Control of Boilers

414 Pages·1991·20.6 MB·English

by  Dukelow, Sam G.

THE
CONTROL
OF
BOILERS
2nd Edition
SAM G. DUKELOW
The information presented in this publication is for the general education of the reader. Because neither
the author nor the publisher has any control over the use of the information by the reader, both the author
and the publisher disclaim any and all liability of any kind arising out of such use. The reader is expected
to exercise sound professional judgment in using any of the information presented in a particular
application.
Additionally, neither the author nor the publisher have investigated or considered the effect of any patents
on the ability of the reader to use any of the information in a particular application. The reader is
responsible for reviewing any possible patents that may affect any particular use of the information
presented.
Any references to commercial products in the work are cited as examples only. Neither the author nor the
publisher endorses any referenced commercial product. Any trademarks or trade names referenced belong
to the respective owner of the mark or name. Neither the author nor the publisher makes any
representation regarding the availability of any referenced commercial product at any time. The
manufacturer's instructions on use of any commercial product must be followed at all times, even if in
conflict with the information in this publication.
THE CONTROL OF BOILERS
Copyright 0 1991 by ISA - The Instrumentation, Systems, and Automation Society
67 Alexander Drive
P.O. Box 12277
Researc h Triangle Park, NC 27709
All rights reserved.
Printed in the United States of America.
10 9 8 7 6 5
ISBN 1-55617-330-X
We would like to thank the many suppliers who provided material for this book, and we regret any we may
have inadvertently failed to credit for an illustration. On notification we shall insert a correction in any
subsequent printings.
Some material herein has previously appeared in Improving Boiler Efficiency by Sam G. Dukelow, produced by
Kansas State University and distributed by ISA.
For information on corporate or group discounts for this book, e -mail: bulksales@ isa.org,
Library of Congress Cataloging-in-PublicationD ata
Dukelow, Sam G., 1917 -
The control of boilers/Sam G. Dukelow. - 2"" ed.
p. cm.
Includes bibliographical references and index.
ISBN 1-55617-330-X
1. Steam-boilers - Automatic control. I. Title.
TJ288.D78 199 1
621.1'83--dc20 91 -3 1399
CIP
Preface to Second Edition
Five years have passed since the first edition of this book, and I have continued to learn
as I have become older and wiser. In the third paragraph of the preface to the first edition, I
implied that what I have done in this second edition was impossible. I want to eat those words.
The cartoon by Gus Shaw on the opposite page tells the story. During his work with Bailey
Meter Co. (prior to Bailey Controls Co.), Gus made several great cartoons on this theme. A
complete study of “The Control of Boilers” must include the “starting up” phase of the
process. One of the purposes of this edition is to include some basic information on that digital
phase of the operation in addition to the modulating “on-line” operation covered in the orig-
inal edition.
This results in the sections and subsections covering interlocks, burner start-up and man-
agement, and the management of the start-up and operation of pulverizers and other fuel-
burning equipment. Along with this is the recognition of applicable safety codes of the Na-
tional Fire Protection Association (NFPA).
Another purpose of this edition is the extension of the “on-line” aspects of boiler control
into the arena of the larger-capacity electric utility boilers. The results of this are new sections
covering the firing rate demand for utility boilers and steam temperature control. The section
covering furnace pressure control has been expanded to include implosion protection. The
section covering the control of pulverized coal firing has been expanded to include cyclone
furnaces and their control and the compartmented windbox boiler and its control.
And, as I said above, I have become older and wiser in the past five years. My ASME
membership card now says 51 years, my ISA membership card says 42 years, and I have had
time to rethink some the things I thought I already knew. In this last five years my work has
taken me to installations involving chain grate stokers, pulverized coal-fired boilers, gas-fired
boilers, the steam power cycle of a nuclear breeder reactor, and process heaters fired with by-
product gas. In the past few months, my work with distributed digital system on an electric
utility unit demonstrated the differences of working with a digital system as compared to an
analog system. Three of the above assignments involved the investigation of furnace explo-
sions that caused major damage. The investigation included considerable dialogue with every-
body involved, as I continued to learn.
In addition, the past five years have brought me approximately 25 teaching assignments
covering boiler control for various types of boilers. This has involved me with approximately
500 students with various industrial and utility boiler backgrounds. In each of these areas, and
i? talking to the people involved, I have gained new insights and thus continued my learning.
So now this second edition of The Control of Boilers. There is less coverage of the field
of “start-up” and utility boiler control than these subjects deserve, but I believe that the basics
included will help tie the whole subject together. There is still much to be written. It is my
understanding that a much more detailed text in the area of burner start-up and management
is in process. An expansion in the area of utility boiler control directed at cogeneration, coal
gasification combined cycles, low NOx control, flue gas scrubbers, fluidized bed boilers and
their control, expert systems, artificial intelligence, and power plant unit performance analysis
would be welcome. As indicated in the preface to the first edition of this book, the whole field
of energy management for least cost operation of boilers and HVAC should also be given
attention.
