Chemical Para Metres

8/3/2019 Chemical Para Metres

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Chemical guidelines for water/steam

cycle of fossil fired units

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Thermal Generation Study Committee

......................................................................................................

April 1997

Ref : 02004Ren9766


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Chemical guidelines for water/steam

cycle of fossil fired units

............................................................................................

Thermal Generation Study Committee

............................................................................................

Paper prepared by:

Geoff Bignold (GB), Stefano Concari (IT), Karol Daucik (DK), Geoff

Fitchett (GB), Richard Harries (GB), Giuliano Magnani (IT), Giovanni

Quadri, (IT), Roger Roofthooft, (BE), Andre Zeijseink, (NLs)

Copyright

Union of the Electricity Industry - EURELECTRIC, 2000

All rights reserved

Printed at EURELECTRIC, Brussels (Belgium)


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TABLE OF CONTENTS

EXECUTIVE SUMMARY ...................................................................................................i

GLOSSARY .........................................................................................................................ii

1. INTRODUCTION ........................................................................................................11.1 Overview .......................................................................................................................... 1

1.2 Review of Current Practice of UNIPEDE Members........................................................... 1

1.3 Scope of the Guidelines..................................................................................................... 2

2. BASICS OF DEPOSITION AND CORROSION PROTECTION.................................32.1 Objectives and Principles ................................................................................................. 3

2.2 Feed-water system ............................................................................................................ 3

2.3 Boiler ............................................................................................................................... 4

2.4 Turbine, superheater and reheater.................................................................................... 5

3. RECOMMENDED CONCEPT OF CHEMICAL CONTROL.......................................5

4. RECOMMENDED CONTROL PARAMETERS..........................................................6

4.1 Feed-water and steam....................................................................................................... 64.2 Boiler water...................................................................................................................... 7

4.3 Condensate....................................................................................................................... 8

5. FUTURE DEVELOPMENTS.......................................................................................8

6. REFERENCES .............................................................................................................8

7. TABLES.......................................................................................................................9TABLE I. Definitions and characteristic of Action Levels..................................................... 10

TABLE II. Key parameters for control of water/steam cycle .................................................. 11

TABLE III. Overview of Action Levels .................................................................................... 11

Appendix 1..........................................................................................................................21

Appendix 2..........................................................................................................................27

Appendix 3..........................................................................................................................32


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EXECUTIVE SUMMARY

The chemical control of water/steam cycles in member countries of UNIPEDE is based on experience published

in world literature, own operational experiences and guidelines issued by different national and international

bodies. The Therchim Committee has made an inquiry into the general practice for chemical control of

water/steam cycles of fossil-fired power units in UNIPEDE members. The inquiry was addressed to membercountries being represented in the committee. The answers were evaluated and on the basis of this evaluation it

was decided to formulate UNIPEDE Chemical guidelines for water/steam cycles of fossil units. A group of

experts from several countries has been constituted to develop generic guidelines.

The guidelines are based on the data assembled from the inquiry. As the practice in single countries is adapted

to the local conditions (type of the installation, operation mode etc.), it was necessary to make the guidelines

applicable to the broadest possible range of units used by utilities in UNIPEDE countries. Furthermore, the

guidelines have in view the present and the near future developments of the design, operation and control of

power plants. In particular, two directions have been considered during development of the guidelines. The

introduction of units operating at higher and higher pressure and temperature and introduction of combined

cycle units operating at several pressure stages, where the low pressure stage is operating at unusually low

pressure. Thus, the pressure range is extended in both directions.

Chemical control of water steam cycle based on the concept of action levels was originally been introduced by

nuclear power utilities. The concept was introduced for the first time for fossil fired plants by EPRI. Several

countries around the world, among them some of the UNIPEDE members, adopted the concept because of its

detailed guidance of operators during chemical disturbances of the cycle. However, the definitions of action

levels have to be adjusted when transforming the concept from the application on nuclear power plants to the

application on fossil plants. The safety considerations on nuclear plants demand very rigid application of the

action level concept, while on fossil plants it is more or less a question of an economic assessment. The length

of time a fossil fired plant should be allowed to operate at a certain action level is a question of cost-benefit

evaluation.

The concept of action levels is introduced in the UNIPEDE Guidelines, but some important adjustments are

made to the concept used by nuclear utilities, as well as to the concept introduced by EPRI for fossil firedplants.

Three action levels and a target for normal operation are defined. Definitions of action levels are slightly

different from the EPRI definitions. A characterisation of each action level is given with an indication of the

risk connected to operation at these conditions. Furthermore, guidance is given for actions whenever an action

level is reached during operation.

The same action levels are recommended for start-up procedure. In this case the unit is in action level 3 region

before start-up. Guidance is given for procedure steps during start-up according to a successive purification of

the cycle and adjustment of parameters.

The UNIPEDE guidelines distinguish between 2 types of chemical parameters - key parameters and diagnostic

parameters. Key parameters are basically purity parameters which should be continuously monitored, if

possible, whereas diagnostic parameters are measured according to operational needs. The most important

difference between parameters is reflected at action level 3, where key parameters call for a forced shut-down of

the unit. The diagnostic parameters call for less radical actions such as load reduction.

Almost all the parameters are specified in diagrams, where the interdependency of parameters is defined. In

some cases it is the interdependency of two chemical parameters (such as pH and oxygen concentration in feed-

water), in others it is dependency of a chemical parameter on full load operating pressure (such as acid

conductivity). In these cases, other parameters, such as heat flux, would be more correct to use, though

extremely inconvenient. There is a certain functionality between the correct parameters and pressure, thus the

most convenient parameter is chosen.

For a successfull implementation of the guidelines, a close cooperation between station chemist and operators,as well as support from the management, is essential.


