Unit2 (Centrifugal Pump)

PUMPS: CENTRIFUGAL PUMPS G 2006/UNIT 2/1

PUMPS: CENTRIFUGAL PUMPS

OBJECTIVES

General Objective

To know the basic operational principles, characteristics, main parts and functions of centrifugal pumps.

Specific Objectives

At the end of this unit, you should be able to describe:

the basic operational principle of centrifugal pumps

the characteristics of centrifugal pumps

the main parts of centrifugal pumps and their functions

UNIT 2

INPUT


2.0 INTRODUCTION

Centrifugal pumps employ centrifugal force and velocity to create pressure. The

mechanical element is an impeller, which is a rotating disc with vanes. The inlet

flow to the pump is directed into the centre of the spinning impeller and the

centrifugal force throws the liquid at high velocity into the surrounding casing.

2.1 BASIC OPERATIONAL PRINCIPLE OF CENTRIFUGAL PUMPS

The energy of a liquid may be increased by pumping it. Centrifugal force is the

force of spinning. When an object is spun around in a circle, it pushes outward

from the centre of the circle. This outward force is called the centrifugal force.

This is how a centrifugal pump works. Centrifugal pumps are machines which

move liquids by means of centrifugal forces. Centrifugal force is the force in a

rotating body which makes particles move away from it centre.

Figure 2.1 Velocity in centrifugal pump


Figure 2.1 shows the velocity in a centrifugal pump. In a centrifugal pump, the

particles of liquids move outward from the eye or the centre or the impeller

toward the rim of the impeller. These particles of liquid, A has two velocities; that

is V1; perpendicular to the radius and V2; in the direction of the radius. The

resulting velocity is V, in the direction of the particles of the liquid leaving the

rim of the impeller.

The resulting V or velocity known as tangential velocity enters the casing of the

pumps where its velocity decreases and where a large part of the velocity energy

is converted to pressure energy to have greater pressure on the pump outlet.

2.2 CHARACTERISTICS OF CENTRIFUGAL PUMPS

Pumps are rated according to their pumping characteristics and according to

things you need to know to operate the pump efficiently. Rating helps you to

select the right pump for your operations.

For example, a certain pump delivers 100 gallons per minute (GPM). The pump

has a rated velocity of 100 GPM. Capacity is usually a factor in rating a pump.

Suction and discharge pressure also affect the rating of pump. For example, a

pump produces a discharge pressure of 30 PSIG. It has a rated discharge pressure

of 30 PSIG.

2.2.1 Capacity

The capacity of a pump is the amount of liquid that the pump moves in a

given length of time. Capacity is usually measured in gallons per minute,

abbreviated as GPM. Pump capacity can be changed by changing the

speed of impeller. Therefore, increasing pump speed also increases pump

velocity. The pump and its prime mover usually run best within a range of


specific speeds. Increasing pump capacity by increasing its RPM is not

always practical.

2.2.2 Pressure and Head

Pressure is the force acting on a unit of area. Head is the height of a liquid.

Suction head is the sum of pressure changed to head, plus the velocity of

head at the inlet to the pump while discharge head is the sum of the

pressure changed to head at the outlet of the pump.

Velocity head is normally small and is not used in pumping calculations.

The total head is the discharge head plus or minus the suction head. Or,

total head can be calculated by reading the pressure at the pump suction

and discharge and converting this pressure measurement to head

measurements.

Figure 2.2 shows suction head, discharge head and total head of the two

different systems. Figure 2.2 (A) shows the static suction head and Figure

2.2 (B) shows the suction lift system.

Figure 2.2 Suction head, Discharge head and Total head

2.2.3 Vapour Pressure


Sometimes when the absolute suction pressure is not high enough, liquids

vaporize or evaporate at the pump suction. Vapour pressure causes the

liquid to vaporize or evaporate. To keep the liquid at the pump from

vaporizing, the absolute suction pressure must be higher than the vapour

pressure of the liquid at that temperature.

2.2.4 Net Positive Suction Head (NPSH)

Net positive suction head (NPSH) is the absolute suction head minus the

vapour pressure head. If suction head is 50 feet and vapour pressure is 35

feet, NPSH is 15 feet.

NPSH available is the absolute pressure at pump suction, changed to head,

minus the vapour pressure of the liquid being pumped and changed to

head. NPSH required is the minimum head needed at the suction to get the

liquid into the impeller without vaporizing.

