Operational Cleaning and Steam Blow of Large Thermal Power Plant Boilers (Kostolac b2 Experience) (1)

Operational cleaning and steam blow of large Thermal Power Plant boilers (Kostolac B2 experience)

THERM-SERVICE für Kraftwerke und Industrie GmbH

by Martin Herberg, dipl. eng., Therm Service GmbH, Seevetal, Germany

Dr. Eng. Zoran Micevic, Energoprojekt-Entel, Belgrade, Serbia Dr. Eng. Slobodan Djekic, Inter Kontakt-Energo d.o.o., Serbia

10.10.2014

Paper for International Conference POWER PLANTS 2014 Zlatibor Serbia, October 28th-31st.

Content

1. Introduction .................................................................................................................................................... 4 1.1 Therm Service GmbH ...................................................................................................................................... 5 1.2 Kostolac B2 Project Data ................................................................................................................................. 5

1.2.1 Boiler design data ......................................................................................................................................... 5 1.2.2 Volumes ........................................................................................................................................................ 5 1.2.3 Key dates acid cleaning ................................................................................................................................ 5 1.2.4 Key dates steam blow ................................................................................................................................... 5

2. Main Part ........................................................................................................................................................ 6 2.1 Project Kostolac B2 (time related) ................................................................................................................... 6

2.1.1 Offer stage .................................................................................................................................................... 6 2.1.2 Contracting stage .......................................................................................................................................... 6 2.1.3 Engineering stage ......................................................................................................................................... 6 2.1.4 Erection and built up of temporary piping ..................................................................................................... 6

2.2 Execution of the chemical cleaning ................................................................................................................. 6 2.3 Execution of the steam blow ............................................................................................................................ 6 3. Basics of chemical cleaning based on VGB R 513 ..................................................................................... 7 3.1 General ............................................................................................................................................................ 7 3.2 Definition ......................................................................................................................................................... 7

3.2.1 Pre-operational acid cleaning of steam boiler plants .................................................................................... 7 3.2.2 Preparatory measures .................................................................................................................................. 8 3.2.3 Pre-operational acid cleaning with mineral acids .......................................................................................... 8 3.2.4 Pre-operational cleaning with organic acids and complexing agents ............................................................ 8 3.2.5 Procedure ..................................................................................................................................................... 9 3.2.5.1 Open circuit (OC) .......................................................................................................................................... 9 3.2.5.2 Closed Circuit (CC) ....................................................................................................................................... 9

3.3 Procedural steps.............................................................................................................................................. 9 3.3.1 Flushing ........................................................................................................................................................ 9 3.3.2 Heating-up .................................................................................................................................................. 10 3.3.3 Acid treatment when cleaning to the open-circuit method .......................................................................... 10 3.3.4 Acid treatment when cleaning to the closed circuit (circulation) method ..................................................... 10 3.3.5 Flushing to low conductivity ........................................................................................................................ 11 3.3.6 Passivation ................................................................................................................................................. 11

4. Operational chemical cleaning of steam boiler plants ............................................................................. 11 4.1 Preparatory measures and functional testing ................................................................................................ 12 4.2 Acid cleaning ................................................................................................................................................. 13 4.3 Procedure ...................................................................................................................................................... 14

4.3.1 Open Circuit (oc) ......................................................................................................................................... 14 4.3.2 Closed Circuit (cc) ...................................................................................................................................... 15

4.4 Procedural steps............................................................................................................................................ 16 4.4.1 Heating up .................................................................................................................................................. 16 4.4.2 Acid treatment when cleaning in open circuit method ................................................................................. 16 4.4.3 Acid treatment when cleaning in closed circuit (circulation) method ........................................................... 16 4.4.4 Flushing to low conductivity ........................................................................................................................ 17 4.4.5 Passivation ................................................................................................................................................. 17 4.4.6 Copper removal .......................................................................................................................................... 17

4.5 Inhibitors ........................................................................................................................................................ 18 4.6 Monitoring ...................................................................................................................................................... 18

4.6.1 Parameters to be monitored ....................................................................................................................... 18 4.6.2 Check for successful cleaning .................................................................................................................... 19

4.7 Effluent (Amount and treatment) .................................................................................................................... 21 5. Steam blowing of steam boiler plants and associated pipework ............................................................ 22 5.1 Introduction .................................................................................................................................................... 22 5.2 Preparatory technical work for steam blowing ............................................................................................... 22 5.3 Safety aspects during steam blowing ............................................................................................................ 22 5.4 Steam blowing processes .............................................................................................................................. 23

5.4.1 Pressure built up method ............................................................................................................................ 23 5.4.2 Sliding pressure method without pressure accumulation ............................................................................ 23


Operational cleaning and steam blow of large Thermal Power Plant boilers (Kostolac B2 experience). Page 2

5.5 The cleaning effect ........................................................................................................................................ 25 5.6 Cleaning force ratio (CFR) ............................................................................................................................. 25 5.7 Target plate ................................................................................................................................................... 26

5.7.1 Dimensioning and surface quality ............................................................................................................... 26 5.7.2 Location of target plate installation ............................................................................................................. 26 5.7.3 Steam blowing results, control, evaluation and records .............................................................................. 27

5.8 Absolute number of impact size per unit area ............................................................................................... 27 5.9 Relative number of impacts ........................................................................................................................... 28 5.10 Control of cleanliness prior to and after steam blowing ................................................................................. 28 5.11 Noise reduction measures ............................................................................................................................. 28

5.11.1 Silencers ..................................................................................................................................................... 29 5.11.2 Silencing through water injection ................................................................................................................ 29

6. Combination of pre-operational cleaning and steam blowing ................................................................. 30 6.1 General .......................................................................................................................................................... 30 6.2 Combination of chemical cleaning and steam blowing .................................................................................. 30

