Ellis,J. Pressure transients in water engineering, A guide to analysis and interpretation of behaviour
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Pressure transients in
water engineering
A guide to analysis and interpretation
of behaviour
John Ellis
Independent Hydraulics Consultant and Honorary Research Fellow,
University of Glasgow, UK
Published by Thomas Telford Publishing, Thomas Telford Ltd, 1 Heron Quay, London E14 4JD.
www.thomastelford.com
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First published 2008
A catalogue record for this book is available from the British Library
ISBN: 978-0-7277-3592-8
# Thomas Telford Publishing Ltd 2008
Whilst every reasonable effort has been undertaken by the author and the publisher to
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Typeset by Academic þ Technical, Bristol
Printed and bound in Great Britain by MPG Books, Bodmin, Cornwall
Contents
Acknowledgements xv
Notation xvi
Introduction 1
Chapter 1 Motivation for hydraulic transient analysis 7
1.1 Primary purpose of analysis, 7
1.2 Secondary objectives, 8
1.3 Permitted pressures, 8
1.4 Maximum pressures, 8
1.5 Pipe materials, 8
1.6 Rigid pipes, 9
1.6.1 Grey cast iron, 9
1.6.2 Asbestos cement, 10
1.6.3 Concrete pipes, 10
1.7 Flexible pipes, 11
1.7.1 Ductile iron, 11
1.7.2 Steel pipe, 11
1.8 Overpressure allowance, 12
1.9 Pipe linings for rigid and flexible pipes, 13
1.9.1 Bitumen, 13
1.9.2 Coal tar enamel, 13
1.9.3 Coal tar epoxy lining, 13
1.9.4 Cement mortar, 13
1.9.5 Paint systems, 14
1.9.6 Polyethylene lining, 14
1.10 Plastic pipes, 14
iii
1.10.1 Thermosetting plastics, 14
1.10.2 Thermoplastics, 15
1.11 Failure modes of pipes, 17
1.12 Maximum pressure and allowable amplitude of surge
in plastic pipes, 18
1.13 Minimum pressures, 18
1.14 When is analysis necessary?, 19
Chapter 2 Derivation of basic equations 21
2.1 The rigid-column approach, 21
2.2 Compressible flow theory, 23
2.2.1 Conservation of force, 23
2.2.2 Conservation of mass, 24
2.2.3 Compressible flow equations in terms of total head
H,25
Chapter 3 Interpretation of a 27
3.1 Fluid properties, 27
3.2 Influence of the conduit wall, 28
3.3 Simple expression for a,29
3.4 Variation of a with conduit shape, 32
3.5 Influence of gas on a,32
3.6 The effect of sewage, 38
Chapter 4 Characteristic equations 41
4.1 Development of characteristic equations, 41
4.2 Significance of the integrals, 44
4.3 Effect of changing pipe elevation, 44
4.4 Pipeline resistance, 45
4.4.1 Corrosion, 47
4.4.2 Sliming, 47
4.4.3 Evaluation of the integral, 48
Chapter 5 Application of characteristic equations 49
5.1 Use of the characteristics, 49
5.2 ‘Natural’ characteristic mesh, 52
5.3 Using variable wave speed a,54
5.4 Use of a larger time step, 55
5.5 Use of a fixed wave speed, 56
5.6 Distribution of free gas along the pipeline, 58
5.7 Model output, 59
iv
Chapter 6 Boundaries 60
6.1 Types of boundary, 61
6.2 Reservoirs and tanks, 62
6.3 Branches and changes in pipe properties, 64
6.3.1 Specific cases — number of pipes ¼1, 67
6.3.2 Specific cases — change of cross-sectional area, 67
6.4 Response of a large pipe or trunk main, 69
6.5 Actuated valves and pipeline fittings, 71
6.5.1 Terminal valves, 73
6.5.2 In-line valve, 74
6.5.3 Automatic control valves, 75
6.6 Use of more than one time step, 77
6.7 Non-reflecting boundary, 78
6.8 Other bifurcation conditions, 82
6.8.1 Bifurcation with operating valves, 82
6.8.2 Isolating valves, 83
6.9 Continuous drawoff, 84
Chapter 7 Valve closure in a simplified system 87
7.1 Instantaneous valve closure at t ¼ 0, 87
7.2 From 0 < t L =a,89
7.3 L=a < t 2L=a,90
7.4 2L=a < t 3L=a,92
7.5 3L=a < t 4L=a,93
Chapter 8 Actual pipelines 95
8.1 Attenuation, 95
8.1.1 Conditions at the wavefront, 97
8.1.2 Conditions when the wave height is of zero
amplitude, 99
8.1.3 Conditions at the closed valve, 100
