Industrial Fired Boilers for General Refinery and Petrochemical Service

API RECOMMENDED PRACTICE 538 FIRST EDITION, OCTOBER 2015

REAFFIRMED, APRIL 2021

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Copyright © 2015 American Petroleum Institute

Foreword

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Reliable boiler operations are necessary to ensure steam production in refineries and petrochemical plants. Boilers are a critical component of U.S. commercial and industrial facilities and operations. Industrial boilers are also a major energy consumer. U.S. refineries and petrochemical plants employ approximately one-third of the total boiler heat input of all U.S. commercial and industrial facilities.

This document is based on the accumulated knowledge and experience of manufacturers and users of industrial fired boilers. This recommended practice (RP) addresses design, operating, maintenance, and troubleshooting considerations for industrial boilers that are used in refineries and chemical plants. This document directly reflects business needs by having API’s Subcommittee on Heat Transfer Equipment (SCHTE) membership, vendors, manufacturers, and contractors tailor these precise requirements. Manufacturers’ input from within and outside the SCHTE was sought and thus the final document reflects prevailing technical expertise. This RP could not have been developed in this manner by any other industry group. Manufacturers’ and contractors’ standards and requirements have individual differences that may not permit a purchaser to understand technical distinctions.

ASME codes focus on a boiler’s mechanical construction and performance testing. National Fire Protection Association codes focus on a boiler’s burner management safety system. API standards applicable to boilers focus on fans/drivers and post-combustion oxides of nitrogen (NOx) control. This SCHTE RP complements rather than duplicates these requirements, focusing on refinery and petrochemical boilers.

API 538 includes information on boiler types, burner management, system reliability/availability, feedwater preparation, BFW and boiler water treatment, waterside control, steam purity, combustion control, boiler burners, emissions, tube cleaning, and more. The information contained in API 538 is not covered by any other standards- writing body. By combining multiple technical subjects related to industrial fired boilers, the boiler user has the benefit of the collective industry experience that this RP provides, rather than having to rely on multiple technical documents.

1. Scope 1

2. Normative References 1

3. Terms, Definitions, Acronyms, and Abbreviations 3

   1. Terms and Definitions 3

   2. Acronyms and Abbreviations 18

4. Boilers—Equipment Overview 20

   1. General Considerations 20

   2. Operations 23

   3. Boiler Configurations 29

   4. Fuels Fired in Industrial Steam Boilers 41

   5. Igniter Management System 41

   6. Burner Management Systems 43

   7. Boiler Feedwater Preparation 45

   8. Boiler Water Quality and Internal Chemical Treatment 45

   9. Steam Purity 45

   10. Boiler Performance 46

5. Water Tube Boiler Components 49

   1. Pressure Parts—Superheaters/Attemperators 49

   2. Pressure Parts—Steam Generating Tubes 53

   3. Pressure Parts of Economizers 55

   4. Pressure Parts—Steam Drum, Mud Drum, and Headers 58

   5. Furnace Design 62

6. Combustion, Burners, and Igniters 64

   1. General Description 64

   2. Igniters 66

   3. Burners 69

   4. CO Boilers 77

   5. Fuels Fired in Industrial Boilers 80

   6. Burner Arrangements 86

7. Instrumentation, Control, and Protective Systems 88

   1. General 88

   2. Applying API 538 to Burner Management Systems 88

   3. Process Measurement 104

   4. Control Systems 123

   5. Protective Systems 147

   6. Pre-ignition Purge Cycle 169

   7. Start-up Sequence 170

   8. Manual Trip (Shutdown) 174

   9. Safety Shutoff Valves 174

   10. Trips and Alarms 178

8. Centrifugal Fans and Drivers 179

   1. General 179

      
      

