A Course on Centrifugal Pumps

A course on Centrifugal Pumps

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A turbo machine where mechanical energy is converted into pressure energy in a liquid by means

of a rotating impeller is called Centrifugal Pump. When the rotating impeller transfers energy to the

liquid, centripetal force is imparted to the liquid. This creates a centrifugal force as a reaction force in the

liquid. Due to this phenomenon, these devices are called Centrifugal Pumps.

Centrifugal Pump can be understood as a reversed inward radial flow reaction turbine.

Main parts of a Centrifugal Pump:

A Centrifugal Pump has the following main parts as shown in the figure 19.1below:

1. Impeller

2. Casing

3. Suction pipe [with foot valve & strainer]

4. Delivery pipe


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Impeller: is the rotating part of a Centrifugal Pump. It is mounted in a shaft that is in turn connected

to an electrical motor. It imparts rotary motion to the liquid. Therefore it imparts kinetic energy to the

liquid. The impeller has an opening at its center called ‘Eye of the impeller’ through which liquid enters

from the suction pipe. Liquid enters radially through the eye of the impeller and passes over the vanes that

are attached to the impeller. Vanes are backward curved and liquid moves over the convex portion of the

vane. In a similar turbine, the water would have passed over the concave portion of the vane1.

Casing2: is an air-tight passage surrounding the impeller. Its shape is called ‘Volute’. This shape

helps in converting the high kinetic energy of the liquid to pressure energy. When the liquid leaves the

outlet of the impeller, the liquid has very high kinetic energy because the impeller would have imparted

rotary motion to the liquid. This liquid with high kinetic energy enters the volute-shaped casing. In this

casing, the high kinetic energy liquid is forced to pass through a progressively reducing cross-sectional

area. This reduces the kinetic energy but increases the pressure energy in the liquid. Thus, the liquid

leaves the outlet of the casing with high pressure energy.

Suction pipe: must have a foot-valve in a Centrifugal Pump arrangement. Foot-valve is a non-return

valve which will allow liquid to only enter the suction pipe and will retain the liquid inside the suction

pipe. This is necessary because Centrifugal Pump has to be primed for working. Without a foot-valve, the

Centrifugal Pump cannot be primed. The suction pipe must also necessarily have a strainer. This prevents

any solid particle from entering the impeller. Solids entering into the impeller could lead to clogging of

the small spaces between vanes and between impeller and casing. Clogging reduces the efficiency of the

pump.

Delivery pipe: will have a non-return delivery valve in long pipes. This prevents back-flow when the

pump stops after working for a considerable time. Back-flow from long delivery-pipes may damage the

electrical motor.

Working: The Centrifugal Pump is primed. The entire volute casing and suction pipe is filled with

liquid. The impeller is completely submerged in the liquid inside the volute casing. The foot-valve

prevents the liquid from getting drained back into the sump. Then the motor is turned on. The shaft rotates

and makes the impeller to rotate. The liquid which is in contact with the vanes gets thrown out of the

impeller with high kinetic energy. This liquid gets collected by the volute casing and is forced along its

volute path. The volute path drastically reduces the kinetic energy which gets converted into pressure

energy. Thus the liquid develops a high head and gets discharged from the volute casing outlet into the

delivery pipe. Since the liquid gets thrown out of the impeller into the volute casing, the lower pressure in

the eye of the impeller sucks more liquid into the impeller and continuous discharge is obtained.

Priming of Centrifugal Pump: is defined as the operation in which the suction pipe and the volute

casing are completely filled with the liquid to be pumped. This filling is done from an external source.

