Heat Recovery Steam Generators Design and Operation (2nd Edition)

Editor: Ch. Daublebsky von Eichhain

HRSG

Heat Recovery Steam Generators Design and Operation 2nd edition

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Heat Exchanger and Condenser Tubes Tube Types – Materials – Attributes – Machining 2004. 311 pages with numerous figures and tables. ISBN 3-934736-08-4. Soft cover E 38,-This book is the english version of the handbook „Wärmeübertrager-Rohre“. It gives a practical oriented and comprehensive overview concerning the different materials and their specifics especially refering to their applications, about the different marks and their advantages. Furthermore the different techniques in manufacturing, surface conditioning and damage removal are describben. Contents: 0. Introduction 1. Tube Types 1.1 Materials 1.2 Optimization with Special Forms 2. Manufacturing of Heat Exchanger Tubes 2.1 Construction/Prefabrication/Machining 2.2 Welding 2.3 Welding/Rolled Tube Joint/Expanding 3. Surface Treatment 3.1 Cathodic Protection 3.2 Pickling/Electrochemical and Chemical Polishing 3.3 Inlet Tube Lining 4. Damages/Damage Removal/Maintenance Bestellungen an:

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HRSG

Heat Recovery Steam Generators Design and Operations 2nd edition Copyright© 2015 PP PUBLICO Publications. All rights reserved. Expected as permitted under the Urheberrecht der Bundesrepublik Deutschland, no part of this publication may be reproduced or distributed in any form or by any means, or stored in a data base or retrieval system, without the prior written permission of the publisher. Direct all inquiries to:

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ISBN: 3-934736-32-7 ISBN-13: 978-3-934-736-32-0 EAN: 978 3 934 736 320

Cover photos: KED, D-München Annotation of the publisher: the quality of figures and tables generally depends on the material made available from the authors. Place of jurisdiction for all matters concerning this book is Essen/Germany. I



III

HRSG Heat Recovery Steam Generators

Design and Operations

Editor: Christian Daublebsky von Eichhain

PP PUBLICO Publications

IV

Compact

Heat Exchangers

Designs - Materials - Applications 2010. 288 Pages with numerous tables and figures ISBN 3-934736-16-5 Hard cover € 44,This handbook presents innovative knowledge concerning designs, naterials and applications of current and future orientated kinds of compact heat exchangers. All authors are recruted from leading scientifical institutions or apparatus producers.

Content: I. Foreword II. Apparatus Designs II.1 Plate Heat Exchangers II.2 Plate & Shell Heat Exchangers II.3 Spiral Heat Exchangers II.4 Block Heat Exchangers II.5 Microstructure Heat Exchanger III. Plate structurization IV. Material Technology IV.1 Copper IV.2 Tantalum

IV.3 Graphite IV.4 Ceramics IV.6 Plastics V. Surface Technology VI. Preventive Measures for Mitigation of Fouling VI.1 Inspection VI.2 Filtration/Mirco Filtration VI.3 Chemical Conditioning VI.4 Cleaning and Reconditioning VII. Applications

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Content

V

1. Introduction 1.1. Abstract 1.1.1. Design 1.1.2. Operation 1.2. Overview 1.2.1. Gas turbine cycle 1.2.2. Rankine- Cycle 1.2.3. Steam turbine 1.2.4. Heat Recovery Steam Boiler 1.2.5. Combined Cycle II 1.2.6. Market of Heat Recovery Steam Generator 1.2.7. History 1.3. Conversion of heat to electrical power 1.3.1. Thermal efficiency 1.3.2. Electrical efficiency

1 2 2 2 2 3 4 5 6 7 8 10 11 11 13

2. Design of a HRSG 2.1. Over all design of a HRSG 2.1.1. Pressure levels 2.1.2. Drum type boiler vs. once through boiler 2.1.3. Pinch Point method 2.2. How to design a boiler 2.2.1. Design of the duct 2.2.2. Tube diameter, fin dimensions and tube pitches 2.2.3. Scaling of fins 2.2.4. Corrosion 2.2.5. Fouling 2.2.6. Fin efficiency and fin material 2.2.7. Pipe wall thickness 2.2.8. Header wall thickness 2.2.9. Drum wall thickness 2.2.10. Gas Side Pressure Drop 2.2.11. Pressure drop on water side 2.2.12. Natural circulation 2.2.13. Forced through circulation 2.2.14. Fin tube heat transfer 2.2.15. Pipe turbulent heat transfer 2.2.16. Pipe evaporation heat transfer 2.2.17. Heat conductivity of steel 2.2.18. Overall heat transfer 2.2.19. Logarithmic mean temperature 2.2.20. Designing of heating surfaces 2.2.21. Noise and vibration problems at heat exchanger 2.2.22. Regenerative feed water preheating vs. condensate preheating 2.2.23. General Remarks 2.2.24. Duct burner

15 16 16 19 19 24 24 24 26 26 28 32 34 35 35 35 36 37 38 38 38 38 38 39 39 40 40 43 44 46

VI

Content

2.2.25. 2.2.26. 2.2.27. 2.2.28. 2.2.29.