If NPSH available is equal to NPSH required, the pump may lose suction

due to slight variations in pump design. Therefore NPSH available must

be then NPSH required.

2.2.5 Friction

During flow, pressure is converted to velocity. As velocity increases

during flow, pressure increases. Pressure between two points in fluids flow

is called the pressure drop. For fluid to flow, the driving force must be

greater than the resisting force. Thus, the pressure drop must be greater

than the amount of friction.

2.2.6 Horsepower


Horsepower is a unit used for measuring rate of work. It is necessary to

overcome friction and other losses and to move the liquid provided by the

prime mover or driver. The amount of useful work that a pump delivers is

the difference between the pressure the liquid has as it enters the pump

and the pressure it has as it leaves the pump.

The horsepower input is always more than the fluid horsepower or

horsepower output. Overall efficiency of a pump is the percentage of the

HP input that is transferred to the liquid leaving the pump or is found by

dividing the HP output by the HP input.

2.3 PERFORMANCES CURVE

All centrifugal pumps come with a set of performance curves. These curves can

be used to find the NPSH, total head, efficiency, and HP for each pump at

different capacities. The performance curves can also show some general

principles of centrifugal pump performance. Figure 2.3 shows performance curves

of centrifugal pumps.

According to Figure 2.3, as the pumping rate increases, the NPSH required

increases. The efficiency of a pump is relatively low at high and low flow rates.

As the flow rate increases, the horsepower required increases.

KEY FACTS:

Centrifugal pumps employ centrifugal force and velocity to create pressure.Pumps are rated according to their pumping characteristics.The performance curve shows some general principles of centrifugal pump

performance.


Fig

ure

2.3:

Per

form

ance

cur

ves


Activity 2A

Let’s test your understanding by answering these questions.

2.1 Fill in the blanks with appropriate answers.

Centrifugal pumps employ (A) and velocity to create pressure.

The mechanical element is an (B) , which is a rotating disc with

vane. Centrifugal pumps are machines which move (C) by means

of centrifugal forces. Centrifugal force is the force in a (D) which

makes particles move away from it centre.

2.2 The particles of liquid have two velocities. They are…

i. _____________________________

ii. _____________________________

2.3 List four characteristics of pumps

i. __________________________________

ii. __________________________________

iii. __________________________________

iv. __________________________________

2.4 What is the purpose of performances curve?

Check your answer on the next page…

INPUT


Feedback to Activity 2A

Here are the answers.

2.1 (A) centrifugal force

(B) impeller

(C) liquids

(D) rotating body

2.2 i. tangential velocity

ii. perpendicular velocity

2.3 Any four of the following

i. Capacity and/or

ii. Pressure head and/or

iii. Vapour pressure and/or

iv. (HPSH) and/or

v. Friction and/or

vi. Horsepower

2.4 The performances curve can be used to find the NPSH, total head, efficiency, and

HP for each pump at different capacities.

The next input will focus on main parts of centrifugal pumps…

Centrifugal pumps are machines which move liquids by means of centrifugal forces. An impeller is one of the important parts in centrifugal pumps. How about the other components?


2.4 MAIN PARTS OF CENTRIFUGAL PUMPS AND THEIR FUNCTIONS

In the previous section we have already introduced the characteristics of

centrifugal pumps that are important to determine the appropriate selection of a

pump for a certain pumping process. In this section we are going to study the

construction and the component of the pumps itself. Figure 2.4 show a cross

sectional view of a centrifugal pump.

Figure 2.4: A cross sectional view of a centrifugal pump

Generally, the parts in centrifugal pumps fall into two main groups; the main

component parts and the latter component parts. The main component consists of

PACK ING BOX

PACK ING RING

SHAFT

VANE

SHAFT SLEEVE

CASING

DISCHARGE NOZZLE

IMPELLER

EY E OFIMPELLER

CASING WEAR RING

IMPELLER WEAR RING


basic component that verifies the construction of pumps itself while the latter

component is supporting component that helps the pumps run efficiently.

2.4.1 Main Component parts of Centrifugal Pump

There are three main component parts of centrifugal pump:

i. Pump casing

ii. Impeller

iii. Stuff or seal box

Pump Casing

The pump casing is the place where an impeller throws the water particles in the

casing and increases the head, causing the liquid to flow out the discharge flange.