The boiler control field, and the rest of the I&C field, has had its revolution. It is now a
distributed digital “microprocessor” world. But as with all control systems, no matter what
hardware or software is used, the control application of the job to be done must be the major
focus and must be defined. This book is a discussion of that application area. The hardware-
software combination of today’s world unleashes the control application engineer from the
xi
bonds of hardware and hardware installation cost contraints. The engineer can now concentrate
on how best to control the boiler and other aspects of the power process.
Again I thank all those who, both knowingly and unknowingly, helped me along the way.
I particularly thank Paul Kenny of Forney Engineering Co.; Ollie Durrant, now retired from
Babcock & Wilcox Co.; and Russ Beal, now retired from Bailey Controls Co., who furnished
me with source documents.
My intent is to emphasize the basic ideas involved in boiler control and thus stimulate the
reader to expand his or her knowledge with more detailed study. If this book provides a jump-
start to beginners in the field of boiler control application, or adds new insights to those ex-
perienced readers, I shall have accomplished my purpose.
S.G. Dukelow
Hutchinson, Kansas
xii
Contents
Chapter Page
.........................................................................
Section 1 Introduction 1
1-1 Content and Objectives ...................................................... 1
1-2 Boiler Control Objectives ................................................... 2
1-3 Control System Diagramming .............................................. 3
1-4 Boiler Control Application in Historical Perspective ................... 4
.......................................
Section 2 Boiler Basics and the Steaming Process 15
2-1 The Basic Steaming Process ................................................ 15
2-2 The Basic Boiler .............................................................. 15
2-3 Heat Recovery from the Flue Gases ....................................... 17
2-4 Boiler Types and Classifications ........................................... 21
2-5 Firetube Boilers ............................................................... 22
2-6 Watertube Drum Boilers ..................................................... 26
2-7 Watertube Once-Through Boilers .......................................... 32
...............................
Section 3 Performance and Input-Output Relationships 35
3-1 Capacity and Performance .................................................. 35
3-2 Input Related to Output ...................................................... 35
3-3 Mass and Energy Balances Involved ...................................... 36
3-4 Efficiency Calculation Methods ............................................ 38
3-5 Boiler Control - The Process of Managing the Energy and Mass
Balances ........................................................................ 39
..................
Section 4 Basic Control Loops and Their System Interconnection 41
4-1 Simple Feedback Control ................................................... 41
4-2 Feedforward-Plus-Feedback Control ...................................... 44
4-3 Cascade Control .............................................................. 46
4-4 Ratio Control .................................................................. 46
4-5 Some Fundamentals of Control System Application and Design ..... 47
4-6 Process Dynamics - Control Response .................................. 48
4-7 Process Factors That Affect the Control System or Loop
Application .................................................................... 48
........
Section 5 Combustion of Fuels. Excess Air. and Products of Combustion 49
5-1 Gaseous Fuels-Their Handling and Preparation ........................ 49
5-2 Liquid Fuels-Their Handling and Preparation .......................... 50
5-3 Solid Fuels-Their Handling and Preparation for Firing ............... 55
5-4 Handling and Delivery of Solid Fuels ..................................... 56
5-5 Fuel Mixtures-Coal.Oi1. Coal-Water .................................... 58
5-6 Physical Combustion Requirements ....................................... 59
5-7 Combustion Chemistry and Products of Combustion ................... 61
5-8 Theoretical Air Requirements and Relationship to Heat of
Combustion .................................................................... 65
5-9 The Requirement for Excess Combustion Air ........................... 67
............................................................
Section 6 Efficiency Calculations 73
6-1 Input-Output or Direct Method ............................................. 73
6-2 Heat Loss or Indirect Method .............................................. 75
V
Chapter Page
........................................................
Section 7 The Steam Supply System 87
7-1 Saturated Steam Moisture Elimination .................................... 87
7-2 Steam Supply Systems ....................................................... 88
7-3 Heat Energy and Water Storage ............................................ 89
...................................
Section 8 Firing Rate Demand for Industrial Boilers 93
8-1 Relationships .................................................................. 93
8-2 Linking the Steam Pressure Change to Changes in Firing Rate ...... 93
8-3 Steam Presssure or Steam Flow Feedback Control ..................... 96
8-4 Feedforward-plus-Feedback-Steam Flow plus Steam Pressure ...... 99
8-5 Load Sharing of Multiple Boilers .......................................... 103
8-6 Automatic Compensation for the Number and Size of Boilers
Participating ................................................................... 104
8-7 Preallocation of Boiler Load Based on Test Results .................... 107
8-8 Energy Management by Boiler Load Allocation on a Least Cost
Basis ............................................................................ 107
8-9 Energy Management Involving Cogeneration Networks ............... 109
.......................................
