Guidelines on the Effects of Cycling Operation on Maintenance Activities

These guidelines are intended to help plant operators and engineers assess the impact of cycling on maintenance activities and take appropriate preventive measures when operating a plant

Guidelines on the Effects of Cycling

Operation on Maintenance Activities

Technical Report

EPRI Project Managers

Guideline on the Effects of Cycling Operation on Maintenance Activities

1004017

Final Report, December 2001

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THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC (EPRI) NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM:

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ORGANIZATION(S) THAT PREPARED THIS DOCUMENT

European Technology Development Limited

Copyright © 2001 Electric Power Research Institute, Inc All rights reserved

CITATIONS

This report was prepared by

European Technology Development Limited

This report describes research sponsored by EPRI

The report is a corporate document that should be cited in the literature in the following manner:

Guideline on the Effects of Cycling Operation on Maintenance Activities, EPRI, Palo Alto, CA:

2001 1004017

REPORT SUMMARY

Cyclic operation can result in an increase in forced outage rates, higher operation and

maintenance (O&M) costs, and further wear and tear on components due to additional overhauls and maintenance Such operation may also increase unforeseen costs due to additional personnel training requirements and the use of more sophisticated evaluation and inspection techniques These guidelines are intended to help plant operators and engineers assess the impact of cycling

on maintenance activities and take appropriate preventive measures when operating a plant in cyclic mode While the guidelines provide important direction on O&M practices when shifting from baseload to intermittent operation, they do not set out to be a comprehensive listing of individual plant maintenance activities

Background

The severity of cyclic operation affects boiler, turbine, electrical, and auxiliary components The effect is largely design dependent, and older plants originally designed for baseload usage fall into the less tolerant category Such units were designed with heavy section headers and piping with a poor response to thermal fatigue, which basically results from temperature changes during startup and shutdown Cycling tends to exacerbate such problems and will lead to an increased incidence of stress corrosion and corrosion fatigue of feedwater heaters, economizers, and

turbine units Clearly, cycling utilities need to understand that events which are tolerable on an occasional basis during baseload operation—in terms of damage to the plant, risk to staff, or impact on the local environment—would be quite intolerable if occurring daily EPRI sponsored development of these guidelines to provide direction for all cycling plants, but particularly those shifting from baseload operation to intermittent operation

• Problems experienced under two-shifting duty

• Engineering modifications to facilitate flexible operation

• The impact of cycling on maintenance practices

• Industry practices as they relate to regulatory codes

Also featured in the guidelines are responses to the survey questionnaire along with interviews concerning routine maintenance practices, condition monitoring, and maintenance scheduling The appendices to the guidelines document several new and interesting developments in repair techniques

In all, the guidelines emphasize that cycling mode presents challenges in the way utilities view O&M procedures Under cyclic operation, nearly every O&M procedure must be analyzed in detail and almost certainly modified to ensure safety, cost-effectiveness and economy,

reliability/repeatability, and minimization of plant damage

EPRI Perspective

These guidelines are part of EPRI’s development efforts under Target 69, Plant Maintenance Optimization (PMO) The PMO mission is to lead the industry by developing and demonstrating products and services that improve the use of power plant maintenance resources and increase profitability for generation businesses

Flexible cyclic operation of large coal-fired units has been successfully carried out while

maintaining high plant availability, without excessive additional costs These guidelines will help utilities increase plant availability by taking a cost-effective, systematic approach to preventive maintenance activities when operating units in cyclic mode

Keywords

Cycling operation

Plant maintenance optimization

Preventive maintenance

ABSTRACT

This overview is mainly based on the UK experience for fossil steam plants with drum boilers However, information from other countries including Ireland, Italy, France, Portugal, South Africa, Hong Kong and the United States has also been utilized with this data including a large population of once through boilers

Some new and interesting developments in repair techniques have also been included in the form

of Appendices

DEFINITIONS

· Two-shifting means synchronising and desynchronising from the grid system once per day,

on a regular basis, although it can also imply a week end shutdown

· Intermittent operation can mean anything from a few hours of shut-down from time to time,

to long periods when the plant is cold but available to be called into operation at agreed notice

