugal pump, the axial flow pump absorbs the maximum power at zero flow.

Sub: Flow through pipes Topic : Axial flow pump & centrifugal pump

Axial-flow Pumps

A mechanical seal prevents leakage where the shaft leaves the casing. A thrust bearing of the tilting pad type is fitted on the drive shaft. The prime mover may be an electric motor or a steam turbine.


Axial-flow Pumps


Axial-flow Pumps

The axial flow pump is used where large quantities of water at a low head are required or example in condenser circulating. The efficiency is equivalent to a low lift centrifugal pump, and the higher speed possible enable a smaller driving motor to be used.


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.



Energy input• 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.



• 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 volute casing or diffuser is gradually increasing area which decrease the velocity and increase the pressure



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 fluid flow causes a vacuum to be formed in the pump suction, which will draw fluid into the impeller suction. Thus fluid flow will occur from the suction to discharge. The liquid enters the centre or 'eye' of the impeller axially, changes direction and flows radially out between the vanes.


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 ...



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 vaporization of the fluid is Cavitation.




• 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.




• To reduce Cavitation we must reduce the 'losses' on the suction side, hence reduce the pipeline friction and NPSH. This means reducing the pump flow rates. To reduce 'losses' on starting, the pump should be started against a closed discharge valve.


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.


Centrifugal pumps - selection

• Viscosity of the medium to be pumped, Type of medium, e.g. corrosive or non-corrosive, this

would affect the choice of material (although for salt and fresh water the difference is often just the casing).

• Materials for salt water could be, casing-gunmetal (cast iron for fresh water), impeller-aluminum bronze, shaft-stainless steel, casing bearing ring seals-leaded bronze.


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• - Friction loss in bearings and glands, surfaces of

impeller and casing. Some impellers are highly polished to minimize friction loss.

• - Head loss in pumps due to shock at entry and exit to impeller vanes and eddies formed by vane edges.

• - Leakage loss in thrust balance devices, gland sealing and clearances between cut water and casing and bearing seals.


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 different discharge rates and total head corresponding to them.


Centrifugal pumps – losses Capacity reduction could be the result of• A damaged sealing ring• Leaking gland• Obstruction (valve partly closed/foreign body)• Incorrect rotational speed Excessive vibration may be caused by• Loose coupling• Loose impeller• Bearing damaged• Impeller imbalance


Centrifugal pumps-component parts


Centrifugal pumps-component parts

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

• 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.


Centrifugal pumps - shaft sealing• To connect the motor to the impeller, the shaft has

to pass through an aperture in the casing.• To allow the shaft to rotate freely in the casing

aperture there needs to be a gap, but this gap needs to be closed off to stop air from being drawn in from atmosphere or liquid from leaking out during operation.

• There are two common methods. • Packing• Mechanical seal• The role of the pump, its speed and the type of

liquid being pumped all play a part in deciding which application works best.


Centrifugal Pumps - Double Entry Pumps



• The incoming liquid enters the double impeller from the top and bottom and passes into the volute casing for discharge in the same way as before. A double entry pump has a lower NPSH required characteristic, which will have advantages in poor suction conditions.

It should be noted that different impeller sizes could be fitted into a basic pumping element. This enables various discharge head characteristics to be provided for the same basic pump frame.

The larger pumps, which are double entry, can achieve flow rates of 10,000 tons/hour.



Illustration shows a cross-section through a double entry centrifugal pump.


Mixed flow pumps

In this type, the pressure is developed partly by centrifugal action and partly by the vanes and, as the name implies, the flow is both axial and radial through the impeller.


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Parul Institute of Technology Subject Code : 151903 Name Of Subject : Fluid Power Engineering Name...

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