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Introduction to Pumps

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Introduction to Pumps

1.0 INTRODUCTION

There are many different types of pump now available for use in pumped

fluid systems. A knowledge of these pump types and their performance

characteristics is extremely useful in selecting the most suitable pump for

a particular application.

2.0 PUMP CHARACTERISTICS AND THEIR

MEASUREMENT

To understand the performance of a pump it is necessary to be familiar

with the following terms:

FLOW – or Discharge or Delivery litres/minute

DELIVERY HEAD ) metres

SUCTION HEAD ) See diagram below metres

TOTAL HEAD ) metres

EFFICIENCY %

SPEED rpm

DRIVING POWER kilowatts

The Driving Power is the output power delivered by the engine or electric

motor driving the pump. Water Power is the energy given to the water by

the pump; it has no significance except in the calculation of efficiency.

The efficiency of a pump is never 100%. What you get out is always less

than you put in.

%100Power Driving

PowerWater

Input

OutputEfficiency

The Head terms are best illustrated by the following installation, where

water is drawn from a well and delivered to an overhead. tank (Figure 1).

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Figure 1

Pump Terms

The Suction Head, A, is the vertical height from the water source to the

pump. The Delivery Head, B, is the vertical height from the pump to the

tank, and the Total Static Head is A + B. On the suction side the pump

has to overcome not only static suction head but also the friction head in

the pipe, including the loss at fittings. This extra head is represented by x

and for convenience is shown as the equivalent suction lift. Similarly

there is a friction head, y, for the delivery side. With a very long delivery

pipe this may represent the larger part of the delivery head, but can be

kept at a minimum by increasing the diameter of the delivery pipe.

The total Working Head (A + B + x + y) determines the load against which

the pump works, and thus the general type of pump required and the

power of the driving motor. To test the performance of a pump the

characteristics listed above are measured under closely controlled

conditions in the laboratory. The pump is run over its full operating range

and the data from the tests are presented either in tabular or graphical

form. Using these results, which are generally included in pump

catalogues, a pump can be selected which will meet the requirements of

any job and which will operate at the best efficiency. Examples of

performance graphs for different types of pump are given in the sections

which follow.

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3.0 TYPES OF PUMP

Of the pumps in use on farms, by far the greatest number belong to one

of three main types, Reciprocating, Rotary and Centrifugal. There are

other types, for example Jet pumps, Airlift pumps and Hydraulic Rams.

• Reciprocating Pumps

Construction

A typical reciprocating pump has a ram or piston working

backward and forward within a cylinder or pump barrel. This

motion is usually obtained from a crank revolving at constant

speed, and a connecting rod. Automatic valves control the flow of

liquid into and out of the cylinder. Such a piston pump may be

single-acting, in which flow occurs only in alternate strokes, or

double-acting where the flow occurs every stroke, and is thus

more uniform. See Figures 2 and 3. Reciprocating pumps are

characterised by the intermittent delivery of the liquid.

Figure 2

Single-acting Piston Pump

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Figure 3

Double-acting Piston Pump

A deep-well reciprocating pump operates in the same way as

described above, but the crank is at ground level above the well,

and the cylinder is placed down the well at the lowest expected

water level. A ‘drop pipe’ carries the pumped water from the

cylinder to the surface, and long rods transmit the reciprocating

motion from crank to piston. See Figure 4.

Figure 4 Figure 5

Deep-well reciprocating pump Diaphragm pump

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The diaphragm pump, commonly used for pumping milk, is also a

positive displacement reciprocating pump. It works in the same

manner as the piston pump, but the piston is replaced with a

flexible rubber diaphragm. This is shown in Figure 5.