Liquid is poured into a funnel provided for this purpose. When the liquid is poured, an air-vent is opened

so that all the air trapped inside the system is replaced by liquid. When the liquid starts coming out of the

air-vent, the priming is complete. It is important that all the air from the entire suction pipe and volute

casing is removed and replaced fully by the liquid. The reasons why priming is needed are:

1 See end of the note for more details on impellers.

2 This is also called as ‘Volute Chamber’ or ‘Volute Casing’.


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1. The head generated in a Centrifugal Pump is given by the equation:

[ ]

Since this equation is independent of the density of the liquid, this equation tells us that if the

Centrifugal Pump runs in air, then the head generated will be in meters of air, and if it is used to

pump water, then the head generated according to this fundamental Centrifugal Pump equation

will be in meters of water. If the impeller is filled with air, then the head generated in terms of

equivalent meter of water will be very low. This is because the density of air is much lower than

water or any other liquid. As a result, if the impeller is filled with air, it won’t be able to

generated sufficient head to lift water or any other liquid. Hence priming is essential in

Centrifugal Pump.

2. If air bubbles get blocked anywhere in the system, the pressure generated by the impeller-casing

system will get dissipated on the air bubbles and discharge will stop. Therefore it is essential to

ensure that priming is done and then the Centrifugal Pump is started.

3. Dry running of Centrifugal Pump may lead to burn out of electrical motor.

Methods of Priming:

1. Small pumps: As explained in the section above.

2. Medium size pumps: Smaller pumps are used to prime the medium sized pumps by using a float

switch.

3. Large pumps: are primed by evacuating the casing and the suction pipe by a vacuum pump or an

ejector. The liquid is thus drawn up the suction pipe from the sump and the pump is filled with

liquid.

4. Self-priming Pump: Some Centrifugal Pumps come with a special arrangement containing a

supply of liquid that is supplied in the suction pipe. This enables automatic priming. Such

Centrifugal Pumps are called Self-priming Pumps. Naturally, these have to be small in size and

output.

Work done by impeller on liquid:

For all practical purposes, a Centrifugal Pump can be considered as a mirror-image of a radial-inward

flow turbine. Consider the figure 19.5:


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We make the following assumptions:

1. Liquid enters the eye of the impeller radially.

2. There is no shock at the entry.

3. There is no energy loss in the impeller due to friction and eddy currents.

4. There is uniform velocity distribution in the narrow passage between two adjacent vanes.

5. For the purpose of easy mathematical calculations, we start by considering that the impeller has

only one vane. The result that we obtain is then generalized for impeller with many vanes.

Let all notations be same as that for a radial-inward flow turbine. From turbine calculations we know that

Work done per second per unit weight of water striking the vane is

[ ]

Since a Centrifugal Pump can be considered as a mirror-image of a turbine, Work done per second per

unit weight of water striking the vane is

[ ]

Since the entry of water is radial, we have .

[ ] meters of water. This is for unit weight of water. If we wish to generalize

this for any liquid, then:

[ ] = [ ], where

Here, is the width of the impeller at the outlet side.

‘Work done/second’ is also called ‘Power at the impeller’ or ‘Power imparted by impeller to liquid’.

Heads in Centrifugal Pump: The various heads in a Centrifugal Pumps are depicted in figure 19.29

below:


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1. Suction head (hs): is the vertical height through which the Centrifugal Pump has to lift the liquid.

It is measured from the center line of the Centrifugal Pump to the foot-valve.

Note: This head is called Suction Lift in case of a submersible pump.

2. Delivery head (hd): is the vertical height through which the pump has to discharge the liquid. It

is measured from the center line of the Centrifugal Pump to the vertical tip of the delivery pipe.

3. Static head ( ): is the sum of the suction head & delivery head for a Centrifugal Pump.

4. Manometric head ( ): is the effective head against which the Centrifugal Pump has to work. It

can be defined in three different ways, as follows:

a. = Head imparted by impeller to liquid – loss of head inside the pump.

if loss is zero.

b. = Total head at outlet of pump – total head at inlet of the pump.

{

} {

}

Where:

c. = suction head + delivery head + friction-head in suction pipe + friction-head in

delivery pipe + velocity-head of liquid in delivery pipe.

