pumps.pdf

FM20

CAPTURE

CENTRIFUGAL PUMP DEMONSTRATION

UNIT

FM20

LECTURERS' NOTES

Parameters File

All numerical constants involved with the Capture unit are included in the file P ARAM. TXT .

The file can be altered with any text editor (eg. the MSDOS ~nv command), should the user wish to change any of the values.

Care should be taken not to damage or corrupt the file. Should this happen, re-install the software from the floppy disks.

Assignment File

The questions given in the student assignment are all stored in the file FM_ASGN.TXT.

The assignment file already has several questions included, but is designed to allow lecturers to enter their own questions, depending on the particular course content.

As for the parameters file, any text editor can be used to edit the questions. The format should be similar to the questions already included. Answers should be placed at the end of the question surrounded by the {} brackets. Font Installation

software, the font To ensure correct operation of the Capture

IGreekMathSymbolsl should be installed in Windows.

This can be checked by double clicking on Control Panel in the Main program group of the Program Manager. SELECT Fonts and check that 'Greek/Math/Symbols' is included in the installed fonts list.

If it is not, click on Add... and select the font 'SYMBOLS' from the list of fonts in the windows\system directory, then click on OK.

The font should now be shown in the installed fonts list.

The following manual is taken from the help screens available within the software.

ARMFIELD LIMITED

OPERATING INSTRUCTIONS AND EXPERIMENTS

FM20

-

CAPTURE CENTRIFUGAL PUMP DEMONSTRATION UNIT

PAGE NO.

1

SAFETY

5 RECEIPT OF EQUIPMENT

MINIMUM COMPUTER SYSTEM REQUIREMENTS 6

INSTALLING THE ARMFIELD INTERFACE CONSOLE IFD 7

INSTALLING THE SOFTWARE 8

DESCRIPI10N

10

PREPARING THE CENTRIFUGAL PUMP

DEMONSTRA nON UNIT FOR USE

13

ROUTINE MAINTENANCE 15

SAFETY IN THE USE OF EQUIPMENT SUPPLIED BY ARMFIELD

Before proceeding to install, commission or operate the equipment described in this instruction manual we wish to alert you to potential hazards so that they may be avoided.

Although designed for safe operation, any laboratory equipment may

.

INJURY THROUGH MISUSE

INJURy FROM ELECfRIC SHOCK (Particularly in presence of water)

.

INJURy FROM ROTAnNG COMPONENTS

.

RISK OF INFEC110N THROUGH LACK OF CLEANLINESS

.

Accidents can be avoided provided that equipment is regularly maintained and staff and students are made aware of potential hazards. A list of general safety rules is included in this manual, to assist staff and students in this regard. The list is not intended to be fully comprehensive but for guidance only.

Please refer to the notes overleaf regarding the Control of Substances Hazardous to Health Regulations.

1

involve processes or procedures which are potentially hazardous. The major potential hazards associated with this particular equipment are listed below.

The COSHH Regulations

Regulations

to Health The Control (1988)

of Substances Hazardous

The COSHH regulations impose a duty on employers to protect employees and others from substances used at work which may be hazardous to health. The regulations require you to make an assessment of all operations which are liable to expose any person to hazardous solids, liquids, dusts, vapours, gases or micro-organisms. You are also required to introduce suitable procedures for handling these substances and keep appropriate records.

Since the equipment supplied by Armfield Limited may involve the use of substances which can be hazardous (for example, cleaning fluids used for maintenance or chemicals used for particular demonstrations) it is essential that the laboratory supervisor or some other person in authority is responsible for implementing the COSffii regulations.

Part of the above regulations are to ensure that the relevant Health and Safety Data Sheets are available for all hazardous substances used in the laboratory. Any person using a hazardous substance must be informed of the following:

Physical data about the substance Any hazard from fire or explosion Any hazard to health

Appropriate First Aid treabnent

Any hazard from reaction with other substances How to dean/ dispose of spillage

Appropriate protective measures Appropriate storage and handling

Although these regulations may not be applicable

in your country

I

it is

strongly recommended that a similar approach is adopted for the protection of the students operating the equipment. Local regulations must also be considered.

Water-Borne Infections

The equipment described in this instruction manual involves the use of water which under certain conditions can create a health hazard due to infection by harmful micro-organisms.

For example, the microscopic bacterium called Legionella pneumophila will feed on any scale, rust, algae or sludge in water and will breed rapidly if the temperature of water is between 20 and 45°C. Any water containing this bacterium which is sprayed or splashed creating air-borne droplets can

produce a form of pneumonia potentially fatal.

called Legionnaires Disease which is

Legionella is not the only hannful micro-organism which can infect water, but it serves as a useful example of the need for cleanliness.

be

must

Under the COSHH regulations,

observed:-the following precautions

Any water contained within the product must not be allowed to stagnate, ie. the water must be changed regularly.

Any rust, sludge, scale or algae on which micro-organisms can feed must be removed regularly, i.e. the equipment must be cleaned regularly.

Where practicable the water should be maintained at a temperature below 20°C or above 4S°C. If this is not practicable then the water should be disinfected if it is safe and appropriate to do so. Note that other hazards may exist in the handling of biocides used to disinfect the water.

the risk A scheme should be prepared for preventing or controlling

incorporating all of the actions listed above.

Further details on preventing infection are contained in the publication "The Control of Legionellosis including Legionnaires Disease" - Health and Safety Series booklet HS (G) 70.

USE OF EARTH LEAKAGE ELECTRICAL SAFETY DEVICE

CIRCUIT

BREAKER

AS AN

The equipment described in this Instruction Manual operates from a mains voltage electrical supply. The equipment is designed and manufactured in accordance with appropriate regulations relating to the use of electricity. Similarly, it is assumed that regulations applying to the operation of electrical equipment are observed by the end user.

However, to give increased operator protection, Armfield Ltd have incorporated a Residual Current Device or RCD (alternatively called an Earth Leakage Circuit Breaker

-

ELCB) as an integral part of this equipment. If through misuse or accident the equipment becomes electrically dangerous, an RCD will switch off the electrical supply and reduce the severity of any electric shock received by an operator to a level which, under normal circumstances, will not cause injury to that person.

