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

7/24/2019 Heat Recovery Steam Generators Design and Operation (2nd Edition)

1/93

Editor:Ch. Daublebskyvon Eichhain HRSG

Heat Recovery Steam Generators

Design and Operation2nd edition

PP PUBLICOPublications

2


2/93

The FINEST way of water treatment!

FINEAMIN inhibitors for water treatment against corrosion

and scale in your systems

FINEAMINis effective and protects the environment

Customer service: fast and competent service

CWB Wasserbehandlung GmbH

Glienicker Weg 95

D -12489 Berlin

Tel.: +49 (0)30 67 89- 37 51

Fax: +49 (0)30 67 89-38 21

info @cwb-berlin.de

www.fineamin.de

h2o facilities sa

8 av. Grandes Communes

CH -1213 Petit Lancy | Switzerland

Tel.: +41 22 879 95 00

Fax: +41 22 879 95 09

info @h2o-f.ch

www.fineamin.com

ABWASSERPOWER PLANTS HEATING SYSTEMS COOLING SYSTEMS WASTE WATER

Your partner inwater chemistry


3/93

Prof. Dr.-Ing. Jovan Mitrovic (Editor)

Heat Exchanger andCondenser Tubes

Tube Types Materials Attributes Machining

2004. 311 pages with numerous figures

and tables.

ISBN 3-934736-08-4.

Soft coverE

38,--

This book is the english version of the handbook

Wrmebertrager-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 manufactu-

ring, surface conditioning and damage removal are describben.

Contents:

0. Introduction

1. Tube Types

1.1 Materials1.2 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:

PP PUBLICO PublicationsWitteringstr. 10 + D 45130 Essen/Germany

Tel.: ++49(0)201/79 12 12Fax: ++49(0)201/79 88 278

e-mail: [email protected]

www.pp-publico.de


4/93

II

HRSG


Design and Operations

2nd edition

Copyright 2015 PP PUBLICOPublications. 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:

PP PUBLICOPublications,

Witteringstr. 10, D - 45130 Essen/ Germany

Phone: +49(0)201/79 12 12


www.pp-publico.de

ISBN: 3-934736-32-7

ISBN-13: 978-3-934-736-32-0

EAN: 978 3 934 736 320

Cover photos:

KED, D-Mnchen

Annotation of the publisher:

the quality of gures 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


5/93

III

HRSG

Heat Recovery

Steam GeneratorsDesign and Operations

Editor: Christian Daublebsky von Eichhain

PPPUBLICOPublications


6/93

IV

Compact Heat ExchangersDesigns - Materials - Applications

2010. 288 Pages with

numerous tables and gures

ISBN 3-934736-16-5

Hard cover 44,-

This handbook presents innovative

knowledge concerning designs, nate-

rials and applications of current andfuture orientated kinds of compact heat

exchangers.

All authors are recruted from leading

scientical 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 TechnologyIV.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 ConditioningVI.4 Cleaning and Reconditioning

VII. Applications

PPPUBLICOPublicationsWitteringstr. 10 + D 45130 Essen/Germany


www.pp-publico.de


7/93

Content V

1. Introduction 1

1.1. Abstract 2

1.1.1. Design 2

1.1.2. Operation 2

1.2. Overview 21.2.1. Gas turbine cycle 3

1.2.2. Rankine- Cycle 4

1.2.3. Steam turbine 5

1.2.4. Heat Recovery Steam Boiler 6

1.2.5. Combined Cycle II 7

1.2.6. Market of Heat Recovery Steam Generator 8

1.2.7. History 10

1.3. Conversion of heat to electrical power 11

1.3.1. Thermal efciency 11

1.3.2. Electrical efciency 13

2. Design of a HRSG 15

2.1. Over all design of a HRSG 16

2.1.1. Pressure levels 16

2.1.2. Drum type boiler vs. once through boiler 19

2.1.3. Pinch Point method 19

2.2. How to design a boiler 24

2.2.1. Design of the duct 24

2.2.2. Tube diameter, n dimensions and tube pitches 242.2.3. Scaling of ns 26

2.2.4. Corrosion 26

2.2.5. Fouling 28

2.2.6. Fin efciency and n material 32

2.2.7. Pipe wall thickness 34

2.2.8. Header wall thickness 35

2.2.9. Drum wall thickness 35

2.2.10. Gas Side Pressure Drop 35

2.2.11. Pressure drop on water side 36

2.2.12. Natural circulation 372.2.13. Forced through circulation 38

2.2.14. Fin tube heat transfer 38

2.2.15. Pipe turbulent heat transfer 38

2.2.16. Pipe evaporation heat transfer 38

2.2.17. Heat conductivity of steel 38

2.2.18. Overall heat transfer 39

2.2.19. Logarithmic mean temperature 39

2.2.20. Designing of heating surfaces 40

2.2.21. Noise and vibration problems at heat exchanger 40

2.2.22. Regenerative feed water preheating vs. condensate preheating 43

2.2.23. General Remarks 44

2.2.24. Duct burner 46


8/93

VI Content

2.2.25. Ductwork and casing 48

2.2.26. Environmental considerations 48

2.2.27. Site conditions 48

2.2.28. Steaming in economizers 49

2.2.29. Important notes 50

3. Operation of steam boiler 51

3.1. Example of a start up 52

3.1.1. Gas turbine mass ow 52

3.1.2. Gas turbine temperature 52

3.1.3. HP Steam mass ow 53

3.1.4. HP Steam pressure 53

3.1.5. Gradients 54

3.2. Start up 54

3.2.1. Deaeration of economizers 54

3.2.2. Purging 54

3.3. Drain 55

3.4. Drum water level 56

3.5. Water running through the economizer 57

3.6. Start up of the gas turbine 57

3.7. Life Cycle Fatigue 58

3.8. Temperature gradients drums and headers 59

3.9. How to start up faster 62

3.10. Control system 623.10.1. Drum water level control 63

3.10.2. Level measurement 63

3.10.3. Swell and shrink 63

3.10.4. Single element control 64

3.10.5. Two element control 64

3.10.6. Three element control 64

3.10.7. Four element control 66

3.10.8. Pressure control 66

3.10.9. Spray cooler control 66

3.10.10. Control Methods 663.10.11. Ziegler-Nichols Methods Facilitate Loop Tuning 67

3.10.12. Load change 68

3.10.13. Sliding Pressure 68

3.10.14. Example of a load change with duct burner 69

3.10.15. Load change of the gas turbine 70

3.10.16. Duct burner 71

3.10.17. Shut down 72

3.10.18. Run out of turbine 72

4. Appendix I Converting factors 72

5. Appendix II Disclaimer 73

6. Literature 76

7. Contact 77


9/93

VII

Prof. Dr.-Ing. H. Mller-Steinhagen

Dr.-Ing. H. U. Zettler (Editors)

Heat Exchanger FoulingMitigation and Cleaning Technologies

2nd revised and enlarged edition

2011. 470 Pages with

numerous tables and gures

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 scientical

institutions, apparatus builders or leading main-

tenance offeres.

