Marine Auxiliary Machinery Chapter 1 Lesson 3 Rotodynamic pumps By Professor Zhao Zai Li 05.2006

Marine Auxiliary Machinery

Chapter 1 Lesson 3

Rotodynamic pumps

By Professor Zhao Zai Li05.2006

Learning objectives

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.

Rotodynamic pumps

There are three different types of rotodynamic pumps:

Axial-flow pumps

Centrifugal pumps

Mixed flow pumps

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 thecentrifugal pump, the axial flow pump absorbs the maximum power at zero flow.

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 PumpsApplicability

with 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.

Axial-flow Pumps

The impeller 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 stainless steel.

Quick quiz

Which part of the pump is the screw propeller?

Click on the screw propeller.

If you are not sure go to the previous screen to refresh your memory.

Centrifugal pumps

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

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

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.


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.


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.


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.


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


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.


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.





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.





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.





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.

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 - 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.

The priming pumps are controlled by the vacuum of central tank, cutting in and out as required according to demand. As a system pump is required the priming connection is opened manually or automatically until good suction is achieved.

The illustration shows a typical central priming system, including tank, valves, gauges and switches.

Centrifugal pumps - selection

The selection of Centrifugal pumps depends mainly upon duty and the space available.

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


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-aluminium bronze, shaft-stainless steel, casing bearing ring seals-leaded bronze.


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.


A typical engine room pump could be a vertical, in-line, overhung (i.e. suction and discharge pipes are in a straight line and the impeller is supported, or hung, from above), either base or frame mounted. From which the impeller can be removed without splitting the casing, breaking pipe joints or removing the electric motor.

Centrifugal pumps - losses

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


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

Capacity reduction

Excessive vibration

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

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 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

Centrifugal pumps - shaft sealing

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.

Packing

Packing

A stuffing box with a soft packing material is the traditional seal for pumps. Normally made from soft impregnated cotton, which takes the form of a length of square cross-section wound spirally onto a tube. This enables the correct length, to suit the external diameter of the shaft, to be manually cut to the correct size.

The stuffing box is then repeatedly filled with sections until almost full, the gland can then be tightened down to provide the axial compressive force. This in turn provides the necessary radial compressive force required to seal the gap due to the sloping bottom face of the aperture.

If the force is insufficient the stuffing box will leak, if the force is too great, the additional friction, and consequently heat generated by the rotating shaft can damage the soft packing and/or shaft.

Mechanical seals

Mechanical seals 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.

Mechanical seals In small pumps this may result in overloading the motor. In addition

maintenance costs are reduced, the rotary seal operating for long periods without wear or attention.

A standard seal consists of a stationary carbon ring insert in the casing, or seal cover where such is provided, and against this a metal ring of easy clearance on the shaft sleeve rotates, contact between the faces being ensured by a lightly loaded coil spring.

The rubbing faces of both carbon and metal rings are independently lapped to give a dead flat surface.

A synthetic rubber ring, of circular cross-section, contained between shaft sleeve and metal ring, in a groove in the latter, effectively prevents leakage between them.

The diameter of the groove is such that a squeeze is exerted on the rubber ring, thus a sufficient frictional force is provided to rotate the metal ring, with certain exceptions.

Mechanical seals

The width of the groove is, however, made considerably greater so that the metal ring is capable of free axial float with accompanying rolling action of the rubber ring.

Materials used for the various seal parts are as follows Carbon stationary ring. Synthetic rubber ring. Bronze rotating ring with bronze spring for standard and

all gunmetal pumps. Stainless steel rotating ring for all iron pumps.

Mechanical seals

For non-lubricating liquids, such as ammonia, section, contained between shaft sleeve and metal ring, in a

groove in the latter, effectively prevents leakage between them.

The diameter of the groove is such that a squeeze is exerted on the rubber ring, thus a sufficient frictional force is provided to rotate the metal ring, with certain exceptions.

The width of the groove is, however, made considerably greater so that the metal ring is capable of free axial float with accompanying rolling action of the rubber ring.

Quick quiz

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.

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 tonns/hour.


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

Centrifugal pumps - hydraulic balance

To control the axial movement of the rotating assembly, a balance piston is arranged to counteract the effect of the thrust of the impellers,especially in the multistage pumps.

Centrifugal pumps - hydraulic balance

The arrangement keeps the rotating assembly in its correct position under all conditions of loading .Water at the approximate pressure of the pump discharge passes from the last stage of the pump between the impeller hub and the balance restriction bush C into the annular space B dropping in pressure as it does so .The pressure of water in the chamber B tends to push the balance piston towards the drive end.When the thust on the balance piston overcomes the drive and the impeller thrust the gap A between the piston and balance ring widens and allows water to escape .This in turn has the effect of lowering the pressure in chamber B allowing the rotating assembly to move back towards the pump end .

Theoretically this cycle will be repeated with a smaller movement each time until the thrust on the balance piston exactly balances the other axial forces acting on the assembly. In practice the balancing of the forces is almost instananeous and any axial movement of the shaft is negligible.

Centrifugal pumps - maintenance

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.

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.

Question 1

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.

Question 2

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.

Question 3

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

Question 4

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

Question 5

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

Question 6

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.

D) Don't know

Documents

Marine Auxiliary Machinery Chapter 1 Lesson 3 Rotodynamic pumps By Professor Zhao Zai Li 05.2006