6.2.1 Weak points of the individual methods ....................................................................................................... 30 6.2.2 Advantages of combining pre-operational cleaning and steam blowing ..................................................... 31 6.2.3 Conclusions from the combination of pre-operational cleaning and steam blowing .................................... 31

7. Method applied at TPP Kostolac B2 .......................................................................................................... 31 7.1 Chemical Cleaning ........................................................................................................................................ 31

7.1.1 Removed copper / iron oxide ...................................................................................................................... 31 7.1.2 Sample tubes / pieces ................................................................................................................................ 32 7.1.3 Examination of chemical cleaning result ..................................................................................................... 32 7.1.4 Treatment of the waste ............................................................................................................................... 33

7.2 Steam Blow ................................................................................................................................................... 34 7.2.1 DPR-factor .................................................................................................................................................. 34 7.2.2 Impact investigation .................................................................................................................................... 35 7.2.3 Target results .............................................................................................................................................. 36

8. Bibliography ................................................................................................................................................. 36



1. INTRODUCTION The main goal of this paper is to show the state of the art of cleaning technology for modernization and reconstruction/life time extension of large Thermal Power Plants based on the example of TPP "Kostolac B2".

Thermal Power Plant "Kostolac B2", 348.5 MW, consists of two pressure stage Sulzer type boiler. It was commissioned in 1992. For this unit a life time extension of about 15 years was decided and therefor a major modernization and reconstruction was performed in 2012.

Before running again into service, an operational cleaning followed by a steam blow had been performed. The paper will further indicate the importance of a structured planning, needed to transact such projects technically and financially effective.

Before it came to the cleaning process itself the most important requirement alongside detailed knowledge about the plant and chemical processes, was a close cooperation between the owner, the project leader and the cleaning service company. Regarding the preparatory work at the plant (e.g. temporary connections, dismantling valve inserts, etc.) approximately fifty points at water- steam cycle were considered at TPP Kostolac B2. Therefore, placing the order at the earliest possible moment means time that enables contracting parties to do a proper engineering, to organize, buy and/or construct material and equipment required for an optimal cleaning process realization.

The chosen combination of chemical cleaning and steam blowing at TPP Kostolac B2 assured the best possible cleaning result and an optimal time schedule to the project. Chemical cleaning was dedicated to remove iron oxides and all mineral deposits up to the metal, including cupper removal. Steam blowing was dedicated to removal of all remaining after installation and creation of proper conditions for introduction of steam into turbine without operational risk. Further advantages of such approach of combining chemical cleaning and Steam blowing were: • Orders integration avoids coordination problems. • Short rebuilding phases between chemical clean and steam blow. • Use of temporary steam blow pipes for execution of chemical cleaning. • Use of same equipment (valves inserts pumps… etc) for chemical cleaning and steam blow (saves

critical path time). • Temporary steam blow pipes are already cleaned up to the target.

Because of the detailed preparation the erection of the temporary chemical clean and steam blow material could start and finish as scheduled. The chemical cleaning process itself was carried out successfully within 4 days, followed by the steam blow of HP-, RH-System and common steam lines within 2 days of effective steam blowing.

The process at TPP Kostolac B2 shows generally that the combination of chemical clean and steam blow is the most effective process to shorten time for commissioning of boiler plants for operational or pre-operational cleaning.

Additionally it shows that the order shall be placed as early as possible to have sufficient time for different arrangements to assure a manageable process and finally to avoid unnecessary time losses and expense.



1.1 THERM SERVICE GMBH

Therm-Service GmbH, was founded in 1966 and has its head office in Seevetal near Hamburg in Germany. The main activities are the pre-operational chemical cleaning of steam boiler plants and the operational chemical cleaning of watersteam circuits of conventional and nuclear power plants as well as the steam blowing during commissioning to include the related pipework needed for steam blowing, and design calculations. The integrated services offered for power and industrial plants as well as with partners in Argentina, England, Spain, Serbia, Belgium, the Netherlands, Kuwait, India, Saudi Arabia, United Arab Emirates, Malaysia, Singapore and China support our customers world-wide.

1.2 KOSTOLAC B2 PROJECT DATA Thermal Power Plant "Kostolac B2", 348.5 MW, consists of two pressure stage Sulzer type boiler. It was

commissioned in 1992. For this unit a life time extension of about 15 years was decided and therefor a major modernization and reconstruction was performed in 2012.

1.2.1 BOILER DESIGN DATA

Steam Out put: 1,000 t/h Steam pressure: 186 bar Steam temperature: 540° C

Reheat pressure: 46.3 / 42.9 bar Reheat temperature: 337.5 / 540° C

1.2.2 VOLUMES

Unit sections Volume HP 416 m3 Volume IP 244 m3 Volume total 660 m3

1.2.3 KEY DATES ACID CLEANING

The chemical cleaning process itself was carried out successfully within 4 days.

1.2.4 KEY DATES STEAM BLOW

The steam blow of HP-, RH-System and common steam lines within 2 days of effective steam blowing.



2. MAIN PART 2.1 PROJECT KOSTOLAC B2 (TIME RELATED)

2.1.1 OFFER STAGE

First offer based on real plant data was prepared on 02nd February 2010.

2.1.2 CONTRACTING STAGE

Contracts for chemical cleaning and steam blow have been finally signed by the customer on March 30st 2012.