Content:

1. Introduction

2. Heat Exchangers for Fouling Duties

2.1 ConstructionalDisposion

2.2 Conditioning Disposion

3. On-Line Mitigation and Cleaning

Methods

3.1 Introductional remarks

3.2 Mechanical Fouling Mitigationand Cleaning

3.3 Chemical Fouling Mitigation andCleaning

3.4 Physical and Energetical Water

Conditioning

4. Off-Line Cleaning Methods

4.1 Introductional remarks

4.2 Chemical Cleaning

4.3 Mechanical Cleaning

5. Fouling Monitoring

PPPUBLICOPublicationsWitteringstr.10+D45130Essene-mail:[email protected]

ww.pp-publico.de


10/93


11/93

Editor:Ch. Daublebskyvon Eichhain HRSG


Design and Operation2nd edition

PP PUBLICOPublications

2


12/93

1. Introduction 1

1. Introduction


13/93

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 ue 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 uids for com-

bined cycles are air and steam. Other working uids (organic uids, potassium vapour,

mercury vapour, and others) have been applied on a limited scale.

2 1. Introduction


14/93

1. Introduction 3

1 - 2 : Isentropic Compression

2 - 3 : Reversible Constant Pressure Heat Addition

3 - 4 : Isentropic Expansion4 - 1 : Reversible Constant Pressure Heat Rejection (Exhaust and Intake in the open cycle)

1.2.1. Gas turbine cycle

Fig. 2: Gas turbine cycle

Fig. 1: Flow diagram of a modern HRSG


15/93

4 1. Introduction

Fig. 3:Enthalpy Entropy (h-s) diagramof a gas turbine cycle

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

1.2.2. Rankine- Cycle

1- 2 Feed Water Pump

2- 3 Economizer Evaporator Superheater

3- 4 High pressure turbine

4- 5 Reheater

5- 6 Low pressure turbine


16/93

1. Introduction 5

4-1 Feed Water Pump

1-2 Economizer Evaporator Superheater

2-3 High pressure turbine

3-4 Condenser

Fig. 5: Pressure Volume diagram of a Rankine Cycle

6- 1 Condensor

1.2.3. Steam turbine

In the steam turbine the transferred heat from ue gas of gas turbine to the water

steam of the HRSG is converted to mechanical power.


17/93

6 1. Introduction

Fig. 6:Steam Turbine

Fig. 7: Cross

section of amodern triple

pressure HRSG

1.2.4. Heat Recovery Steam Boiler

1. Inlet with inside insulation covered by stainless steel liner panels.

2. High pressure superheater (HP).

3. Reheater section (RH).

4. Gas or distillate oil fueled duct burner

5. High pressure boiler section and required downcomer piping.

6. High pressure steam drum with internals to meet steam purity requirements.

7. Carbon monoxide (CO) converter and selective catalytic reduction (SCR) System.8. Intermediate pressure (IP) superheater section.


18/93

1. Introduction 7

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

1.2.5. Combined Cycle II

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


19/93

8 1. Introduction

Fig. 9: Market shares of gas turbine OEMs

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.

1.2.6.2. Prices of CCPP economics of scale

With increasing capacity the prices per kW drops signicantly.

Until ca. 450MW installed capacity the size of gas turbine and steam turbine is increas-

ing then the economics of scale is much lower because then there are more gas tur-bines and HRSGs required, the size of steam turbines can get bigger.


20/93

1. Introduction 9

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-red 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 somecountries only, e.g. the U.K., but is on the way for the rest of the EU.

Fig. 10: Prices of Combined Cycle power plants


21/93

10 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 sufcient

and reliable power generation in the past led to capacities above the actual needs. Even

though these plants, mostly fossil red, need replacement in the coming one or two

decades. In addition coal red plants, in several European countries, serve great publiceconomic benets 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) tar-

geted for the utility power generation market. Over the last decade, large combustionturbines have been developed with higher efciency and dramatically improved emis-

sions proles. More efcient water/steam cycles have been developed to take advan-

tage of higher exhaust temperatures from advanced combustion turbines installed in

combined cycles. Capital costs of gas red combined cycle are about 40% of coal red

steam plants [Got1]. Gas price and availability support a life cycle cost advantage in

many regions of North America and Europe. The net efciency of the combined cycle

power plant (up to 60% expected in the near future, at the time being 58% for high end

CCPPs) 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 improve-ments 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 un-

derstanding 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 efciently mate the Rankine steam cycle with high-temperature gas turbines, new

HRSGs had to be developed that could operate at substantially higher uegas temper-

atures. New HRSG designs also were required to match each incremental jump in gas-

turbine 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 pro-

duction. This change, which enabled lower stack temperatures and thus greater recov-

ery of thermal energy from the gas-turbine ex-haust, increased thermal efciency of acombined-cycle plant by nearly four percentage points. Later designs went one step


22/93

1. Introduction 11

further, from dual- to triple-pressure steam production, and yielded approximately one

more percentage point gain for the overall cycle. Today, virtually all HRSG manufactur-

ers offer triple pressure reheat steam systems to maximize efciency [Swa1].

1.3. Conversion of heat to electrical power

The main purpose of a HRSG is to convert the hot ue gas of the gas turbine to electri-

cal power. In some cases the HRSG converts a part of the input energy in district heat-

ing.

The thermal efciency 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 red steam generators has efciencies up to 95% based on LHV. The low-

er thermal efciency of the HRSG is caused by the rather low input ue gas temperature

and the big ue gas mass ows causes high stack losses.

1.3.1. Thermal efciency

There are different methods to calculate the thermal efciency.

The thermal efciency is dened:

Fig. 11:Sankey energydiagram of a HRSG


23/93

12 1. Introduction

1.3.1.1. Input Output Method

=

Useful_heat

Input_heat

1.3.1.2. Heat loss Method

= 1Heat_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 efciency 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 efciency 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 efciency of the HRSG doesnt give an answer how much electrical powerthe steam can produce.

It is possible to have a boiler with a very high thermal efciency and the electrical ef-

ciency 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

Newtons second law:

W mech= m ( Steam_in Steam_out) Turbine_Blade

W mech= m ( Steam_in Steam_out)SAxel-Turbine_Blade

rev Turbine

SAxel-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])


24/93

1. Introduction 13

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_outand the turbine produces the

mechanical power out of the velocity differences.

Enthalpy and velocity has a close connection:

hin hout = v2out

v2in

2 2

The mechanical power of the turbine is converted in the generator in electrical energy.

The efciency 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 efciency

The electrical efciency is much lower.

Electrical efciency is dened

el =

electrical_Power

Input_heat


25/93

14 1. Introduction


26/93


2. Design of a HRSG


27/93

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 tur-

bine 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 = pres-

sure) on a turbine blade increases dramatically and may destroy it.

The stress is:

lokal= F

= m (in out)

= V (in out)

= inA (in out)

A A A A

lokal= in (in out)

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 pres-sure 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 per-

formance. 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 efciency

i.e. ca. 85%).

So another very important advantage of the reheating is, that the efciency of the ther-

modynamic process is increasing dramatically. So introducing multiple pressure stages

minimize the exergy losses. The exergy it this part of the input energy that cant betransformed to mechanical engergy.

The minimum of the exergy losses in the HRSG is, if the heating of the working uid (in

this case Water) has a minimum temperature difference to the cooling of the other (hot)

uid (ue gas of the gas turbine).

Increasing efciency

There are 3 main ways to decrease the temperature differences between ue gas and

water:

1. Multiple pressure stages2. Once through boiler

3. Binary uids (e.g. H2O NH3Kalina process)

16 2. Design of a HRSG


28/93


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


29/93


Fig. 15: Temperature Transferred Heat (T-Q) Diagram singlepressure diagram

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


30/93


2.1.2. 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.