8.1.4 Conditions downstream of a pump or valve, 100
8.2 A uniform gravity main, 100
Chapter 9 Valve operations 106
9.1 Treated water main, 107
9.2 Improving valve operation, 113
9.3 Two-stage valve closure, 113
9.4 Submerged discharge valve, 117
9.5 In-line valves, 118
9.5.1 Isolating valves, 118
v
9.5.2 Actuated valve, 119
9.6 Control of transient pressures and estimation of valve
operating time, 122
Chapter 10 Pumps 125
10.1 Types of pump, 125
10.1.1 Pumps which produce transient behaviour only when
changing their mode of operation — that is, starting,
stopping or changing speed, 125
10.1.2 Pumps which generate surge effects as a by-product of
their operation, 126
10.1.3 Pumps which produce transient events in order to fulfil
their function, 126
10.1.4 Pumps which do not by themselves produce surge
effects, 126
10.2 Turbine pumps, 126
10.2.1 Centrifugal or radial flow pumps, 127
10.2.2 Mixed or semi-axial flow pumps, 128
10.2.3 Axial flow or propeller pumps, 128
10.3 Turbine pump performance curves, 128
10.4 Including turbine pumps in hydraulic transient
analyses, 132
10.4.1 Transfer pump, 134
10.4.2 Booster pump, 136
10.4.3 Other pumping station and pipeline configurations, 137
10.4.4 Station losses, 139
10.5 System curves and pump duty, 139
10.6 Turbine pump start, 140
10.6.1 Direct start, 140
10.6.2 Star/Delta and transformer starting, 140
10.6.3 Variable speed or ‘soft’ start, 141
10.7 Case studies of pump start, 141
10.7.1 Simulation of direct start in solo pumping, 141
10.7.2 Direct start in multi-pump operation, 143
10.8 Initial conditions of flow, 146
10.9 Pump failure or ‘trip’, 146
10.10 Other pumps, 150
10.10.1 Reciprocating pumps, 150
10.10.2 Pneumatic ejector, 152
10.10.3 The hydraulic ram, 155
10.10.4 The jet pump, 156
vi
Chapter 11 Flywheels 159
11.1 Moment of inertia, 159
11.2 Flywheels, 160
11.3 Limitations on flywheel size, 161
11.4 Pipeline limitations, 162
11.5 Case study with different pump speed options, 163
11.6 Flywheels on a larger system, 167
11.7 Booster pump installations, 170
11.8 Multi-pump installations, 170
11.9 Advantages of flywheels, 171
Appendix Moment of inertia, 171
Chapter 12 Pressure vessels 173
12.1 Modelling a pressure vessel, 173
12.1.1 Polytropic relationship, 174
12.1.2 Rational heat transfer (RHT) equation, 176
12.2 Role of a pressure vessel in surge suppression, 176
12.3 Initial estimation of required pressure vessel volume, 177
12.3.1 Graphical techniques, 177
12.3.2 Simple numerical method, 178
12.3.3 More detailed numerical assessment, 178
12.3.4 Subsequent investigations and criteria, 178
12.4 Case study of a sewage pumping system, 179
12.5 Worst-case conditions, 181
12.6 Reversed flow and refilling a pressure vessel, 183
12.7 Low-lift systems, 189
12.8 Vessels at a booster pumping station, 193
12.8.1 The upstream pumping station, 194
12.8.2 The downstream pumping station, 196
12.9 Summary of response with a pressure vessel
included, 199
Appendix Equations for estimating air vessel parameters, 200
A12.1 Equation of motion, 201
A12.2 Solution ignoring resistance to flow, 203
A12.3 Including resistance to flow, 205
A12.4 Complete equations, 207
A12.5 Application of the equations, 207
A12.5.1 Maximum expanded gas volume, 207
A12.5.2 Peak upsurge pressure head, 209
A12.5.3 Required throttling, 212
A12.6 Pipeline system of varying cross-section, 214
vii
Chapter 13 Further aspects of pressure vessels 215
13.1 Pressure vessel types and their fittings, 215
13.2 Vessels having an air—water interface, 215
13.2.1 Air compressors, 215
13.2.2 Control of gas charge/liquid level, 217
13.2.3 Other vessel fittings, 218
13.3 Bladder vessels, 219
13.4 Positioning a pressure vessel, 221
13.5 Installation with air valves, 225
Chapter 14 Surge tanks and related structures 230
14.1 Purpose of a surge tank, 230
14.2 Simple analysis, 232
14.3 Long connection to a chamber, 233