   2. Mechanical 181

   3. Steam Turbine and Electric Motor Drives 182

   4. System Resistance Curves 183

   5. Control Systems 185

9. Boiler Feedwater Preparation 186

   1. Makeup Water Type and Quality 186

   2. Condensate Return and Treatment 187

   3. Deaeration 188

   4. Chemical Treatment 189

   5. Boiler Feedwater Quality 192

   6. Deaerator/BFW Monitoring and Control 194

10. Boiler Water Quality and Internal Chemical Treatment 197

    1. Boiler Water Quality 197

    2. Internal Chemical Treatment 197

    3. Boiler Cycles 201

    4. Boiler Water Monitoring and Control 203

11. Steam Purity 207

    1. Considerations 207

    2. Steam/Water Separation 207

    3. Attemperation—Surface or Direct Contact 207

    4. Monitoring and Control and Sampling 208

12. Boiler Piping 210

    1. Interconnecting Piping 210

    2. Downcomers 212

    3. Vents and Drains 212

    4. Blowdown 213

13. Boiler Trim and Instruments 214

    1. Water Column and Gauge Glasses 214

    2. Safety Valves 218

    3. Flow Meters 220

    4. Control Valves 223

14. Tube Cleaning 225

    1. Internal—Chemical, Mechanical 225

    2. External—Sootblowers 231

15. Structure, Setting, and Casing 240

    1. General 240

    2. Platforms, Stairways, and Ladders 240

    3. Observation Ports 241

    4. Access Doors 241

16. Flue Gas Sampling Connections 241

17. Computational Fluid Dynamics and Cold Flow Modeling 242

Annex A (informative) Equipment Data Sheets 244

Annex B (informative) Purchaser’s Checklist 280

Annex C (informative) Air Preheat Systems for Boilers 287

Annex D (informative) Performance Measurement 289

Annex E (informative) Emission Controls 290

Annex F (informative) Procedures 299

Annex G (informative) Inspection Test Plans 327

Annex H (informative) Functional Diagrams 331

Bibliography 347

Figures

1. Boiler Steam and Water Circulation 21

2. Water Tube Package “D” Boiler 30

3. Water Tube Package “O” Boiler 31

4. Water Tube Package “A” Boiler 31

5. Modular Boiler 32

6. Single Steam Drum Boiler 33

7. Field Erected Boiler 34

8. Carbon Monoxide Combustor and Heat Recovery Steam Generator 35

9. Thermal Oxidizer 37

10. Modular Waste Heat Boiler Configurations 39

11. Typical Circulation Ratio vs Steam Drum Operating Pressure 54

12. Raw Gas Igniter—First Type 67

13. Raw Gas Igniter—Second Type 67

14. Premix Igniter 68

15. Boiler Burner 71

16. Burner Igniter with Flame Rod 123

17. Typical Steam Generator Load Controls 127

18. Single-element Feedwater Control System 129

19. Two-element Feedwater Control System 130

20. Three-element Feedwater Control System 131

21. Single Point Positioning Control 133

22. Parallel Positioning Control 135

23. Metering Control 137

24. Metering Control with O2 Trim 139

25. Feedforward-Feedback Control System Example 141

26. Example Bypass Testing Arrangement, Option 1 176

27. Example Bypass Testing Arrangement, Option 2 177

28. Fan System Resistance Curve 184

    
    

29. Fan Characteristic Curves 185

30. Boiler Feedwater, Boiler Water, Condensate, and Steam Sample Conditioning System 196

31. pH vs PO4 Graph at Different Na/PO4 Ratios 201

32. pH vs log PO4 Graph at Different Na/PO4 Ratios 202

33. Boiler Blowdown vs Cycles of Concentration 202

34. Typical Steam Flow Rate for Retractable Sootblowers 234

35. Typical Steam Flow Rate for Fixed Rotary Sootblowers 237

E.1 Illustration of Lean Excess Air on Boiler NOx Production 292

1. Legend Sheet—Single Point Positioning Control 332

2. Boiler Master/Flue Gas Recirculation Controls 333

3. Discrete—Controls 334

4. Legend Sheet—Parallel Positioning Control 335

5. Boiler Master/Steam Pressure Controls 336

6. Combustion Controls 337

7. Discrete—Combustion 338

8. Draft Controls 339

9. Legend Sheet—Fully Metered Control Systems 340

10. Boiler Master/Steam Pressure Controls 341

11. Combustion Controls 342

12. Fuel Flow 343

13. Discrete—Combustion 344

14. Discrete—Combustion 345

15. Draft Controls 346

Tables

1. Igniter Properties Summary 42

2. Troubleshooting Superheaters 52

3. Troubleshooting Steam Generating Tubes 55

4. Typical Economizer Fin Parameters 57

5. Troubleshooting Economizers 59

6. Troubleshooting Steam and Mud Drums 61

7. Troubleshooting Furnaces 64

8. Boiler Firebox Design Limits 65

9. Typical Gas Fuel Composition and Properties (Examples) 81

10. Typical Analyses and Properties of Fuel Oils (Examples) 83

11. Pressure Sensors (Examples) 107

12. Gross Heating Values of Fuel Gas Compounds 110

13. Bands of Control 124

14. Speed of Control Response 143

15. Typical Alarms and Shutdown 178

16. Makeup Water Source to Boiler Pressure 188

17. Guidelines for Mechanical Deaeration Performance 189

    
    