2.1.3 ENGINEERING STAGE

The basic engineering for chemical cleaning and steam blow on May 15th 2012. The final engineering for chemical cleaning/Steam blowing dated on

September 11th/18th 2012

2.1.4 ERECTION AND BUILT UP OF TEMPORARY PIPING

2.1.4.1 Chemical Cleaning

02.10.-09.11.2012 Erection of temp piping

2.1.4.2 Steam Blow

18.11.-23.11.2012 Erection of remaining steam blow equipment

2.2 EXECUTION OF THE CHEMICAL CLEANING

11.11.2012 Tightness test of temporary piping Filling and tightness test of system to be cleaned 14.11.2012 Flushing of system to be cleaned 16 – 17.11.2012 Cleaning of Feedwater, HP – and RH-System 18 – 19.11.2012 Treatment of waste water of acid cleaning

2.3 EXECUTION OF THE STEAM BLOW

02.12.2012 Final walkdown for insulation of temporary piping protocol ready for steam blow

03.12.2012 Steam blow HP - system 04.12.2012 Steam blow IP - system 05.12.2012 Start rebuilding temp. steam blow piping



3. BASICS OF CHEMICAL CLEANING BASED ON VGB R 513 3.1 GENERAL

The purpose of each internal cleaning is to reach steam purity stipulated in VGB-R 450 L as soon as possible after the start up of the unit. In principle each internal cleaning is based on two different processes (physical/chemical) independent of the method used. Removal of insoluble substances • The insoluble substances in the system to be cleaned like swarf, welding beads, wood and fibre of

insulating sheets…etc will be discharged out of the system by flushing. Removal of soluble substances • By use of chemicals substances like salts, grease, rust, scale…etc will be dissolved. The dissolution

rate of iron oxides increases proportionally to the acid concentration and exponentially due to increase in temperature; as a rule, the dissolution rate doubles per 10 K temperature increase.

• By combination of these steps it is possible to discharge scales including soluble substances, solved

or dissolved, and the remaining insoluble substances.

3.2 DEFINITION Pre-operational chemical cleaning Use of acids in water and steam-wetted plant components for the purpose of creating defined clean surface conditions so that protective layers can be built-up.

Operational chemical cleaning Operational cleaning is the removal of deposits accumulated during plant operation.

3.2.1 PRE-OPERATIONAL ACID CLEANING OF STEAM BOILER PLANTS

For decades, pre-operational acid cleaning of new plants has been a reliable method to obtain a high degree of system cleanliness and clearly defined clean surfaces thus allowing the formation of uniform and dense protective layers and permitting an as rapid as possible steam turbine operation. Fluids engineering, the determination of the right procedure in consideration of the materials used, and the selection of the respective chemicals are decisive for successful cleaning. It is advised to perform pre-operational cleaning whenever the amount of deposits exceeds 100 g/m2.



3.2.2 PREPARATORY MEASURES

The development of a suitable procedure, comprehensive planning, co-operation of the parties involved, the determination of responsibilities for personnel as regards the operation of system and temporary valves, and the permanent monitoring of the cleaning procedure will be decisive for successful cleaning of the plant. Following need to be considered: • valves not resistant to acid cleaning (e.g. chromium clad and nitrided steels) shall be protected or

otherwise be dismounted. • provision of storage capacity for water and waste water. • procurement of acid-resisting valve trim. • disassembly of check valves, spray water control valves, pumps (circulating pumps), strainers,

distributor nozzles, flow-metering equipment installed (where required), etc. • fabrication of temporary pipework, control of materials for resistance to acid cleaning and of plant

components for possible chemical pre-treatment and/or coatings, the cutting-off of tube samples (dry saw-cutting) is also of importance prior to the beginning of cleaning operations to ensure that the cleaning is successful.

• orifices to be replaced by adapters or remain in the system. Agreement between the purchaser and the cleaning firm must be found as early as possible.

• The local authorities responsible shall be consulted as early as possible with respect to waste water treatment and effluent disposal conditions required after such cleaning procedures.

3.2.3 PRE-OPERATIONAL ACID CLEANING WITH MINERAL ACIDS

Hydrofluoric acid • Hydrofluoric acid is the acid with the highest efficiency of dissolving natural rust. • Hydrofluoric acid is capable of partially or fully dissolving silicates, this will lead to a low SiO2 content

in the steam and therefore this reduces the bypass operation time. • Handling of 1 % diluted hydrofluoric acid has proved to be uncritical. • Hydrofluoric acid cleaning requires operating temperatures of 50 - 80 °C. • Simpel waste water handling.

Other mineral acids In some applications, other acids such as hydrochloric acid and sulphuric acid may also be used. Contrary to the use of hydrofluoric acid, waste water with a strong salt content will be obtained after neutralisation. When using hydrochloric acid and sulphuric acid it shall be taken into account that improved (e.g. nitrided, stellited or chromium-plated) surfaces may be subject to corrosive attacks.

3.2.4 PRE-OPERATIONAL CLEANING WITH ORGANIC ACIDS AND COMPLEXING AGENTS

While in Europe pre-operational cleaning with hydrofluoric acid has gained acceptance, pre-operational cleaning with organic acids (citric acid) and complexing agents (EDTA/NTA) is still of importance in the Asian and Anglo-American areas. Operating temperatures of 90 °C, when using organic acids and temperature ranging between 120 and approx. 200 °C, when using complexing agents are required. Due to such high temperatures specific measures with respect to protection of personnel and safety at workplace have to be taken.



When disposing off organic acids, the increased COD value of the effluent shall be taken into account. In most European countries it is not permitted to directly discharge waste water containing EDTA which shall be disposed off separately.

3.2.5 PROCEDURE

The use of demineralised water for all procedural steps of pre-operational cleaning is the rule. Principally, distinction is made between the open-circuit (OC) method and the closed circuit (CC) or circulation method.