[Fran1]

2.1.3. Pinch Point method2.1.3.1. Pinch Point

The pinch point is dened 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.

Fig. 17: Once through HRSG


31/93


2.1.3.2. Approach Point

The approach point is dened as the temperature difference between saturation tem-

perature in the drum and economizer outlet temperature. If the approach point is de-

creased, less steam must be condensed to preheat the economizer outlet water tosaturated temperature.

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

The pinch point and the approach point have a big inuence to the steam ow, if it is as-

sumed that the other parameter are xed e.g. gas turbine ue gas ow 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 efciency 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 dened. Some

small gas turbines dont produce ue gas with high temperatures (lower than 930 F[500C]), so the HP Steam temperature is determined as ue gas temperature minus

ca. 18 F [10C] (There must be always a temperature difference to transfer heating pow-

er. 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 ques-

tion 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. Ferritic-

martensitic steels are preferred because of their lower coefcient of thermal expansionand higher thermal conductivity compared to austenitic steels.


32/93


Among the 9% Cr steels fully commercialised, the P91 steel has the highest allow-

able stress and has been extensively used all over the world as a material for head-

ers, steam pipes and superheater tubes operating at steam temperatures up to 1103 F

[595C] - nominal, 1139F [615C] as a maximum for HRSG applications according to

the German TRD Code or up to 1202F [650C] tube metal temperature according toASME. 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

[635C]. 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 [600C] 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 [650C] in the conventional

red power station of RWE in Germany. Therefore it must be remarked that the limit-ing factor for efciency increasing high steam temperatures is the high end steam tur-

bine, which is commercially available for steam temperatures at a maximum of 1049 F

[565C], only [Nes1].

With the material of the superheaters, reheaters and headers respectively the live steam

a reheat steam temperature is xed.

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)) dened and the efciency of the tur-

bine (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 theenthalpy difference with the efciency of the turbine and search for points with the same

entropy with the condenser pressure, the SH or RH temperature and the enthalpy dif-

ference (see example). So the start point for expansion is xed too.


33/93


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


34/93


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


35/93


2.2. How to design a boiler

2.2.1. Design of the duct

The ue 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 rst dimension of the duct must be guessed and during an iterative calculation

adapted. With the length of the duct the ne 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, n dimensions and tube pitchesIt must be decided which outer diameter and n 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. Simi-

lar anomalies exist for other geometry parameters such as n type, n geometry, tube

length etc. For this reason it is not prudent to eliminate designs which may have toomuch or too little surfaces.


36/93


Transversal pitch is recommended, cause higher pressure drop but higher heat transfer,

lower number of rows in ue gas ow direction necessary.

Typical values are

Parameter Traditional What is used value today

Tube OD inch [mm] 2.0 [50.8] 1.25 [31.8]

Fin type Solid Serrated

Fins/inch [Fin/m] 5 [200] 0,1 8 [10-315]

Arrangement Inline staggered

Tube pitches inch [mm] 3 6 [76-152] 2.5 [63]

Fin height inch [mm] 0.75 [19.1] 1.00 [25.4]

Tab. 1: Typical design values of HRSGs

Out of the outer diameter and n height the transversal and longitudinal tube spacing

can be calculated. It is recommended to have distances between n tips of about 0.5

inch 0.25 inch [12.7mm 6.4mm].

Fig. 22: Serrated ns

The n density can be chosen between 0.5 ns/in [20 ns / m] and 7.5 ns/in [300 ns

/ m] [Br1] depended on the needed heat transfer, maximum ue gas velocities and

pressure drop.

There are different methods to manufacture the nings 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 shouldnt be any mechani-cal or pitting corrosion dismantling. If there is only a tiny gap between ns and tube, the

ns dont transfer any heat and can start scaling.


37/93


2.2.3. Scaling of nsThe n tips has a much higher temperature than the pipe wall. So there is a danger of

scaling of ns. The n tip temperature can be calculated by hand or computer program.

TFin-Tip= T Wall+ (TFlueGas T Wall)

Carbon steel ns can have n tip temperatures up to 1112 F [600C] [Berg1], if there is

no chlorine, vanadium, very less sulphur and sodium in the ue gas.

2.2.4. CorrosionPotential 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). Cor-

rosion and fatigue damage are cumulative and can not be reversed. Using HRSG ini-

tially designed for base load operation in cycling operation denes 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 FatigueSince 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

inuences 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.

2.2.4.2. Flow Accelerated Corrosion

First, the HRSG designer has to consider ow velocities lower than the known limits todissolve protective Magnetite layers in water and/or lines carrying two phases, water

Fig. 23: Fin efciency diagram


38/93


and steam. Second, the chemistry regime has to be maintained in a way that the Oxy-

gen 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 Corro-

sion 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 wa -

ter) considering the above.

2.2.4.3. Gas Side CorrosionCold 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 ue

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 conse-

quence. This results in decrease of thermal efciency and increase of draft losses at the

long term, n and tube corrosion, if the deposits are moistened - by air humidity or wash-ing. 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


39/93


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 = 2.98810-3K-1

B = 5.9710-5K-1

C = 1.16110-4K-1

D = 6.210-6K-1

P H2O = x H2O p

P H2SO4= x H2SO4p

[Ver1]

Unfortunately the sulphuric trioxide content in ue gas is not known. Normally it is as-

sumed that the converting rate form SO2to SO3is up to 5% [Gan2] [Ras1] but other

articles say, it can be up to 50% [Wic1].

2.2.5. Fouling

Outside Fouling Factors Minimum Fin Spacings

Fuel hr ft F/Btu [mK/kW] in [mm]

Dry Air 0.000 - 0.001[0.000-0.176] 0.05 [1.27]

Natural Gas 0.001 - 0.003[0.176-0.528] 0.07 [1.78]

Propane 0.001 - 0.003[0.176-0.528] 0.07 [1.78]

Butane 0.001 - 0.003[0.176-0.528] 0.07 [1.78]

No. 2 Fuel Oil 0.002 - 0.004[0.352-0.704] 0.12 [3.05]

No. 6 Fuel Oil 0.003 - 0.007[0.528-1.233] 0.18 [4.57]

Crude Oil 0.008 - 0.015[1.409-2.642] 0.20 [5.08]

Residual Oil 0.010 - 0.030[1.761-5.283] 0.20 [5.08]Coal 0.010 - 0.050[1.761-8.805] 0.34 [8.64]

Wood Wastes 0.010 - 0.050[1.761-8.805] 0.34 [8.64]

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


40/93


TEMA fouling resistances for cooling water (hr ft F/Btu [mK/kW])

Type of cooling water Fouling resistance

Seawater (Tout< 113 F [45C]) 0.001- 0.002 [0.18 0.35]

Brackish water (Tout < 113 F [45C]) 0.002-0.003 [0.35 0.53]

Treated cooling tower water 0.002-0,003 [0.18 0.53]

(Tout< 122 F [50C])

Treated recirculated water 0.002 [0.18]

Fluvial water 0.002-0.003 [0.35 0.53]

Engine cooling water 0.001 [0.18]

Distilled water or condensate 0.0005 0.001 [0.09 0.18]Treated boiler feedwater 0.0005 [0.09]

Boiler blowdown 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 nned heat

transfer surfaces(Wei[1])