Ductwork and casing Environmental considerations Site conditions Steaming in economizers Important notes

3. Operation of steam boiler 3.1. Example of a start up 3.1.1. Gas turbine mass flow 3.1.2. Gas turbine temperature 3.1.3. HP Steam mass flow 3.1.4. HP Steam pressure 3.1.5. Gradients 3.2. Start up 3.2.1. Deaeration of economizers 3.2.2. Purging 3.3. Drain 3.4. Drum water level 3.5. Water running through the economizer 3.6. Start up of the gas turbine 3.7. Life Cycle Fatigue 3.8. Temperature gradients drums and headers 3.9. How to start up faster 3.10. Control system 3.10.1. Drum water level control 3.10.2. Level measurement 3.10.3. Swell and shrink 3.10.4. Single element control 3.10.5. Two element control 3.10.6. Three element control 3.10.7. Four element control 3.10.8. Pressure control 3.10.9. Spray cooler control 3.10.10. Control Methods 3.10.11. Ziegler-Nichols Methods Facilitate Loop Tuning 3.10.12. Load change 3.10.13. Sliding Pressure 3.10.14. Example of a load change with duct burner 3.10.15. Load change of the gas turbine 3.10.16. Duct burner 3.10.17. Shut down 3.10.18. Run out of turbine 4. Appendix I Converting factors 5. Appendix II Disclaimer 6. Literature 7. Contact

48 48 48 49 50 51 52 52 52 53 53 54 54 54 54 55 56 57 57 58 59 62 62 63 63 63 64 64 64 66 66 66 66 67 68 68 69 70 71 72 72 72 73 76 77



VII

Prof. Dr.-Ing. H. Müller-Steinhagen Dr.-Ing. H. U. Zettler (Editors)

Heat Exchanger Fouling

Mitigation and Cleaning Technologies 2nd revised and enlarged edition 2011. 470 Pages with numerous tables and figures ISBN 3-934736-20-3 Soft cover € 58,This handbook presents innovative knowledge concerning designs, preventive measures, maintenance services and monitoring. All authors are recruted from leading scientifical institutions, apparatus builders or leading maintenance offeres.

Content: 1. Introduction 2. Heat Exchangers for Fouling Duties 2.1 Constructional Disposion 2.2 Conditioning Disposion 3. 3.1 3.2

On-Line Mitigation and Cleaning Methods Introductional remarks Mechanical Fouling Mitigation and Cleaning

3.3 3.4

Chemical Fouling Mitigation and Cleaning Physical and Energetical Water Conditioning

4. 4.1 4.2 4.3

Off-Line Cleaning Methods Introductional remarks Chemical Cleaning Mechanical Cleaning

5. Fouling Monitoring

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Editor: Ch. Daublebsky von Eichhain

HRSG

Heat Recovery Steam Generators Design and Operation 2nd edition

2

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1. Introduction

1

1. Introduction

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1. Introduction

1. INTRODUCTION 1.

Introduction

1.1. Abstract This book is about the design and operating of a Heat Recovery Steam Generator (HRSG)

1.1.1. Design • • • • • • • • •

How many pressure stages are taken and why How to determine the pressure of each pressure stage How to design the superheater, evaporator, economizer Tube dimension of the heating surfaces Fin dimension Tube arrangement Velocities of flue gas side, watersteam side and piping Pressure drop How to design a natural circulation system

1.1.2. Operation

• • •

Start up with purging, drain, considering the temperature gradients of drum and headers Start the duct burners Load change

1.2. Overview Combined Cycle The combined cycle is the combination between a gas turbine thermodynamic cycle (Brayton- Cycle) and a steam cycle (Rankine- Cycle). The Brayton Cycle has high source temperature and rejects heat at a temperature that is conveniently used as the energy source for the Rankine Cycle. The most commonly used working fluids for combined cycles are air and steam. Other working fluids (organic fluids, potassium vapour, mercury vapour, and others) have been applied on a limited scale.