· Load following is varying load to match grid requirements and requires ramping up and down

between unit peak capacity and minimum load capability as required Many plants which are two shifting will also be required to load follow, rather than running at a constant rate during the two shift period

ACKNOWLEDGMENTS

This is to acknowledge the input from a number of European and U.S.A colleagues in the

preparation of this Report and in the provision of information contained herein Responses from, and discussions with, a number of plant operators has provided further insight in to the

maintenance practices carried out in countries outside Europe and North America This input is gratefully acknowledged

CONTENTS

1 INTRODUCTION 1-1

2 OBJECTIVES OF THE STUDY 2-1

3 METHODOLOGY USED TO CONDUCT THE STUDY 3-1

4 RELIABILITY/REPEATABILITY OF CYCLIC OPERATION 4-1

5 MINIMIZATION OF PLANT DAMAGE AND OPTIMIZATION OF OPERATION 5-1

Turbine 5-1 Boiler 5-2 Stressing of Thick Section Components 5-3 Drainage 5-3 Drum Level Control 5-3 Furnace Wall Tubes 5-3 Gas Side Isolation 5-4 Desuperheater Sprays 5-4 Sootblowing 5-4 Steam Temperature Raising 5-4 Light-up Burners 5-6 Auxiliary Steam Supplies 5-7 Coal Bunker Control 5-7 Mills 5-7 Other Plant Considerations 5-7 Switchgear 5-7

xiv

6 OPERABILITY 6-1

7 OTHER CONSIDERATIONS 7-1

Noise 7-1 Efficiency 7-1 Maintenance 7-2 Chemistry 7-2 Water Treatment Plant/Purified Water Provision 7-2 Alarms and Protection 7-2 Running Auxiliary Equipment 7-3 Operator and Maintainer Training 7-3

8 PROBLEMS EXPERIENCED UNDER TWO SHIFTING DUTY 8-1

High Temperature Headers (mainly outlets) 8-1 Attemperators 8-1 Tubing 8-2 Steam Drum 8-2 Economiser Inlet Header 8-2

ID Fans 8-2 Main Steam Pipework 8-2 Rotors 8-2 Valves 8-3 Inner Casings 8-3 Feed Heaters 8-3 Generator and Stator 8-3

9 ENGINEERING MODIFICATIONS TO FACILITATE FLEXIBLE OPERATION 9-1

10 IMPACT OF CYCLING ON MAINTENANCE PRACTICES – RESPONSES TO THE

QUESTIONNAIRE AND INTERVIEWS 10-1

Routine Maintenance Practices 10-1 Condition Monitoring and Maintenance Scheduling 10-2 Rotating Equipment and associated plant components 10-2 Maintenance Scheduling 10-2 Major Outages 10-2 Minor Outages 10-2

11 INDUSTRY PRACTICES/CODES 11-1

12 NEW DEVELOPMENTS AND RELATED R&D EFFORTS 12-1

13 CONCLUSIONS 13-1

14 GENERAL REFERENCES 14-1

A EXACERBATION OF THERMAL FATIGUE, CORROSION FATIGUE AND STRESS

CORROSION IN WATER AND STEAM SYSTEMS UNDER CYCLING CONDITIONS A-1

Thermal Fatigue of Heavy Section Headers, Steam Chests and Related Components A-1 Corrosion Fatigue of Feedheaters, and Economisers A-2 Stress Corrosion in Turbines A-2

B ADVANCES IN REPAIR WELDING TECHNIQUES B-1

B-1 Weld Repair Without Stress Relief B-1 B-2 Major Weld Repairs Without Pressure Testing B-1 B-3 In-Situ Repair of Hard Facings on Main Steam Valves B-2 B-4 References B-2