At least once each month, check that the RCD is operating correctly by pressing the TEST button. The circuit breaker MUST trip when the button is pressed. Failure to trip means that the operator is not protected and the equipment must be checked and repaired by a competent electrician before it is used.

RECEIPT OF EQUIPMENT

SALES IN THE UNITED KINGDOM .1.

The apparatus should be carefully unpacked and the components checked against the Advice Note. A copy of the Advice Note is supplied with this instruction manual for reference.

Any omissions or breakages should be notified to Armfield Ltd wi thin three days of receipt.

SALES OVERSEAS

2.

The apparatus should be carefully unpacked and the components checked against the Advice Note. A copy of the Advice Note is supplied with this instruction manual for reference.

Any omissions or breakages should be notified immediately to the Insurance Agent stated on the Insurance Certificate if the goods were insured by Armfield Ltd.

Your own insurers should be notified immediately if insurance was arranged by yourselves.

MINIMUM COMPUTER SYSTEM

REQUIREMENTS

Appropriate CAPTURE Demonstration Unit.

CAP'IURE Interface Device (Cat. Ref.IFD)

Disk containing software (~") entitled "FM20 Centrifugal Pump

"

IBM 386 Microcomputer (or 100% compatible)

Windows 3.1

1.44MB 3 1/2" floppy drive.

Printer (if copies of results are required).

An SWAt Integrating Wattmeter may be used to measure the electrical power supplied to the electric motor associated with this hydraulic machine. Since this Wattmeter takes readings of the current and voltage supplied to the electric motor, any noise on the electrical supply or earth connection will result in noisy readings from the Wattmeter. Excessive noise on the electrical supply will therefore cause the Wattmeter readings displayed on the computer monitor to be unstable. A stable/noise free electrical supply is therefore required for optimum results.

INSTAlliNG THE ARMFIELD INTERFACE CONSOLE IFD

The ARMFIELD POD is the interface between the sensors and the software in the PC It plugs into a parallel port on the PC, and any printer that would occupy this port may be plugged into the output printer port on the POD.

A program (sortport.exe) is supplied as part of the software, and will run during the installation process. The program will identify the parallel port(s) , and, in the case of computers with more than one such port, it will offer the user the choice of locations for the POD.

The sortport.exe program may be run after the initial installation of the software should the user wish to change the port being used.

INSTAlliNG THE SOFTWARE

Each item in the Armfield CAPTURE range of equipment is supplied with a program which runs under Microsoft Windows 3.1.

The application disk contains a set-up program which will install the software onto your hard disk. The default condition will install the software into a Windows Group named ARMFIELD. Should you wish to change this, you will have the opportunity during the set-up.

To install the software, place the applications disk into the floppy drive A:. Choose 'File' from the menu bar in the Windows Program Manager, and then choose 'Run' from the drop-down menu. Type

A:SETUP

in the command box, and then choose 'OK'.

The installation procedure may take a little while, as the files have to be decompressed.

.

. .

Installing the software for the first time:

the software, To ensure correct operation of the Capture

'GreekMathSymbols' should be installed in Windows.

font

This can be checked by double clicking on Control Panel in the Main program group of the Program Manager. Select Fonts and check that 'GreekMathSymbols' is included in the installed fonts list.

If it is not, click on Add... and select the font 'SYMBOLS' from the list of fonts in the windows \ system directory, then click on OK.

1]

7

7~

~

It

0

0

~

6 5

2

9 7

DESCRIPTION

All numerical references in brackets relate to the diagram on page 9.

The equipment comprises of a centrifugal water pump (6) driven by an electric motor (19) which is mounted on a support plinth (2) together with a clear acrylic reservoir (11) and associated pipework for continuous circulation. Clean water is used as the operating fluid and a drain valve (10) at the base of the reservoir allows the water to be drained after use. Appropriate sensors are incorporated on the unit to facilitate analysis of the pump performance when connected to the par~el port of a suitable microcomputer via an Armfield 'POD' interface (IFD).. In addition to the tappings required by the pressure sensors, additional tappings (5, 15 and 18) are included in the pipework to allow appropriate calibration instruments to be connected.

The flow of water through the centrifugal pump is regulated by a flow control valve (16) installed in the discharge pipework of the pump. Adjustment of this valve allows the head/flow produced by the pump to be varied. A valve (9) in the inlet pipework of the pump allows the effect of suction losses to be investigated.

A spare impeller (8) is installed on the plinth to allow visual inspection of the impeller which is installed inside the volute of the water pump.

the performance of the The following sensors are used to monitor

centrifugal water

pump:-.

Differential pressure sensor SPWI connected to Channell on IFD:

This comprises of a pressure sensitive piezoresistive device with appropriate signal conditioning all contained in a protective

case

(13) and is used to measure pressure developed across the orifice plate (14) installed in the discharge pipework of the pump. .The volume flow rate of water through the pump can be calculated using this measurement.

The sensor is connected to the appropriate tappings in the pipework using flexible tubing. Additional tappings (15) are provided for the connection of appropriate instrumentation (not supplied) to facilitate calibration of the differential pressure sensor.

Qifferential pressure sensor SPW3 connected to Channel 2 on IFD:

This comprises of a pressure sensitive piezoresistive device with appropriate signal conditioning all contained in a protective case (4) and is used to measure the difference in pressure between the inlet

and outlet of the centrifugal pump. The head developed by the pump can be calculated from this measurement.

The sensor is connected to the appropriate tappings in the pipework using flexible tubing. Additional tappings (5 and 18) are provided for the connection of appropriate instrumentation (not supplied) to facilitate calibration of the differential pressure sensor.

Rotational speed sensor 5501 connected to Channel 3 on IFD:

This comprises of a reflective infra-red opto switch (1) on a remote lead with appropriate signal conditioning in a protective case (3) and is used to measure the rotational speed of the motor/pump impeller.

The opto switch is mounted on a support bracket adjacent to the end of the motor shaft which incorporates a reflective strip to facilitate measurement of the rotational speed. An appropriate non-contacting optical tachometer (not supplied) may be used to calibrate the rotational speed sensor.

.

A tem~erature sensor STSl connected to Channel 4 on IFD:

This comprises of a temperature sensitive semiconductor device (17) on a remote lead with appropriate signal conditioning in a protective case (7) and is used to measure the temperature of the water entering the centrifugal pump.