Section 9 Firing Rate Demand for Utility Boilers 115
9-1 Matching Firing Rate Demand to Electrical Load (Boiler-Turbine
Coordination) .................................................................. 116
9-2 Boiler Load Measurement ................................................... 118
9-3 Unit Load Demand Development .......................................... 121
9-4 Boiler Following-Firing Rate Demand Development ................. 123
9-5 Turbine Following-Throttle Pressure Control with the Turbine
Valves .......................................................................... 129
9-6 Boiler-Turbine Coordinated Control ....................................... 133
9-7 Sliding or Variable Pressure Control ...................................... 136
9-8 Heat Rate Optimization with Sliding Pressure Control ................. 137
9-9 Digital Interlock and Tracking Control Modes .......................... 137
Section 10 Main Steam and Reheat Steam Temperature Control ..................... 139
10-1 Temperature vs . Boiler Load .............................................. 139
10-2 Mechanisms for Control of Superheat Temperature ................... 139
10-3 Basic Steam Temperature Control Strategies ........................... 144
10-4 Steam Temperature and Reheat Temperature Control Strategies .... 147
10-5 A Reheat Temperature Control Arrangement for a Combustion
Engineering Boiler .......................................................... 149
10-6 The Corresponding Superheat Temperature Control for the
Combustion Engineering Boiler .......................................... 152
10-7 Spray Water Sources-Steam and Water Flow Measurements ....... 154
10-8 Interactions ................................................................... 156
10-9 Pumping and Firing Rate for Once-Through Boilers .................. 156
10-10 Steam Temperature Control for Once-Through Boilers ............... 160
.......................................................
Section 11 Boiler and Unit Interlocks 165
11 . 1 Applicable Codes ........................................................... 165
1 1-2 Logic Diagramming for Motor Starting and Trip Protection ......... 165
11-3 Digital Interlocks within the Control System ........................... 173
11- 4 Classification of Trip Interlocks Relative to Potential Consequence 174
11-5 Limits and Runbacks ....................................................... 176
vi
Chapter Page
..............
Section 12 Feedwater Supply and Boiler Water Circulation Systems 177
12-1 The Basic System ......................................................... 177
12-2 Heating and Deaeration .................................................. 177
12-3 The Boiler Feedwater Pump ............................................. 180
12-4 The Flow Regulation System ........................................... 181
12-5 Shrink and Swell and Boiler Water Circulation ...................... 183
12-6 Feedwater Chemical Balance and Control of Boiler Blowdown ... 187
....................................................
Section 13 Feedwater Control Systems 189
13-1 Measurement and Indication of Boiler Drum Level .................. 189
13-2 Feedwater Control Objectives ........................................... 191
13-3 Single-Element Feedwater Control ..................................... 194
13-4 Two-Element Feedwater Control ....................................... 198
13-5 Three-Element Feedwater Control ...................................... 202
13-6 Control Refinements and Special Control Problems ................. 204
13-7 Control of Feedwater for Once-Through Boilers ..................... 207
.............................................................
Section 14 Boiler Draft Systems 211
14-1 Draft Losses in Boilers ................................................... 211
14-2 Natural Draft and Forced Draft ......................................... 212
14-3 Pressure-Fired Boilers .................................................... 213
14-4 Balanced Draft Boilers .................................................... 215
14-5 Dampers and Damper Control Devices ................................ 216
14-6 Draft and Air Flow Control Using Variable-Speed Fan ............. 221
14-7 Minimum Air Flow ....................................................... 224
...............................
Section 15 Measurement and Control of Furnace Draft 227
15-1 Measurement of Furnace Draft .......................................... 229
15-2 Furnace Draft Control Using Simple Feedback Control ............ 229
15-3 Furnace Draft Control Using Feedforward-plus-Feedback
Control ...................................................................... 230
15-4 Furnace Draft Control Using Push-Pull
Feedforward-plus-Feedback Control ................................... 231
15-5 Protection Against Implosion ............................................ 233
Section 16 Measurement and Control of Combustion Air Flow plus Related
...........................................................................
Functions 239
16-1 Differential Pressure Measurement of Air Flow ...................... 239
16-2 Non-Inferential Measurement of Air Flow ............................ 244
16-3 Control of Air Flow ....................................................... 246
16-4 Flue Gas Dew Point Control ............................................. 249
16-5 Soot Blowing ............................................................... 253
.........