Fuel Fouling resistance Flow velocity ft/shr ft F/Btu [mK/kW] [m/s]

Natural gas 0.0005-0.003 [0.09-0.53] 98 131 [30 40]

Propane 0.001-0.003 [0.18-0.53]

Butane 0.001-0.003 [0.18-0.53]

Clean turbine gas 0.001 [0.18]

Moderately clean turbine 0.0015-0.003 [0.27-0.5] 82 98 [25 30]

gas 0.002-0.004 [0.36-0.7]

Light fuel oil 0.003 [0.53]

Diesel 0.003-0.007 [0.53-1.24] 59 79 [18 24]

Heavy fuel oil 0.004-0.015 [0.7-2.7]

Crude oil 0.005-0.050 [0.89-8.85] 49 69 [15 21]

Coal

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


41/93


Fouling through evaporating liquids

Fouling problems in evaporators are caused by corrosion or local concentration or pre-

cipitation 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

coefcients 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

Tab. 5: Feed water requirements

Pressure Iron Copper SiO2 Hardness Alkalinity Conductivity

PSI [bar] ppm ppm ppm ppm CaCO3 1/( in)[S/cm]

0-290 [020] 0.100 0.050 150 0.300 700 1.78 [0.7]

290-435 [2030] 0.050 0.025 90 0.300 600 1.52 [0.6]

435-580[3040] 0.030 0.020 40 0.200 500 1.27 [0.5]

580-725 [4050] 0.025 0.020 30 0.200 400 1.02 [0.4]

725-870 [50 60] 0.020 0.015 20 0.100 300 0.76 [0.3]

870-1015 [6070] 0.020 0.015 8 0.050 200 0.51 [0.2]

1015-1450 [70100] 0.010 0.010 2 0.000 0 0.038 [0.015]

1450-2031 [100140] 0.010 0.010 1 0.000 0 0.025 [0.01]


42/93


a. Boiler feed water

Natural-circulation Forced circulation boilers

boilers

80 bar1

General demands Clear and colourless

Oxygen 0.03 ppm max; in continous operation < 0.02 ppm

PH at 20C 7 9.5

SiO2 < 0.02 ppm2

Hardness3 n.d.4 < 1 ppm < 0.5 ppm n.n.

Total iron < 0.02 ppm If possible < 0.05 < 0.03

Copper < 0.003 ppm < 0.01 < 0.005Total CO2 < 1 ppm if possible < 20 < 1

Conductivity < 0.2 S/cm < 0.3

Permanganate if poss. < 5 ppm if possible < 10 < 5

Oil < 0.3 ppm if possible < 1 < 0.5

b. Boiler water

Pressure5 bar 20 40 65 80 125 160

p-Value6

ppm < 500 < 300 < 150 < 50 < 15 < 5SiO2 ppm < 70 + 7 p < 30 + 3p < 10 < 4 < 1.2 < 0.4

Phosphates7 ppm < 25 < 10 < 10 < 3 < 3 < 3

Conductivity S/cm < 8000 < 5000 < 2500 < 1500 < 250 < 50

Density Bc < 0.4 < 0.25

Tab. 6: Feed water requirements

German feed water specications for water-tube boilers

1If the local heat ux > 230000 W/m2the guide values for pressures > 80 bar must be taken.

2This value applies only if there is no blowdown. Otherwise, the only values to observe are those

for boiler feedwater.3mg CaCO2/l

4n.d. = not detectable

5If the local ux > 230000 W/m2the guide values for 160 bar are recommended for all pressure

stages6The alkalinity is obtained from the cm3of N / 10 hydrochloric acid consumed in titration With

phenol-phthalein as indicator. If the pressure is higher than 60 bar, alkali hydroxides should be

added.7Can be left out completely if sudden changes in hardness can be reliably avoided


43/93


Fig. 25: Rise in temperature of heated surfaces in evaporators due to depositions of Fe2O3

The rise in temperature of an evaporator wall that results from magnetite (Fe3O4) scale

on the heated surfaces was measured by MacBeth [Mac1], [Mac2], [Mac3] and results

are shown in g. 25.

Since the heat uxes transferred in conventional steam generators do not signicantly

exceed 317,000. Btu/(hr ft) [1000 kW/m], rises in temperature higher than 18 F [10 K]

ought to occur. MacBeth also reported [Mac1] that magnetite deposits reduce the critical

heat ux by 5% - 10% and increased the frictional pressure drop by as much as 50%.

2.2.6. Fin efciency and n material

The n height should be dened in such a way that it makes sense to use ns in the rst

place. The efciency of the ns drops with growing n heights, because an ever larger

temperature difference is needed which consequently leads to higher n tip tempera-

tures (for a n efciency of 0%, the temperature of the n tip equals the medium on the

gas side).

The n efciency is calculated as follows :

l f - n height [ft (m)]

t f - n thickness [ft (m)]b = l f +

tf 2


44/93


m =2h

o(t f + w s )

k ft f ws

ho- outside heat transfer coefcient

[Btu/(hr ft F) (W/m K)]

ws- serration (segment) width[ft (m)]

k f- n thermal conductivity [Btu/(hr ft F) (W/m K)]

Obviously, the coefcient of thermal conduction greatly inuences the heat transfer ef-

ciency and changing to austenitic steel grades should be considered very carefully.

A very efcient way to increase the heating surface is to increase the number of ns

per meter. Until today, because of fabrication and technical constraints, the maximum

number of ns for a n thickness of 0.039 in [1 mm] was limited to approx. 88 ns/ft [290

ns/m] .

Fig. 26: Fin efciency as a function of n height

X = tanh(mb)

mb

E = X(0,9 + 0,1X)


45/93


Fig. 27: Serrated ns

Tab. 7: Stress calculation according ASME

2.2.7. Pipe wall thickness

See ASME UG 27


46/93


Pipe wall thickness

2.2.8. Header wall thicknessSee ASME UG 27 or EN 12952

2.2.9. Drum wall thicknessSee ASME UG 27 or EN 12953

2.2.10. Gas Side Pressure Drop

Pressure drop ue gas side is a direct loss in electrical power

[kW]

[mbar]

[m/s]

[kW]

[in of Water]

[ft/min]

Example

1,080,898. [ft/min] GT volume ow

15.781 [inch of Water] HRSG pressure Drop

70% [-] efciency of GT

P loss = 1,401. kW

Gas side pressure drop has a role in determining the surface area but its signicance

is limited. Previously it was normal to have at least 0.5 decrease in efciency of gas

turbine for every inch of gas side pressure drop in the HRSG. Todays advanced gas

turbines have reduced this by about 25%. The Optimum design seems to be between

10- 14 inwc [25-35 mbar], depending of the numbers of pressure stages and the kind

of boiler. Lower pressure drops increase the area requirements rapidly but at higher al-

lowable pressure drops the area decrease is not very large. The gas velocity changes

with the square root of the pressure drop. Hence high pressure drop results in moderate

velocity and heat transfer increase, because the heat transfer is a bit lower than linear

with the velocity. Consequently, the surface area reductions are small. So the reason

tP= SE1 0.6P

P d

2

P= p VGT exp 0.1

P= p VGT exp 0.000117346

p

V

p

V


47/93


for keeping the pressure drop in the 10-14 inwc [25 - 35 mbar] region is that this velocity

region is the most economical. On the other hand, very high pressure drop would lead to

higher gas velocities, which may be detrimental to the integrity of the outside surfaces.