3.2.5.1 OPEN CIRCUIT (OC)

This procedure is mainly used for once-through type boilers. The treatment to the open-circuit method comprises several procedural steps as follows: • Pre-flushing • Pre-treatment of the system to be cleaned by means of chemicals, e.g. H2O2, wetting agents or

ammonium. • Removal of chemicals, where required. • Heating-up. • Treatment with inhibited acid (e.g. 1 % hydrofluoric acid)

o dynamic phase o static phase

• Removal of acid. • Flushing to low conductivity. • Alkaline treatment with ammonia and passivation with hydrogen peroxide. • Removal of passivation solution with treated DI water (ammonia pH value approx. 10), where

required.

3.2.5.2 CLOSED CIRCUIT (CC)

This procedure is mainly used for drum type boilers. The treatment to the circulation method comprises several procedural steps as follows: • Pre-flushing. • Where required, pre-treatment of the system to be cleaned by means of chemicals, e.g. of H2O2,

wetting agents. • Removal of chemicals, where required. • Heating-up. • Treatment with inhibited acid (e.g. 1% hydrofluoric acid). • Removal of acid. • Flushing to low conductivity. • Removal of initial rust, if any. • Alkaline treatment with ammonia and passivation with hydrogen peroxide.

3.3 PROCEDURAL STEPS

3.3.1 FLUSHING

To remove coarse contaminants or the agents, flushing with demineralised water at a flow rate >0.5 m/s shall be effected until a turbidity <0.2 has been obtained. In general the higher the velocity the better is the result of the flushing.



3.3.2 HEATING-UP

Via a temporary pipe system it must be ensured that the whole system to be cleaned can be circulated during the heating process.

3.3.3 ACID TREATMENT WHEN CLEANING TO THE OPEN-CIRCUIT METHOD

The process step "acid treatment" comprises a dynamic and a static phase. When cleaning the system to the open-circuit method, samples shall be taken at each sampling point until measurements show an iron content ≤ 2g/ and an outlet acid concentration more or less equal to the inlet concentration. The time required to perform the open-circuit method depends on the reaction rate of the acid used. A steady phase of 2 to 3 hours shall follow the dynamic phase to remove the remainder of residual deposits.

Open-circuit method

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3.3.4 ACID TREATMENT WHEN CLEANING TO THE CLOSED CIRCUIT (CIRCULATION) METHOD

When applying the circulation method for pre-operational acid cleaning, the required temperature shall be obtained. During heating-up, in-situ temperature control measurements shall ensure that all tubes are flown through. If the required acid concentration has been obtained, auxiliary pumps shall maintain circulation until a constant iron content with free acid has been achieved.

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3.3.5 FLUSHING TO LOW CONDUCTIVITY

Flushing to low conductivity at flow rates > 0.5 m/s shall follow the displacement of acid to ensure that dissolved mill scale particles are flushed out. Flushing shall be finished if a conductivity of less than 20 µS/cm has been obtained referring to the conductivity measured at the inlet.

3.3.6 PASSIVATION

Alkaline pH adjustment of the system fill water shall then be made with ammonia to obtain a pH value of ≥ 10. Where passivation is performed, a 30 to 35 % hydrogen peroxide solution shall additionally be injected shortly after having started the alkalisation. In the system a concentration of 0.1 to 0.3 % by volume shall be obtained. The temperature shall not exceed 40 °C. When applying the circulation method the clean surface formed in the acid phase will corrode again depending on the process and duration of flushing. Citric acid may be used for dissolving initial rust by complexing the iron. Where citric acid is used, the high COD content of the solvent effluent should be taken into account. Sodium nitrite (danger of nitrous fumes formation) and hydrazine should not be used for environmental protection and safety and health reasons.

4. OPERATIONAL CHEMICAL CLEANING OF STEAM BOILER PLANTS

Depending on the extent and composition of deposits, material condition and operational requirements, the plant user, in each specific case, shall make a choice between chemical cleaning and re-tubing. As other chemical cleaning media are often used in chemical cleaning, the suitability of the specific materials shall be considered and evaluated separately. In practice, it was found out if the inner deposit thickness at evaporator exceeds 0.1 mm (corresponds to approx. 500 g/m2) it is recommended to cut tubes and to determine the type and composition of the deposits. For the superheaters and reheaters at an amount of deposits > 1000 g/m2 considerable problems such as risk of plugging, hydrogen formation etc. be expected when performing chemical cleaning. To avoid unplanned shutdowns it is recommended to check the layer thickness of the heating surfaces at regular intervals (e.g. by taking tube samples or by non-destructive examination) and, if required, to have chemical cleaning performed. In the case of once-through boilers, the pressure loss through the evaporator will additionally indicate the necessity of performing chemical cleaning. Experience has shown that, besides the zones of high heat flux density, other areas may show an increase in deposits, e.g. at the evaporator inlet, in dependence of the mode of operation. Chemical cleaning is aimed at removing deposits, improving heat transfer, reducing pressure losses, and avoiding overheating of materials and corrosion risks. The removal of deposits from the water-steam cycle of operational plants requires careful selection of chemicals and cleaning methods. Apart from protective layers, the deposits may also contain hardening salts, phosphates and silicates, carbon, but also metals and metal oxides (copper, manganese, chromium, aluminium, etc.). To determine the chemical and process cycles, tube samples shall be examined by all means and the results of the examination be documented. Prior to chemical cleaning the samples should be taken at locations where the highest amount of deposits is to be expected with respect to the operational characteristics and the water-steam analyses. Experience gained from former tube damage, if any, as well as from observations made during other repair work or change in mode of operation should be considered. Such locations are e.g. the high heat-release surfaces of the evaporator, the first superheater tubes as well as the heating surface portions downstream of the spray attemperator, and the final superheater and reheater.