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Fig. 1: Flow diagram of a modern HRSG

1.2.1. Gas turbine cycle

Fig. 2: Gas turbine cycle

1 - 2 : Isentropic Compression 2 - 3 : Reversible Constant Pressure Heat Addition 3 - 4 : Isentropic Expansion 4 - 1 : Reversible Constant Pressure Heat Rejection (Exhaust and Intake in the open cycle)

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1. Introduction

Fig. 3: Enthalpy – Entropy (h-s) diagram of a gas turbine cycle

1.2.2. Rankine- Cycle

Fig. 4: Flow diagram and Temperature – Entropy (T-s) diagram of a Rankine Cycle

1- 2- 3- 4- 5-

2 Feed Water Pump 3 Economizer – Evaporator – Superheater 4 High pressure turbine 5 Reheater 6 Low pressure turbine

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6- 1 Condensor

Fig. 5: Pressure – Volume diagram of a Rankine Cycle

4-1 Feed Water Pump 1-2 Economizer – Evaporator – Superheater 2-3 High pressure turbine 3-4 Condenser

1.2.3. Steam turbine In the steam turbine the transferred heat from flue gas of gas turbine to the water – steam of the HRSG is converted to mechanical power.

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1. Introduction

Fig. 6: Steam Turbine

1.2.4. Heat Recovery Steam Boiler

Fig. 7: Cross section of a modern triple pressure HRSG

1. 2. 3. 4. 5. 6. 7. 8.

Inlet with inside insulation covered by stainless steel liner panels. High pressure superheater (HP). Reheater section (RH). Gas or distillate oil fueled duct burner High pressure boiler section and required downcomer piping. High pressure steam drum with internals to meet steam purity requirements. Carbon monoxide (CO) converter and selective catalytic reduction (SCR) System. Intermediate pressure (IP) superheater section.

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9. High pressure economizer section. 10. Intermediate pressure boiler section and required downcomer piping. 11. Intermediate pressure economizer section. 12. Low pressure boiler section with downcomer piping. 13. Carbon steel or stainless steel condensate preheater section. 14. Intermediate pressure steam drum. 15. Low pressure (LP) steam drum with internals adapted for integral deaerator arrangement. 16. Deaerator tank with required pegging steam and equalizer lines. 17. Outlet stack with required environmental monitoring connections and test Ports. 18. Access platforms, ladders and stairway

1.2.5. Combined Cycle II

Fig. 8: Flow diagram and Temperature- Entropy (T-s) Diagram of a combined cycle

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1. Introduction

1.2.6. Market of Heat Recovery Steam Generator 1.2.6.1. Market survey GT Europe According Gas Turbine World 2003 Handbook Foster Wheeler is not mentioned in this survey.

Fig. 9: Market shares of gas turbine OEMs

1.2.6.2.

Prices of CCPP economics of scale

With increasing capacity the prices per kW drops significantly. Until ca. 450MW installed capacity the size of gas turbine and steam turbine is increasing then the economics of scale is much lower because then there are more gas turbines and HRSGs required, the size of steam turbines can get bigger.

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Fig. 10: Prices of Combined Cycle power plants

1.2.6.3.

Market CCPP

The market for Combined Cycle Power Plants (CCPP) has experienced rapid growth in the last years. This growth has been driven by different reasons: • Deregulation in the U.S. and Europe. • Low prizes • Fast to build up (in some cases the gas turbine is installed very quickly and the HRSG is installed later) • Rather low fuel cost of natural gas • Less problem with environmental requirements • Very good cycling behaviour Due to this, e.g. independent power producers (IPP) rose and have induced both, a growth in new power production and a shift from coal and solid-fuel-fired conventional steam plants to gas turbine (GT) plants and CCPP leading to economically interesting returns of investment (ROI). In the U.S. alone, while gas turbine and combined cycle plants represent only 10% of the existing base of 860 GW, they currently provide well over 90% of all new capacity [Got1]. In Europe the markets seem to hesitate. Until now deregulation has taken place in some countries only, e.g. the U.K., but is on the way for the rest of the EU.

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1. Introduction

Expectations in Europe are rather for a consistent growth, than a boost like that in the U.S., which is unlikely, due to the fact that governmental responsibilities for sufficient and reliable power generation in the past led to capacities above the actual needs. Even though these plants, mostly fossil fired, need replacement in the coming one or two decades. In addition coal fired plants, in several European countries, serve great public economic benefits as a result of large own resources. Same applies to hydro power, e.g. Norway, Austria and Switzerland. Nevertheless growth expected in Europe – selecting France, Germany, Spain, Sweden, UK and Finland – as an average number, is 70 GW for new capacity until 2005 [FTE1] The deregulation-driven growth is expected to fall off in North America, while at the same time, combined cycle power plants will support continued HRSG growth in the recovering Asia market. Another key driver is the aggressive technical development of large frame combustion turbines (170 to 250 MW, even 370 MW in a test stage) targeted for the utility power generation market. Over the last decade, large combustion turbines have been developed with higher efficiency and dramatically improved emissions profiles. More efficient water/steam cycles have been developed to take advantage of higher exhaust temperatures from advanced combustion turbines installed in combined cycles. Capital costs of gas fired combined cycle are about 40% of coal fired steam plants [Got1]. Gas price and availability support a life cycle cost advantage in many regions of North America and Europe. The net efficiency of the combined cycle power plant (up to 60% – expected in the near future, at the time being 58% for high end CCPP’s) is much higher than – with conventional steam plants (typically 35% to 40%, up to 50% for high end plants). Combined cycle plants also continue to offer improvements in permitting and Installation time thereby reducing the capital cost and risk to plant developers. Combined cycle plants are able to provide lowest levels of NOX and CO emissions per kWh of electricity produced, especially if low NOX burners and SCR, CO catalysts are considered. This all results in a necessary development in HRSG technology, as well as a new understanding of the HRSG supplier delivering a less priced, though key component of a plant gaining more and more shares in power generation and economic success of the owner.