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GLOSSARY

Acid conductivity Conductivity measured after exchange of cations to H+ passing an acid regeneratedcation exchanger. Thus, alkalinity is neutralised and all salts are converted to acids.

AVT All Volatile Treatment. Conditioning concept, where ammonia is used with or

without addition of hydrazine as oxygen scavenger.

CPP Condensate polishing plant.

CWT Combined Water Treatment (in this document called Oxygenated Treatment, OT).

Conditioning concept, where ammonia and oxygen are added. Very high purity of

water is required for succesful application of this concept.

EPRI Electric Power Research Institute, USA.

Equivalent Lifetime Operation time at reference conditions causing the same wear or damage as realoperation time at actual conditions.

OT Oxygenated Treatment (also called CWT). Conditioning concept, where ammonia

and oxygen are added. Very high purity of water is required for succesful

application of this concept.

Quality Index Index expressing rate of lifetime consumption relative to lifetime consumption at

reference conditions

Strong Mineral Acids Sum of chlorides and sulphates expressed as mg chlorides / kg water.

Therchim UNIPEDE committee of chemical experts.

VGB Technische Vereinigung der Grosskesselbetrieber, German founded international

organisation of boiler owners.


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1. INTRODUCTION

1.1 Overview

The integrity of water/steam circuits of thermo-electric power plants is critically dependent onchemistry issues to ensure the avoidance of excessive corrosion damage and deposition. The consequences of

inappropriate chemistries can be severe irreversible plant damage requiring extensive repairs and remedial

actions. Accordingly there has been widespread development, generally on a country by country and in some

cases on a company by company basis, of guidelines for the chemistry of fossil fired plants. Discussion of this

subject on a seminar (at the Israel meeting of THERCHIM, Group of Experts meeting in 1995) showed a

convergence of national guidelines comparing to the earlier inquiry [1] in 1970. Because of differences in

national standards at that time, joint UNIPEDE guidelines could not be prepared. The current agreement

between countries motivated formation of a Working Group for the preparation of the UNIPEDE guidelines for

the chemical control of steam water circuits. This includes the establishment of guidance for the optimisation of

plant integrity and availability when operating for periods outside the normal specification range.

The approach adopted has been to encompass all of the national and company reference guidelines for thechemical control of large generating plant available from the utilities represented in UNIPEDE. These

guidelines have been developed and defined taking account of both operational experience and the

recommendations of the plant suppliers.

The concept of action levels for important chemical control parameters has been adopted, and a procedure for

further development of the use of these in terms of plant lifetime assessment is suggested. In some of the

national and company guidelines action levels are already identified for some parameters for the out of

specification operation of the water, steam circuit. These set out, in broad terms, the time limits for operation

out of specification. In general the action level approach is not supported by the plant suppliers who tend to

specify normal operating parameters only. In Europe the VGB recommendations[2], which apply to industrial

plants in addition to power systems carry international authority.

The concept of action levels has also been adopted by the Electrical Power Research Institute (EPRI) in the

USA[3]; these are probably the most widely adopted guidelines world wide.

The UNIPEDE guidelines will only address fossil fuelled power plants. The action level approach adopted

covers chemical control parameters from normal acceptable operation, through minor perturbations, to more

serious deviations and ultimately plant shut down. Control parameters are chosen as those which are directly

influenceable by operator intervention.

1.2 Review of Current Practice of UNIPEDE Members

Guidelines from several countries/utilities have been collected together and compared. Only the most

significant chemical parameters for feed-water, boiler water, and steam have been considered, and although afew UNIPEDE members already use the action level approach, the normal operating values in individual

guidelines have been found to be quite similar (see Appendix 1). It has therefore proved feasible to propose

action levels which are broadly applicable across UNIPEDE members.

Data from the inquiry relate to high pressure plants, including both once through and drum type boilers. The

following operational regimes are covered:

- All volatile treatment, AVT, - reducing regimes in which pH is controlled with ammonia and an

oxygen scavenger (usually hydrazine) may be added.

- Combined water treatment, CWT, or oxygenated treatment, OT, - ammonia under slightly oxidising

conditions usually achieved by oxygen dosing with very restrictive limits on acid conductivity.(In these guidelines the acronym OT will be used.)


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- Non-volatile alkali - sodium hydroxide or sodium phosphate boiler water treatment applied under

reducing conditions. (This regime is limited to drum boiler circuits only and ammonia is still used to

adjust pH in feed-water and steam).

All the plants operate with high purity water and steam conditions and almost all national and company

guidelines define the target parameter values to be aimed for (either below or within which to operate) and themaximum values (or the maximum range) not to be exceeded.

The individual guidelines are based largely on the experience gained in the last twenty years as a result of many

earlier plant failures resulting from inadequate control of water and steam purity.

1.3 Scope of the Guidelines

These guidelines are intended for the operation of fossil fired generating plants. They do not cover low

rated industrial equipment, that operate with softened water. They include multi-pressure combined cycle

plants, where each stage is considered as an individual boiler with respect to limits, but the control strategy

have to take account the interdependency of stages.

Existing guidance, reflecting good practice, takes into account avoidance of corrosion and deposition within

power plant components. In specifying action levels an attempt is made to relate deviation from good practice

to possible plant damage.

In implementing the guidelines, it must be borne in mind that :

- Plant chemistry provides the manager with information for decision making. Commitment of

management, to make economic and safety decisions based on risk assessment, backed by high quality

technical guidance, is essential.

- Action levels must be credible to those running the plant, and making operational decisions, not just

those with specialist chemistry knowledge.

- Operators are normally opposed to shutting-down plant as a consequence of loss of chemical control.

- Sufficient information must be available on which to base a request for intervention.

The guidelines recommend conditions for both continuous operation and for plant start-up. A target range and

three levels of action are defined in simple terms below, and in more detail in table 1.