Where copper is found in layer examinations, specific additional copper removal steps shall be taken since copper cannot be removed with the standard cleaning procedure. In the case of power plant equipment made of copper-containing materials, often an increased copper ingress into the boiler system is observed. Copper may partially enrich in the protective layers and lead to considerable problems during chemical cleaning where necessarily performed. In addition, copper may be transported via the steam to the high-pressure steam turbine and be deposited on the blades of the high-pressure steam turbine section thus leading to considerable loss of turbine efficiency. On evaporator tube samples the amount of deposits on the heat-loaded surface is generally clearly higher than on the other part of the furnace walls. These examinations result in the choice of the cleaning chemicals and duration of chemical treatment which, when compared to laboratory studies made, may lead to deviations due to differing conditions between volume and surface, flow rate and temperature.

4.1 PREPARATORY MEASURES AND FUNCTIONAL TESTING

• For all measures to be taken the respective responsibilities should be contractually agreed between the parties involved.

• An operational acid cleaning program shall be established to cover all preparatory work, treatment steps, safety precautions and effluent disposal. The contractor performing acid cleaning is always dependent on the co-operation of the plant user and manufacturer. Especially where large steam boiler plants have to be cleaned the parallel operation of several feed pumps at short periods of only a few minutes requires a well-coordinated team.

• All plant parts required for acid cleaning shall be ready-to-operate prior to the beginning of cleaning. • If this is not the case, the pressure of time during plant re-starting often leads to measures which are

contrary to the purpose of acid cleaning, i.e. to obtain uniform clean surfaces. It shall be ensured that test runs of feed pumps, incomplete control devices and lacking plant components do not delay or even interrupt the cleaning program. The flushing phases prior to acid cleaning should not be considered test phases for the putting-into-operation of the pumps. I&C status reports on gate valve positions should be available so that the position of the gate valves need not be controlled at situ. New valves should have been tested before so that the acid cleaning program schedule can be adhered to.

• The isolation of boiler sections by seal water and temporary dampers will bear several risks. In the case of parallel tube arrangements, back-flowing is to be expected despite seal water isolation. Isolating dampers are rarely leak-tight. Alternatively, the following measures can be taken:

• Isolation by cutting-off of connecting lines.



• Acid cleaning of downstream systems.

An important prerequisite to successful chemical cleaning is the treatment of upstream parts (pre-boiler system), such as auxiliary steam system, condenser, de-ionised water storage tanks, feed water tanks, and LP feed heating trains unless included in the operational cleaning cycle.

• Where the open-circuit method is applied, all welded joints of the temporary acid dosing line shall be fully radiographed.

• In the case of large steam boiler plant cleaning, the sampling points for analytical control of the acid cleaning solution should be arranged horizontally. The sampling lines should be provided with ball valves and the connected hose lines be safeguarded with clamps.

• The handling of concentrated acids requires careful attention. Medical care shall be ensured. Eye washers and safety showers shall be available, and e.g. where cleaning is performed with hydrofluoric acid, calcium gluconate solutions shall be provided.

• The availability of elevators and the provision of functionally fit communication equipment are important.

• When first filling and venting the system to be treated, the emergency shut-off specifically defined for chemical cleaning, of the feed pump shall be tested at the latest. The plant shall be checked for leak-tightness. Non-drainable components (e.g. pendant superheaters) shall be completely filled.

4.2 ACID CLEANING

In most cases, the acid mixture to be determined by laboratory studies is a mixture of hydrochloric acid and hydrofluoric acid. This solution should, however, be renewed at a total iron content of approx. 15g/. Practice has shown that at higher iron contents the inhibitor protective function is not guaranteed anymore and thus an incalculable risk of corrosion attack up to plant damage exists. The first use of hydrochloric acid and, in a second stage, of hydrofluoric acid, may also become necessary. Hydrochloric acid should not be used for the operational cleaning of austenitic materials as there will be the risks of chloride-induced stress corrosion cracking and crevice corrosion. Copper removal methods may be used individually or in combination: • copper removal prior to chemical cleaning,

• copper removal during chemical cleaning,

• copper removal upon chemical cleaning,

• copper removal along with passivation.

For these methods oxidising agents (e.g. hydrogen peroxide, atmospheric oxygen, bromates) and, where required, additional complexing agents (EDTA, NTA, EDA, thioamines) will be used.



4.3 PROCEDURE

Principally, distinction is made between the open-circuit and the closed circuit (circulation) method.

4.3.1 OPEN CIRCUIT (OC)

In the open-circuit method the chemicals are injected into the water circuit by means of an injection pump at the inlet of the plant part to be cleaned and after a single passage are discharged, often at several intermediate discharge points. The feed water pressure shall be reduced at the pressure control system (usually to < 25 bar). The Feed water pump discharge gate valves often not designed for this purpose shall be equipped with temporary throttling elements. The treatment to the open-circuit method comprises several procedural steps as follows:

• Pre-flushing, where required. • Pre-treatment of the system to be cleaned by means of chemicals, e.g. of H2O2, wetting agents or

ammonia. • Displacement of chemicals, where required. • Heating-up. • Treatment with inhibited acid

o dynamic phase o static phase

• Removal of acid. • Flushing to low conductivity. • Alkaline treatment with ammonia and passivation with hydrogen peroxide. • Removal of passivation solution with treated DI water (ammonia pH value approx. 10), where

required.



4.3.2 CLOSED CIRCUIT (CC)

According to their design, natural-circulation boilers can only be treated to the circulation method. Circulation is effected with acid-resisting auxiliary pumps, often supported by the injection of nitrogen to achieve internal circulation (auto-circulation method) in the evaporator. The drains usually installed will not suffice for the return of sufficient volume flows from the bottom evaporator headers to the acid cleaning tank and an as rapid as possible draining of the evaporator. Therefore, the existing inspection nipples should also be used for draining. The connected temporary pipework should be attached to ensure safe cleaning operation. A typical acid cleaning cycle of a natural circulation boiler is shown hereafter:

The treatment to the circulation method comprises several procedural steps as follows: Depending on the measurement for copper the copper removal has to be fixed either/or or even combined.