C ALTERNATORS C-1

References C-1

D THE QUESTIONNAIRE D-1

Survey Form D-1 Survey of Plant Cycling Effects on Maintenance Practices D-1

To cover only the Coal Fired Steam Plant D-1 Your Contact Details D-3

E CYCLING EFFECTS ON MAINTENANCE ACTIVITIES IN THE UK – SUMMARY OF

TYPICAL CYCLING PROBLEMS AND THEIR POTENTIAL IMPACT E-1

LIST OF TABLES

Table 10-1 Variation of Valve Maintenance Intervals Detailed by Some Stations 10-3 Table 10-2 Variation in National Boiler Inspection Intervals 10-4

1

INTRODUCTION

Power plant maintenance was originally controlled in the UK by the 1926 Factories Act which stated that boiler plant should be inspected within a time interval of 26 months For power plant this implied a major shut down every two years and the opportunity to link this in with general maintenance This changed with the Pressure Systems and Transportable Gas Regulations of

1989, which handed over responsibility for the time interval to the Inspecting Authority The impact of this change has been rather less than might be expected, since although in principle the period between the boiler unit itself has been extended, the need to do maintenance on ancillary equipment such as valves, turbines, pumps and alternators has tended to shorten this interval The other factor, which has tended to prevent inspection intervals being increased, is the switch from base load to cycling or two shifting To combat this, companies are turning to more refined approaches to estimating the impact of cycling on the life of high temperature components

This Report is intended to help plant operators and engineers assess the impact on maintenance and take appropriate preventive measures when operating a plant in cyclic mode This is done with specific reference to European plant operating and maintenance practices The Report therefore provides guidance as to what needs to be done when moving from base load to

intermittent operation However, it does not set out to be a comprehensive guide as to what should be done on individual plants

Any station moving from base load operation to intermittent operation will have done cold, hot and warm starts, and will have procedures for so doing In moving to more frequent starting and shutting down, as in the case of cyclic operation, these procedures must be analyzed in detail and almost certainly modified to ensure:

Introduction

1-2

matter if the safety valve were to blow once a day This type of issues are addressed in this Report

Coal fired units vary immensely in design Different coals have significantly different

characteristics Hence it is only possible to offer generalizations here Whatever the design,

intermittent operation should be possible In the UK, 660MW coal-fired units have been

regularly two-shifted, and some 500MW coal-fired units have been cycled more than once per day on a regular basis The same pertains to oil and gas fired steam units which of course are spared the problems associated with handling a solid fuel and its waste by-products

Much of the emphasis, hitherto, on two-shifting, has been with thermal stress and fatigue of large plant items operating at high temperature In short, two-shifting has been regarded as another facet of normal plant operation, where a unit may be cycled just a few times a year Nevertheless, two-shifting can bring other problems too For example, superheaters and reheaters, due to the need to start up “against the clock”, can experience temperature excursions significantly above design

The UK represents a good example of how attitudes have changed towards the operation of generating plant and the ability to two-shift it efficiently and quickly The original specification for 500 MWe plants, built in the 1960’s and 70’s, required that units be capable of both

continuous base load and two-shift operation In terms of two-shifting, after a six-hour overnight shutdown, when the temperature was expected to be of the order of 480°C, units were to be capable of being brought to full load within 60 minutes It was also essential that the temperature

of the steam to the turbine, on start up, should not be lower than the turbine metal temperature From ‘synchronized no load’, the boiler was to be capable of attaining its full rated capacity in

20 minutes, with a steady rise in superheater outlet conditions In practice these objectives were never fully achieved A more typical figure for hot start was 100 to 150 minutes, with loading rates of 10 MW/min, giving a time to full load of around 50 minutes That is, 150 to 200 minutes from request to full load Improved operation was needed and two-shifting trials were carried out

in the 1970’s and 1980’s to assess two-shift operability It was concluded that faster two-shifting was possible, albeit with a need for some changes to plant equipment and instrumentation

Since that time, privatization of the electricity supply industry in the UK in 1989, combined with the competition from new low cost (and hence base load) CCGT plant, has acted as an incentive for further improvements Today, a typical 500 MW machine can be brought on line within about 35 minutes of notice and run up to full load within about 40 minutes That is about