5. Net Positive Suction head (NPSH): is the total suction head. It is the head required to make the

liquid flow from the foot-valve, through the suction pipe up to the impeller.

NPSH is an important value because all the minimum suction conditions of a Centrifugal

Pump are generally expressed in terms of NPSH. NPSH is the minimum head required to avoid

cavitation in Centrifugal Pump. NPSH depends on the vane outlet angle, speed of impeller and

discharge of the pump.

All Centrifugal Pump manufacturers specify the required NPSH (NPSHR) for the given

pump. Refer to figure 19.29 above. The available NPSH (NPSHA) is calculated by the equation:


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NPSHA = {

[

]

}

NPSHA = {Ha – Hs – Hv} where Ha = Absolute pressure head (

)

Hs = Total Suction head {

Hv = Vapor pressure head for liquid (

=

Efficiencies of Centrifugal Pump: Since a Centrifugal Pump has many losses, we can consider various

kinds of efficiencies for a Centrifugal Pump. The losses can be mechanical losses, leakage losses and

hydraulic losses. Corresponding to each of these losses, we can calculate an efficiency number for the

pump.

1. Mechanical efficiency ( ): is the ratio of power at the impeller to the power at the shaft.

.

2. Volumetric efficiency ( : is the ratio of discharge per second from the pump to the total

quantity of liquid passing per second through the impeller.

Where q = loss of liquid/second.

3. Manometric efficiency ( ): is the manometric head to head imparted by the impeller.

.

4. Overall efficiency (no): is the ratio of the power output of pump to the power input into the

pump.

. [This equation is used for calculating the Shaft power

3.]

An alternate expression for no is:

.

Minimum speed for starting a Centrifugal Pump:

The Centrifugal Pump will discharge liquid only if the pressure increase is equal to or greater that

the manometric head. Hence, there is a minimum speed of the Centrifugal Pump above which discharge

occurs, and below which there will be no discharge. This minimum speed is given by:

Nmin =

[

]

3 Shaft Power: is the power that the electric motor gives to the pump.


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Specific speed of Centrifugal Pump: is defined as the speed of a geometrically similar pump which

would deliver one cubic meter of liquid per second against a head of one meter. It is given by:

Ns = √

Maximum suction head possible:

[

]

is limited to 7.8 meters of water. If the suction head is more than this, then rapid vaporization

may occur, which will lead to cavitation.

Cavitation4: When the pressure of a liquid becomes equal to or falls down below its vapor pressure

5, the

liquid undergoes a phase change into its vapor state. Thus, even though the liquid may be at standard

temperature, the liquid would be effectively ‘boiling’. Bubbles are formed as a result. These bubbles

travel further downstream and burst in a region of higher pressure. This entire phenomenon is called

‘Cavitation’. Thus cavitation involves formation of vapor bubbles and the bursting of those bubbles.

In a Centrifugal Pump, cavitation is a dangerous situation and it has to be avoided. On the one

hand, separation occurs and discharge stops. On the other hand, damage occurs on the surface of vanes

and casing, which reduces efficiency and may eventually lead to mechanical failure.

Effects of cavitation on Centrifugal Pump:

1. Due to sudden collapse of vapor bubbles, considerable noise and vibrations are produced.

2. Vapor bubbles hit the metallic surface of vanes and casing and collapse there. This bursting

creates a localized hammer-effect. Although the force released in this bursting is relatively small,

at the molecular level, these are enough to chip off a micro part of the metallic surface. The

metallic surfaces are thus damaged and cavities are formed on the surface. This phenomenon is

called ‘Pitting’. Pitting has to be avoided.

3. Sudden drop in head, efficiency and power transferred to liquid.

4. The efficiency of a Centrifugal Pump decreases considerably. This occurs because pitting makes

the surface of the vanes very rough. Hence the flow velocity imparted by vanes to liquid reduces.

So, liquid leaves the impeller with less kinetic energy than the maximum possible by the pump.