The sensor is inserted through the wall of the pipe using a waterproof gland. The sensor may be removed from the gland for the purpose of calibration using appropriate equipment (not supplied).

In addition to the above sensors. which are all ~ermanentl~ attached t,Q, the

FM20 unit. an IntegIating Wattmeter (SWA1) ma~ be connected to

ChannelS on IFD:

The Wattmeter is connected between the mains lead (20) from the pump and a suitable power supply to facilitate measurement of the electrical power supplied to the motor. The Integrating Wattmeter may be calibrated using a suitable twin trace oscilloscope (not supplied).

When using the FM20 program in conjunction with a suitable microcomputer, measurements from the above sensors are displayed and used to compute appropriate calculated variables. These allow the following performance curves to be displayed on the monitor or copied to a printer:

Volume Flow Rate

1. Rotational Speed versus

Volume Flow Rate

2. Motor Input Power versus

Volume Flow Rate

3. Pump Total Head versus

Volume Flow Rate

4. Pump Power Output

versus

Volurne Flow Rate

UNIT

PREPARING THE CENTRIFUGAL PUMP DEMONSTRATION

FOR USE

All numerical references in brackets relate to the diagram on page 9

Before using the unit for the first time attach the two sections of delivery pipework (12) between the discharge of the centrifugal..pump (6) and the reservoir (11). Ensure that all unions are tight before filling with water. Connect the flexible tubing from sensor SPWI (13) to the tappings on the orifice plate (14) with LOW PI connected to the top tapping the ffiGH P2 connected to the bottom tapping.

Place the Centrifugal Pump Demonstration Unit in a suitable location adjacent to a compatible microcomputer.

Place the Interface IFD alongside the microcomputer. Place the Integrating Wattmeter SWAI (if available) alongside the IFD as convenient.

Ensure that all tappings in the pipework of the Pump Unit are connected to appropriate sensors or blanked.

Open the inlet valve (9) and close the outlet control valve (16).

Ensure that the drain valve (10) at the base of the reservoir is fully closed then fill the reservoir with clean, cold water.

The pressure sensors on the unit require priming with water before initial operation (and whenever the tank has been emptied and refilled). A hypodermic syringe and micro-bore tubing are supplied for this purpose.

To prime the tubes with water remove the flexible tubing from the PVC pipe by removing the pipe clip and gently pulling the tube from the stainless steel tapping. Fill the syringe with water and gently insert the micro-bore tubing into the sensor's flexible tubing until it is a few millimetres away from the sensor. Hold the flexible tubing vertically.

Slowly inject water into the tube until it is completely filled, then remove the syringe and micro-bore tubing, and replace the fleXIble tubing on to the stainless steel tapping.

There are two stainless steel tappings at each tapping point. One is connected to the sensor, whilst the other is used to connect a manometer for calibration. It should be noted that there is a difference between the two points. The sensor tapping is fitted with a nylon restrictor to dampen pressure fluctuations. The calibration tapping has no restrictor. Ensure the sensor is connected to the correct tapping point.

Connect the mains lead (20) from the motor of the centrifugal pump to the Integrating Wattmeter SWAI. Connect the Wattmeter to the POWER OUTPUT of IFD.

Connect the mains supply lead from an appropriate electrical supply to the MAINS INPUT socket on IFD ensuring that the voltage of the electrical supply is compatible with the console (indicated on the rear of the console).

Switch on the mains supply. Switch on the IFD. Check that the pump operates. Open the outlet flow control valve fully and allow water to circulate until all air bubbles are expelled. Switch off IFD.

Connect each of the sensor conditioning boxes to the appropriate SENSOR SOCKETS on the front of IFD, using the numbered connecting leads, as

follows:-Channell to sensor 5PWl (13) Channel 2 to sensor 5PW3 (4) Channel 3 to sensor 5501 (3) Channel 4 to sensor ST51 (7)

Channel 5 to the Integrating Wattmeter SW Al

The equipment is ready for use with the Armfield Windows software.

NOTE: The apparatus is classified as Education and Training

Equipment under the Electromagnetic Compatibility (Amendment) Regulations 1994. Use of the apparatus outside the classroom, laboratory or similar such place invalidates conformity with the protection requirements of the Electromagnetic Compatibility Directive (89 /336/EEq and could lead to prosecution.

ROUTINE MAINTENANCE

To preserve the life and efficient operation of the equipment it is important that the equipment is properly maintained. Regular servicing/maintenance of the equipment is the responsibility of the end user and must be performed by qualified personnel who understand the operation of the equipment.

the following notes should be

In addition to regular maintenance

observed:-The equipment should be disconnected from the electrical supply when not in use.

1.

Water should be drained from the equipment when it is not in use.

2.

The exterior of the equipment should be periodically cleaned. 00 NOT use abrasives or solvents.

3.

The reservoir should be periodically cleaned to remove debris and deposits on the walls. DO NOT use abrasives or solvents.

FM20

INDEX TO EXPERIMENTS Page No Experiment Warning! ii

Introduction - Instructional Objectives.

III

Introduction

-

Energy Transfer in a Pump.

Practical Exercise No 1

-

Using Engineering Units

PE2-1 Practical Exercise No 2

-

Pump Inherent Characteristics

PE3-1 Practical Exercise No 3

-

Pump Constant Speed Characteristics

PE4-1 Practical Exercise No 4

-

Introduction to Scaling

PEs-I Practical Exercise No 5

-

Pump Suction

.

Warning!

Each of the practical exercises described in the Labsheets help file requires the equipment to be set up and in working condition according to the instructions given in the menu-bar 'Install'. If you have not done this, or are not sure whether the unit is correctly set up, go back to the 'Install' pull-down screens, and follow the instructions given there to completion. The final test of readiness is made under the selection button 'Diagrm' when the effect on the measured variables of changing the various pump settings can be seen numerically on screen. For example, adjusting the position of the rotary dial by hand on the SW Al Integrating Watt meter will cause the pump speed to change, as well as the flow rate and head developed by the pump. Similarly, adjusting the valves on the pump inlet and outlet pipes by hand will also cause changes in pump flow and head. Check that the changes in these pump settings give trends in the measured variables, as displayed in the boxes on screen, which you would intuitively expect. If no change in pump speed or measured power or flow or head occurs whatever changes you make to the power input or to the valve positions, then clearly something is wrongly set up and you need to establish what the problem is by working through the 'Install' procedures again.