75 minutes from request to full load It is probable that some units achieve 60 minutes This reduction in time has been achieved by a combination of further modification to units, including inter-stage drains, improved instrumentation of critical components,improved automated control

and anticipatory sequencing using current computer control software (Note: Inter-stage drains

are usually located between the various sections of the superheater in the boiler to promote progressive establishment of flow through the boiler as the boiler is fired A higher firing rate is possible allowing condensate to boil out in the inverted sections whilst the flow prevents

overheating of the hotter sections A bypass drain can usefully be installed around the HP turbine

to the reheater to provide a ‘cooling’ steam flow to protect the reheater sections which could

otherwise overheat until the turbine stop valves are opened Critical components will include

Introduction

most of the later stages of superheater headers, possibly the main steam pipework, boiler stop valves and HP turbine casing - basically any thick sections susceptible to the generation of

thermal gradients)

Nevertheless in spite of these changes cyclic operation can result in an:

· Increase in forced outage rate due to the increased component failure frequency

· Increase in operation and maintenance (O&M) costs to keep units in operation

· Increase in wear and tear of components due to additional overhauls and maintenance

· Increase in unforeseen costs due to greater personnel training requirements, and more

sophisticated evaluation and inspection techniques

The severity of cyclic operation affects boiler, turbine, electrical and auxiliary components The effect is largely design dependent, and older plant, which was originally designed for base load usage, is in the less tolerant category Such units were designed with heavy section headers and pipe work with a poor response to thermal fatigue which basically results from temperature changes during start up and shut down Hence cycling tends to exacerbate such problems

Cycling will also lead to an increased incidence of stress corrosion and corrosion fatigue of feedheater, economiser and turbine units which are further explained in Appendix 1

Advice, information and experience have been sought from a number of plant owners, operators, researchers and associated organizations in Europe, Asia and South Africa This Report is thus based on the reviewers’ own knowledge and understanding of plant issues, published literature, and information accessed from plant operators and technical experts

There are five Appendices with this Report Appendix 1 describes some practical aspects of thermal stress, corrosion fatigue and stress corrosion as applied to critical plant items

Appendix 2 describes a few innovative repair processes which the user of this Report may find helpful Appendix 3 describes a few ‘Alternator’ related issues and Appendix 4 reproduces the Questionnaire sent out to some of the plant operators Appendix 5 is perhaps the most significant appendix in that it gives an overview of the maintenance related issues concerning various

components of cycling plant and the significance of these problems

2

OBJECTIVES OF THE STUDY

This was to assist EPRI in the above study by supplying information on maintenance practices and activities necessitated by the cyclic operation of fossil steam plants in Europe with the

emphasis on coal fired plant

3

METHODOLOGY USED TO CONDUCT THE STUDY

Because of the range of technical issues involved, a multi-disciplinary team was engaged in

carrying out this study The team has conducted surveys of plant operators and R&D

organizations The surveys were conducted through the questionnaire shown in Appendix 3 and

through interviews and discussions/meetings with plant operators and experts The countries

involved in the survey were the UK, Italy, Ireland, Hong Kong, Portugal and South Africa Input

to the report also includes UK experience in maintenance through ETD staff/consultants’ own

experience

A survey of available literature was carried out with reference to sources which included:

· Ex-CEGB UK

· International Conferences, Authors & Private Communications Worldwide

increased plant damage

The first condition of avoiding commercial penalties is reliability; can the start procedures be gone through regularly with an acceptably low percentage of failures? The second condition is repeatability: can the procedures deliver synchronization and load-up within the target times despite changing conditions (e.g with wet coal and dry coal, with the plant’s best coal and its worst, with the best shift team and the worst) or can simple variations be devised which will cater for known different conditions?