1.2.7. History To efficiently mate the Rankine steam cycle with high-temperature gas turbines, new HRSGs had to be developed that could operate at substantially higher flue- gas temperatures. New HRSG designs also were required to match each incremental jump in gasturbine size as combined cycle units grew larger and larger. Perhaps the most important development in HRSG design was the move from single- to dual- pressure steam production. This change, which enabled lower stack temperatures and thus greater recovery of thermal energy from the gas-turbine ex-haust, increased thermal efficiency of a combined-cycle plant by nearly four percentage points. Later designs went one step

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further, from dual- to triple-pressure steam production, and yielded approximately one more percentage point gain for the overall cycle. Today, virtually all HRSG manufacturers offer triple pressure reheat steam systems to maximize efficiency [Swa1].

1.3. Conversion of heat to electrical power The main purpose of a HRSG is to convert the hot flue gas of the gas turbine to electrical power. In some cases the HRSG converts a part of the input energy in district heating. The thermal efficiency of the HRSG is rather low, according EN 12952- 15 based on higher heating value (HHV) or ASME PTC it is about 70% - 77%. According EN 12952- 15 based on lower heating value (LHV) or DIN 1942 it is 80% -88%. Direct fired steam generators has efficiencies up to 95% based on LHV. The lower thermal efficiency of the HRSG is caused by the rather low input flue gas temperature and the big flue gas mass flows causes high stack losses.

Fig. 11: Sankey energy diagram of a HRSG

1.3.1. Thermal efficiency There are different methods to calculate the thermal efficiency. The thermal efficiency is defined:

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1. Introduction

1.3.1.1.



η = Useful_heat Input_heat

1.3.1.2.



Input – Output Method

Heat loss Method

η = 1 – Heat_Losses Input_heat

Because of

Input_heat = Useful_heat + Heat_Losses both methods must lead to the same results. Which method is used for testing the efficiency depends on the meas-urement data. For example it is not so easy to get the radiation and con-vection losses, so it is better to calculate the efficiency according the In-put- Output – Method. One hint: The blow down is not a loss, it is a part of the useful heat. The thermal efficiency of the HRSG doesn’t give an answer how much electrical power the steam can produce. It is possible to have a boiler with a very high thermal efficiency and the electrical efficiency is very low. For converting heat in electrical power very often hot steam with high pressure is used. A turbine converts the hot steam with high pressure in mechanical power according Newton’s second law:

W mech = m• (ν Steam_in – ν Steam_out ) ν Turbine_Blade W mech = m• (ν Steam_in – ν Steam_out ) •S S Axel-Turbine_Blade rev Turbine

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Axel-Turbine_Blade

•

rev Turbine

distance turbine axle to middle of turbine blade [m] revolution of turbine per second (normally US: 60 1/s [Hertz] Europe 50 1/s [Hertz])

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Fig. 12: Velocity triangles of a steam turbine

In a nozzle the hot steam with high pressure will be expanded and accelerated to the velocity v Steam_in (turbine Inlet). In the turbine the steam will be decelerated to v Steam_out and the turbine produces the mechanical power out of the velocity differences. Enthalpy and velocity has a close connection: 2 2 h in – h out = v out – v in 2 2

The mechanical power of the turbine is converted in the generator in electrical energy. The efficiency of converting mechanical power in elec-trical power in a generator is rather high (about 98%). But even if the losses are rather low, the generators must be cooled (a 1000 MW gen-erator has losses of about 20 MW!) by hydrogen or water. In some cases the generator is cooled by air.