2.2.11. Pressure drop on water side

The pressure drop on the water side should be not too low because of bad mass ow

distribution. For example, if the nal superheater has a too low pressure drop, it is pos-

sible, that due to an small additional resistance in one single pipe (e.g. sharp edge in a

hole of the header because of not complete drilling) there is too low steam ow in that

pipe, so the pipe is not cooled enough and could cause damages. If the pressure drop

in the pipes is high enough, the inuence of an additional resistance is not as big. An -

other reason is, that high pressure drop means higher heat transfer coefcient in thepipe. Inside heat transfer has much bigger inuence to the overall heat transfer at heat-

ing surfaces with ned tubes than heating surfaces with bare tubes.

Also there should be not too high pressure drops on the water / steam side: Too high

velocities can cause damages:

Erosions corrosion

Flow accelerated corrosion (FAC)

Some hints for velocities:

Unit Value ReasonHP Superheated steam velocities ft/s 230 Sound,

[m/s] [70] economics

HP Saturated steam velocities ft/s 66 Erosion

[m/s] [20] Corrosion,

Two Phase velocities ft/s 33 FAC, EC,

[m/s] [10] economics

Water Velocities ft/s 6-13 FAC, EC,

[m/s] [2-4] economics

Tab. 8: Recommended velocities in tubes

Like for the pressure drop on the ue gas side, the pressure drop of the water side must

be compensated from the feed water pump. This also causes directly loss of electrical

power:

Example:

357,149. lb/hr [162. t/hr]

100 PSI [6.89 bar]pressure drop pumped by the feed water pump

219 F [104C] 70% efciency

need 46 kW electrical power.


48/93


The higher the pressure drop in the superheater, the higher is the pressure in the drum

and the lower is the steam ow.

An other reason not to have too high pressure drops is, that the drum pressure would

increase too much and that would mean higher wall thickness lower cycling gradients;

The friction pressure drop in natural circulation system must be very low to have a high-er natural circulation ratio.

2.2.12. Natural circulationThe natural circulation works with the difference of density of the water in the downcom-

er and the water steam mixture in the evaporators and risers.

The pressure difference is the same as the pressure drop of friction and acceleration.

The natural circulation calculation must full two conditions:

Pressure at outlet of riser must be the same as at the inlet of downcomer.

At each junction must be the same pressure

So the natural circulation calculation is a pressure drop calculation and a mass ow dis-

tribution calculation.

The friction of the two phase ow in the evaporator and risers is much higher than the

friction of the water in the downcomers.

The natural circulation ratio (NCR) is dened:

NCR = 1 / mass steam content in riser

Or more simply:NCR = Mass ow downcomer / steam mass ow out of the drum

The NCR should be bigger than 5 !

The velocity in the downcomer (one phase ow) should not be bigger than

13.1 ft/s [4m/s] lower than 8 ft/s is recommended.

The velocity in the evaporator and riser should not be bigger than 32.8 ft/s [10m/s].

In some cases (mostly with horizontal evaporator pipes) it is recommended to install a

siphon at the downcomer, so the natural circulation cant start in the wrong direction

at the start up. The wrong side start up can be happen because at the rst steam bub-ble production the steam water mixture is pressed in both directions: to the risers and

to the downcomers if there are too much steam bubbles in the downcomer, the down-

comer can become a riser and the circulation goes in the wrong direction, at increasing

load the direction of circulation can change, that can cause very high drum water level

changes. In this case it can happen that the drum water level goes over the maximum

or under the minimum, that triggers a boiler trip.


49/93


2.2.13. Forced through circulation

A modern HRSG has normally no forced through circulation. There are three advan-

tages of having none: Saving invest cost because the circulation pump is rather expensive. The pump

must run with the rather high evaporation temperature, the feed water pump runs

with a much lower water temperature.

Saving electrical power for running the pump.

Decreasing of the reliability of the boiler because most designs of the circulation

system is so, that it doesnt run without the circulation pump, if the circulation pump

is damaged, the boiler must trip.

2.2.14. Fin tube heat transfer

There are different heat transfer methods for nned tubes. The methods according[Kri1] and [Esc1] are recommended.

2.2.15. Pipe turbulent heat transferThe formulas according Gnielinski are recommended [VDI1] chapter Gb.

2.2.16. Pipe evaporation heat transferThe formulas according VDI Heat Atlas [VDI1] chapter H.

2.2.17. Heat conductivity of steel

Temp F 32 68 212 392 572 752 932 1112 1292 1472 1652 1832

ASTM DIN ca.

CS St 35.8 32.9 32.9 32.9 31.2 28.9 26.0 24.3 21.4 21.4 21.4 21.4 21.4

T9 15 Mo 3 29.5 29.5 29.5 28.3 26.0 24.3 22.5 20.8 20.8 20.8 20.8 20.8

T11 13 CrMo 4 4 26.6 26.6 26.6 26.6 24.8 23.7 22.0 20.8 20.8 20.8 20.8 20.8

T22 10 CrMo 9 10 20.2 20.2 21.4 22.0 22.0 21.4 20.2 19.1 19.1 19.1 19.1 19.1

T91 X 10 CrMoVNb 9 1 15.0 15.0 15.6 16.2 16.2 16.8 17.3 17.3 17.3 17.3 17.3 17.3

T304 X 5 CrNi 18 10 8.7 8.7 9.2 10.4 11.0 12.1 12.7 13.9 14.4 15.0 16.2 16.8T321 X 6 CrNiTi 18 10 8.7 8.7 9.2 10.4 11.6 12.1 12.7 13.9 14.4 15.6 16.2 16.8

T309 1.4833 7.2 7.3 8.1 9.0 10.0 10.9 11.8 12.8 13.7 14.7 15.6 16.5

T310 1.4841 7.2 7.3 8.1 9.0 10.0 10.9 11.8 12.8 13.7 14.7 15.6 16.5

T409 10 CrMo 9 10 13.9 14.0 14.3 14.8 15.3 15.7 16.2 16.6 17.0 17.5 18.0 18.4

Tab. 9: heat conductivity of steel Btu / hr ft F


50/93


Temp C 0 20 100 200 300 400 500 600 700 800

ASTM DIN ca.

CS St 35.8 57 57 57 54 50 45 42 37 37 37

T9 15 Mo 3 51 51 51 49 45 42 39 36 36 36T11 13 CrMo 4 4 46 46 46 46 43 41 38 36 36 36

T22 10 CrMo 9 10 35 35 37 38 38 37 35 33 33 33

T91 X 10 CrMoVNb 9 1 26 26 27 28 28 29 30 30 30 30

T304 X 5 CrNi 18 10 15 15 16 18 19 21 22 24 25 26

T321 X 6 CrNiTi 18 10 15 15 16 18 20 21 22 24 25 27

T309 1.4833 12.4 12.7 14 15.6 17.3 18.9 20.5 22.1 23.7 25.4

T310 1.4841 12.4 12.7 14 15.6 17.3 18.9 20.5 22.1 23.7 25.4

T409 10 CrMo 9 10 24.1 24.2 24.8 25.6 26.4 27.2 28.0 28.7 29.5 30.3

Tab. 10: heat conductivity of steel W / m K

2.2.18. Overall heat transfer

2.2.19. Logarithmic mean temperature

The logarithmic mean temperature in cross ow:

Outlet H2O temperature

Outlet ue gas temperature

h = 1

1+ R fo

+

SW+

Ao+

1

h Ai hiR f( )

v1O= v1i (v1i v2i )

(v1i v1o )

1 NTU2 1

1 e NTU1+

NTU1(1 e NTU2)

NTU1

tlog Crossow=NTU2

OutH2O= inH2O +mcp

Q

OutFG= inFGmFGcpFG

Q


51/93


2.2.20. Designing of heating surfacesThe purpose is clear: transfer of heat from one medium to another. In most cases, the

medium ow is continuous so that we will consider this particular case only.