To achieve repeatability, the procedures must allow for varying conditions and therefore the standard procedure to be adopted for each type of start will be neither the quickest nor most economical achievable It is usually easier to delay the late stages of a start-up that is going well

so as to avoid being too early, than to try to accelerate a too ambitious start procedure However, some designs of steam turbine run into differential expansion problems if run-up or load-up is delayed

The reliability of a unit depends on the product of the reliabilities of the individual critical-path items Given the large number of critical-path items (e.g switches, actuators, motors, valves) on

a big unit, each critical path item needs a very high reliability for the overall reliability to be satisfactory RCM (Reliability Centered Maintenance) practices can be applied to focus

predictive and preventive measures on critical components to better meet a unit’s mission at minimum cost and highest availability (footnote reference to appropriate EPRI report #)

5

MINIMIZATION OF PLANT DAMAGE AND

OPTIMIZATION OF OPERATION

Reliable instrumentation in the right place with output made available in the control room is the

major requirement to minimize damage Very often the original instrumentation package

provided by the plant manufacturer is not adequate for intermittent operation Deducing plant conditions (e.g turbine differential expansion) from an envelope of input conditions and time can be very risky when abnormal conditions arise However, instrumentation is expensive and it may be economically necessary to install extensive instrumentation on only one of a type of unit

so that the other units may be instrumented only in areas found to be critical

Turbine

As a sweeping generalization, steam turbines can usually be cycled without significant damage

if attention is paid to temperature matching and drainage For a running regime involving

shut-downs of up to 8-10 hours (often the definition of “hot start” for turbine starting), the best procedure is usually to deload the turbine quickly, thus preventing significant cooling During start up steam temperatures should be matched on the positive side with the temperature of the turbine That is the steam before inlet valves should be towards the upper limit for matching, as set by the manufacturer This temperature should take into account the throttling loss through the valve (i.e typically about 50oC above the turbine metal temperature) This will permit the turbine

to be run up and loaded up

Drainage is critical, as water in the wrong place can do severe damage Reliance should not

be placed on steam traps or opening drains for a specific period of time The ideal drainage arrangement on important steam lines is for two drain valves (one for isolation, one for

controlling the drainage) in series together with a reliable thermocouple Drains should not be closed until a temperature margin above saturation has been achieved, and then may need to be blown through again as a precaution at a later stage of the run-up/load-up The length of drain line between the item being drained and the first drain valve should be minimized to prevent blow-back of collected water onto hot components in the event of a trip or sudden deload

Drain locations will vary from plant to plant and will be provided by the Original Equipment Manufacturer All that is being suggested here is two drain valves in series on each drain line, one to be used for isolation and the other for throttling the drainage (the latter one clearly subject

Minimization of Plant Damage and Optimization of Operation

(180oF) was a typical figure These were aimed at achieving a certain design life (e.g a particular number of hot, cold or warm starts) Where the manufacturer does not provide a limit or the operator desires to change from the manufacturer’s design criteria (e.g by changing the speed or frequency of starts), an independent engineering study will be needed These days of course, it is possible to come up with a scientific value by the use of finite element analysis when it should be possible to optimize the through wall limit This technique can also be used to optimize the geometry of the component to improve its tolerance to temperature differentials

Top to bottom temperatures for turbine HP and IP cylinders may be desirable Bearing

temperatures and vibrations are essential, and differential rotor/casing expansions highly

recommended A reading of the critical metal temperature for steam matching must be available, together with a fall-back temperature measurement that can reliably be used to deduce the critical temperature should that thermocouple fail

The failure mechanism is thermal fatigue and creep fatigue A common failure mechanism would

be the overheating of the inner surface of a thick metal section relative to the outer section The outer section constrains the expansion of the inner section, causing the inner section to go into plastic deformation (yield) in compression When the section temperature is re-equalised (on reaching steady state temperature or during or after cool-down), the inner surface will be in tension If the tension is sufficient, the inner surface may then crack under the tension Once cracks are formed, further temperature cycling (even within the normal limits) may propagate the cracks leading to eventual failure by loss of cross-section Many cracks will not propagate, but this must be established by monitoring and/or engineering analysis

Turning gear to prevent bowing of the shaft during cooling down and jacking oil pumps to lift the rotor shafts off the bearings will spend much more time in service and have many more starts Attention should be given to provision of spares and to eliminating poor reliability or excessive manual input requirement An alternative means of turning the shaft in emergencies is desirable There should be a standard procedure dealing with failure to go on turning, including a time limit beyond which attempts to re-establish barring will not be continued