Section 17 Flue Gas Analysis Trimming of Combustion Control Systems 255
17-1 Useful Flue Gas Analyses ................................................ 255
17-2 Methods of Flue Gas Analysis .......................................... 256
17-3 Pros and Cons of Measurement Methods and Gases Selected
for Measurement ........................................................... 260
17-4 Flue Gas Analysis vs . Boiler Load ..................................... 262
17-5 PPM CO vs . PPM Total Combustible Gases ......................... 264
17-6 Control Applications Used for Flue Gas Analysis Trimming ...... 265
17-7 Limiting Factors in Reducing Excess Air ............................. 272
vii
Chapter Page
.................................
Section 18 Fluid Fuel Burners for Gas. Oil. and Coal 275
18-1 Burners for Gaseous Fuel ................................................ 275
18-2 Pulverized Coal Burners .................................................. 279
18-3 Fuel Oil Burners ........................................................... 282
....................................................
Section 19 Solid Fuel Burning Systems 291
19-1 Types and Classification of Stokers .................................... 291
19-2 Special Stoker Control Problems ........................................ 297
Section 20 Burner Management and Flame Safety Interlocks for Gas- and
...............................................................
Fluid-Fired Boilers 299
20-1 Basic Cause of Furnace Explosions .................................... 299
20-2 Boiler Purge Logic ........................................................ 301
20-3 Ignitor Header Valve Management ..................................... 303
20-4 Main Gas Header Valve Management ................................. 303
20-5 Gas Burner Management Logic ......................................... 304
20-6 Main Fuel Trip ............................................................. 307
20-7 Degree of Burner Automation ........................................... 308
20-8 Reliability of Interlock Circuitry ........................................ 308
.............
Section 21 Combustion Control for Liquid and Gaseous Fuel Boilers 311
21-1 Single-Point Positioning Control ........................................ 311
21-2 Parallel Positioning Control .............................................. 315
21-3 Metering Control Systems ................................................ 317
2 1-4 Effects of Fuel Btu Variation ............................................ 328
.....................
Section 22 Pulverized Coal and Cyclone Coal Burning Systems 333
22-1 The Coal Feeder ........................................................... 333
22-2 The Pulverizer and Classifier ............................................ 336
22-3 The Primary Air Fan or Exhauster Fan and the Coal Drying
System ....................................................................... 338
22-4 Pulverizer Control Systems .............................................. 341
22-5 Compartmented Windbox Pulverized Coal Boilers .................. 350
22-6 Start-up and Management of Pulverizers and Their Burners ....... 352
22-7 The Cyclone Furnace ............................... 355
22-8 Start-up and Management of Cyclone Furnaces ..................... 358
Section 23 Combustion Control for Cyclone and Pulverized Coal-Fired
...............................................................................
Boilers 359
23-1 Coal Btu Compensation .................................................. 359
23-2 The Use of Multiple Pulverizers ........................................ 362
23-3 The Combustion Control System for Pulverized Coal as a Single
Fuel .......................................................................... 363
23-4 Pulverized Coal in Combination with Liquid or Gaseous Fuels ... 364
23-5 Compartmented Windbox Pulverized Coal Control Systems ....... 367
23-6 Control Systems for Cyclone Furnace Boilers ........................ 369
.............................
Section 24 Combustion Control for Stoker-Fired Boilers 373
24-1 Parallel Positioning Control System for Stoker-Fired Boilers ...... 373
24-2 Inferential Measurement of Combustion Conditions in Boilers .... 375
24-3 Parallel Positioning Control System with Steam Flow/
Air Flow Readjustment ................................................... 376
24-4 Series Ratio Control Systems for Stoker-Fired Boilers .............. 379
...
Vlll
Chapter Page
24-5 Applying Flue Gas Analysis Trim Control to Stoker-Fired
Boilers ....................................................................... 381
24-6 Combustion Control for Combination of Stoker and Liquid or
Gaseous Fuel Firing ....................................................... 383
24-7 NFPA Purging and Interlock Requirements for Stoker-Fired
Boilers ....................................................................... 386
...........................................
Section 25 Atmospheric Fluidized-Bed Boilers 389
25-1 Bubbling Bed Fluidized-Bed Boilers ................................... 390
25-2 Circulating Bed Fluidized-Bed Boilers ................................. 392
25-3 NFPA Requirements for Atmospheric Fluidized-Bed Combustion
System Boilers ............................................................. 395
Section 26 Control System Complexity and Future Directions for Boiler
..............................................................................
Control 397
26-1 Complex Control Systems for Electric Utility Boilers Using
Embedded Process Models ............................................... 397
26-2 Improving Control Precision and Stability without Process
Modeling .................................................................... 398
26-3 Artificial Intelligence and Expert Systems ............................. 402
26-4 A General Observation Relative to Boiler Modeling ................ 402
26-5 General Observations Relative to Boiler Control Application ...... 404
........................................................................................
Index 407
ix
X