1.3.2. Electrical efficiency The electrical efficiency is much lower. Electrical efficiency is defined



ηel = electrical_Power Input_heat

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2. Design of a HRSG

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2. Design of a HRSG

2. Design of a HRSG 2.1. Over all design of a HRSG 2.1.1. Pressure levels Why to make different pressure levels ? The steam turbine works with high velocities (about 985ft/s [300 m/s]). The steam turbine “takes” the energy of the steam, so if the steam transferred too much energy, the steam starts to condense to water. Thus the local stress (= compressive stress = pressure) on a turbine blade increases dramatically and may destroy it. The stress is: •

•

F m (νin – νout ) Vρ (νin – νout ) νin Aρ (νin – νout ) σ lokal = = = = A A A A

σ lokal = νin ρ (νin – νout ) The difference of density of water and steam is the difference of local compressive stress. The density of water is more than 1,000. times higher than of steam (in low pressure stages up to 50,000. times higher) The “reheating” of the steam can prevent, that there is too much water in the steam. So the reheating can avoid erosion of the turbine blades and of course increases the performance. If the steam has a high enough pressure, nearly all the energy transferred to the reheat steam can be recovered by the turbine (multiplied with the turbine efficiency i.e. ca. 85%). So another very important advantage of the reheating is, that the efficiency of the thermodynamic process is increasing dramatically. So introducing multiple pressure stages minimize the “exergy” losses. The exergy it this part of the input energy that can’t be transformed to mechanical engergy. The minimum of the exergy losses in the HRSG is, if the heating of the working fluid (in this case Water) has a minimum temperature difference to the cooling of the other (hot) fluid (flue gas of the gas turbine). Increasing efficiency There are 3 main ways to decrease the temperature differences between flue gas and water: 1. Multiple pressure stages 2. Once through boiler 3. Binary fluids (e.g. H2O – NH3 Kalina process)

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You can see the temperature difference in the QT diagram

Fig. 13: Temperature – Transferred Heat (T-Q) Diagram triple pressure diagram

Fig. 14: Temperature – Transferred Heat (T-Q) Diagram dual pressure diagram

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2. Design of a HRSG

Fig. 15: Temperature – Transferred Heat (T-Q) Diagram single pressure diagram

Fig. 16: T-Q Diagram diagram theoretical ideal steam generator

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2.1.2.

19

Drum type boiler vs. once through boiler

Advantages drum type boiler • Easy to control • More safety and longer possible feed water stop (i.e. switch feed water pump) because of the much higher water mass in drum and circulation system Advantages of once through boiler • Faster reaction of load change (less water mass and steel mass) • Faster to start up, no drum preheating Disadvantage of once through boiler • Maldistribution of water in the pipe • Can cause gas side temperature streams • Very expensive water treatment necessary • Very fast reaction to the changing of heat input, because of this the control system must be very fast, reliable and sophisticated.

Fig. 17: Once through HRSG

[Fran1]

2.1.3.

2.1.3.1.

Pinch Point method

Pinch Point

The pinch point is defined as the difference between the gas temperature exiting the last evaporator section and the saturation temperature in that drum. That means with a lower pinch point more steam is produced at that pressure stage.

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2.1.3.2.

2. Design of a HRSG

Approach Point

The approach point is defined as the temperature difference between saturation temperature in the drum and economizer outlet temperature. If the approach point is decreased, less steam must be condensed to preheat the economizer outlet water to saturated temperature.

Fig. 18: Pinch Point and Approach Point in a T- Q Diagram

The pinch point and the approach point have a big influence to the steam flow, if it is assumed that the other parameter are fixed e.g. gas turbine flue gas flow and temperature, superheater steam temperature and pressure, feed water temperature etc. To decrease the pinch point it is normally necessary to increase the transferred heating power. That means often to increase the heating surface or the gas side pressure drop. So there is a search for the optimum with higher efficiency and lower costs. After the decision how many pressure stages there should be, the pressures of each pressure stages can be determined: First of all: The temperature of the HP Steam an RH Steam must be defined. Some small gas turbines don’t produce flue gas with high temperatures (lower than 930 F [500°C]), so the HP Steam temperature is determined as flue gas temperature minus ca. 18 F [10°C] (There must be always a temperature difference to transfer heating power. The lower the temperature difference the bigger must be the heating surface area) If the gas turbine produces higher temperatures the superheating temperature is a question of the pressure and tube material. The higher the temperature the lower should be the pressure and the more expensive is the material. The key components, whose performance is critical, are high-pressure steam piping, headers, and super heater tubing. All these components have to meet creep strength requirements, but thermal fatigue resistance and weldability are important, too. Ferriticmartensitic steels are preferred because of their lower coefficient of thermal expansion and higher thermal conductivity compared to austenitic steels.