Target range, no action required; this range covers the practicable values which plant managers will

normally achieve without excessive cost.

Action level 1, minor disturbance requiring investigation, diagnosis and optimization.

Action level 2,serious disturbance in chemical control requiring diagnosis and action to eliminate the

cause.

Action level 3, very serious disturbance requiring substantial operator intervention, such as load

reduction, or plant shut-down.

The limits are the same for start-up and continuous operation. For start-up however, the action levels should be

used for optimising the start-up procedure.


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2. BASICS OF DEPOSITION AND CORROSION PROTECTION

2.1 Objectives and Principles

Corrosion is a process of unwanted and uncontrolled attack on the materials of construction.Deposition occurs as a byproduct of this and also from the process of supersaturation of impurities in the

water/steam cycle; it may be equally harmful.

The objectives of chemical control of the water/steam circuit are to minimise corrosion damage and deposition

on the internal surfaces of water/steam circuit as far as is economically prudent:

The basic principles involved in minimising corrosion and deposition are:

- Minimisation of the ingress of impurities into the water/steam circuit.

- Control of redox potential to stabilise oxide films and to minimise transport of metal oxides (usually

pracised by control of oxygen).

- Control of the pH to counter corrosion effects, to stabilise oxide films and to minimise transport of

metal oxides.

Other major influences on the corrosion/deposition process include plant operating history, heat flux and the

impact of phase transitions.

Control of the corrosion and deposition processes are consideredfor the following areas of plant:

- Feed-water and condensate system

- Boiler

- Turbine and steam circuits

2.2 Feed-water system

The basic approach to feed-water conditioning is maintenance of sufficient water purity to limit

corrosion of feed train material and to minimise the transport of corrosion products and corrosive contaminants

to the boiler. For once through boilers the only conditioning applied will generally be to the feed-water. In this

cases the quality of the steam, is directly determined by the quality of the feed-water.

For drum boiler circuits, although further control measures may be applied in the evaporator, it remains good

practice for modern power units to have the same target with respect to feed-water and steam.

Corrosion rarely threatens the integrity of the feed system as such. Erosion-corrosion of mild steel components,

where water velocities and turbulence are high and oxygen levels are low, can cause damage and will also lead

to enhanced iron levels which are fed forward to the boiler. Corrosion of copper alloys can be stimulated by the

combined effect of dissolved oxygen and ammonia; this can cause copper to be transported from the feed system

into the bolier and turbine.

Traditionally the chemical systems for conditioning feed-water fall into two groups:

- The reducing (ammonia or an amine with hydrazine) all volatile treatment, (AVT), where the

protection steel is based on low solubility of iron oxides at elevated pH

- The oxidising (oxygen with a low concentration of ammonia) treatment (OT), with very low anionconcentrations (low acid conductivity), where the protection of steel is based on low solubility of iron

oxides at elevated oxidation-reduction potential.


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Although individual national and company guidelines generally specify limited concentration ranges, overall

experience indicates that these two protection mechanisms act simultaneously and there are no distinguished

border lines between these types of conditioning. On the contrary, there is seen to be a continuum of suitable

operation conditions in a broad range with high pH and low oxygen concentration at one end, and low pH and

high oxygen concentration at the other. Achievable purity of feed-water determines the degree of freedomavailable to operators within this range (high oxygen concentrations are incompatible with chloride and

sulphate contamination).

Choice of the optimal chemical conditions within this broad range will be influenced by the boiler type,

operational conditions, design and materials of construction. The presence of the following materials is

particularly important:

- Carbon steels are particularly compatible with mildly oxidising conditions in the absence of

contamination anions (chloride, sulphate, etc.)

- Copper and copper alloys may suffer oxide transport problems in oxidising regimes in some plants and

are vulnerable to attack by high levels of ammonia.

- Other materials, such as titanium, high chromium steel and chromium nickel steel are relatively

indifferent to the conditioning regime.

2.3 Boiler

Two general classes of boilers are in use:

- Once through boilers in which water is evaporated to a high steam content. These are not tolerant of

nonvolatile dosing chemicals and generally operate without further dosing downward the feedwater

chemical dosing.

- Drum boilers in which steam separation takes place in an unheated vessel. Boiling occurs in tubes

through which water from the drum is recirculated, preventing dryout at the boiling surfaces. Such

boilers may be tolerant of addition of low levels of non-volatile alkalis to prevent any risk of acidic

corrosion.

The major objectives of boiler water treatment are to minimise deposition and corrosion of the boiler and to

ensure that steam is of the appropriate quality. During initial operation or post chemical cleaning, the boiler

steel reacts with the water and steam to produce a protective film of iron oxides. The rate of reaction decreases

with time as the thickness of the protective oxide film increases.

Boiler integrity can be prejudiced by a number of corrosion mechanisms or by overheating due to excessive

thickness of oxide layers.

Nonvolatile impurities can concentrate in boilers and can increase the risk of corrosion. A number of factors

influence this. The build up of porous oxides by deposition onto heat transfer surfaces is particularly

detrimental. Other important factors include details of design, construction and operating regime.

The optimum boiler water condition is mildly alkaline. Deviation either to acidic or to highly alkaline

conditions carries a risk of damage.

- Acid forming species (particularly chlorides, but also sulphates and organic anions) if present and

able to concentrate at boiler tube surfaces can result in very rapid rates of general corrosion. This type

of corrosion is often accompanied by hydrogen damage in mild steels which can lead to large sudden

tube failures. Acids can be generated from neutral salts particularly under oxidising conditions, and soit is particularly important to minimise ingress of chlorides and sulphates when using oxidising

treatments and during oxygen transients at start-up for reducing treatments.