Minimization of Plant Damage and Optimization of Operation

The following are checkpoints, but there is no substitute for a thorough engineering study,

including additional thermocouples and a test program by an experienced engineering team; many such are available commercially

Stressing of Thick Section Components

Careful consideration needs to be given to each header to ensure the inner to outer temperature limits are not exceeded The economizer inlet header tends to be vulnerable as it will stay hot during a short shut-down, but may be chilled during the start-up until the feed heaters come into service On drum boilers, the drum is generally not a limiting factor for hot starts unless

excessive feeding is required such that only cool water is available

superheater banks will require a routine of blowing down

Drum Level Control

Testing will be necessary to determine the optimum program of drum level control, starting with the level to be achieved when the boiler is boxed up on the previous shut-down; this must maximize the amount of hot water available for the restart but avoid the need for expensive blowing down to control the level as the water swells under the start-up firing The need to run

a feed pump to top up during the shut-down should be avoided if possible All steam and water valves on the boiler must be maintained such that leakage and loss of pressure and water is minimized Operators must make proper use of master/slave valve arrangements

Furnace Wall Tubes

In a natural circulation boiler, there may be difficulties in getting even flow distribution

established and this will be exacerbated by uneven slag formations Uneven flow distribution can lead to differential expansion problems between tubes, overheating of tubes, stressing of

Minimization of Plant Damage and Optimization of Operation

5-4

Gas Side Isolation

The main boiler outlet isolating dampers must be maintained so that they can be operated

reliably and kept gas-tight to minimize heat losses during shut-down However, regular checks during shut-down to ensure that start-up burners are not leaking oil or gas into the furnace are strongly recommended, and there must be an effective furnace purging procedure before any attempt is made to light the boiler

Desuperheater Sprays

Superheater and reheater desuperheater sprays are a potential source of dangerous leakage into the boiler and steam leads Desuperheater control valves should not be relied on for isolation but must be kept in good condition so that controlled low flows can be achieved during start-up Spray isolation valves should be shut until the desuperheater spray is required and the use of the feed pump during shut-down should be minimized

Sootblowing

The regular expansion and contraction of the boiler will often help to control slag build-up Selective blowing of either the water walls or the superheaters may be necessary before shut-down to assist in achieving a good pressure/temperature raising balance on start-up Airheaters are likely to accumulate some unburned fuel during start-up and should be blown at the earliest opportunity Sootblowers supplied from reheat steam may need an alternative superheater source

to allow early use during start-up

Steam Temperature Raising

Some units are provided with turbine bypass systems which greatly facilitate drainage for

temperature raising compared with those that rely on superheater and leg drains In either case, superheater drains are likely to be used for initial temperature raising until the main steam lead metal temperatures can be matched and flow established to turbine For regular hot starting, drain line sizes (and the capacity of the downstream systems) may need to be increased as, in general, the more the flow the greater the rate of temperature raising Boiler pressure needs to be kept down to increase the temperature raising and to minimize the throttling loss that must be allowed for establishing steam flow to the turbine However, sufficient pressure must be maintained to allow block load to be applied to the turbine on synchronization

UK practice has been to discharge the main start-up drains from the legs and the boiler into blowdown vessels The steam is released to atmosphere The water from the boiler drains is led

to waste and the water from the leg drains is recovered (the difference being in the chemical purity) The mainland European practice has been to take the steam leg drains via a throttling valve into the main turbine condenser thereby condensing the steam and recovering the water when the desired purity was achieved The bypass systems were generally designed to open quickly in the event of the generator becoming detached from the grid system and allowing the

Minimization of Plant Damage and Optimization of Operation

boiler/turbine unit to be stabilised at a low load, supplying its own auxiliaries Thus the bypasses served two functions