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Among the 9% Cr steels fully commercialised, the P91 steel has the highest allowable stress and has been extensively used all over the world as a material for headers, steam pipes and superheater tubes operating at steam temperatures up to 1103 F [595°C] - nominal, 1139F [615°C] as a maximum for HRSG applications according to the German TRD Code or up to 1202F [650°C] tube metal temperature according to ASME. The steel P-92, developed by substituting part of the Mo in P-91 by W, has even higher allowable stress values and can be operated up to steam temperatures of 1175F [635°C]. P-92 is already approved by the ASME boiler code, but no approval according to the German rules is available for the time being. Further developments are E-911, which is already approved in Germany (material number 1.4905) and P122, which was developed in Japan and has been approved by ASME. The allowable creep strength of these new steels at 1112 F [600°C] is about 25% higher than that of P-91 [Vis1]. As an example for application, a super heater made of E-911 and steam loops made of E-911 and P-92 are operating at steam temperatures of 1202 F [650°C] in the conventional fired power station of RWE in Germany. Therefore it must be remarked that the limiting factor for efficiency increasing high steam temperatures is the high end steam turbine, which is commercially available for steam temperatures at a maximum of 1049 F [565°C], only [Nes1]. With the material of the superheaters, reheaters and headers respectively the live steam a reheat steam temperature is fixed. The condensate pressure should also be known (e.g. an air cooled condenser has an higher pressure than an sea cooled or river cooled condenser (ca. 0.75 PSI [0,05171 bar])) Then there must be the maximum water content in steam (ca. 5% - 10% mass fraction water in the steam (= 95% - 90% steam content)) defined and the efficiency of the turbine (The data is normally received of the turbine manufactory). So the end- point of the graph in the h- s (enthalpy – entropy) can be determinate (see end point 1 in picture). In a computer calculation the enthalpy (h) and entropy (s) of steam water mix is a function of the pressure and water content h(p,x) s (p,x). Then determinate the enthalpy differences between this point and the point with the same entropy and the superheating or reheating temperature respectively. Divide the enthalpy difference with the efficiency of the turbine and search for points with the same entropy with the condenser pressure, the SH or RH temperature and the enthalpy difference (see example). So the start point for expansion is fixed too.

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2. Design of a HRSG

Fig. 19: Turbine Expansion in a h- s diagram

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Fig. 20: Triple pressure turbine expansion in a h- s diagram

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2. Design of a HRSG

2.2. How to design a boiler 2.2.1. Design of the duct The flue gas duct of the boiler should be longer along the pipes than across the pipes, because with a smaller width of the boiler, less pipes must be welded in the headers. The ratio can be 3 to 4 times along the tubes to the width.

Fig. 21: Cross section of a HRSG

The first dimension of the duct must be guessed and during an iterative calculation adapted. With the length of the duct the fine tuning of gas side pressure drop and the heating surface area can be made very easy, e.g. 10% length of the duct means 10% more heating surface and 17% decrease of pressure drop.

2.2.2.

Tube diameter, fin dimensions and tube pitches

It must be decided which outer diameter and fin height should be used. The geometry effect is the apparent anomaly in heat transfer surface between various vendors for the same performance. As an example, a vendor with 2.0" [51mm]OD tubes may propose 25 % more surface than the competitor who uses 1.5" [38mm] OD tube. This does not mean that the lower surface is the result of high technology heat transfer equipment design. This happens simply because of the nature of heat transfer itself. Lower diameter tubes give the same amount of heat absorption with less surface. Similar anomalies exist for other geometry parameters such as fin type, fin geometry, tube length etc. For this reason it is not prudent to eliminate designs which may have too much or too little surfaces.

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Transversal pitch is recommended, cause higher pressure drop but higher heat transfer, lower number of rows in flue gas flow direction necessary. Typical values are Parameter

Traditional value

What is used today

Tube OD inch [mm] Fin type Fins/inch [Fin/m] Arrangement Tube pitches inch [mm] Fin height inch [mm]

2.0 [50.8] Solid 5 [200] Inline 3 – 6 [76-152] 0.75 [19.1]

1.25 [31.8] Serrated 0,1 – 8 [10-315] staggered 2.5 [63] 1.00 [25.4]

Tab. 1: Typical design values of HRSGs

Out of the outer diameter and fin height the transversal and longitudinal tube spacing can be calculated. It is recommended to have distances between fin tips of about 0.5 inch – 0.25 inch [12.7mm – 6.4mm].

Fig. 22: Serrated fins

The fin density can be chosen between 0.5 fins/in [20 fins / m] and 7.5 fins/in [300 fins / m] [Brü1] depended on the needed heat transfer, maximum flue gas velocities and pressure drop. There are different methods to manufacture the finings on the tubes. A very dearly (close) mounting with a continuous welding (very seldom soldering) is recommended. There should be no spot welding. During the whole live there shouldn’t be any mechanical or pitting corrosion dismantling. If there is only a tiny gap between fins and tube, the fins don’t transfer any heat and can start scaling.