Removal of copper (optional) • Pre-flushing, where required • Heating-up • Treatment with inhibited acid/mixture of acids

Removal of copper during chemical cleaning • Displacement of acid • Flushing to low conductivity • Removal of initial rust, if any

Removal of copper, if any • Alkaline treatment with ammonia and, where required, passivation with hydrogen peroxide and

Removal of copper if any



4.4 PROCEDURAL STEPS

4.4.1 HEATING UP

Via a temporary pipe system it must be ensured that the whole system to be cleaned can be circulated during the heating process.

4.4.2 ACID TREATMENT WHEN CLEANING IN OPEN CIRCUIT METHOD

The process step "acid treatment" comprises a dynamic and a static phase. When cleaning the system to the open-circuit method, samples shall be taken at each sampling point until measurements show an iron content ≤ 2g/ and an outlet acid concentration more or less equal to the inlet concentration. The time required to perform the open-circuit method depends on the reaction rate of the acid used. A steady phase of 2 to 3 hours shall follow the dynamic phase to remove the remainder of residual deposits.

Open-circuit method

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4.4.3 ACID TREATMENT WHEN CLEANING IN CLOSED CIRCUIT (CIRCULATION) METHOD

When applying the circulation method for acid cleaning, the required temperature shall be obtained. Heating-up may start prior to dosing chemicals by injection of steam; however, upon chemicals dosing this is only possible to a limited extent. The limit values specified by the inhibitor supplier shall definitely be observed. Heating-up through boiler firing system of the plant filled with acid is to be rejected. During heating-up, in-situ temperature measurements shall be made to control whether all tubes are flown through. If the required acid concentration has been obtained, auxiliary pumps shall maintain circulation until a constant iron content with free acid has been obtained. The majority of acid cleaning procedures require auto-circulation for the evaporator to maintain the flow in the evaporator tubes.

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Iron



4.4.4 FLUSHING TO LOW CONDUCTIVITY

Flushing to low conductivity at flow rates > 0.5 m/s shall follow the displacement of acid. For the removal of acid and suspended contaminants approx. 3 to 4 plant volumes of DI water are required. Flushing shall be finished if a conductivity of less than 20 µS/cm has been obtained at all measuring points. Care should be taken to ensure that all auxiliary lines, water level indicators, blowdown lines, injection lines, drains and measuring lines are flushed (to avoid back diffusion of acid solution as well as deposits).

4.4.5 PASSIVATION

Alkaline pH adjustment of the system fill water shall then be made with ammonia to obtain a pH value of ≥ 10. Where passivation is performed, a 30 to 35 % hydrogen peroxide solution shall additionally be injected shortly after having started the alkalisation. In the system a concentration of 0.1 to 0.3 % by volume shall be obtained. The temperature shall not exceed 40 °C. When applying the circulation method the clean surface formed in the acid phase will corrode again depending on the process and duration of flushing. Citric acid may be used for dissolving initial rust by complexing the iron. By the alkalisation with ammonia and addition of hydrogen peroxide a thin oxide layer will be formed to prevent further corrosion of the plant for approx. 4 weeks in dependence of the degree of dryness of the system. Plants to be started up within a few days need not be passivated. Here, alkaline flushing with ammonia will generally suffice. Where citric acid is used, the high COD content of the solvent effluent should be taken into account.

4.4.6 COPPER REMOVAL

In order to be able to perform chemical cleaning in plants showing copper-containing deposits, extensive examinations and dissolution tests shall be performed on tube samples with deposits. Only by this way can the chemicals to be used and the most effective procedure be determined.

The following cleaning procedures have proved to be effective in practice:

• Copper removal in alkaline solutions

This copper removal method is to be performed prior to and upon operational acid cleaning. Copper will be removed at pH values of approx. 10.0. Amines or bromates are used as complexing agents.

• Copper removal with EDTA

At pH values of approx. 5.5, EDTA may also be used for chemical cleaning. When the absorption of iron (from the magnetite layer) is completed, the pH value of the cleaning solution shall be increased to approx. 10.0 upon which copper will be removed.



• Copper removal in acid solutions During chemical cleaning thiocarbamide will be dosed at the end of the cleaning process in order to remove the copper film from the deposits and to complex it in the solution. The Fe content in the cleaning solution will again be increased upon addition of thiocarbamide and should be constant after approx. 2 hours.

• Copper removal upon passivation

Upon completion of the methods described above and also upon chemical cleaning without use of copper removal methods, but with slight amounts of copper remaining in the layers, a slight copper film will remain on the metal surfaces. This copper film may then be removed during passivation in the alkaline range (ammonia) by increased addition of H2O2.

4.5 INHIBITORS

To protect materials against acid attacks, corrosion inhibitors are used depending on type of inhibitor and acid used. The inhibitors shall prevent the acid from attacking the metal without preventing the removal of deposits. The maximum allowable operating temperature fixed for each inhibitor shall not be exceeded since otherwise the inhibitor will decompose and become ineffective. Therefore, heating up of steam boiler plants filled with acid cleaning solution by means of the plant firing system shall be refused.