The actual heat transfer can be obtained by the following equation :

Q - Heat ow rate [W]

h - heat transfer coefcient [W/mK]

A - Area [m]

- log mean temperature difference [K]

To change the heat transfer capacity, following variations in heating surface geometry

are available:

1. Length of the tubes (a function of duct height)2. Number of pipes in ue gas direction

3. Number of tubes in transversal direction (is a function of the duct width and

transversal spacing)

4. Tube diameter and tube wall thickness (is a function of stress calculation)

5. Fin height

6. Fin pitch

7. Fin thickness

8. Velocity of the gas medium (is a function of item 1, 3, 4, 5, 6, 7)

9. Velocity of the medium in the tube (is a function of item 3, 4 and how many rowscarrying ows)

10. Changing the difference in temperature

To meet the requirements, these days computer simulations are used and the different

possibilities in a ranges are tried out.

Outlet H2O temperature

Outlet ue gas temperature

Q = h A tlog

tlog

OutH2O= inH2O +mcp

Q

OutFG= inFGmFGcpFG

Q

2.2.21. Noise and vibration problems at heat exchangerAt serrated ns it may happen, that due to a rather high ue gas velocity a high-pitched

whistle can occurs, that can increase to a threshold noise of pain.

The velocities in the graph is the velocity before the bundle. This velocity was mucheasier to obtain in the test facility


52/93


Fig. 28: Noise Bulk ue gas velocity diagram

Also tube bundles in cross ow are often subject to vibration and noise problems. Vibra-tion can lead to wear and consequent tube failures. Noise problems can be a nuisance

to operating personnel.

Fig. 29:Amplitude as a functionof the uid velocity

Flue gas ows over a tube bundle in inline or staggered arrangement, vortices are

formed and shed beyond the wake of the tubes, resulting in harmonically varying forces

perpendicular to the ow direction. It is a self excited vibration and the frequency of vi-

bration is called vortex shedding frequency. If the frequency of vibration of the von Kr-

man vortices, as they are called, coincide with the natural frequency of vibration of thetubes, resonance occurs leading to bundle vibration.


53/93


a- negative pressure areab- origin of vortex

Another phenomenon that occurs with vortex shedding is acoustic vibration, leading

to noise and high gas pressure drop. Standing waves are formed inside the duct. The

acoustic pressure uctuations are a maximum where the uid motion is zero; hence the

walls of the enclosure are subject to pressure pulsations and may distort outwardly. The

duct or the bundle enclosure vibrates when the vortex shedding frequency coincides

with the acoustic frequency.

There are ve rules to reduce the danger of noise and vibration:

Not too high velocities of ue gas between the pipes

Install support sheets in vertical HRSG or support (pipe xing) construction in

horizontal HRSG (to double the (eigen-) frequency)

Install the support sheets or support construction not symmetrically to have different

(eigen-) frequencies of one pipe

To have possibilities to install acoustic bafes to eliminate noise concerns. When a

bafe is inserted in the tube bank, reducing the width by half or a third etc.

Here again, not a symmetric of the bafes.

Vary the n density of the tube rows a bit to alter the frequency

Fig. 30:Vortexes aftera tube

Fig: 31: Forcesdue to vortex

shedding


54/93


There are some possibilities to calculate the vibration for bare tube bundles [Gan1],

but for nned tubes bundle there are existing very few papers with different sources

[Chen1].

2.2.22. Regenerative feed water preheating vs. condensatepreheating

At a big direct red boiler the regenerative feed water preheating increases the efcien-cy of the power plant signicantly. Despite of taking steam of the turbine for preheating,

the effect is, that the most transferred heat from the steam of the turbine to the feed wa-

ter would be condensate in the condenser. So the heating power for feed water preheat-

ing can be saved and has not to be taken from the ue gas. The ue gas can be used for

air preheating. This is the reason for the increasing of the efciency. A thermodynamic

explanation would be, that the average input heating power temperature is increasing.

At a combined cycle process this is not the case, because there doesnt exist an air

preheater, so if the ue gas would be not used for feed water preheating, the ue gas

heating power would be lost through the stack. Therefore decrease of the steam ow

through the turbine due to the steam extraction at the turbine would only decrease theperformance of the turbine instead of increasing the live steam ow.

Fig. 32: Eigenfrequency =Resonance Frequency


55/93


2.2.23. General Remarks

Typically, for large GT CCPP the clients and HRSG manufacturers request for the fol-

lowing:

Triple Pressure Single Reheat HRSGs - the present existing economic optimum

High pressure (HP) level - the existing economic optimum is 1885.5 psi [130 bar],

although the thermal optimum lies well above (2610.6 [180 bar] [Eis1]) for triple pres-

sure reheat HRSG.

Steam Temperatures - economic optimum, dened by the steam turbine, is 1049 F

[565C].

Steam Output - dened by the economic determination of the Pinch Point (10.8 to

14.4 F[6 to 8 K]) at the HP Evaporator and the Approach Point at the Economiser

(3.6 to 7.2 F[2 to 4 K]), typically 163 lb/s [74 kg/s] without supplementary ring (SF).

265 lb/s [120 kg/s] using SF. Feed water/Condensate Inlet temperature with respect to the type of fuel used,

above 122 F [50C] for natural gas, at no sulphur content, and above 230 F [110C]

for light distillate oil to ensure Operation above the acid or the water dew point.

Stack temperature minimum 176 F [80C]

Steam Purity - entering the Super Heater at 99,9%, especially important if the client

requests for solid alkalisation in addition to the all volatile treatment (AVT), being

state of the art for HRSG design in Europe.

HRSG ue gas draft losses - approx. 0.36 psi [25 mbar], 0.51 psi [35 mbar] if cata-

lysts are required. The spray cooler never should spray so much water, that the steam purity goes

under 100% (i.e. saturated steam) because the water droplets in the steam will be

separated in the next heater. Some pipes can get thermo shocks. Be careful at some

supplementary ring cases. Sothere must be a right location of spray cooler

HRSG manufacturers also offer a choice between a horizontal or vertical uegas path.

Vertical designs - which have originally been developed in Europe where the major sup-

pliers of this kind still are located - offer a smaller footprint and are less vulnerable to

thermal cycling problems than the horizontal designs commonly applied and originated

in North America. Since the vertical HRSG no longer require forced-circulation pumps,not even for Start ups, due to design improvements of the evaporator systems both

HRSG types offer the Same overall efciency, although the decision may be directed to

one type of HRSG:


56/93


Horizontal HRSG Vertical HRSG

Output and Efciency Equal Equal

Surface Area for equal Output Similar, except the reheater and Base superheater Section, which might

require slightly more heating surface

area mainly due to less advantageousue gas ow distribution with regard totemperatures and mass ow

Purging Worse to purge because unburned light Base hydro carbons could not be purged out

as consequence of maldistribution of airdue to high opening angle of inlet ductand low air velocoity

Plot Plan Area for equal Output Up to 30% more, mainly due to the Base opening angle of the inlet duct and the

stack. Also if supplementary ring systems,

SCRs, CO Catalysts, etc. are requiredEmisson control Requires more HRSG length Requires more HRSG height,

cleaning of downstream fouledsurfaces has to be carried outcarefully, not to poison the catalyst.