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2.2.3. Scaling of fins

The fin tips has a much higher temperature than the pipe wall. So there is a danger of scaling of fins. The fin tip temperature can be calculated by hand or computer program.

Fig. 23: Fin efficiency diagram

T Fin-Tip = T Wall +θ (T FlueGas – T Wall ) Carbon steel fins can have fin tip temperatures up to 1112 F [600°C] [Berg1], if there is no chlorine, vanadium, very less sulphur and sodium in the flue gas.

2.2.4.

Corrosion

Potential problem areas as a result of load cycling or on-off cycling include: gas turbine exhaust dew point corrosion, corrosion fatigue, and consequences of not maintaining proper steam cycle chemistry (i.e., on-line, off-line storage and return to service). Corrosion and fatigue damage are cumulative and can not be reversed. Using HRSG initially designed for base load operation in cycling operation defines the need to carefully evaluate several occurrences with regard to HRSGs. Special attention has to be paid to three of them at least:

2.2.4.1.

Stress Corrosion Fatigue

2.2.4.2.

Flow Accelerated Corrosion

Since cycling means temperature and pressure gradients from ambient to operational level and air ingress during longer outages, stress corrosion fatigue as a result of these influences will occur. A proper chemistry regime, i.e. maintaining low dissolved oxygen, pH within the required range and proper feed water quality (VGB, O2 < 0,1 mg/kg), is a must. From the HRSG operating side, the boiler should be kept under pressure as long as possible, e.g. no forced cooling and closing of the stack damper to prevent rapid natural draft cooling. First, the HRSG designer has to consider flow velocities lower than the known limits to dissolve protective Magnetite layers in water and/or lines carrying two phases, water

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and steam. Second, the chemistry regime has to be maintained in a way that the Oxygen content is not too low to prevent a proper magnetite layer from forming - erosion corrosion is increasing - and on the other hand not too high to accelerate Stress Corrosion Fatigue. In Europe this has been taken into account by the increased maximum O2 content (VGB, TRD, etc.) for boiler feed water (from 0,02 mg/kg to 0,1 mg/kg for pH > 9). Best choose is not to fall below 0,05 mg/kg (VGB minimum for pH neutral feed water) considering the above.

2.2.4.3.

Gas Side Corrosion

Cold end corrosion is a well known phenomenon. It can be prevented by increasing the water inlet temperature, e.g. condensate recirculation, above the dew point of the flue gases. Cycling leads to a situation at each start up, when the inlet temperature can not be properly increased - deposits on the cold end of the HRSG surfaces are the consequence. This results in decrease of thermal efficiency and increase of draft losses at the long term, fin and tube corrosion, if the deposits are moistened - by air humidity or washing. To prevent or limit the effect of cold end corrosion during cycling Operation, regular inspections and cleaning of the boiler surfaces is recommended. This is usually done by air blasting (little deposits), dry ice blasting (up to 6 layers affected) and washing with large amounts of low pressure water (entire surfaces). The water washing is the most effective, although special considerations have to be made and actions set to prevent corrosion of the casing (horizontal type HRSG) or poisoning a catalyst (vertical type HRSG). Start up after performing water washing is recommended to prevent corrosion of other HRSG parts. The ultimate solution to cold end corrosion is the use of corrosion resistant materials - the only reliable and lasting but expensive solution.

Fig. 24: Sulphur acid dew point diagram

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Dew point of sulphuric acid as a function of partial pressure of sulphuric trioxide and water vapour T_S = (A – B • ln ( p H2O) – C • ln ( p H2SO4 ) + D • ln ( p H2SO4) • ln ( p H2O))-1 A = 2.988 • 10-3 • K-1 B = 5.97 • 10-5 • K-1 C = 1.161 • 10-4 • K-1 D = 6.2 • 10-6 • K-1 P H2O = x H2O p P H2SO4 = x H2SO4 p [Ver1] Unfortunately the sulphuric trioxide content in flue gas is not known. Normally it is assumed that the converting rate form SO2 to SO3 is up to 5% [Gan2] [Ras1] but other articles say, it can be up to 50% [Wic1].

2.2.5.