18. Oxygen Scavenger Chemistry 190

19. Deaerator/BFW Amine Chemistry 191

20. Suggested Boiler Feedwater Chemistry Limits 193

21. Deaerator Mechanical Performance Monitoring and Control 194

22. Deaerator/Boiler Feedwater Monitoring 195

23. Suggested Boiler Water Chemistry Limits 198

25 Complexing Internal Treatment 199

24 Residual Phosphate Internal Treatment 199

1. Congruent pH/PO4 Treatment 200

2. Boiler Water Cycle Control and Monitoring Methods 204

3. Boiler Water Monitoring 205

4. Chemical Feed Locations 206

5. Attemperation Water Quality 208

6. Flow Elements Advantages and Disadvantages 221

7. Alkaline Boilout 226

8. Cleaning Guideline Based on Boiler Pressure and Deposit Weight Density 226

9. Chemical Analysis of Internal Tube Deposits 227

10. Deposits and Typical Cleaning Solutions 229

1. Relative Effectiveness of NOx Reduction Techniques 291

   
   

   Users of this recommended practice (RP) should be aware that further or differing requirements may be needed for individual applications. This RP is not intended to inhibit a vendor from offering, or the purchaser from accepting, alternative equipment or engineering solutions for the individual application. This may be particularly applicable where there is innovative or developing technology. Where an alternative is offered, the vendor should identify any variations from this RP and provide details.

   
   

   In API RPs, the metric (SI) system of units is used. Where practical in this RP, U.S. customary (USC) units are included in brackets for information. In Annex A, separate data sheets are provided in SI units and USC units.

   
   

   A bullet ( ● ) at the beginning of a section or subsection indicates that either a decision is required or further information is to be provided by the purchaser. This information should be indicated on data sheets (see examples in Annex B) or stated in the enquiry or purchase order.

   
   

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   Industrial Fired Boilers for General Refinery and Petrochemical Service

   
   

   1. Scope

      1. This recommended practice (RP) specifies requirements and gives recommendations for design, operation, maintenance, and troubleshooting considerations for industrial fired boilers used in refineries and chemical plants. It covers waterside control, combustion control, burner management systems (BMSs), feedwater preparation, steam purity, emissions, etc.

         
         

      2. This RP does not apply to fire tube boilers, gas turbine exhaust boilers, or fluidized bed boilers.

         
         

         • 1.3 This RP does not cover boiler mechanical construction. Purchaser or owner shall specify codes such as ASME, ISO, etc.

         
         

         1.4 This RP does not cover forced circulation boilers.

         
         

   2. Normative References

The editions of the following standards, codes and specifications that are in effect at the time of publication of this recommended practice shall, to the extent specified herein, form a part of this recommended practice. Changes in referenced standards, codes and specifications shall be mutually agreed to by the purchaser and the supplier.

API Manual of Petroleum Measurement Standards (MPMS) Chapter 14.3.3, Orifice Metering of Natural Gas and Other Related Hydrocarbon Fluids—Concentric, Square-edged Orifice Meters—Part 3: Natural Gas Applications

API Specification 6FA, Specification for Fire Test for Valves

API Recommended Practice 534, Heat Recovery Steam Generators

API Recommended Practice 535, Burners for Fired Heaters in General Refinery Services

API Recommended Practice 536, Post-combustion NOx Control for Fired Equipment in General Refinery Services

API Standard 541, Form-wound Squirrel Cage Induction Motors—375 kW (500 Horsepower) and Larger

API Standard 547, General-purpose Form-wound Squirrel Cage Induction Motors—250 Horsepower and Larger

API Recommended Practice 551, Process Measurement Instrumentation

API Recommended Practice 553, Refinery Valves and Accessories for Control And Safety Instrumented Systems

API Recommended Practice 555, Process Analyzers

API Standard 560, Fired Heaters for General Refinery Service, 5th Ed., 2015

API 570, Piping Inspection Code: In-service Inspection, Rating, Repair, and Alteration of Piping Systems

API Recommended Practice 574, Inspection Practices for Piping System Components

API Standard 607, Fire Test for Quarter-Turn Valves and Valves Equipped with Nonmetallic Seats

API Standard 611, General Purpose Steam Turbines for Petroleum, Chemical, and Gas Industry Services

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