A major consideration in plant start-ups and shut-downs is to avoid overheating or chilling thick metal components Hence, for a hot start, the steam temperature at the boiler final superheater outlet must be raised close to the temperature of the main steam legs to the turbine before steam

is admitted to the legs Then steam will have to be passed down the steam legs into drains until the steam temperature at the turbine end of the legs matches the metal temperature (depending on the exact layout of the plant) between the throttle valves and the stop valves in the steam chest Again depending on the plant design, the next metal temperature to match will be a

representative temperature (probably the inlet belt metal) in the hottest section of the turbine, allowance being made for the temperature loss caused by throttling through the turbine

admission valves

The pressure/temperature raising characteristics will depend greatly on the balance between radiant and convection superheaters in the boiler (for each fuel type) Experimentation will be needed to determine the target boiler pressure to be achieved at shut-down to optimize the start-

up, though this will need to be modified to allow for passing valves and dampers A tight boiler will retain its pressure better and will not need to be topped up with cold feed water Similarly if the dampers pass, there will be a cooling draft through the boiler to conduct the heat away

water/steam-In general, less stress is caused to a hot steam turbine by running it up and loading it quickly rather than slowly The problem, particularly with coal-fired boilers, is to achieve a fast, but steady increase in firing and to ensure that the firing is matched to the steam flow (to avoid steam and boiler metal temperature excursions) The steam boundary layer effect in the superheaters means that maximising the steam flow through the boiler is necessary to achieve the desired steam temperatures.

To ensure that the desired run-up and load-up rate of the steam turbine can be achieved, it is useful to have stored energy in the boiler (in the form of high pressure) so that delays in

increasing the firing rate (perhaps because of problems starting a mill) do not immediately

require reduction in the turbine loading rate

There are three factors which limit the desirable boiler pressure during start-up:

· one is that if the firing rate increases faster than targeted, the boiler safety valves will be lifted causing a number of undesirable effects

· The second is that the higher the boiler pressure, the more throttling is necessary to control the turbine run-up and load up rates The increased throttling increases the temperature drop

in the steam Since most boilers have difficulty in achieving high steam temperatures at low

Minimization of Plant Damage and Optimization of Operation

5-6

There is therefore an ideal boiler pressure to be achieved at the start of the steam to legs, steam to turbine process To achieve this ideal pressure at the end of the initial boiler fire-up process most economically, a target boiler pressure should be established to be achieved when the boiler is boxed up at the end of the previous shut-down The relationship between the ideal start-up

pressure and the shut-down pressure is determined by the shut-down time, the cooling and

leakage rates of the boiler during shut-down and the pressure/temperature raising characteristics

of the boiler during start-up

It will generally be found that different mills have a markedly different effect on the rate at which furnace and steam temperatures can be raised Because of the different heat emission characteristics of oil and coal flames, it will probably be necessary to fire a mill as soon as it is safe to do so However to prevent the risk explosions, pulverized coal should not be admitted to a boiler until oil or gas start-up fuel has reached safe temperature limits

Light-up Burners

These must be maintained for high reliability, particularly those that ignite the start-up mill burners otherwise excessive delays and/or extensive use of back-up staff will be incurred The problems will depend on the manufacturer and type Light up burners generally work better when used frequently Other than the fact that they are used more and therefore require more maintenance, they may well be more reliable!

Some of the precautions to be taken and points to be considered are listed below:

· During shut-down, the lighting-up oil will typically be on recirculation at a low rate

· During start-up, the pumping and heating will need to increase quickly and reliably to allow the firing of a lot of burners in quick succession

· For a hot start, a high percentage of the boiler’s light-up burners will be needed, so they need

to be reliable

· Usually, not all the mills are useable for initial light-up and temperature raising For safety reasons, the early mills probably need all of the relevant oil burners to be in service; this requires a very high reliability of these particular burners

· Older designs of light-up oil burner involved the ignition and oil lances to be pushed forward into position by rams as part of the starting process Boiler casing distortion and ram failures caused reliability problems with such burners

· Recirculating tip oil burners avoid the need for oil lance purge sequences Where other types

of burner are used, steam or air purging has to be effective or the burner tip may get

carbonised deposits during the shut-down and therefore fail to ignite when called for

· Many big, coal-fired units with low NOx burners are successfully two-shifted It should be stated here that there is nothing unique with low Nox burners and the light up burners are generally the same for all types of burner