Introduction

-

Instructional Objectives.

The objects of the practical work exercises described in the 'Labsheet' help screens, are to understand the operating characteristics of a centrifugal pump.

In this type of pump

(Fig 1), the fluid is drawn

into the

centre

of a

rotating

impeller and is thrown outwards by centrifugal action. As a result of the

high speed of rotation,

the

liquid acquires a high kinetic energy. The

pressure difference between the suction and delivery sides arises from the conversion of this kinetic energy into pressure energy.

~

EfficiaM:yE N-lOOO N-lSOO ~ N'".2000 rev/mia 1500 ~p 1000 HaclH :ttt1" ~ ACentrifuaalPmnp Fig. Disc8geQ

F II- 2 ()pa8iDc CI8KtaiIIicI of . C=rifup1 ~

The operating characteristics of a pump are often conveniently shown by plotting head H, power P, and an efficiency E against discharge flow Q for a series of constant speeds N, as shown in Fig 2. It is important to note that the efficiency reaches a maximum and then falls, whilst the head at first falls slowly with Q but eventually falls off rapidly. The optimum conditions for operation occur when the required 'duty point' of head and flow coincides with a point of maximum efficiency.

This Armfield 'Capture' unit, ref FM20, is designed to allow students to determine the operating characteristics of a centrifugal pump rapidly and meaningfully, using 'on-line' data acquisition and analysis. Test results may be displayed in tabular and graphical forms, and it is a simple matter to repeat or add to the data to cover areas of the pump performance of particular interest

At the conclusion of the work, students are asked a series of questions 0 n an interactive basis, to ensure that a true understanding of pump characteristics has been gained.

[

Introduction - Energy Transfer in a Pump.

Fluid machines are usually characterised in two distinct classes: rotodynamic or positive displacement. In the former of these, relative motion is required between the rotating element of the machine (the 'rotor') and the fluid stream, whereas in the latter case the machine components mechanically displace a set volume of fluid. In a rotodynamic machine, therefore, the changes in fluid velocity and pressure between inlet and outlet are of considerably greater significance in determining performance than for a positive displacement machine, where essentially machine speed is the key operating parameter.

The centrifugal pump, of which the Armfield FM20 unit is a small-scale example, is a radial flow rotodynamic machine, wherein fluid enters the rotor or impeller at one radius and leaves at a larger radius. In so doing, changes in kinetic, potential and pressure energy occur, and any understanding of pump behaviour and performance assessment requires measurement or calculation of these quantities.

The general relationship between the various forms of energy, based on the 1st Law of Thermodynamics applied to a unit mass of fluid flowing through a 'control volume' (such as the pump itself) is expressed

as:-(1)

voLdp + F

-WI = d(V2/2) + g.dz +

where:-is the mechanical shaft work performed on the fluid d(v2/2) is the change in kinetic energy of the fluid

is the change in potential energy of the fluid

is the change in pressure energy, where 'vol' is the volume per unit mass of the fluid. For an incompressible fluid of constant density Rho , this term is equal to jdp/Rho or (P2-PI where P2 refers to the pump discharge outlet andpI to the pump inlet.

F

is the frictional energy loss as heat to the surroundings or in heating the fluid itself as it travels from inlet to outlet.

(2) where subscript 2 refers to the pump outlet and subscript 1 to the inlet. The term Wa represents the actual work performed in changing the energy stages of a unit mass of the fluid. This may alternatively be presented as the total dynamic head H of the pump, by converting the units from work per unit mass to head expressed as a length

:-(3)

It can be assumed for the purposes of the following practical experiments that the fluid is incompressible (ie. Rho is constant).

FM20

Practical Exercise No 1

-

Using Engineering Units

Qbjective:-To ensure users fully understand the conversion of measured units of quantity to those of the variables necessary to calculate pump performance.

Theoretical

Background:-The basic tenns used to define, and therefore measure, pump performance include

i) ii)

iii) power input and efficiencies.

Each of these is considered in turn.

i) Discharge Qv

The discharge, or flow rate or capacity, of a pwnp is the volume of fluid pwnped ~er unit time. In 51 units, this is expressed in cubic metres per second m Is, or, for convenience with small flows, in cubic decimetres per second dm3/s.

The Armfield FM20 unit employs an orifice plate in the pump discharge

pipeline to measure

QYI

according to the conventional relationship

between the measured pressure drop dpo across the orifice and the flow rate

:-(4) Where Cd is the orifice discharge coefficient.

(This applies when the orifice diameter d is no more than 50% of the pipe diameter.)

The appropriate constants needed to use this equation for deducing

discharge

Qv

from dpo are given in the "Params" section of the menu-bar.

Similarly, the calibration of the orifice necessary to confirm these values of parameters is described in the "Calibrt" section of the menu-bar.

ii) Head H

The term 'head' refers to the elevation of a free surface of water above or below a reference datum. Terms specifically applied to the analysis of

L

discharge, head.

FM20

pumps and pumping systems are illustrated graphically in Fig 3, and are briefly defined below.

T

-

-

I I VI T

-~~ff~~

Lioe

--H

---Hydraulic Gradient I. Pwnp Inlet 2. Pwnp <Altlet H Total Head (m) v Flow velocity (m/~) 2 P Pressure (N/m )_{- -2- -} .!!- --~ I Rho.1 r PI &g 1 - ~ r=- h:::~Da1um Line F1ow- - +-~ -:I" --+- - ' '' / z-O

\ 1-1- :.-- -

3>--2

"-v Pump

Fig 3 Definitions of Head across a pump.

Manometric suction head ~1 is the suction gauge reading (metres) measured at the suction nozzle of the pump referenced to the impeller centre line datum.

1)

Manometric discharge head Hm2 is the discharge gauge reading (metres) measured at the discharge nozzle of the pwnp referenced to the impeller centreline.

2)

Velocity head relates to the kinetic energy of the fluid when flowing at a velocity v, ie. v2 /2g

3)

therefore, the suction velocity head is v~ /2g and the discharge velocity head is v~ /2g.