Supplementary Firing Readily installed in the HRSG inlet Readily installed in the HRSG inletduct or within the boiler surface area duct, difcult to install within the

boiler surface area

HRSG enclosure / boiler house Free Standing, self supporting Attached to and supported by the enclosure HRSG structure, light enclosure

Natural Circulation State of the art Special design considerations,though state of the art

Modularized/Standard concepts Better modularising possible Base

Support sheets No tube support sheets needed Support sheets needed; There is alimit of n temperatures becausethe nned tubes are supported bysheets, at too high n temperaturesthe ns are bending

Erection Area, prefabrication Equal, though more crane area is Equal, though heavy transportationon site required for pressure part (harps) 265,000.lb[120 ton] may be mounting which typically lasts 5 weeks required at site, typical time needed

for large GT CCPP for boiler surface mounting:3 weeks for large GT CCPP.

Cycling State of the art design experiences Less vulnerable if properly designed severe cycling problems at superheater designed e.g. because of less

and reheater stages, design considera- headers tions cost effective

HRSG cost Equal Equal(ready to run)

O&M cost Higher number of and larger textile Replacement and blocking of tubes expansion joints, boiler surface replace- possible ments not possible, repair by blocking

of tubes, cost effective

Tab. 11: Horizontal HRSG vs Vertical HRSG


57/93


Fig. 33: Vertical HRSGduring erection

2.2.24. Duct burner

In the case that the electrical power output should be increased or if there is a need for

an occasional power peak, supplementary fuel is red in the HRSG by utilizing the duct

burners. Normally the heat input through the burner is fully recovered in the HRSG. But

in addition, more heat from the gas turbine exhaust gas will also be recovered. So the

net effect of ring is to make the HRSG more efcient than the unred case. This is the

reason for the apparent burner efciency of greater than 100% or more heat extraction

than the amount put in through the burner. For example if 107,85 MW of heat (fuel LHV)

is input through the burner the steam turbine output should increase by about 34,1 MW

(31,66% efciency). The steam turbine power increase is about 52,0 MW. The extra

17,9 MW are obtained from the gas turbine exhaust because under red operation, thestack temperature decreases causing more heat recovery.


58/93


Fig. 34:T-Q diagramwith duct burnerin operation

This increase may not be possible if there is an LP boiler operating at a very low steam

pressure at the tail end. For example, if the gas turbine input is increased by about 8%,the steam turbine power would also go up by the same percentage. If the same amount

of fuel is burned in the HRSG via a duct burner, the ST power will increase by about

9,5%, from the gas turbine exhaust has been recovered over the unred operation. The

red Operation generally can be classied in three distinct groups:

1. HRSGs designed for red Operation only

2. HRSGs designed for occasional ring for peak loads and

3. HRSGs designed for red and unred Operation equally.

Of these the rst two are relatively easy to design because they will be designed for oneconditions only. The continuous red Operation boiler, because of higher temperatures,

needs more consideration in the selection of metallurgy to withstand higher tempera-

tures. Today it is feasible to design to a ring temperature of about 1500 F [820 C] with

convection sections only. Higher temperatures up to 2000 F [1100 C] can be designed

with a waterwall furnace section [Pas1].

An unred HRSG is also more easy because design temperatures may not be very high

and can be accommodated with normal materials with normal thicknesses. It should be

noted that todays advanced gas turbines have about 1200 F [650 C] at the gas turbine

exhaust with a superheat steam temperature of 1055 F [570 C]. Care is needed in ma-terial selections.


59/93


When the HRSG needs to be designed for both red and unred operation, the design

becomes difcult. Because of the wide range of operating condition, the steam ows in

red case may be three times that of the unred case. The HRSG needs to be designed

with particular attention to the critical areas. Critical areas consist of attemperator sizing,

superheater and reheater velocities, drum internals, non-steaming economizer design,valve sizing and circulation connections. Detailed specications of all operating condi-

tions is necessary for a dual, red and unred design as they have a signicant impact

on the design and operation of the HRSG. Interstage ring, double or triple attempera -

tors, bypassing of the part or full economizer, dual valves are some of the means em-

ployed to optimize the red and unred designs.

There are some cases where the HRSG is designed to have the same capacity with out

the gas turbine exhaust ow. This is done by providing a fan and burner System to du-

plicate the gas turbine exhaust conditions. Since it will be very costly to reproduce the

100% gas turbine ow and temperature, the Fresh Air red units are designed to oper-ate at reduced capacity. In any event this alternative is a very costly one and should be

used only when it is very critical to have uninterrupted steam ow and when other alter-

natives outside the HRSG are not available.

2.2.25. Ductwork and casing

The ue gas ductwork uses an internally insulated, cold casing design. In this design a

combination of ceramic bre and mineral wool insulation is sandwiched between a du-

rable alloy or carbon steel internal liner and external casing. HRSGs designed for high

rates of supplemental ring may have ring temperatures which are unsuitable for usewith alloy internal liners. In these cases a special rigidised ceramic bre liner is em-

ployed [Pas1].

2.2.26. Environmental considerations

Environmental considerations, such as emissions have considerable inuence an the

HRSG design and operation. Generally speaking, controlling the NOxand CO emis-

sions are of highest mportance. For NOx reduction, a Selective Catalytic Reduction

(SCR) is the applicable technology today. In addition to the capital cost for the hardware

and recurring cost for the injected ammonia, SCRs also increase gas turbine back pres-

sure. This results in the lower gas turbine output and increased HRSG cost since theHRSG must be designed for a lower pressure drop. The cost of the optional duct burner

for red cases, has to be increased to provide for a Low NOxburner.

2.2.27. Site conditions

The site conditions have a inuence to the gas turbine performance and also the HRSG

output is affected. In cold areas the gas turbines and HRSG produce more power when

compared with the units operating in hot environments. Conversely at higher altitudes,

the capacities are reduced, because at higher altitudes the HRSG pressure drop will be

higher for the same amount of gas ow. This reduces the overall capacity.


60/93


For example a HRSG in Mexico City 7,350. feet [2240 meter] over see level would have

about 33% more volume ow due to the reduced pressure at this altitude.

Environment air born chemicals also have an effect an HRSG performance. HRSG near

oceans or chemical factories may experience higher level of chlorine. In such places,

series 300 stainless steel for feedwater heaters can not be used due to the corrosiveeffects of chlorine. For these cases the design is either made low efcient by exhaust-

ing at a higher stack temperature or stainless steel is replaced by other higher order

material.

2.2.28. Steaming in economizers

One of the problems

often encountered in

HRSGs is economizer

steaming or steam for-mation in economiz-

ers, particularly at low

loads or low steam

generation levels. This

may result in vibration,

noise problems, de-

posit formation inside

tubes and consequent

fouling and poor per-formance.

Steaming in econo-

mizers normally oc-

curs in HRSG, at

direct red boilers the

economizer outlet wa-

ter temperature de-

creases at part load.