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- If strong alkalis concentrate at surfaces, corrosion at unacceptable rates can also occur. Hydrogen

damage is not normally caused by this type of attack, but some alloys are vulnerable to stress corrosion

cracking and grooving in very high pH environments.

The required benign boiler water which is mildly alkaline at operating temperatures and pressures is achievedusing either an AVT or solid alkali treatment. The choice of regime may be limited by heat flux considerations,

since this has a strong effect on concentration of involatile materials at boiling surfaces. Furthermore all

substances that are added to control boiler water corrosion will inevitably impact upon steam quality.

Ideally the aim is to have a zero concentration of impurities, but this is impractical and realistic targets for both

acceptable operation and limited out of specification operation are needed.

2.4 Turbine, superheater and reheater

No direct conditioning of steam is normally applied, and hence the chemical quality of steam derives

from the measures applied to control feed and boiler water. Thus, one of the objectives of feed-water and boilerwater conditioning is to avoid deposition and corrosion in the steam pipework and turbine.

Steam purity must be high and actual quality is determined by:

- The concentration and solubility of salts in steam. The solubility is a function of pressure, temperature

and of other chemical components

- Carry over of droplets of boiler water

- Injection of contaminated feed-water into steam for attemporation.

Both acidic and alkaline contaminants are important :

Sodium hydroxide, hydrogen sulphates and chlorides at certain concentrations present a stress corrosion

cracking risk to steels, particularly with austenitic structures.

Salts deposited in steam pipework on-load can result in the development of concentrated solutions off-load

following condensation of residual steam. This effect is particularly significant for reheaters and some types of

feedheaters.

Decomposition products of organic impurities (organic and carbonate anions) may be implicated in turbine

damage.

The early condensation zone of the turbine is particularly sensitive to low volatility contaminants. These

impurities can concentrate on surfaces and in the very first droplets of condensate to form an aggressiveenvironment.

Silica is the most soluble of the common boiler water contaminants in high pressure steam and can become

supersaturated during expansion in the turbine. This results in deposition on the blades causing loss of turbine

efficiency, and in severe cases, loss of output.

3. RECOMMENDED CONCEPT OF CHEMICAL CONTROL

The chemical control is based on specifications of target and 3 action levels for out of target

concentrations of chemical species. The most significant parameters are defined as key parameters and

stringent control of them is required. If possible continuous monitoring must be applied.

Other chemical measurements will frequently provide valuable diagnostic data. Laboratory support is required

for periodical extended analysis and check of monitors.


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The action levels are defined which allow the operator to use the same set of limits for continuous operation

and for start-up. The detailed definitions of action levels are specified separately for these situations in table 1.

Action levels are time related and the combination of concentration and time are set to minimise damage to

feed water systems, boiler and turbine components from corrosion and deposition processes.

The limits for action levels are defined as a function of pressure. This is a simplified approach; there are otherparameters which affect the "true" limits (e.g. heat flux). Nevertheless, pressure has been chosen as the most

convenient parameter for operators. Boilers with extraordinary high heat flux (some designs of oil fired boilers)

may require more stringent limits particularly in regard of boiler water quality.

- Operation in action level 1 regularly requires extended analysis for diagnostic and optimisation

purpose.

- Operation in action level 2 requires qualified interpretation of laboratory and monitor data to enable

the operators to take appropriate corrective steps.

- Operation in action level 3 with respect to the key parameters requires immediate action to shut-down

the unit. In cases when less critical parameters (i.e. results of diagnostic measurements) exceed actionlevel 3, load reduction will generally be required until the fault is rectified.

It is the aim of the guidelines to avoid the shut-down requirement as long as there is any realistic chance to

eliminate the source of trouble. This should be managed by such actions as load reduction to reduce heat flux

(i.e. reducing the risk of damage whilst remedial actions are being undertaken) before action level 3 limits are

exceeded. Load reductions may also be essential when feed-water contamination is encountered in order to

allow the flow of this water to attemporator sprays to be terminated without risk of overheating.

When a drum boiler on AVT dosing is exposed to high levels of impurities, it can be temporarily conditioned

with solid alkali (giving it higher tolerance of impurities) and thus delaying or avoiding action level 3.

Some designs of once-through boilers, with water filled level holding vessel having drain facilities, approach

the conditions of drum boiler (though without moisture separators) during low load operation.

The key parameters for action level 3 decisions are set out in table II.

4. RECOMMENDED CONTROL PARAMETERS

Chemical parameters are specified for the following sampling points:

- Condensate at the condensate pump discharge

- Feed-water at the economizer inlet

- Boiler water (preferably at a downcomer sampling point)

- Steam (saturated and/or superheated)

Table III summarises the specifications of chemical control parameters. Most of these are detailed in the

diagrams in chapter 7, with parameters characterising the purity of the system expressed as function of

operating pressure.

4.1 Feed-water and steam

Purity specifications for feed-water at the economizer inlet and for steam are the same, as no

distinction is drawn between units with drums and those with once-through boilers. Operation with condenser

leakage on a drum boiler unit without a condensate polishing plant (CPP) is considered as operation at

particular action level.

The specification of pH and oxygen concentration in feed-water is given as a broad range for information infigure 1 and 2 (for systems with and without copper alloys). This specification does not suggest random

operation within this range, but identifies the limit at which action level 3 becomes applicable.


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To find a suitable target operational feed-water pH and oxygen range for a particular unit it is necessary to

optimize conditions for this unit according to its design, materials of construction, operation mode and achie-

vable purity of the water/steam cycle. Part icularly when copper alloys are used in the water/steam circuit, the

optimisation must take into account the enhancement of copper oxide solubility in the presence of higher levels

of oxygen and ammonia. Having determined the optimum operational pH, the target operating range is defined

as within 0.2 of this value, action level 1 is defined as within 0.4 and action level 2 is defined as within 1. Limits for oxygen are station specific and are thus individually estimated. Generally the purer feed-water is,

the more relaxed specifications on oxygen can be accepted.