14.4 Full-size connection, 236
14.5 Extent of protection, 236
14.6 Other aspects, 239
14.7 Initial estimates of surge tank parameters, 240
14.8 Related structures, 240
14.8.1 Service reservoir as a one-way surge tank, 241
14.8.2 Operation of an existing service reservoir, 242
14.8.3 Filtration plant, 243
14.8.4 Seawater intake system, 245
14.8.5 Seal weir, 248
14.8.6 Water towers, 249
14.8.7 Special structures, 253
Chapter 15 Feeder tanks or volumetric tanks 259
15.1 Components and location of a feeder tank, 259
15.2 Mode of operation, 261
15.3 Abnormal behaviour, 264
15.4 Mains duplication: Example 1, 267
15.5 Mains duplication: Example 2, 270
15.6 Aspects of feeder behaviour to consider, 276
15.7 Preliminary estimation of feeder tank volume, 277
Chapter 16 Discharge conditions 279
16.1 Vertical bellmouth, 279
16.2 A tank or chamber of finite area, 280
16.3 Back-flow connection, 282
16.4 Siphon breakers, 284
viii
16.4.1 Above-ground storage tanks, 284
16.4.2 Vacuum disconnecting valves, 286
16.5 Air valve operation, 292
16.6 Summary of influence of discharge arrangements, 293
Chapter 17 Air valves 295
17.1 Normal air valve locations, 297
17.2 Air valves for surge alleviation, 298
17.3 Events following flow reversal, 302
17.4 Air valve closure, 308
17.5 Case study of a sewage pumping station, 310
17.6 Pump restart with air in a pipeline, 314
17.7 Other considerations, 318
17.7.1 Uncertainties in simulation, 318
17.7.2 Liquid being conveyed, 319
17.7.3 Inspection, 320
17.7.4 Valve chamber and cover, 320
17.7.5 Valve materials, 321
17.8 Buffer tanks and estimation of required volumes, 321
Chapter 18 Air and gas 325
18.1 Pump start-up with an air-filled riser, 325
18.1.1 A more restricted air outflow device, 332
18.1.2 Soft-start of the pump, 333
18.1.3 Use of an accumulator, 334
18.2 Pump start with slow valve closure, 334
18.2.1 Air venting through a standard air valve, 336
18.2.2 A butterfly valve for air venting, 336
18.3 Air venting through a ‘sparg’ line, 339
18.4 Gas evolution, 339
18.5 Gas pockets in a pipeline, 340
18.6 Throttled outflow air valves, 343
18.7 Case study of a sewage rising main, 345
18.8 Pump blockage, 351
18.9 Pumped outfall pipeline, 354
18.9.1 Pipeline configuration, 354
18.9.2 Viking-Johnson coupling failure, 356
18.9.3 Hydrodynamic forces, 356
Chapter 19 Relief valves 359
19.1 Relief valve types, 359
ix
19.2 Initial valve sizing, 362
19.3 Valve positioning, 363
19.4 Analysis of behaviour, 363
19.5 Automatic surge control valve, 365
19.6 Surge anticipation valve, 366
19.7 Pumping station pressure relief valve, 366
19.8 Grove regulator, 369
19.9 High head relief valves, 371
19.10 Bursting disk, 375
Chapter 20 Check valve dynamics 376
20.1 Check valve response, 376
20.2 Pumping station check valves, 377
20.3 Consequences of an unsuitable check valve
installation, 377
20.4 Prediction of pumping station hydraulic
transients, 380
20.5 Reopening of a check valve door, 382
20.5.1 Check valve reopening due to pressure wave
reflections, 383
20.5.2 Valve reopening in longer term, 385
20.6 Check valve response in a multi-pump
installation, 388
20.7 Surge behaviour as a check valve shuts, 388
20.8 Modelling a pumping station, 390
20.8.1 Non-reflecting boundary with allowance for external
pipeline resistance, 390
20.8.2 System curve boundary, 393
20.9 Reduction of transient pressures following valve
closure, 394
20.10 Maximum pressures at a check valve, 396
20.10.1 Initial valve closure, 396
20.10.2 Cavitation upstream of the valve and resulting peak
pressures, 397
20.11 Other applications of check valves, 400
20.11.1 Check valve at the start of a rising main, 400
20.11.2 Check valve on vessel connection, 400
20.11.3 Bypass check valve, 400
20.11.4 Check valves along a rising main, 401
20.11.5 Inclusion of air valves with in-line check
valve, 406
x