Fig 35 Pressure drop Mass ux diagram with instability

Fig. 36: Pressure drop Mass ux diagram for stabilization


61/93


2.2.28.1. Methods of minimizing Steaming Concerns in a HRSG

Lower the pinch point

To avoid instability increase the pressure drop of economizer either to use less par-allel pipes or insert orices at the inlet of the pipes. Because of the steaming the

volume and the velocity of the water steam mixture is increasing rapidly, so the pres-

sure drop increases to the square of the velocity, in consequence it can happens

that some pipes of the economizers have not a proper water mass ow, because

the water goes through this pipes with lower pressure drop. This effect is called in-

stability [Hel1]

Last leg of economizer coils should have vertical ow upstream in order to ensure

that the steam bubbles ow smoothly up. Downward motion of steam bubbles can

cause ow stagnation and ow instability problems. The last legs of the economizer

may be designed with multipasses to accomplish this.

If steaming occurs for a very short duration only, the situation can be handled by

increasing the continuous blow down, though it is not recommended for continuous

operation as treated water is wasted.

The steaming problem is associated with low steam ows in the HRSG. Hence if

you have auxiliary ring capability, use it to increase the steam ow when steaming

2.2.29. Important notes

Reduces bypasses of ue gas at the heating surfaces with bafes etc.

Fired HRSGs are in most cases more efcient than unred units

Higher the n density and surface respectively, lower the overall heat transfer coef-

cient.

With a lower the tube side heat transfer coefcient, there should be a smaller exter-

nal n surface area HRSG can be optimised using HRSG simulation methods

Water temperature affects economizer tube wall temperature much more than

the gas temperature and hence for corrosion prevention consider raising water

temperature

Fouling inside tubes is more serious in nned tube surfaces than in bare tube sur-

faces

Surface areas should not be the basis for selecting HRSGs

Understand the difference in efciency based on higher and lower heating values


62/93


3. Operation of steam boiler


63/93

3. Operation of steam boilers

3.1. Example of a start up

3.1.1. Gas turbine mass ow

52 3. Operation of steam boiler

Fig. 37: Gas turbine mass ow time diagram during start up

Fig. 38: Gas turbine temperature time diagram during start up

3.1.2. Gas turbine temperature


64/93


3.1.3. HP Steam mass ow

3.1.4. HP Steam pressure

Fig. 39: High pressure steam mass ow- time diagram during start up

Fig. 40: High pressure steam pressure- time diagram during start up


65/93


3.1.5. Gradients

Fig. 41:High pressuretemperature and

pressure gradi-ent- time diagramduring start up

3.2. Start upStart-up and shutdown operations have little impact to units designed for base load op-

erations. For units with cyclic operation, number of start-ups and shutdowns, the condi-

tion of the HRSG at start-up, the nature of shutdown, all inuence the life of the boilerand hence need to be taken into consideration while designing the HRSG. Generallythicker components such as HP drum are considered for life time studies. If the HRSG is

exposed to extreme conditions and the frequency of changes is high, dynamic analysis

and life time study is necessary for the HRSG.

3.2.1. Deaeration of economizersIt is very important to take care and attention for a complete deaeration of the economiz-

ers. If there are too big air bubbles after lling the economizers with water the perform-ance of the economizers can be very low.

3.2.2. PurgingThe Purging required as a precondition to start the GT through the boiler, is a common

requirement of all boiler codes to ensure safety operation of the plant.

Germany:

TRD 411 and TRD 412 [TRD1]. Europe: EN 12952 UK : British Gas USA : NFPA 8506, NFPA 8606

This rules are historically evolved, since in the beginning of boiler Operation severe ac-

cidents occurred. Today purging a hot HRSG strains all involved boiler parts to a highextent, special considerations and design features have to be taken into account (see

above) to cope with the requirement of daily start ups for a lifetime of 25 years.


66/93


Fig. 42: Condensate forming during warm and hot start

The stratication of air ow in horizontal boilers during prestart purge of the HRSG alsosuggests that the purge is not accomplishing its intended purpose--to remove combus-

tible gases from the HRSG before turbine ignition. Because natural gas and most vola-tiles released from distillate fuel oil are lighter than air, it is particularly important for the

purge to ush out any combustibles that have accumulated in the dead spaces at thetop of the duct.

The industry needs

a thorough review

of the purpose of

HRSG purging and

of the circumstanc-

es in which there

is risk of ignition of

combustibles in theupper duct. So it is

recommended to in-

crease the transition

angles of the inlet

duct of a horizontal

boiler.

3.3. Drain

Condensation oc-

curs in superheater

tubes during every

purge of the HRSG

at warm or hot start

prior to gastur-

bine ignition. This

is because turbine

exhaustgas tempera-ture falls below satu-

ration temperature.

Quantities of con-

densate are substan-

tial during hot and

warm starts. A repeatpurge can actually llthe front panel tubes

of the superheater.

1. Extensive temperaturemonitoring conrmed that asubstantial quantity of condensateformed in superheater tubesduring gasturbine purging,even in large-bore headers

2. Condensate began to clearfrom superheater tubes oncesteam ow commenced

3. A single, small-bore drain,opened during purging, reduces

the quanity of condensate, butdoes not completely eliminate it

4. Condensate clears rst fromthe tubes closest to the end-pipe

connections, creating temperaturedifferences between individualtubes along the headers


67/93


Condensate should be removed from lower headers at the peak rate at which it forms

to prevent pooling and ooding. But this requires adequately sized and correctly oper-ated drains elements that have been overlooked at many large combined-cycle installa-

tions. Many units have no blowdown vessel for high-pressure/temperature drains from

the superheater. Others have a blowdown vessel inadequately rated for ow, pressure,and temperature of drains from the superheater during hot restart purges. The drain

installed an most superheater designs was sized for maintenance purposes and is too

small for clearing condensate at the rate it collects. In [Eis2] it is reported how to esti-mate the drain water ow and taking the right nozzles.

Even where superheater drains are installed and connected to a blowdown tank, noguidance has been given by HRSG manufacturers or EPC contractors as to when andhow they are to be used. Not surprisingly, they often are incorrectly used or not used atall during hot starts.

To remove condensate from lower headers of vertically tubed HRSGs, the lower head-

ers must have adequate bore in relation to their length and number of attached tubes to

ensure that tubes cannot ood [Pea1].

Drain arrangement [Als1]

3.4. Drum water levelBefore start up it is recommended to decrease the drum water level, because at the rstevaporation there will be water displaced by steam in the evaporators, causing a water

swell in the drum. So a lower drum water level can prevent a too fast increasing over

the high water level mark, that trigger a boiler trip.The drum water level before start up can be controlled by the blow down valve.

Fig. 43:Drainage arrangement


68/93


3.5. Water running through the economizer

Also it is recommended to run water through the economizers before and during startup. The water comes out through the blow down valve. The point of it is to have lower

and constant temperature and to pour steam and air bubbles out of the economizer. Iffor example there are too much steam bubbles in the economizer, then it can happens

that a fast increasing pressure can cause the collapsing of the bubbles and then the

feed water has to ll rst the eco and in worse case the eco sucks water out of thedrum. For that reason it is recommended that the connection of the eco to the drum is

above the drum water level.

3.6. Start up of the gas turbine

Fig. 44: Typical Start up of a gas turbine


69/93


3.7. Life Cycle Fatigue

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

Documents

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