4) Total dynamic head (H) is the head against which the pump must

work when fluid is being pumped. For the pump shown in Fig 3, H is given by:

(5)

Equation (3) relates precisely to Equation (5) for a control volume enclosing the pump outlet and inlet, as

~

and ~1 are the measured pressures equal to

Z2 + (P2/RhO) and ~ + (PI/RhO) respectively.

The Armfield FM20 instrumentation is such that (Hm2

-

HmJ is actually

FM20

subsequent calculations to dpp/Rho.g metres (see 'Formulae Used' in this Help File).

iii) Power Input and Efficiencies

The power P consumed by the fluid in producing the total dynamic head H

at a discharge

Qy

is given by Equation

(6):

(6)

[Nm/s

=

Watts]

P = Rho.g.Qy.H

However, the fluid friction 'losses' in the pump itself, represented as Fin

Equation

(1), require a hydraulic efficiency

En

to be defined

as:-'Useful' fluid power absorbed (P u)

x

1000/0

E =_h

Power supplied by the impeller (Ph)

Further, the mechanical losses in the bearings etc. require a mechanical

efficiency Em to be defined as

The Armfield FM20 Centrifugal Pump Unit does not include the direct measurement of mechanical power Pm for cost reasons, but instead measures electrical power P Sf to the pump motor. A further efficiency is therefore required, expressing the electro-mechanical losses in the motor

E:-E

=

Power supplied to the impeller (P m)xl000/o e Power supplied to the motor (P gr)

The overall efficiency Egr is

thus:-It will be seen that

Egr = ~.~.Ee

FM20

EQuipment

Set-U~:-As described in the 'install' help section, with the pump running at a maximwn speed setting (100% setting on SWA1). The 'Diagrm' screen button from the menu bar should be selected, in order that the measured variables are displayed on-line in the appropriate boxes of the pump schematic diagram.

Procedure:-Open inlet valve VI fully. Close discharge valve V 2 then start the pump

(pump motor started under minimum load). Open discharge valve V

2

fully, and allow the water to circulate until all air bubbles have dispersed. Select 'Diagmt' and note the value of the volume flow indicated at the

bottom of the screen.

Gradually close discharge

valve V

2

until the volume

flow is approximately half of the maximum reading.

When the indicated readings of the 5 measured variables are reasonably constant, select the 'Take Sample' button from the menu-bar. It is only necessary to take one set of results for this exercise. Now select the Tables' button from the menu-bar, and you will see the results of your test sample laid out as one row of a Tab.1e under one of two headings:- 'Measured Variables' and 'Calculated Variables'. Write down on a piece of paper the values of the measured variables your sample took, as

follows:-Differential pressure across orifice dpo Differential pressure across pump dpp Motor rotation speed N

Motor Input Power P

f

Water Temperature w [kPa](Hz] (W] [DegC]

:-From your own classroom notes, or using the appropriate theory and equations in the 'Theory' section of the Help menu, you can calculate the various pump performance variables for yourself and compare your calculated figures with those computed in this software and displayed as 'Calculated from Measurements' in the row of tabulated results from your sample. In your own calculation, you will have to use the values of the various physical constants listed in the menu choice button 'Params'.

FM20

FM20

Objective:-To obtain a head-flow curve for a centrifugal pump operating at inherent speed.

Theoretical

Background:-The best way to describe the operating characteristics of a Centrifugal Pump is through the use of characteristic curves. (Fig 4). This figure shows the interrelation of discharge pressure or head H , capacity

~

, and efficiency Egr , and power input P gr , for a given pump at inherent speed (motor speed changes with load). The H - Qy curve shows the relation between total head and capacity. The pressure increase created by a centrifugal pump is commonly expressed in terms of the head of the fluid following. This discharge head H is independent of the density of the fluid.

In Fig 4, the

head increases

continuously as the capacity

is decreased;

this

type of curve is referred to as a rising characteristic curve. A stable head-capacity characteristic curve is one in which only one head-capacity can be obtained at anyone head. Pump selection should be made such that stable operating characteristics are available.

Fig 4. Characteristic curves for a single FM20 centrifugal pump.

The P gr

-

Qy curve of Fig 4 shows the relation between power input and

FM20

pump having the characteristics of Fig 4, maximum efficiency would occur at a volume flow rate of 0.7 dm3 / sec, and a total head of 6.75 metres.

Equipment

Set-Up:-Exactly the same as that for Practical Exercise No 1. Valve Vt should be fully open, and remain so for this exercise.

Procedure:-i) Select maximum pump speed N 1 by adjusting the power controller

to 1000/0.

ii)

Open inlet valve V

1

fully. Close discharge

valve V

2

then start the

pump (pump motor started under minimum load). Open discharge valve V 2 fully and allow the water to circulate until all air bubbles have dispersed. Select 'Diagrm' and note the value of the volume flow indicated at the bottom of the screen. Decide on suitable increments in flow to give adequate sample points (typically 15 points between zero and maximum flow).

iii) Close Valve V 2 to correspond to the condition of no flow ie. Qy = O.

When the measured readings as indicated in the boxes on the schematic diagram are sufficiently steady, select 'Take Sample'. This represents the first point on the characteristic curve. 00 NOT leave the pump in this condition of a closed outlet valve V 2 as the water will heat up and so change in viscosity as to invalidate the results! Go on to the next point (iv) as soon as

possible:-iv)

Open valve V

2

slightly, to give the first increment in volume flow

at the bottom of the screen. When readings are steady enoughi select 'Take Sample'.

v) Repeat step (iv) above for a gradually increasing set of valve V 2 openings, ie. increasing values of flow Qy. The final sample point will correspond to valve V 2 being fully open.

vi) The recorded set of head-flow data may now be examined and

assessed in the various selectable options of 'Graphs', 'Tables' or downloaded into a spreadsheet. (see 'Software' help screens if necessary).

FM20

Speed

Pump

Constant

Practical

Exercise

Characteristics

No

3

-

Objective:-To obtain the head-flow curves for a centrifugal pump at a range of pump speeds, and to relate these parameters to pump efficiency.