Boiler water pH of drum boilers operating on AVT treatment is determined by the pH of feed-water. A careful

control of low limits in feed-water pH is therefore essential (specified in table III).

Control of pH of feed-water (and hence of steam) may be based on direct conductivity measurements as a

convenient reliable alternative for many plants.

Figure 3 specifies the action levels for acid conductivity of steam and feed-water.

Figure 4 shows action level limits for sodium in steam. For once through boilers this limits apply tosuperheated steam as well as to feed-water. For drum boilers this applies to saturated steam.

4.2 Boiler water

For boiler water, the action levels for units with drum boilers vary depending upon whether the

applied treatment is AVT or non-volatile alkali. Because of the enhanced ability of sodium hydroxide and

phosphate dosed systems to maintain alkaline conditions at the boiler tube surface, greater concentrations of

impurities than under AVT conditioning are tolerable in the bulk water and this is reflected in a higher

acceptable acid conductivity.

Action levels and target ranges for boiler water pH are given in figure 5 . The figure expresses the dependence

of the high and low pH limits on the pressure of the boiler and on the acid conductivity of boiler water. The

optimal pH of boiler water increases with increase of acid conductivity, but falls with increasing pressure. The

ratio of acid conductivity to boiler pressure can be used to derive specific pH limits for any individual boiler.

Some examples of this procedure are shown in Appendix 2.

The broad range of normal operation is for general guidance only. The normal range of operation for a given

unit is dependent upon the chosen chemical conditioning regime and on factors specific to that unit.

Boiler water pH for AVT dosed plant is determined by feed-water treatment. The control of feed-water pH is

thus essential not only for condensate and feed-water circuit, but also for the boiler. Because of the high

volatility of ammonia, the pH of boiler water is considerably lower than the pH of feed-water. Particularly at

low pressure a very high ammonia concentration is necessary to reach sufficient high pH in boiler water.

Therefore, AVT treatment of boilers below 8MPa, particularly those with copper alloy components in the

condenser and/or feedwater circuit, is not recommended.

The control of boiler water pH in units conditioned with sodium hydroxide is usually based on the measurement

of conductivity and acid conductivity. These two parameters give the operator a safe and reliable method to

controlling boiler water purity by blow down and pH by either dosing additional NaOH or blowing down the

excess.

Boiler water pH in units alkalized with a phosphate is controlled by direct measurement of pH and phosphate.

On units where phosphate hide-out occurs, a measurement of sodium is also necessary to assure adequate

controll.

Guideline concentrations for the most important impurities in boiler water are given in figures 6, 7 and 8. The

anionic impurities are controlled via acid conductivity measurements of boiler water, when AVT or sodiumhydroxide is used for pH control. When phosphate treatment is chosen for pH control, this measurement would

be disturbed by phosphate contribution to the acid conductivity. Limits for concentration of strong mineral


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acids (fig. 9) are then used to ensure purity of boiler water. The practical definition of strong mineral acid

concentration is the sum of chloride and sulphate concentrations.

4.3 Condensate

Because condenser leakage is the major source of impurities in circuits, monitoring of the condensate

is particularly important as an early indicator of the need for action. As station circuits vary, consideration for

each plant on an individual basis is necessary to ensure that contaminated condensate is not fed to vulnerable

components (such as attemporator sprays, etc.).

Units with once through boiler are always equipped with a condensate polishing plant (CPP) to take care of this

problem. The CPP should be designed, maintained and operated on a standard which is able to cope with

condenser leakages.

Drum boiler units often do not have a CPP and in case of condenser leakage precautions must be taken to avoiddamage.

The specific action level responses (action level determined by feed-water specifications) appropriate to

contamination of condensate are as follows:

Action level 1 - reduce flow to attemporators sufficiently to maintain sodium in steam within target. This may

imply either feeding attemporators from an uncontaminated alternative source, or minor load reduction in order

to avoid overheating.

Action level 2 - terminate flow to attemporators. Adjust load and provide alternative uncontaminated water

source as appropriate. Prepare to increase boiler blow-down or condensate polisher regeneration frequency

(where fitted). Plan to address condenser leakage at first convenient opportunity.

Action level 3 - reduce load and address condenser leakage as soon as practicable.

5. FUTURE DEVELOPMENTS

The action level concept is seen as positive progress towards the development of optimal control

routines. Operation outside the target region will cause damage and its impact will depend on both the size and

the duration of the excursion. EPRI has addressed this relationship, giving time limits for operation at each

action level during one year. This is a rather rigid approach, and does not take into consideration the actual

operating conditions and lifetime prognosis for the installation. A system of chemistry evaluation on the basis

of economic/scientific evaluation of all possibilities for damage is not possible. A somewhat more pragmaticsystem based on quality indices is suggested in Appendix 3 of these guidelines. This system is based on a

simple mathematical presentation of general long time operational experience. However, it must be considered

as a rough preliminary guide and may benefit from future development.

6. REFERENCES

[1] Report on the chemical standards of the water-steam cycle in Power Stations as in use by the member-

Countries of the UNIPEDE. KEMA, Arnhem, november 1970. Made for UNIPEDE SUB-

COMMITTEE for the study of electric Power Station Chemistry.

[2] VGB-R 450 L - Richtlinien fr Kesselspeisewasser, Kesselwasser und Dampf von Dampferzeugern

ber 68 bar zulssigem Betriebsdruck, 1988[3] Interim Consensus Guidelines on Fossil Plant Cycle Chemistry, EPRI CS-4629, June 1986


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99

7. TABLES


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1100

TABLE I. Definitions and characteristic of Action Levels

Action

level

Characterisation Risk Action during operation

(specific actions on condensate in chapter 4.3)

Action during start-up

Target Normal stable operation,

where everything is

under control

The maintenance of chemical control through the

monitoring of key parameters should be continued.