Performance

The Reciprocating pump delivers a volume of liquid which changes

in proportion to speed, but which is almost independent of delivery

head. So for a particular pump speed, the head/discharge

characteristic is nearly vertical:

TOTALHEAD(metres)

Slow Medium Fast

DISCHARGE, (l/min)

Piston pumps are capable of pumping to high heads, 200 metres

or more, and are quite widely used in water supply work for small

flows, but because of their high cost they are not economical for

low heads. They operate at low speeds, usually 50-250 rpm, so a

large speed reduction is necessary where a petrol engine or electric

motor is used.

• Rotary Pumps

Construction

A rotary pump consists of two cams or gears which mesh together

and rotate in opposite directions inside an oval casing. The

rotating parts fit the casing closely, and the liquid trapped

between them and the casing is forced through the pump as they

rotate. A definite amount of water depending on the size and shape

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of the gears, is passed with each revolution – the gear pump is

shown in Figure 6.

Figure 6

Gear Pump

Performance

Rotary pumps have a head/discharge characteristic similar to

reciprocating pumps but provide a steadier flow, do not require

priming, and, having no valves, are of simpler construction.

They have almost the same suction characteristics as

reciprocating pumps, and will pump against high heads. They

have not proved popular for water supply work as they wear badly

if the water contains sand or grit. Their main use is in hydraulic

systems and sprayers, or for the pumping of viscous fluids like

molasses.

• Centrifugal Pumps

Construction

The rotating part of this type of pump is called the impeller. This

impeller may be shaped either to force the water from the inlet at

the centre of the impeller outwards, at a right angle to the pump

axis – this is called radial flow – or to force the water in the

direction of the pump axis – axial flow. Axial flow machines are

more correctly referred to as propeller pumps.

The impeller, fitted with blades or vanes usually curved backwards

towards the tips, is fixed to a spindle, and rotates rapidly inside a

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casing. The impeller may be of the closed type where the vanes are

mounted between discs, or of the open type consisting of a hub to

which the vanes are attached. The open impeller does not have

such a high efficiency as the closed type, but is less likely to

become clogged and hence is better adapted to handling liquids

containing solids.

The casing of centrifugal pumps may be of the volute or turbine

type. In the volute the casing gradually increases in cross-section,

causing the velocity head of the water leaving the impeller to be

changed into pressure head as the velocity is reduced towards the

outlet. In the turbine, or diffuser type, a series of fixed blades has

the same effect: this type is not used much in water supply or

irrigation, except in deep well pumps, where the turbine casing

shape lends itself to a cylindrical body. The difference in casing

shape is shown in Figures 7 and 8.

Figure 7 Figure 8

Volute Centrifugal Pump Turbine Centrifugal Pump

Centrifugal pumps give their best performance at a certain rate of

discharge and head, and their efficiency falls off when these

conditions are varied. The head against which a centrifugal pump

with a single impeller – called a Single-Stage Pump – will pump

satisfactorily is usually not very great, less than 50 metres without

undesirably high speeds. For higher heads a Multi-Stage Pump is

used, with two or more impellers arranged in such a way that the

discharge from one impeller enters the centre, or eye, or the next

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impeller so that delivery head is increased in stages – see Figure 9.

This principle is used in many submersible pumps, where pump

and motor are both placed below the water surface. Delivery to the

surface is through a riser pipe on which the assembly is

suspended.

Figure 9

Multi-stage Centrifugal Pump

Centrifugal pumps need to be primed before starting unless they

are located below the source of water. The suction performance of

centrifugal pumps is not very good, generally less than 5 metres,

and they should be placed as near to the water as possible. Proper

arrangement of the piping is essential if the pump is to operate at

its greatest efficiency. Figure 10 shows a typical centrifugal pump

installation.

Figure 10

Typical Centrifugal Pump Installation

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A propeller-type pump has only two or four blades, and thus has

large unobstructed passages which permit the handling of water

containing debris without clogging. On larger pumps the blades

are adjustable for maximum efficiency. A typical installation is

shown in Figure 11.