Precaution against cavitation in Centrifugal Pump:

1. Foot-valve design to be done in such a way that suction doesn’t get restricted.

2. Pressure of the liquid should not be allowed to fall below its vapor pressure in any part of the

pump. (For water this means the absolute pressure should not be below 2.5 m of water).

4 Please refer to ‘A course on Reciprocating Pump’ for short note on this topic

5 Vapor pressure: A liquid may change its phase under two conditions. If the temperature is raised above its boiling

temperature, phase change occurs. Or, if the pressure falls below a limit that is characteristic of the liquid, even at

standard temperature, phase change will occur.


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3. Use of special materials like aluminum-bronze & stainless steel which are corrosion resistant

materials. If cast iron or cast steel is used for impeller, then they are surface-hardened.

4. If hot liquids are being pumped, temperature has to be maintained in such a way that it doesn’t

boil off into vapors.

Impellers:

1. Types of impellers:

a. Shrouded impeller: is the most common type of impeller. This is basically a cylinder

where the vanes are covered on both sides. This makes it easy to manufacture. Most

liquids can be pumped with this type of impeller.

b. Propeller: is also called unshrouded impeller. The outer cover for the vanes is not

present here. High viscosity liquids and mixture of liquids & solids such as sludge can be

pumped with this type of impellers.

c. Mixed flow impeller: is always associated with diffuser vanes. These are used in

submersible pumps for low viscous liquids.

2. Materials for impellers: depends on the liquid being pumped.

Sl No Liquid to be pumped Material of impeller

1 Water Cast Iron

2 Hot water (≥ 150oc) Cast Steel

3 Acids Stainless Steel or Cast Steel with corrosion-resistant coating

4 Milk/edible oils Stainless Steel

3. Vane design: The of a Centrifugal Pump is closely related to the vane outlet angle . As

varies from 90o to 20

o, varies from 50% to 75%. However, cannot be reduced further

lower than 20o for practical reasons, as the vanes will become extremely thin and weak.

Similarly, the depends on the number of vanes on the impeller. It has been found

empirically that if the number of vanes is equal to or greater than 24, then Euler’s fundamental

equation for pumps holds good. But it is not practical to have a large number of vanes since

fabrication will become difficult and the impeller may get clogged.

Multi-stage pumps:

Centrifugal Pumps can be connected in series and in parallel in order to achieve better results. If

the pumps are connected in series, we can get very high heads. Hence multi-stage Centrifugal Pumps in

series are also called ‘Booster Pumps’. If the pumps are connected in parallel, we can get very high

discharge.

Pumps in series:

For obtaining a high head for a small discharge, a number of impellers are mounted in series on the same

shaft as shown in figure 19.12 below. Discharge from the first impeller enters the eye of the second

impeller. The liquid entering the second impeller already a certain pressure-head imparted to it by the first

impeller.


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The second impeller further increases the head and discharges the liquid at a much higher

pressure. If the number of impellers is increased, the pressure can be increased further. If ‘n’ number of

identical impellers is used and if each impeller imparts a head of at each stage, then the total head

from the multi-stage pump will be . However, total discharge will not be greater than the output

of the first impeller.

Pumps in parallel:

When a large quantity of liquid has to be pumped against a small head, two or more pumps are

arranged as shown in figure 19.13 below. Each of these pumps works separately but delivers its discharge

to a common delivery pipe. The head imparted to the liquid will be common. But the discharge gets added

up. If this parallel arrangement has ‘n’ number of pumps and if each of them delivers a discharge Q, then

the total discharge will be ‘nQ’.