Theoretical

Background:-Pump Operating Characteristics

Pump manufacturers provide information on the performance of their pumps in the form of characteristic curves, such as those shown in Fig 5 for a typical, variable speed centrifugal pump.

Eft:.:icll:y % 'i 5 b.l 0.2 DiIcb.F Q (cu.m/~) 8.3 0

Fig 5 Typical characteristic curves for a 375mm dia impeller pump

(courtesy of Smith &; Loveless, quoted in 'Environmental

Engineering' by HS Peavy, DR Rowe &; G Tchobanoglous, McGraw-Hill 1985)

The curves allow engineers to see the maximum efficiency of a particular design of pump for a range of operating speeds, and hence helps with the selection of pumps for a required pressure-flow. Other charts may show the influence of change in impeller diameter, or altering the blade angle. Equipment

Set-Up:-Exactly as Practical Exercise No 2, with pump suction valve V 1 fully open

FM20

Procedure:-Choose a relatively low pump speed ego 10Hz, and repeat the procedure of Exercise No 2 above. Remember to take the series of sample points in ascending steps of increased flow, and also to correct the power controller setting to ensure the pump speed is the same for all head-flow readings.

i)

ii) Now increase the pump speed to a new, somewhat higher setting

ego 15Hz or even 20Hz, and repeat the taking of samples for gradually increasing values of flow rate, as in i) immediately above.

iii) Your recorded tables and corresponding graphs of head-flow at

different but fixed pump speeds should look like those in Fig 2 of the 'Introduction' section. It is instructive to superimpose the efficiency curves on to the graph, to indicate those combinations of head, flow and speed where efficiency is at a maximum.

1

FM20

Practical Exercise No 4

-

Introduction to Scaling

Objective:-To predict the head-flow characteristic at one pump speed from measured at another speed.

Theoretical

Background:-It is not practicable to test the performance of every size of pump in a manufacturer's range at all speeds at which it may be designed to run. Hence a mathematical solution is r~uired whereby assumptions can be made as to the operating characteristics of a pump running at one speed, impeller size, etc from experimental results taken at another.

The multiplicity of such curves, which result from dimensional plotting, can be reduced to a single curve if appropriate dimensionless groups are used. It turns out that, provided that the effects of fluid viscosity on pump performance are small and that cavitation (see later) is not occurring, the characteristic of a given type and shape of pump is represented by:

function of

JgL

N.D3

gH

N2I)T

= where N and D = pump speed (rpm or Hz) = impeller diameter (m)

For a single curve of the type suggested by Equation (9) to represent more than one operating condition of the particular type of pump, the criterion of 'dynamic similarity' must be fulfilled. That is, all fluid velocities at corresponding points within the machine are in the same direction and proportional to impeller speed. When this is the case, as for a particular pump operated at different speeds, a simple graph of data is formed, as depicted in Fig 6:

FM20

Flow coefficient Q/(ND)

'if

0+ +

Key: X<;o o<>~ x

x 2500 rpm + '<> 0

3500

<>

4500

+

5000

~ + \,9;.~ ~ _~

,

Fig 6 Dimensionless head-discharge characteristic of a particular

centrifugal pump operated at different speeds. (ack. to IIFluid

Mechanics, Thermodynamics of Turbomachinery" , SL Dixon,

Pergamon 1966.)

Use of these so-called 'affinity laws' allows performance of geometrically similar pumps of different sizes or speeds to be predicted accurately enough for practical purposes. Exact accuracy would require that effects of surface roughness of the pump, the viscosity of the fluid, etc. to be taken into account.

The methods used for forming these dimensionless groups will not be entered into here, but the groups themselves are known by the following names:

P / pN3Ds

= P

Q/ND3=cj) gH/N2D2 = 'II

the power coefficient the flow coefficient the head coefficient

These laws are most often used to calculate changes in flow rate, head and power of a pump when the size, rotational speed or fluid density is changed. The following formulae are derived from the above considerations, and allow calculation of head H, power P and efficiency E at one speed N] to be deduced from those measured at another speed N2:

Q) Q2 Nt

---N2

N2 -1. N2 2 HI H2 = (10) PE4-2 I I I

0.02

0.04

0.06

°50-~

.

~4.0.

bb

-

.~3.0-(.)

e

82.0-(.) -0 /:+- x

~

₁₀

I"\L det . . . ,i"/:+- <)

::c . - ~rve enoraDon m y-

.

.

perfonnance at high speeds (effect due to cavitation)

FM20

N3₁ N3 ₂

Pt

-

--P2

More generally, the relationship between two geometrically similar machines with characteristic diameters Dt and D2 operating at rotational speeds Nt and N2 is shown in Fig 7. For any two points at which values of

~gH

_{N2- D2 and}

₎

are the same (these are termed 'corresponding points'), it follows

that:-2 2

~

Nt

O2

-OJ

and

H2=H1 3 N2

(

~

₎

Q2

=

Qt°N-; Dt ₍₁₁₎ :I: ~ (U Q) :I:

~

NO~

_.~~ ;2 y v FQt Q~ (W;° )3

O

/ N.1 1 01 -~ ~ Volume F~ Q q; geometrically

Fig 7 Relationship of performance characteristics

similar machines operating at different speeds.

FM20

Equipment Set-Up:.

Exactly as Practical Exercise No 2, with pump suction valve V 1 fully open

throughout the exercise.

Procedure:-i) As discussed, there is no need to perform further test runs to gain

all pump performance characteristics, as in Experiment 3, as the results so far obtained can be used as they are to test the 'scaling laws'.

From the Equation (10) in the Theory section, the Head H and Flow Qy at one speed N 1

are, for dynamically similar pumps (here of

course the same pump), related to those at another speed N2 by:

ii)

H1/H2 = N~ /N~

Ql/Q2 =N1fNz and

iii) Therefore take anyone pair of readings of H and

~

at a chosen

speed

N

1

and predict the values of H and

Qy

at another speed for

which you happen to have the measured

values of H and

Qy

as well.

A comparison between predicted and measured values will be instructive. It is of course possible to predict the entire characteristic curve of H

-

Qy at one speed from the measured results at another.

FM20

Practical Exercise No 5 - Pump Suction

Objective:-To investigate the effect on pump performance of different suction conditions at the pump inlet.