Target should be reached within 24 hours. If not,

improvements in system control are required.

1 Periodic or minor

disturbances in chemical

control.

Long term damage and

reduction in remaining

life of power cycle

components.

Monitoring of the circuit chemistry should be

extended to diagnostic components to identify the

source of the problem. Strategic considerations

should be made to avoid similar occurrences in the

future.

Action level 1 for key parameters should be achieved in 2

to 8 hours for warm and cold starts respectively.

2 Serious loss of chemical

control.

Serious damage to

components due to

deposition and

corrosion. Significant

reduction in the

component life

Immediate action should be taken to find and

eliminate the cause within hours and/or act ions

should be taken to minimise the damage (e.g.

decrease load).

Fire the boiler. Check the steam quality. At least action

level 2 for all parameters should be reached before

turbine is brought into service.

3 Chemistry out of

control.

Component failure. The unit should be shut down within 1 hour using

the normal shut down procedure if one of the key

parameters deteriorate to action level 3. If one of the

diagnostic parameters deteriorates to this action

level, reduce load to prevent immediate damage and

to gain time to restore chemical control.

Purge the boiler until all the key parameters are below

action level 3.


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1111

TABLE II. Key parameters for control of water/steam cycle

Circuit Sampling point Conditioning Key parameter

Drum Boiler Boiler Water- AVT Conductivity after cation exchange

Drum Boiler Boiler Water NaOH Conductivity after cation exchange

Specific conductivity or pH

Drum Boiler Boiler Water Phosphat Strong mineral acids and pH

Drum Boiler Feed-water AVT pH

Once through boiler Feed-water All Conductivity after cation exchange

All Steam All Conductivity after cation exchange and

sodium

TABLE III. Overview of Action Levels

Sample Economizer Inlet

Steam

Boiler Water

Action level lower limit 1 2 3

pH pHopt0.2 pHopt0.4 pHopt1 Fig. 5

Oxygen

(g/kg)

Fig. 1, 2

Acid Conductivity

(S/cm)

Fig. 3 Fig. 6, 7

Strong Mineral Acid

(g/kg)

Fig. 9

Silica

(g/kg)

10 20 50 Fig. 8

Sodium

(g/kg)

Fig. 4

Iron

(g/kg)

5 20 100


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1133

Figure 2

pH and Oxygen in Feed-water

Copper a lloys present

0

100

200

8 9 10pH

O2(g/l)


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1144

Figure 3Feed-water and Steam

Action Levels for Acid Conductivity

0,01

0,1

1

10

0 5 10 15 20 25 30

Pressure (MPa)


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Figure 4Feed-water and Steam

Action Levels for Sodium

0,1

1

10

100

1000

0 5 10 15 20 25 30

Pressure (MPa)


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Fig. 5Boiler Water - Solid Alkaliser

Action Levels for pH

0,01

0,1

1

10

100

1000

7,00 7,50 8,00 8,50 9,00 9,50 10,00 10,50 11,00 11,50

pH


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Figure 6Boiler Water treated with NaOH


1

10

100

1000

0 2 4 6 8 10 12 14 16 18 20

Pressure (MPa)


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Figure 7Boiler Water - AVT


0,1

1

10

100

0 2 4 6 8 10 12 14 16 18 20

Pressure (MPa)


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Figure 8Boiled Water

Action Levels for silica

0,01

0,1

1

10

100

0 2 4 6 8 10 12 14 16 18 20

Pressure (MPa)


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Figure 9Boiler Water treated with phosphate

Action Levels for Mineral Acids

0,1

1

10

0 2 4 6 8 10 12 14 16 18 20

Pressure (MPa)


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2211

Appendix 1

Comparison of significant parameters for control of water/steam cycle in UNIPEDE countries


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2222

A 1 - TABLE I. Comparison of Significant Parameters for Feed Water of Drum and Once Through Boilers (AVT)

Country BE DE DK ES FI FR GB IE IL IT NL PL PT SE

Acid Conduc-tivity

(S/cm)

N123

0,2

0,15

0,30

0,1>0,2

0,2

0,65>2


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A1 - TABLE II. Comparison of Significant Parameters for Feed Water at OT of Once Through Boilers


AcidConduc-tivity

(S/cm)

N123

0,1>0,2

0,15


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A 1 - TABLE III. Comparison of Significant Parameters for Boiler Water of Drum Boilers on Solid Alkalizer (16 MPa)

Country BE DE DK ES FI FR GB IE IL IT NL PL PT SE FI

Alkal. agent Ph-NaOH NaOH Ph Ph NaOH NaOH Ph NaOH Ph Ph Ph

(mg/kg)


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A 1 - TABLE IV. Comparison of Significant Parameters for Boiler Water of Drum Boilers on AVT (16 MPa)


Acid Conduc-tivity

(S/cm)

N123

5

pH N123

7 - 9 > 8,5 8,9-9,1 8,5 9-9,2

200

20-50>200


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A 1 - TABLE V. Comparison of Significant Parameters for Steam


Acid Conduc-tivity

(S/cm)

N123


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Appendix 2

Examples of Action Level Limits for pH of boiler water at different pressures


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A2 Figure 1

0.5 MPa - Boiler Water treated with solid alkaliser


0,1

1

10

100

1000

7,00 7,50 8,00 8,50 9,00 9,50 10,00 10,50 11,00 11,50

pH

AcidCondutivity(S/cm)


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A2 Figure 22 MPa - Boiler Water treated with solid alkaliser