Figures 11

Typical Propeller Pump Installation

Performance

For a centrifugal or propeller pump, the delivery, head and power

input all increase as speed is increased. The delivery is

proportional to speed; head is proportional to the square of the

speed; power input is proportional to the cube of the speed. This

means that if we double the speed, the delivery doubles, the head

is increased 4 times, and the power input is increased 8 times.

A typical performance curve for a centrifugal pump, as obtained in

a laboratory test, would appear as Figure 12.

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H e a d(M e t r e s )

P o w e r (k W )

E f f i c i e n c y(% )

P o w e r

E f f i c i e n c y

H e a d

D i s c h a r g e (1 / m i n )

Figure 12

Typical Performance Curve Centrifugal Pump

Repeating the test over the complete speed range of the pump

gives an overall picture of the operational characteristics of the

pump.

Unlike positive displacement pumps, the head developed by a

centrifugal pump varies with the flow or discharge produced by

the pump. In other words the pump can pump a lot of water at a

lower head or a smaller amount of water at a higher head.

The exact flow and head is determined by the interaction of the

pump and pipe system (more about this next year).

It must be noted also that there is a particular flow and head at

which the pump operates most efficiently.

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3.1 Summary of Pump Characteristics

Advantages Disadvantages

Reciprocating

Pumps

Positive action.

Efficient over a wide range of delivery

and head.

Discharge pulsates.

Subject to vibration.

Sometimes noisy.

Rotary Pumps Positive action.

Occupy little space.

Wide range of speed.

Steady discharge.

Subject to abrasion.

Likely to get noisy.

Centrifugal Pumps Simple design.

Quiet operation.

Steady discharge.

Efficient for pumping large volumes.

Suitable for direct connection to

electric motor.

May be either horizontal or vertical.

Can be multi-staged.

Propeller Pumps Generally as above.

Usually vertical mounting.

Pump very large quantities at low

heads.

Widely used for land drainage and

flood control.

4.0 PUMP CONTROLS

4.1 Pressure Tanks

Historically pressure tanks were the only means of controlling pumps.

With the advent of other control systems, pressure tanks are losing

favour but there are still large numbers in use today.

Pressure tanks operate by pumping water into the tank and compressing

the air trapped at the top of the tank. The pumping operation continues

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until the air pressure reaches the preset level on the pressure switch and

the pump cuts out.

During the initial stages of water draw-off the air in the tank forces the

water out of the tank into the pipe system. When the air pressure drops

to a preset point the pressure switch starts the pump. The time between

stopping and starting of the pump is known as the cycle time. Most

people assume that the buffer storage capacity of the pressure tank is

equal to the volume of the tank. This, in fact, is incorrect.

The volume of water stored between cut in and cut out is controlled by

Boyle’s Law. This law states that the product of the initial pressure and

volume is equal to the product of the final pressure and volume, i.e.

Pi Vi = Pf Vf

where i = initial

f = final

For example:

Assuming that a 300 litre pressure tank is empty at atmospheric

pressure, and that the cut in pressure is 5 bars absolute (i.e. not gauge

pressure). Therefore:

Pi = 1 bar absolute

Vi = 300 litres of air

Pf = 5 bar absolute

Vf = ?

from Pi Vi = Pf Vf

Vf = P V

P

i i

f

= 1 x 300

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= 60 litres of air

That is the air which did occupy 300 litres now occupies only 60 litres. So

the volume of water in the tank is = 300 - 60 litres.

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= 300-60 litres

= 240 litres of water stored

If the cut out pressure is set at 7 bar absolute, then:

Pi = 5 bar absolute

Vi = 60 litres of air

Pf = 7 bar absolute

Vf = ?

from Pi Vi = Pf Vf

Vf = P V

P

i i

f

= 5 bar x 60 litres

7 bar

= 42.8 litres of air

That is the air now occupies 42.8 litres, so the volume of water in the

tank =

= 300-42.8 litres of air

= 257.1 litres

The amount of water stored between cut in and cut out is the difference

between these two numbers, i.e. volume of water stored

= 257.1 – 240.0

= 17.1 litres

The volume stored is known as the Effective Water Storage (EWS) and in

this case is considerably smaller than the volume of the tank. These

calculations can be represented graphically (Figure 13). Note that

precharging the cylinder with air significantly increases the EWS.