Theoretical

Background:-CA VITAllON AND SUcnON HEAD

In both the design and operation of a rotodynamic machine, careful attention has to be paid to the fluid conditions on the suction side. In particular, it is important to check the minimum pressure that can arise at any point to ensure that 'cavitation' does not take place.

i) Cavitation.

If the pressure at any point is Jess than the vapour pressure of the liquid at the temperature at that point, vaporisation will occur. This is most likely to arise in the suction side and , if so, the pump may not be capable of developing the suction head necessary to ensure the correct operating point is achieved. Moreover, the vaporised liquid appears as bubbles within the liquid, and these subsequently collapse with such force that mechanical damage may be sustained. This condition, known as cavitation, is accompanied by a marked increase in noise and vibration in addition to the loss of head.

Suction head ii)

For any pump, manufacturers specify the minimum value of the 'net positive suction head' (NPSH) which must exist at the suction inlet point of the pump. NPSH is the amount by which the pressure at this point must exceed the vapour pressure of the liquid. For any installation this must be calculated, taking into account the absolute pressure of the liquid, the level of the pump, and the velocity and friction heads in the suction line. The NPSH must allow for the fall in pressure occasioned by the further acceleration of the liquid as it flows onto the impeller and for losses in the pump itself. If the required value of NPSH is not reached, partial vaporisation may occur, with the result that both suction and delivery heads may be reduced. The loss of suction head is more important because it may cause the pump to be starved of liquid.

FM20

Equipment

Set-Up:-As for the previous Exercises, but choose a pump speed N and outlet valve

setting V

2

(and thus flow rate) corresponding

to one of the data points of

Exercise No 3. This makes comparison easier. Probably a pair of mid range

values of N and

Qy

are the most suitable.

Procedure:-This procedure is as Exercise 2 but changes the inlet valve, and leaves the discharge valve fully open.

Initially, start with the inlet valve V fully open. Take Sample, which should produce a head and efficiency result corresponding precisely to that from Exercise 3.

i)

Now dose the inlet valve V 1 somewhat, and Take Sample.

ii)

Close the inlet valve a further amount, restore the pump speed and Take Sample. Continue this restriction of the inlet, taking data samples at the same constant speed, until the pump no longer draws any water from the feed tank. Then stop the pump operation immediately to prevent damage.

iii)

iv) The resulting tabulation and graphs of pump head and efficiency

under the gradually worsening conditions at the inlet can then be compared with those for the same speed and flow with an unrestricted inlet.

FM20

- duty

Characteristic

Practical point

Exercise

No 6 - System

Objective:-To obtain a Head

-

Flow curve for the piping system through which the fluid has to be pumped.

Theoretical Background:-Analysis Of Pump Systems

System analysis for a pumping installation is conducted to select the most suitable pumping units and to define their operating points. System analysis involves calculating head-capacity curves for the pumping system (valves, pipes, fittings etc.) and the use of these curves with those of available pumps. The s~tern curves are a graphic representation of all possible duty points in so far that the total dynamic head (static lift plus kinetic energy losses) is plotted against discharge flows from zero to the expected maximum, and a typical set are shown in Fig 8.

ISI 20'

51

0 101

Discharge Qv

Fig 8 Typical head-discharge curves for a pumping installation (pipes, valves etc).

As noted previously, pump characteristic curves illustrate the relationship between head, discharge, efficiency and power over a wide range of possible operating conditions, but they do not indicate at which point on the curves the pump will operate. The operating point (or duty point) is found by plotting the pump discharge curve with system head-discharge curve, as in Fig 9. The intersection of the two curves represents the head and discharge that the pump will produce if operated in the

FM20

given piping system. It will be seen that the optimum operating condition is achieved if this operating point coincides with the maximum point in the efficiency-discharge curve of the pump.

40""1 PumpbeedoCap8citycurve ~~

=

I

)

'-~. 5 10 15 Discharge Q

Fig 9 Definition sketch for determination of pump operating point.

r90

'--SO

Equipment Set-

Up:-It is necessary to use the pump outlet pressure tapping to measure the head against which the fluid must be pumped relative to atmospheric pressure. The connection between the pump inlet pressure tapping and the low pressure port on differential pressure sensor SPW3 must therefore be temporarily broken. To do this, remove the small bore tubing from the pressure tapping adjacent to the inlet of the pump then temporarily blank the tapping using a short length of flexible tubing with clamp to prevent leakage through the tapping. The free end of the flexible tube which is still attached to the low pressure side of sensor SPW3 should be left open to atmosphere with the end positioned approximately at the same height as the centreline of the pump (height datum for measurements).

The pump motor may now be switched on and water circulated as in the previous Exercise set-ups. Inlet valve VI should be fully open.

Procedure:-i)

Select

a position for the outlet valve V

2 such that it is partly closed

and forming a significant resistance to flow ego 2/3rds shut. This setting will be maintained throughout this Exercise (unless you have to repeat the results at a different setting to achieve a better range, for example).

ii) Flow control of the pump will be made by altering the speed setting

on the power controller rather than by altering the outlet valve as in the previous Exercises.

FM20

Iii) Select maximum speed as the starting point (5WA1 set to 1000/0). When the readings of pump outlet pressure and orifice pressure drop are constant, Take Sample. (The pump outlet pressure, now measured relative to atmospheric pressure, represents the 'system pressure', as conditions in the vessel to which the water is returned remain constant.)

iv) Decrease the speed slightly by adjusting SW AI, ie. decrease the flow rate Qv, and Take Sample for the new conditions of system pressure. Continuation of this measurement at gradually decreased flow will provide the data in Tables or Graphs as the 'system' head-flow curve, as described in Theory section 6.

Note: It will not be possible to reduce the speed continuously down to zero.

GENERAL SAFETY RULES

1 _Follow Relevant Instructions

a

b

Before attempting to install, commission or operate equipment, all relevant suppliers/manufacturers instructions and local regulations should be understood and implemented.

It is irresponsible and dangerous to misuse equipment or ignore instructions, regulations or warnings.

Do not exceed specified maximum operating conditions (eg.

temperature, pressure, speed etc. '

( 2 Installation a b d

e

f

Use lifting tackle where possible to install heavy equipment. Where manual lifting is necessary beware of strained backs and crushed toes. Get help from an assistant if necessary. Wear safety shoes where appropriate.