0,1

1

10

100

1000

7,00 7,50 8,00 8,50 9,00 9,50 10,00 10,50 11,00 11,50

pH


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A2 Figure 3

8 MPa - Boiler Water treated with solid alkaliser


0,1

1

10

100

1000

7,00 7,50 8,00 8,50 9,00 9,50 10,00 10,50 11,00 11,50

pH

AcidConductivity(S/cm)


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A2 Figure 416 MPa - Boiler Water treated with solid alkaliser


0,1

1

10

100

7,00 7,50 8,00 8,50 9,00 9,50 10,00 10,50 11,00

pH


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3333

IP

P L

L=

+

103

322

*for P > L3

where IP is the index for parameter pP is the monitored value of parameter p

L1 is the threshold for action level 1



The index can be used for relative evaluation of the chemical performance with respect to the lifetime

consumption of components for which the parameter p is the key parameter. It has to be emphasized, that it is

an evaluation relative to reference conditions and if more than one key parameter is relevant for the component

in question, a combination of indices described on page 33 should be used.

As examples of key parameters for some components are:

1. Acid conductivity of feed-water for the evaporator of once through boiler with respect to the operation timebetween acid cleanings.

2. Acid conductivity and pH of boiler water for the evaporator of a drum boiler on caustic treatment withrespect to the operation time between acid cleanings.

3. Acid conductivity of boiler water and pH of feed-water for the evaporator of a drum boiler on AVT withrespect to the operation time between acid cleanings.

4. Acid conductivity of steam for the turbine blades.

5. Conductivity of the effluent from condensate polishing plant (CPP) as a key parameter for performance ofCPP. In this case there is no direct connection to lifetime of any particular component.

Some examples are illustrated in more details .

For a typical cold start-up a good practice is to reach L1 (coming from L3) for acid conductivity in feed-water

within 24 hours. The calculation shows, that the mean value of the index I is about 30. This indicates, that the

chemical load on the evaporator during this day of start-up is similar to the load of 30 (720 h) days of operation

at L1. For a unit, which is not performing very well, the purification period during the start-up can easily take 2

days, which makes the life consumption for the whole period 2*30=60 days (1440 h).

Similarly it can be calculated, that a hot start-up of a good unit taking 16 hours will be equivalent to roughly

200 h lifetime consumption, while unattended unit will use 400 h equivalent operation at reference condition.

These values are used in examples below.

Table A 3.1 shows some examples of units with different operation mode (1. base load, 2. cycling load, 3. peak

load) and different quality of chemical control (A. Ideal purity, B.Good practice, C. Unattended control). In

the following the layout and calculation procedure are explaned:

At the top of the table the limits L 1, L2, L3 are given.

Column 1 defines the unit operation mode and the purity standard according to the code specified above. As

reference a unit operating 7000 hours/year at L 1 , having 1 cold and 5 warm starts-up/year is taken.

Columns 2 and 3 specify the number of cold (n 1) and warm (n 2) starts-up/year.

Columns 4 - 6 specify operating hours at different levels of acid conductivity (key parameter). For simplicity

there are 3 groups only: 0,07 S/cm characterise excellent chemical conditions; 0,1 S/cm characterizereference conditions and 0,15 characterise operation in action level 1. In the first row values of indeks I

matching these three acid conductivities are calculated.

Column 7 shows the average value of Index for the whole year. Here the contribution from start-ups is not

included. The calculation for row 1B is as follows:


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II t

t

i i

i

= =+

+=

* , * *,

0 501 3000 1 4000

3000 40000 786

Column 8 show the calculated equivalent lifetime consumption T c including contributions from start-ups. The

calculation for row 1B is as follows:

Tc = I * ti + n1 * 720 + n2 * 200 = 0,786 * 7000 + 1 * 720 + 0 * 200 = 6222

The registered lifetime consumption is of course 7000 h, but chemically the evaporator has received more load

due to start-ups, but decreased load because of the excellent purity in part of the operation time. In calculation

of examples with purity standard C, a higher life time consumption is used for start-ups (1440, 400).

Columns 9 and 10 show a prognosis for acid cleanings if the same operating conditions would be maintained

all the time. This calculation is based on experience with units operating close to the conditions of the reference

unit. These units used to be acid cleaned after 100 000 operating hours.

Tables A 3.II - A 3.V show similar exercises for drum boilers on NaOH and AVT respectively. Two key

parameters are used for separate calculation in each case, and in the table A 3.VI a combined evaluation of

indices calculated from these two key parameters is shown. As the best combination of indices was found to be

their sum -1. Mathematically:

I12 = I1 + I2 - 1

More generaly:

I I ii ii

i

11 1= +=

On the basis of these combined indices the expected acid cleaning frequency is calculated.

It must be emphasised that the method is designed for evaluation of operational lifetime consumption. The

impact of the off-load damage on lifetime must be evaluated separately. It may be possible to use the same

concept, but a careful consideration should then be given to reference condition, key parameters and action

levels.