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P r e s s u r e t a n k v o l u m e ( l i t r e s )

0

1

2

3

4

5

6

7

0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0

N o p r e c h a r g i n g .

C u t i n

C u t o u t

C y l i n d e r p r e -c h a r g e dt o 3 b a r a b s o l u t e .

E W S E W S = 1 7 l i t r e s

Figure 13

Pressure/Volume Relationship in a 300 1 Pressure tank

The disadvantage of pressure tanks system is that air at high pressure

dissolves into the water with the result that over a period of time the

water will fill the air space. This is known as water logging and reduces

the EWS, decreases the cycle time and increases the cycle frequency.

Rapid cycling (pump starting and stopping frequently) increases the wear

and tear on the pump motor and can lead to overheating problems.

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4.2 Aquacells

The low EWS and water logging characteristic of the pressure tank led to

the development of the aquacell (Figure 14). Since the aquacell only

provides a means of operating the pressure switch, its size is kept to a

minimum. The rubber bladder prevents water logging.

The effect of a small EWS (3-5 litres) with large capacity pumps means

that cycling times will be small. The inclusion of a modified check valve

(Figure 15) will help overcome the problem.

Figure 14

Aquacell Pressure Unit

Figure 15

Operation of Modified Check Valve

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The modified check valves allow full flow when the aquacell is draining

but restricts the flow during filling, therefore increasing filling time and

cycling time.

P r o b e s i g n a la c t i v a t e s s t a r t /s t o ps y s t e m

P r o b e C a b l e 0 -1 0 k m

6 v o l t A / C p o w e r s u p p l y

N e u t r a l

L o w

H i g h

M o t o r

P u m p

3 ¿ p o w e r

Figure 16

Probe Control System

4.3 Probe Control (Figure 16)

In situations where water is being pumped to a storage tank the water

level in the tank can be monitored using probes. The probes are set to

detect maximum level (pump cut out) and minimum level (cut in). The

third probe is a neutral probe. Electrical contact between the probes

relies on the conductivity of the water.

4.4 Flow Sensing Control

The low flow control has a valve which isolates the pressure switch from

the main line at flows greater than 3-5 litres/minute, i.e. at flows above

these, regardless of mainline pressure, the pump will not switch off. The

potential exists for the pump to operate at or near maximum head and

therefore pipelines must be able to withstand these pressures.

Motor

3 phase power

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Recent developments have produced a flow sensing control unit which

can eliminate the need for storage vessels. The use is made of two

electronic sensors one of which detects the initial drop in pressure within

the line when a demand for water is made. This first sensor starts the

pump which satisfies the flow requirements but raises the pressure. But

as soon as flow starts from the pump a lightly spring loaded valve is

moved to a position where the second sensor can electronically override

the pressure sensor and so flow will continue regardless of the demand,

down to a minimum flow of about 20 litres/hr – these units are only

suitable for centrifugal pumps. A strong advantage of these units over a

pressure switch is that if no water is able to flow for some reason, the

pump will only run for a few seconds before switching off so preventing

pump damage. This protective switching requires manual resetting after

the flow problem is solved.

Figure 17

Flow Sensing Pump Control

4.5 Float Switch Control

Modern float control switches enable tank levels to be controlled by a

simple plastic float with a mercury switch within and attached to a

waterproof electrical cable. This cable is secured to the roof of the tank

with an adjustable clamp which allows level control. When the level of

water is low the float hangs from the cable and the switch causes the

pump to start. As the water level rises the unit floats on the water and

when it eventually comes to a horizontal position the pump is switched

off.

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Figure 18

Float Switch Pump Control