The casings determine the rotation. There are two different types of casings. There

are circular casing and volute casing. Figure 2.5 shows circular and volute casing.

Figure 2.5 Circular casing and Volute casing

i. Circular Casing

Circular casings have a uniform space outside the impeller. The distance

between the edge of the impeller and casing is uniform and the size of the

space is equal. The pressure is constant at the beginning as it is transferred by

the vane to the end at the discharge nozzle.

ii. Volute casing


The volute casing has an increasing space or a spiral wall outside the impeller.

The spiral is called a volute. Liquid pressure increases because the shape of

casing gets larger at the end of discharge nozzle but the velocity of the liquid

gets slower and flow uniformly.

Impeller

The impeller is one of the most important parts of the centrifugal pump. This

impeller is a disc with large vanes. The valves are curved to the rear to help push

liquid out, so this impeller turns clockwise if viewed from the motor side. Figure

2.5 shows an impeller.

Figure 2.6: An impeller

Never run it backwards. The screw on impeller may come off and it has less than

half of the capacity when turning wrong way. The liquid has a certain speed on

entering the volute.

This speed is converted to become pressure by a gradual increase of the bore of

the volute. The shape of the impeller depends not only on the relations between

the capacity and pressure at a certain speed of revolution, but also the nature of

the liquid to be pumped.


Types of Impellers

i. Shrouded impellers

This is the most frequently used type of impeller. The vanes in this case are

enclosed between two walls, together with which they form a whole. This

kind of impeller is used exclusively in pumps which is used to transport

clean or only slightly contaminated liquids.

Figure 2.7 Shrouded impeller

ii. Semi-open impeller

The construction of semi-open impeller is the same as that of shrouded

impellers except one the side wall at the inlet side which is omitted.


Figure 2.7 Semi open impeller

iii. Open Impeller

Both the inlet and the rear walls are omitted. These impellers are generally

used in pumps suitable for liquids containing a high concentration of

solids. The permissible concentration obviously depends on the minimum

passage in the impeller.

Figure 2.8 Open impeller

iv. Channel impeller

This consists of two or three curved channels located in one plane at right

angles to the central axis of the shaft and meeting at a common liquid

inlet. Because of their wide passages these impellers are used in pumps

which have to transport more liquids with high concentration of solids

such as sewage.


Figure 2.9 Channel impellerStuffing or Seal Box

There are two types of seals in pumps:

i. Packing Ring

ii. Mechanical Seal

Packing Ring

The packing rings are formed around the shaft to minimize leakage of liquids

from the pump. A drop of liquid between the packing ring and the shaft is

necessary for lubrication. It is done by adjusting the packing nuts.

Figure 2.10 Packing ring and lantern ring

Lantern rings are used in refinery hazards like acid or corrosive liquids where

lubrication liquid is from out side the pump.

Mechanical Seal

Mechanical seals are used to help keep corrosive or erosive materials out of the

seal. It is made of two antifriction surfaces, stationary seal (carbon) and a rotating

seal of a special metal where it comes in contact with stationary seal ring.

The spring pushes against the compression ring and compresses the flexible O-

ring, the shaft and rotating seal members, to prevent leakage at this point.


Figure 2.11 Mechanical Seals

Seal becomes one of the most important parts in centrifugal pumps. Its function is to prevent leakage from pumps. Between these two seals, which type do you think you would prefer to used?

Mechanical seal is an option to packing ring. It’s very rarely used because of its cost and the difficulties of maintenance compare to packing ring. But practically, in operation it prevents leakage better than packing ring.


2.4.2 Latter components of centrifugal pumps

There are three latter components of the centrifugal pump. They are:

i. Shaft

ii. Bearing

iii. Wear plate or Wear Ring

Shafts

The shaft is a smooth and straight solid material where it fits into bearings to

transmit power to the impeller.

Bearing

Bearings are use to support the shaft and allow it to rotate freely without

dragging. It also controls radial and axial movements of the shaft that may reduce

the time of failure of the packing or mechanical seal. Bearings are set nearest the

couplings and impeller with locking collars.