Fouling

Fuel

Outside Fouling Factors hr ft² F/Btu [m²K/kW]

Minimum Fin Spacings in [mm]

Dry Air Natural Gas Propane Butane No. 2 Fuel Oil No. 6 Fuel Oil Crude Oil Residual Oil Coal Wood Wastes

0.000 - 0.001[0.000-0.176] 0.001 - 0.003[0.176-0.528] 0.001 - 0.003[0.176-0.528] 0.001 - 0.003[0.176-0.528] 0.002 - 0.004[0.352-0.704] 0.003 - 0.007[0.528-1.233] 0.008 - 0.015[1.409-2.642] 0.010 - 0.030[1.761-5.283] 0.010 - 0.050[1.761-8.805] 0.010 - 0.050[1.761-8.805]

0.05 [1.27] 0.07 [1.78] 0.07 [1.78] 0.07 [1.78] 0.12 [3.05] 0.18 [4.57] 0.20 [5.08] 0.20 [5.08] 0.34 [8.64] 0.34 [8.64]

Tab. 2: Fouling and fin spacing as a function of fuel

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TEMA fouling resistances for cooling water (hr ft² F/Btu [m²K/kW]) Type of cooling water

Fouling resistance

Seawater (Tout < 113 F [45°C]) Brackish water (Tout < 113 F [45°C]) Treated cooling tower water (Tout < 122 F [50°C]) Treated recirculated water Fluvial water Engine cooling water Distilled water or condensate Treated boiler feedwater Boiler blowdown

0.001- 0.002 [0.18 – 0.35] 0.002-0.003 [0.35 – 0.53] 0.002-0,003 [0.18 – 0.53] 0.002 [0.18] 0.002-0.003 [0.35 – 0.53] 0.001 [0.18] 0.0005 – 0.001 [0.09 – 0.18] 0.0005 [0.09] 0.002-0.003 [0.35 – 0.53]

Tab. 3: Fouling as a function of water type

Fouling resistance in heat transfer from gaseous combustion products to finned heat transfer surfaces(Wei[1]) Fuel Natural gas Propane Butane Clean turbine gas Moderately clean turbine gas Light fuel oil Diesel Heavy fuel oil Crude oil Coal

Fouling resistance hr ft² F/Btu [m²K/kW] 0.0005-0.003 [0.09-0.53] 0.001-0.003 [0.18-0.53] 0.001-0.003 [0.18-0.53] 0.001 [0.18] 0.0015-0.003 [0.27-0.5] 0.002-0.004 [0.36-0.7] 0.003 [0.53] 0.003-0.007 [0.53-1.24] 0.004-0.015 [0.7-2.7] 0.005-0.050 [0.89-8.85]

Flow velocity ft/s [m/s] 98 –131 [30 – 40]

82 – 98 [25 – 30]

59 – 79 [18 – 24] 49 – 69 [15 – 21]

Tab. 4: Fouling and maximum flue gas velocity as a function of fuel

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Fouling through evaporating liquids Fouling problems in evaporators are caused by corrosion or local concentration or precipitation of components with a lower vapour pressure than that of the carrier liquid. In situ corrosion of heated surfaces presents much less problems than the deposition of products of corrosion formed upstream [Som1], [Goo1]. Since fouling is furthered by bubble formation and severely affects the high hest transfer coefficients normally encountered in evaporation, very strict codes apply the purity of boiler feed water. The values recommended in 1975 by the ASME Research Committee for Water in Thermal Power Stations for operating cycles of one year [Sim1] are listed in tab. 5. For Germany the values are given in tab. 6 [VGB1]. Guide values for boiler feed water Pressure PSI [bar] 0-290 [0–20] 290-435 [20–30] 435-580[30–40] 580-725 [40–50] 725-870 [50 – 60] 870-1015 [60–70] 1015-1450 [70–100] 1450-2031 [100–140]

Iron Copper ppm ppm 0.100 0.050 0.050 0.025 0.030 0.020 0.025 0.020 0.020 0.015 0.020 0.015 0.010 0.010 0.010 0.010

SiO2 ppm 150 90 40 30 20 8 2 1

Hardness Alkalinity ppm CaCO3 0.300 700 0.300 600 0.200 500 0.200 400 0.100 300 0.050 200 0.000 0 0.000 0

Conductivity 1/(μΩ in)[μS/cm] 1.78 [0.7] 1.52 [0.6] 1.27 [0.5] 1.02 [0.4] 0.76 [0.3] 0.51 [0.2] 0.038 [0.015] 0.025 [0.01]

Tab. 5: Feed water requirements

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a. Boiler feed water

Natural-circulation boilers

General demands Oxygen PH at 20°C SiO2 Hardness3 Total iron Copper Total CO2 Conductivity Permanganate Oil

Forced circulation boilers 80 bar1

Clear and colourless 0.03 ppm max; in continous operation < 0.02 ppm 7 – 9.5 < 0.02 ppm2 n.d.4 < 0.02 ppm < 0.003 ppm < 1 ppm < 0.2 μS/cm if poss. < 5 ppm < 0.3 ppm

< 1 ppm < 0.5 ppm If possible < 0.05 < 0.01 if possible < 20 < 0.3 if possible < 10 if possible < 1

n.n. < 0.03 < 0.005