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3355

Table A 3.I Examples of Lifetime Evaluation of Evaporator Protective Layer

Once Trough boiler - Acid conductivity of FeedwaterL1 L2 L3 3*L3

0,1 0,2 0,5 1,5

No. of Operation time (h) Average Used eq. Acid cleaning

Start-ups at acid conductivity (S/cm) Index lifetime frequency

Cold Warm 0,07 0,1 0,15 I (h/year) (years) (h)

I 0,501187 1 3,1622777

Ref. 1 5 7000 1 8720 11,5 100000

1A 1 0 7000 0,50 4228 23,7 165551

1B 1 0 3000 4000 0,79 6224 16,1 112476

1C 1 0 1000 3000 4000 2,02 17590 5,7 45480

2A 1 5 4000 1000 0,60 4725 21,2 105826

2B 1 5 2000 3000 0,80 5722 17,5 87376

2C 1 5 2000 3000 2,30 14927 6,7 33497

3A 3 50 1000 2000 0,83 14661 6,8 20462

3B 3 50 2000 1000 1,72 17322 5,8 17319

3C 3 50 3000 3,16 33807 3,0 8874


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3366

Table A 3.II Examples of Lifetime Evaluation of Evaporator Protective Layer

Drum boiler - Acid conductivity of Boiler water - NaOH treated

Limits L1 L2 L3 3*L3

S/cm 5 15,8 50 150


Starts-up at acid conductivity (S/cm) Index lifetime frequency

Cold Warm 3 5 10 I (h/year) (years) (h)

I 0,65285 1 2,90378

Ref. 1 5 7000 1 8720 11,5 100000

1A 1 0 7000 0,65 5290 18,9 132326

1B 1 0 3000 4000 0,85 6679 15,0 104813

1C 1 0 1000 3000 4000 1,91 16708 6,0 47881

2A 1 5 4000 1000 0,72 5331 18,8 93784

2B 1 5 2000 3000 0,86 6026 16,6 82978

2C 1 5 2000 3000 2,14 14151 7,1 35332

3A 3 50 1000 2000 0,88 14813 6,8 20253

3B 3 50 2000 1000 1,63 17064 5,9 17581

3C 3 50 3000 2,90 33031 3,0 9082


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3377

Table A 3.III Examples of Lifetime Evaluation of Evaporator Protective Layer

Drum boiler - pH of Boiler water - NaOH treat.Limits L1 L2 L3 3*L3

9,4 9,1 8,6 7

10,4 10,6 10,8 11,8


Start-ups pH Indeks lifetime frequency

Cold Warm 9,4 9,2 8,8 I (h/year) (years) (h)

I 1 4,64159 39,8107

Ref. 1 5 7000 1 8720 11,5 100000

0

1A 1 0 6990 10 1,005202 7756 12,9 90248

1B 1 0 6500 500 1,260113 9541 10,5 73369

1C 1 0 6000 900 100 2,022643 14879 6,7 47048

0

2A 1 5 6990 10 1,005202 8756 11,4 79941

2B 1 5 6500 500 1,260113 10541 9,5 66409

2C 1 5 6000 900 100 2,022643 15879 6,3 44085

3A 3 50 6990 10 1,005202 19196 5,2 36465

3B 3 50 6500 500 1,260113 20981 4,8 33364

3C 3 50 6000 900 100 2,022643 26319 3,8 26597


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3388

Table A 3.IV Examples of Lifetime Evaluation of Evaporator Protective Layer

Drum boiler 16 MPa - Acid conductivity of Boiler water - AVT treat.

Limits L1 L2 L3 3*L3

S/cm 1 2,29 5,23 150


Start-ups at acid conductivity (S/cm) Index lifetime frequency

Cold Warm 0,8 1 2 I (h/year) (years) (h)

I 0,69978 1 5,95928

Ref. 1 5 7000 1 8720 11,5 100000

1A 1 0 7000 0,70 5618 17,8 124589

1B 1 0 3000 4000 0,87 6819 14,7 102649

1C 1 0 1000 3000 4000 3,44 28977 3,5 27608

2A 1 5 4000 1000 0,76 5519 18,1 90594

2B 1 5 2000 3000 0,88 6120 16,3 81705

2C 1 5 2000 3000 3,98 23318 4,3 21443

3A 3 50 1000 2000 0,90 14860 6,7 20189

3B 3 50 2000 1000 2,65 20119 5,0 14911

3C 3 50 3000 5,96 42198 2,4 7109


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3399

Table A 3.V Examples of Lifetime Evaluation of Evaporator Protective Layer

Drum boiler 16 MPa - pH of feed-water - AVT treat.Limits L1 L2 L3 3 * L3

9 8 7 6


Start-ups pH Index lifetime frequency

Cold Warm 9 8,5 7,5 I (h/year) (years) (h)

I 1 3,16228 31,6228

Ref. 1 5 7000 1 8720 11,5 100000

1A 1 0 6990 10 1,003 7742 12,9 90420

1B 1 0 6500 500 1,15 8801 11,4 79535

1C 1 0 6000 900 100 1,72 13448 7,4 52051

2A 1 5 6990 10 1,00 8742 11,4 80077

2B 1 5 6500 500 1,15 9801 10,2 71420

2C 1 5 6000 900 100 1,72 15448 6,5 45312

3A 3 50 6990 10 1,00 17061 5,9 41028

3B 3 50 6500 500 1,15 17209 5,8 40677

3C 3 50 6000 900 100 1,72 30015 3,3 23322


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4400

Table A 3.VI Evaluation of drum boiler lifetime consumption on basis of combined Index

Treatment NaOH AVT

Purity pH Combined Used eq. Purity pH Combined Used eq.

Index Index Index lifetime Index Index Index lifetime

I 1 I 2 I12 (h) I 1 I 2 I12 (h)

Ref. 1 1 1 7000 1 1 1 7000

1A 0,65 1,01 0,66 4606 0,70 1,00 0,70 4920

1B 0,85 1,26 1,11 7779 0,87 1,15 1,03 7180

1C 1,91 2,02 2,93 20518 3,44 1,72 4,16 29103

2A 0,72 1,01 0,73 5092 0,76 1,00 0,76 5340

2B 0,86 1,26 1,12 7849 0,88 1,15 1,03 7241

2C 2,14 2,02 3,16 22154 3,98 1,72 4,69 32837

3A 0,88 1,01 0,89 6226 0,90 1,00 0,90 6321

3B 1,63 1,26 1,89 13263 2,65 1,15 2,81 19653

3C 2,90 2,02 3,93 27485 5,96 1,72 6,67 46723


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