Figure 2.12 Radial and thrust bearing


Figure 2.13 Shaft movement

Wear ring or Wear Plate

Both sides of the impeller maintains close clearance with the casing. As a result,

wear at the eye minimizes leakage from the discharge back to the suction and the

wear ring between the collar and the casing minimize leakage into the collar.

Figure 2.14 Wear ring

A wear plate is installed to protect from mud and sand abrasion and to balance

against the wearing on the impeller.

Wear ring is installed at both of the pumps. It minimizes leakage of liquids to discharge back to the suction. Beside the above reason is there any other functions wear rings are placed there?

Yes, there is. Beside as minimizes leakage, wear rings hold impeller thus it can rotate without touching other parts of the pumps. They are fixed to the guide pins so they could not rotate together with the impeller during the operation!


To the next activity…

KEY FACTS:

The parts in centrifugal pumps fall into two main groups; the main component parts and the latter component parts.

There are three main component parts of centrifugal pump:Pump casing ImpellerStuff or seal boxThere are three latter components of the centrifugal pump. They are:ShaftBearingWear plate or Wear Ring


Activity 2B

Once again, stop for a while and test your performance here. Answer all the questions in this activity without referring to the notes.

2.5 Complete the table below

Main Parts of Centrifugal PumpsMain Component Latter Component

i. i.ii. ii.iii. iii.

2.6 There are two different types of casings i.e.…..

i. _______________________________

ii. _______________________________

2.7 Names the four (4) types of impellers.

2.8 List two types of seals used in pumps.

i. _____________________________

ii. _____________________________

2.9 Define what is shaft?

2.10 Name the two types of bearing used in centrifugal pumps:

i. _____________________________

ii. _____________________________

2.11 What is the difference between wear rings and wear plate?

Are you centrifuging enough right now? Then we’ll see next page…

Types of impellers

B.

C.

A.

D.


Feedback to Activity 2B

Let count how well you’ve score…

2.5 The answers are:

Main Parts of Centrifugal PumpsMain Component Latter Component

i. Casing i. Shaftii. Impeller ii. Bearingiii. Stuffing or Seal Box iii. Wear plate or wear rings

2.6 The casings are:i. Circular casingii. Volute casing

2.7 Types of impellers

2.8 Two types of seals are:i. Packing ringsii. Mechanical seal

2.9 The shaft is a smooth and straight solid material where it fits into bearings to transmit power to the impeller.

2.10 Name the two types of bearing used in centrifugal pumps:

i. Radial ball bearingii. Thrust ball bearing

Types of impellers

B. Semi-Open

C. Open

A. Shrouded

D. Channel


2.11 Wear rings are located both at the end of the impeller to minimize leakage from discharge back to the suction while wear plate is installed to protect from mud and sand abrasion and to balance against the wear rings on the impeller.

SELF-ASSESSMENT UNIT 2

The previous activities are so simple. Now try the more challenging questions in this self assessment. Answer all the questions. If you need extra revision, you are encouraged to do so.

1. What is a centrifugal pump?

2. Describe how are centrifugal force and velocity applied in centrifugal

pumps. Draw a figure to assist your answer.

3. Look at figure 1.

Figure 1

a. Define

i. Suction head

ii. Suction lift head

iii. Discharge head

iv. Total head

b. Find the value of the following :

i. Figure A : Suction head, discharge head, total head

ii. Figure B: Suction lift, discharge head, total head


4. Look at the performances curves in page 7 in unit 2.

At the capacity of 500 GPM, find these values.

i. Total head

ii. NPSH

iii. Horsepower

iv. Efficiency

5. Label the components A-G in the cross sectional view of a centrifugal pumps in

Figure 2.

E

G

H

F

VANE

B

IMPELLER WEAR RING

APACK ING BOX

D

C

Figure 2

6. What are the differences between circular casing and volute casing?

7. Explain impellers in the perspective of:

i. Design

ii. Assembly

iii. Principles of operation

iv. Types of impeller

8. Differentiate between packing ring and mechanical seal


9. Figure 3 show typical type of bearing used in a centrifugal pump. Explain the

functions of each type of them.

Figure 3: Radial and thrust bearings

Tired..??? Rest for a few minutes and check your score in the next page…


Feedback to Self-Assessment Unit 2

Let check your answer now.