4.6 MONITORING

4.6.1 PARAMETERS TO BE MONITORED

The success of pre-operational/operational chemical cleaning largely depends on the careful execution and monitoring of the process. The parameters to be monitored are:

Parameter Applicable measurement procedure Turbidity Extinction in 5 cm layer at a wavelength of 435/440 nm. Iron Determination of soluble iron e.g. volumetrically (titration) or photo-

metrically. Copper Determination of soluble copper e.g. volumetrically (titration) or photo-

metrically. Acid concentration Volumetrically (titration) or in the case of hydrofluoric acid, determination

via conductivity, but always in connection with titration. Temperature Contact thermometer and thermo-well. Conductivity Conductometre (acid-resisting electrodes). pH value Glass electrode or indicator paper. Redox potential Platinum electrode against reference system (silver/silver chloride). Inhibitor test Upon application of acid solution, steel wool shall not show any hydrogen

development and shall not rise.



Schematic for monitoring of operational cleaning with e.g. hydrofluoric acid and hydrochloride acid:

Procedural steps Condition Parameter Guide value Open-circuit method Circulation

method

Functional check

Operating supplies Plant components

Available Leak tightness

Pump shut-off device shall trip if pressure is exceeded

Acid treatment

Flow rate < 0.5 m/s Temperature 50 - 80 °C

Iron < 2g/ Constant

(< 15 g/) Acid 1 – 10 % 1 – 10 % Corrosive attack on metal (Steel wool test)

Inhibitor test ok Inhibitor test ok

Metal loss Weight loss

max. 40 g/m2 per treatment

max. 40 g/m2 per treatment

Flushing to low conductivity

Flow rate > 0.5 m/s

Conductivity difference

< 10 µS/cm < 20 µS/cm

Removal of initial rust

Temperature < 40 °C

Iron < 1 g/ Citric acid > 0.5 %

No visible sign of flocculation during alkalisation

Alkalisation pH value > 10 > 9.5

Copper removal

Copper Redox potential H2O2

positive verifiable

constant positive verifiable

Passivation Platinum electrode against silver/silver chloride or Indicator sticks

Redox potential Oxidation means (preferably H2O2)

positive verifiable

positive verifiable

Post-flushing pH value > 10 > 10

Required Optional

4.6.2 CHECK FOR SUCCESSFUL CLEANING

• The surfaces obtained upon pre-operational or operational cleaning should be free from deposits. Where passivation is performed, initial rust as thin layer or individual agglomerates may be found on the surface upon draining of the passivation solution. This is due to the influence of the humid or wet fluid on the relatively sensitive passivation layer of carbon steels. What is important is that the passivation layer under the rust is integer and that the visible thin dust-like "rust layer" can be easily wiped off (e.g. by means of a handkerchief). This does not mean a loss of quality and therefore is permitted.



• On completion of copper dissolution treatments the surfaces shall be clean and free from copper

(visual examination). • A QA plan should be agreed between all parties involved prior to performing the cleaning process. • Visual examinations shall be performed on visually inspectable plant components e.g. drum,

headers, connections for temporary pipework) and on tube specimens. Residual deposits may be determined analytically on tube specimens.

Parameter Measurement procedure Metal loss Determination of weight loss on the materials used in the system circuit

Residual deposit Gravimetric or chemical test procedure in consideration of acid attack

The procedure shall not cause damage on the component subjected to cleaning and adjacent plant parts. It shall be verified that the materials used in the plant circuit are not endangered by the chemical treatment and the process applied. The corrosive attack on metals shall be determined by the weight loss method. Upon operational chemical cleaning the treated surfaces shall be free from deposits and other contaminants. Residual layers, if any, shall be guaranteed to be less than 20 g/m2. As a rule, thin and easy-to-dissolve layers are to be removed during pre-operational cleaning so that the risk of a great extent of residual layers remaining in the system is relatively low. Since, on the other hand, welding work is required to determine the amount of residual layers, which may cause danger on the previously inspected pressurised body, the determination of residual layers can be renounced during pre-operational cleaning. The weight loss upon repeated acid treatments shall be determined excluding losses due to corrosive attack. The allowable corrosive attack shall be determined using specimens/tube specimens to be installed externally to the circuit. In addition, further specimens may be taken from the pressurised body; however, this will mean additional welding work on the pressurised body.

4.6.2.1Pre-operational acid cleaning The metal loss during pre-operational chemical cleaning depends on the chemicals used. The following maximum values can be adhered to as experience has shown.

Cleaning agent (inhibited) Metal loss Hydrofluoric acid ≤ 20 g/m2 EDTA ≤ 40 g/m2 NTA ≤ 40 g/m2 Hydrochloric acid ≤ 20 g/m2 Citric acid ≤ 20 g/m2

4.6.2.2Operational chemical cleaning Metal loss due to operational chemical cleaning depends on the composition of the deposits and the acid used. As experience has shown, the values can be higher than those obtained by pre-operational cleaning.



4.7 EFFLUENT (AMOUNT AND TREATMENT)

Depending on the procedure and plant design up to 14 filling volumes are required. For the collection of chemical solvents the storage tanks and collecting basins, temporary tanks and foil-lined excavated basins or the cooling tower basin or pond may be used. To blend the solution, the injection equipment (lances, perforated hoses) for pressurised air shall be used. Where hydrofluoric/hydrochloric acid has been used for chemical cleaning, the acid, for the purpose of waste water treatment, shall be neutralised by the addition of lime (calcium hydroxide) as well as fluoride and metal hydroxides be precipitated. In addition, the COD value may be reduced by adding pulverised activated carbon. Upon analysis and release of the effluent by the competent body or authority, the waste water may be pumped off. The remaining thin slurry, upon desiccation, e.g. in a chamber filter press, shall be disposed off at a dumping ground in accordance with the relevant dumping plan. Where the solution is discharged into the public sewage system, the national local discharge conditions shall be observed.