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Comparison of Centrifugal Pumps with Recip Pumps:

Sl No Parameter Centrifugal Pump Recip Pump

1 Cost Cheap Costly

2 Installation & Maintenance Easy & cheap Difficult & Costly

3 Discharge capacity High Low

4 Size for same power & capacity Small Large

5 Performance characteristics Superior Inferior

6 Type of liquids handled All kinds Only low viscosity

7 Operating speeds Very high Very low

8 Type of coupling of drive shaft Direct Flexible only

9 Torque on motor Uniform Sinusoidal

10 Output from pump Uniform Intermittent

Pump Characteristics Charts6: contain various curves that help in predicting the performance of a

Centrifugal Pump under different flow rates, heads and speed. There are many types of pump

characteristics charts out of which we shall study three main types. These charts have great practical

application when we have to select a suitable Centrifugal Pump based on the head and discharge required.

Types of Pump Characteristics Charts:

1. Main characteristics Chart: contains variation of manometric head ( ), power ( &

discharge ( ) against speed as shown in the figure 19.14:

This chart will have three curves. Discharge curve is a straight line. Manometric head curve is a

parabolic curve. Power curve is a cubic curve.

In these charts: For plotting against , is kept constant. [ ]

For plotting against , is kept constant. [ ]

For plotting against speed, & are kept constant.

[ ]

6 Care must be taken not to be confused between these charts and Turbine Characteristics Charts. The charts

appear similar, but have a vital difference. Turbine charts are generally plotted for unit speed and unit discharge.


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Use: These charts are used for comparing various Centrifugal Pumps under different parameters.

2. Operating characteristics Chart: contains variation of manometric head ( ), power ( &

efficiency ( ) against discharge ( ), keeping speed as constant, as shown in the figure 19.15

The main features of this chart are:

a. Input curve will start on y-axis, above origin. This is because even when , power is

required for overcoming mechanical losses.

b. Output curve will start from origin since .

c. curve will have maximum value when .

d. curve will also start at the origin since output power is zero when . However,

this curve will rise up to a certain value of and then it will drop. This is because of

cavitation.

Use: These charts are used for obtaining the design head & discharge for maximum efficiency.

3. Constant Efficiency Charts: makes use of two charts simultaneously as shown in figure 19.16:

The against chart is used along with against chart. A constant efficiency line is

drawn on the – curves. The points of intersection at various speeds are projected onto


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the – curves for corresponding speeds. The resulting curves are called Iso-efficiency7

curves. Some Centrifugal Pump manufacturers provide a combined chart which already has these

constant efficiency or iso-efficiency curves for easy selection.

Use: These charts help us determine the range of speeds for a given head and discharge in which

the Centrifugal Pump will have maximum efficiency.

Steps in Pump Selection:

1. Discharge (Q), Head (Hm) and Speed (N) are the main criteria that are determined from in-situ

requirements.

2. Corresponding to these values, Specific Speed of the pump is calculated. From this we get the

specific NPSHR. From these two parameters, we can determine the specific pump model and size.

3. From this value, along with the other three parameters, the Performance Characteristics Charts

are referred to and optimum working ranges for Q, Hm, N & P are determined.

4. Based on the type of liquid being pumped, the type and material of impeller is determined.

5. Based on the values obtained from steps 2 and 3 above, we can determine the size range and

dimension range of impeller and vane. In fact, for a given set of Specific Speed & Specific

NPSHR, we can get a number of different impeller diameters. The choice of geometry & type of

impeller depends on the operating conditions, properties of the liquid and chemical composition

of the liquid.

6. Select an impeller that is one size smaller than the maximum size for the given pump casing, so

that it can be replaced with a larger one without replacing the casing.

7. Law of affinity in Centrifugal Pumps: When we change the speed or impeller diameter in a

pump, then, from the MC Chart, we know that:

a. Discharge or Capacity varies directly as the change in speed or diameter.

b. Head varies as the square of the change in speed or diameter.

c. Power varies as the cube of the change in speed or diameter.

Any choice of the impeller diameter has to consider this Law of Affinity while selecting the

pump.

***************

7 Iso-Efficiency Curves are also called ‘Muschel Curves’.

Documents

A Course on Centrifugal Pumps