1. Centrifugal pumps are machines which move liquids by means of centrifugal

forces. Centrifugal pumps employ centrifugal force and velocity to create

pressure. The mechanical element is an impeller, which is a rotating disc with

vanes. The inlet flow to the pump is directed into the centre of the spinning

impeller and the centrifugal force throws the liquid at high velocity into the

surrounding casing.

2. Centrifugal force is the force in a rotating body which makes particles move away

from it centre.

Velocity in centrifugal pump

The figure above shows the velocity in a centrifugal pump. In a centrifugal pump,

the particles of liquids move outward from the eye or the centre or the impeller

toward the rim of the impeller. These particles of liquids, A has two velocities;

that is V1; perpendicular to the radius and V2; in the direction of the radius. The

resulting velocity is V, in the direction of the particles of the liquid leaving the

rim of the impeller.


The resulting V or velocity known as tangential velocity enters the casing of the

pumps where its velocity decreases and where a large part of the velocity energy

is converted to pressure energy to have greater pressure on the pump outlet

3.

a. According to Figure 1 (A) and (B)

i. Suction head is the water height measured from the suction tank A that

located relatively above the pumps centre.

ii. Suction lift is the water height measured from the suction tank A that

located relative below the pumps centre.

iii. Discharge head is water height measured relatively from the pumps centre

to the higher level in discharge tank B.

iv. Total head is the addition of subtraction between suction head or suction

lift and discharge head.

b.

i. Figure A : Suction head : 50 m, Discharge head : 75 m, Total head : 25 m

ii. Figure B : Suction lift : 75 m, Discharge head : 20 m, Total head : 55 m

4. The values are:

i. 40 feet

ii. 5 feet

iii. 6.4 BHP

iv. 80 % (percent)

5. The components name are:

A. Eye of impeller

B. Casing wear ring

C. Discharge nozzle

D. Packing ring

E. Shaft

F. Shaft sleeve

G. Casing

H. Impeller

6. Circular casing:

Circular casings have a uniform space outside the impeller. The distance between

the edge of the impeller and casing is uniform and the size of the space is equal.

The pressure is constant at the beginning as it is transferred by the vane to the end

at the discharge nozzle.

Volute casing:


The volute casing has an increasing space or a spiral wall outside the impeller.

The spiral is called a volute. Liquid pressure increases because the shape of the

casing gets larger at the end of discharge nozzle but the velocity of the liquid gets

slower and flow uniformly.

7. The impellers in the perspective of:

i. Design

The impeller is a disc with large vanes that curved to the rear to help push liquid

out of the impeller. It has an eye of suction where the liquid enter into the pumps.

Some of the impeller had wall or shrouded and some of them does not.

ii. Assembly

An impeller turns clockwise if viewed from the motor side. Never run it back

wards. The screw on impeller may come off and it has less than half of the

capacity when turning wrong way.

iii. Principles of operation

The liquid forced into the pumps through the eyes of suction and transfers outside

from the pumps by the blades. The liquid has a certain speed on entering the

volute. This speed is converted to become pressure by a gradual increase of the

bore of the casing or volute.

iv. Types of impeller

There are four type of impeller i.e. shrouded impeller, semi-open impeller, open

impeller and channel impeller. Each of this impeller has their own purpose and

character in the perspective of construction and capacity of liquid pumped.

8. Packing Ring

The packing rings are formed around the shaft to minimize the leakage of liquid

from the pump. A drop of liquid between the packing ring and the shaft is

necessary for lubrication and it is done by adjusting the packing nuts. Lantern ring


is used in a refinery hazard like acid or corrosive liquid where lubrication liquid is

from out side the pump.

Mechanical Seal

Mechanical seals are used to help keep corrosive or erosive materials out of the

seal. It is made of two antifriction surfaces, stationary seal (carbon) and a rotating

seal of a special metal where it comes in contact with the stationary seal ring.

The spring pushes against the compression ring and compresses the flexible O-

ring, the shaft, and rotating seal members to prevent leakage at this point

9. Radial bearings are use to support the shaft and allow them to rotate freely

without dragging. Thrust bearing controls radial and axial movements of the shaft

that may reduce the time of failure of the packing or mechanical seal. Bearings

are set nearest the couplings and impeller with locking collars.

The destination is still far away. I need to complete this journey...6 units to go...I need to take a rest…should I proceed to the next unit afterwards….????


Step to the next unit if you are willing to do so…

Unit2 (Centrifugal Pump)

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