Extreme care should be exercised to avoid damage to the equipment during handling and unpacking. When using slings to lift equipment, ensure that the slings are attached to structural framework and do not foul adjacent pipework, glassware etc. When using fork lift trucks, position the forks beneath structural framework ensuring that the forks do not foul adjacent pipework, glassware etc. Damage may go unseen during commissioning creating a potential hazard to subsequent operators.

Where special foundations are required follow the instructions provided and do not improvise. Locate heavy equipment at low level.

Equipment involving inflammable or corrosive liquids should be sited in a containment area or bund with a capacity 500/0 greater than the maximum equipment contents.

Ensure that all services are compatible with the equipment and that independent isolators are always provided and labelled. Use reliable connections in all instances, do not improvise.

Ensure that all equipment is reliably earthed and connected to an electrical supply at the correct voltage. The electrical supply must incorporate a Residual Current Device (RCD) (alternatively called an Earth Leakage Circuit Breaker

-

ELCB) to protect the. operator from severe electric shock in the event of misuse or accident.

Potential hazards should always be the first consideration when deciding on a suitable location for equipment. Leave sufficient space between equipment and between walls and equipment.

g

A voiding fires or explosion 7 a

b

c

d

e

f

Ensure that the laboratory is provided with adequate fire extinguishers appropriate to the potential hazards.

Where inflammable liquids are used, smoking must be forbidden. Notices should be displayed to enforce this.

Beware since fine powders or dust can spontaneously ignite under certain conditions. Empty vessels having contained inflammable liquids can contain vapour and explode if ignited.

Bulk quantities of inflammable liquids should be stored outside the laboratory in accordance with local regulations.

Storage tanks on equipment should not be overfilled. All spillages should be immediately cleaned up, carefully disposing of any contaminated cloths etc. Beware of slippery floors.

When liquids giving off inflammable vapours are handled in the laboratory, the area should be ventilated by an ex-proof extraction system. Vents on the equipment should be connected to the extraction system.

Students should not be allowed to prepare mixtures for analysis or other purpose without competent supervision.

g

Handling poisons, corrosive or toxic materials 8

a

b

c

d

Certain liquids essential to the operation of equipment, for example mercury, are poisonous or can give off poisonous vapours. Wear appropriate protective clothing when handling such substances. Clean up any spillage immediately and ventilate areas thoroughly using extraction equipment. Beware of slippery floors.

Do not allow food to be brought into or consumed in the laboratory. Never use chemical beakers as drinking vessels.

Where poisonous vapours are involved, smoking must be forbidden. Notices should be displayed to enforce this.

Poisons and very toxic materials must be kept in a locked cupboard or store and checked regularly. Use of such substances should be supervised.

When diluting concentrated acids and alkalis, the acid or alkali should be added slowly to water while stirring. The reverse should never be attempted.

e

9 A voiding cuts and bums

a b

Take care when handling sharp edged components. Do not exert undue force on glass or fragile items.

Hot surfaces cannot in most cases be totally shielded and can produce severe burns even when not "visibly hot'. Use common

3 _{Commissioning}

Ensure that equipment is commissioned and checked by a competent member of staff before permitting students to operate it. a Operation 4 a b d

Ensure that students are fully aware of the potential hazards when operating equipment.

Students should be supervised by a competent member of staff at all times when in the laboratory. No one should operate equipment alone. Do not leave equipment running unattended.

Do not allow students to derive their own experimental procedures unless they are competent to do so.

Serious injury can result from touching apparently stationary equipment when using a stroboscope to 'freeze' rotary motion.

5 Maintenance

a

b

Badly maintained equipment is a potential hazard. Ensure that a competent member of staff is responsible for organising maintenance and repairs on a planned basis.

Do not permit faulty equipment to be operated. Ensure that repairs are carried out competently and checked before students are permitted to operate the equipment.

Using Electricity 6 a b

c

d

At least once each month, check that ELCB's (RCCB's) are operating correctly by pressing the TEST button. The circuit breaker must trip when the button is pressed (failure to trip means that the operator is not protected and a repair must be effected by a competent electrician before the equipment or electrical supply is used).

Electricity is the commonest cause of accidents in the laboratory. Ensure that all members of staff and students respect it.

Ensure that the electrical supply has been disconnected from the equipment before attempting repairs or adjustments.

Water and electricity are not compatible and can cause serious injury if they come into contact. Never operate portable electric appliances adjacent to equipment involving water unless some form of constraint or barrier is incorporated to prevent accidental contact.

Always disconnect equipment from the electrical supply when not in use.

e

Eye protection

10 a

b

Goggles must be worn whenever there is a risk to the eyes. Risk may arise from powders, liquid splashes, vapours or splinters. Beware of debris from fast moving air streams. Alkaline solutions are particularly dangerous to the eyes.

Never look directly at a strong source of light such as a laser or Xenon arc lamp. Ensure that equipment using such a source is positioned so that passers-by cannot accidentally view the source or reflected ray.

Facilities for eye irrigation should always be available.

c

Ear protection

11

Ear protectors must be worn when operating noisy equipment. a

Clothing

12

a

b

Suitable clothing should be worn in the laboratory. Loose garments can cause serious injury if caught in rotating machinery. Ties, rings on fingers etc. should be removed in these situations.

Additional protective clothing should be available for all members of staff and students as appropriate.

13

Guards and safety devices a

b

c

d

Guards and safety devices are installed on equipment to protect the operator. The equipment must not be operated with such devices removed.

Safety valves, cut-outs or other safety devices will have been set to protect the equipment. Interference with these devices may create a potential hazard.

It is not possible to guard the operator against all contingencies. Use common sense at all times when in the laboratory.

Before starting a rotating machine, make sure staff are aware how to stop it in an emergency.

Ensure that speed control devices are always set at zero before starting equipment. e First aid 14 a

b

If an accident does occur in the laboratory it is essential that first aid equipment is available and that the supervisor knows how to use it. A notice giving details of a proficient first-aider should be prominently displayed.

A 'short list' of the antidotes for the chemicals used in a particular laboratory should be prominently displayed.