Minimization of Plant Damage and Optimization of Operation

Auxiliary Steam Supplies

Steam may be needed to heat and/or atomize the light-up oil, to seal turbine glands and to

operate condenser air extractors A single auxiliary boiler may be adequate for occasional

start-ups, but may need augmentation for frequent starts An alternative to additional auxiliary boilers might be to install a steam supply from the unit itself or to allow adjacent units to supply each other if not all units are shut down at the same time

Coal Bunker Control

Regular shut-downs make it possible for reduction of the coal yard manning periods

Close liaison between the coal handling staff and operations staff is necessary so that the bunker filling can be done appropriately to the load regime There must be enough coal in the bunkers to allow the start-up and operation until the time when more coal can be put up, but not too much if the bunkers are prone to losing flow Poor quality or wet coal should not be put in the bunkers such that it will be at the bunker mouth during the start and load-up If such coal has to be put

up, the operations staff must be made aware so that they can modify their procedure or allow more time

Mills

Here the avoidance of fires and explosions and coal level control are the key issues Good

operating practices must be maintained Modern control systems and regular attention to the level sensing devices may be necessary to avoid over and under filling However, since through-put is lower with intermittent operations than with base-load, lower maintenance or wear parts can be expected But potential problems with frequent starts and stops must closely observed

Other Plant Considerations

Switchgear

Some high voltage switchgear, e.g oil-filled, has a high maintenance requirement if used

frequently Vacuum-filled circuit breakers need much lower maintenance and are generally easier to isolate and earth (the opportunity of frequent shut-downs might be taken to do more maintenance) A cost/benefit analysis for replacement of older switchgear should be considered Any unreliable low/medium voltage switchgear should also be considered for replacement

Feed Heaters

Minimization of Plant Damage and Optimization of Operation

5-8

Dust Plant

If the fly ash dust is sold, it may be necessary to dispose of the dust created during start-up separately because the initial boiler combustion will be poor, leading to carbon in dust levels which may not be acceptable to the purchaser

Feedwater Regulating Valves

Sustained low feed flow rates will need to be achieved during start-up This requires careful maintenance of the main valves or the provision of a start-up valve bypassing the main valves The usual failure mechanism is cutting of the seat through sustained, heavy throttling with the valve nearly closed Vibration can be a problem The important issue is to have a valve that is specifically designed for this sort of application, but maintenance is important and problems can

be alleviated sometimes by e.g hardening of the seats A small, start-up feed regulator valve is recommended

Condenser Air Extraction System

This must be able to raise vacuum reliably and repeatably despite the strong possibility of greater than normal air in-leakage arising from the more frequent expansion and contraction of the relevant joints Vacuum raising should be fast enough not to be on the critical path for the start-

up

6

OPERABILITY

Unless labor is cheap and can be deployed plentifully, all valves and dampers must be power operated and with control and indication on the operator’s panel, likewise electric motors

All key instrumentation must also be readable at the operator’s panel

The next major step is to make it possible for the operator to be able to take an overview and monitor the key elements of the process and not be overwhelmed with the volume of information presented and number of individual actions he has to perform However many staff are deployed

to operate the unit, one person must have the overview and be able to spot the variations that could lead to commercial loss or plant damage Here, modern control systems and VDU

(Visual Display Unit) displays can make an immense difference compared with old control systems and chart recorders To give some examples:

· All the turbine bearing vibrations and shaft eccentricities can be displayed on a single,

histogram VDU display where the color of each block changes to yellow then red when its value approaches the warning or danger level

· A target critical path line can be displayed on a screen for start-up, run-up and load up, with the actual achievement also displayed live during the process In this way, the operator

quickly becomes aware of variations and can make timely corrections

· Elements of the task can be combined into single-button-start subroutines, e.g firing mills, commissioning feed-water heaters, steam to turbine/generator set and run-up

· Alarms can be graded according to their relevance at any point in the process Early

computerized systems often churned out huge amounts of irrelevant alarm state information

Complete replacement of existing control and information systems can be very expensive, but there are now available commercial systems that will extract and process information from the existing system and provide high level information and control facilities

The demands for tight combustion control to meet environmental standards are likely to

necessitate a modern, automatic system