Marine Auxiliary Machinery. Chapter 1 Lesson 3 Rotodynamic pumps. By Professor Zhao Zai Li 05.2006. Learning objectives. After successfully completing this lesson, you will be familiar with: 1. Different types of rotodynamic pumps
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Chapter 1 Lesson 3
By Professor Zhao Zai Li
After successfully completing this lesson, you will be familiar with:
1. Different types of rotodynamic pumps
2. The component parts of rotodynamic pumps
3. Maintenance of rotodynamic pumps
The lesson will end with a test and your score will be recorded.
There are three different types of rotodynamic pumps:
Mixed flow pumps
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.
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.
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 fs possible enable a smaller driving motor to be used.
The axial-flow pump is also suitable for supplementary use in a
condense scoop circulating system, since the pump will offer little
resistance to flow when idling.
With scoop circulation, the normal movement of the ship will draw in
water; the pump would be in use only when the ship was moving slowly
or stopped. The pump is reversible and this, in conjunction with high
capacity flow, makes it suitable for trimming and heeling duties as well.
The pump casing is of gunmetal for condenser cooling duties and cast iron for heeling and trimming pumps. The impellers are of aluminium bronze and guide vanes of gunmetal are arranged immediately after the impeller, the pump shaft being of
Which part of the pump is the screw
Click on the screw propeller.
If you are not sure go to the previous screen to refresh your memory.
In this part of the lesson we will take a closer look at Centrifugal pumps
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 transformations inside the Dump
Velocity and pressure levels Fluid flow
Click on an item to jump to it.
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.
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.
This causes the fluid to accelerate out of the impeller with this increased velocity.
The volute casing or diffuser provides gradually widening passages i.e. an expansion of the flow area, which reduces the fluid velocity and this energy is converted into 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.
Velocity and pressure
levels Fluid flow
The diagram illustrates that velocity and pressure levels vary as the fluid
moves along the flow path in a centrifugal pump.
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.
If the pipeline leading to the pump inlet contains a non-condensable gas such as air, then the pressure reduction at the impeller inlet merely causes the gas to expand, and suction pressure does not force liquid into the impeller inlet
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.
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.
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.
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 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 selection of Centrifugal pumps depends mainly upon duty and the
The duty points are:
Flow and total head requirements. This will govern the speed of rotation, impeller dimensions, number of impellers and type e.g. single or, double inlet,
Range of temperature of fluid to be pumped. If suction capability is insufficient to accommodate supply conditions due for example to high inlet temperature Cavitation can occur
The selection of Centrifugal pumps depends mainly upon duty and the space available.
The space points are:
With vertically arranged pumps less floor space is required, this usually means that no hydraulic balance is necessary, impeller access is simple and no pipe joints have to be broken.
When assessing the amount of power needed to operate a centrifugal
pump you must always take into account the various 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.
The illustration shows the characteristic curves for three different types of pumps.
Losses can be caused by:
Failure to deliver caused by loss of suction may be due to
Capacity reduction could be the result of
Excessive vibration may be caused by
The provision of rotary shaft seals instead of the usual stuffing box and gland, where conditions are suitable, possesses many advantages. The power absorbed is lower and is constant, whereas a gland excessively tightened causes a considerable increase in power absorbed.
where is the packing material located?
Click on the packing material.
If you are not sure go to the previous
screen to refresh your memory.
Illustration shows a cross-section through a double entry centrifugal pump.
When the pump is due for overhaul, it will be necessary to dismantle it to its component parts to examine them for wear. The following procedures are intended as a general guide only, and your attention should be drawn to the manufacturer\'s operational instructions regarding specific pump requirements before commencing to dismantle the pump.
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.
1: Cavitation of the fluid in a centrifugal pump is caused by?
A) Too high a speed of impeller rotation creating adverse heat
B) The vapour pressure in the suction pipe falls below the vapour pressure of the liquid at a given temperature.
C) The viscosity of the fluid is too high, the extra power absorbed being converted into heat.
D) Do not kown.
Why is the axial flow pump ideal for trimming and heeling duties?
A) It is reversible and has a high capacity flow.
B) It has excellent suction lift.
C) Its discharge pressure is increased by the speed of the ship.
D) I do not know.
What is the advantage of a double entry centrifugal pump?
A) It has a lower NPSH required characteristic, giving advantages in poor suction conditions.
B) It gives double the flow rate.
C) It uses only half the input power for the same flow rate.
D) Don\'t know
Which one of these options is NOT a cause of excessive vibration in centrifuga pumps?
A) Bearing damaged.
B) Impeller imbaianced.
C) Discharge valve partly closed
D) Don\'t know
Centrifugal pumps need priming because?
A) An excellent suction lift causes the surface of the liquid to vaporise
B) They must be started with the discharge valve open to reduce the starting load, but this causes the pump to run backwards.
C) It is the movement of the liquid from the eye of the impeller to the discharge that causes a low-pressure region at the suction, if the pump is started full of air this movement of liquid does not occur therefore no suction pressure is created.
D) Don\'t know
The energy transformation within a centrifugal pump is as follows?
A) Pressure is converted into kinetic energy by the impeller; this kinetic energy is converted to an increase in velocity by the volute casing.
B) The impeller creates centrifugal force, which increases the liquid velocity, an increase in velocity means an increase in kinetic energy, the increased kinetic energy is converted into pressure by reducing the velocity in the volute casing.
C) Decreasing the velocity in the impeller decreases the kinetic energy, decreasing the kinetic energy whilst increasing the velocity of the fluid in the volute casing increases it\'s pressure.