5. STEAM BLOWING OF STEAM BOILER PLANTS AND ASSOCIATED PIPEWORK

5.1 INTRODUCTION The purpose of each steam blowing is to protect the downstream steam turbine. This process requires that the pertinent disturbance factors K are obtained and mechanical steam purity is proved by means of target plates. Other processes where disturbance factors are not adhered to and mechanical steam purity is not verified (by means of target plates), such as blowing-off of steam via start-up lines and other operational systems, etc. are not considered steam blowing operations for the purpose of VGB R513 guideline.

5.2 PREPARATORY TECHNICAL WORK FOR STEAM BLOWING Instrumentation and control devices as well as internals such as strainer inserts, orifices, steam traps and water separators, valves, check valves, or the like which may impair the blowing through of debris and solid matter, shall also be checked and be dismounted, where required, prior to steam blowing. For drum type boilers, the separators may remain in the drum during steam blowing.

5.3 SAFETY ASPECTS DURING STEAM BLOWING Steam blowing of steam generators means working under increased safety risks and high noise nuisance. It shall also be considered that the plant has not yet proved its effectiveness, space is restricted (due to erection scaffoldings etc.), and a high number of personnel will be employed to perform numerous working activities within restricted time schedules. To ensure safe steam blowing operations, the following safety aspects should be

additionally: • steam blowing operations shall be organised and performed by expert personnel. • prior to performing steam blowing operations, a written program and check lists shall be

established and be distributed to the parties involved. • strength analyses shall be made for temporary pipework and equipment. • the erector’s and plant user’s personnel as well as authorities and the public shall be informed

in writing. • during steam blowing operations the steam generator and turbine house shall be evacuated

and be cordoned off by a sufficient number of instructed expert and control personnel. • between the control personnel and the control room permanent and reliable connections (by

radio or cable) should be established. • at suitable locations danger signs shall be installed in sufficient number. • the steam blow exhaust jet should expand safely into a cordoned-off area and not hit any

objects such as buildings, equipment etc. • as a rule, temporary pipework will not be thermally insulated, or where required, will only be

provided with personnel protection insulation. • the area of non-insulated temporary pipework shall be specifically identified (by flutter band,

chains, etc. and signs) so that personnel cannot be injured during the cooling phase of the pipework.

• combustible or other dangerous or sensible materials shall not be stored in the area of temporary pipework and equipment during steam blowing.

• personnel present in the area of increased noise level shall wear suitable ear protection equipment.

Water quality during steam blowing The water fed to the water-steam system during steam blowing should have the same make-up water quality as that used later during operation. Alkalisation of the feed water shall only be performed with steam-volatile agents such as ammonium, hydrazine or amines, etc. (pH value in feed water > 9.5, preferred value 9.8). Due to risk of water droplet carry-over solid conditioning agents such as sodium hydroxide, trisodium phosphate, shall not be used.

Lecture of operational cleaning and steam blow of large Thermal Power Plant boilers based on Kostolac B2 experience 22

5.4 STEAM BLOWING PROCESSES

In general there are two different steam blow methods available on the market Pressure built up method (huff and puff method), • Sliding pressure method. Prior to steam blowing from cold condition the entire piping system which must have been completely drained before it should be warmed up with the allowable temperature gradients. This should especially be considered for thick-walled components (headers, valves and the like). Condensate, if any, shall be completely discharged. At the beginning of steam blowing, the first steam blowing operation (initial blowing step) should not be made at maximum blowing velocity so that problems, if any (restraint to expansion, etc.) in the steam blowing system can be detected and removed.

5.4.1 PRESSURE BUILT UP METHOD

In the past, steam blowing at pressure built up method has preferably been performed especially in natural and assisted-circulation boilers. When applying this process, the steam generator will be started-up and operated to meet the boiler manufacturer’s instructions. The steam blowing pressure shall be specified by the boiler manufacturer and, as a rule, should amount to 40 – 60% of the operating pressure. In this case, steam blowing in accordance to the pressure built up method, can be effected with the firing system being shut down or operating. Water shall be fed such that at the end of a steam blow the water level is maintained at visible level in the drum or separating vessel, respectively. The allowable pressure and temperature gradients should not be exceeded. Proper steam blowing results can only be expected if the process and the steam parameters correspond to the statements made before. Steam blowing in pressure built up will subject thick-walled components (drum, headers, etc.) to higher loadings. Care shall be taken to ensure that the pressure and temperature gradients as designed are not exceeded. Steam blowing at pressure built up is not recommended for combined cycle plants since the gas turbine then will be subject to high start-up and shutdown cycles.

5.4.2 SLIDING PRESSURE METHOD WITHOUT PRESSURE ACCUMULATION

In principle, the steam generating plant shall be operated while by-passing the turbine and blowing the steam exhaust into the atmosphere or into the condenser. The average operational values range between 5 to 25 bar steam pressure and 380 to 510 °C steam temperature at 20 to 40 % load. The main steam stop valves shall be fully open during this operation.



The steam blowing operation described lasts approximately two to five hours, including start-up and shutdown of the steam generating plant. Normally not more than two steam blowing operations should be effected daily to ensure adequate cooling of the system. Steam blowing with fully flown-through reheater shall also start during continuous start-up from cold condition to a given load. The HP steam temperature shall be adjusted by means of the attemperators to a temperature which shall be adapted to meet the strength requirements for the cold-leg reheat line. The HP turbine shall be by-passed by means of temporary pipework. Upon approx. 10 to 20 minutes of effective steam blowing the plant shall be shut down to outage without interruption so that it can cool down. With the plant having cooled down steam blowing shall be repeated.

5.4.3 Combination of pressure built up and sliding pressure method Where the required disturbance factor cannot be obtained with the firing rate available at the time of steam blowing, the steam blow flow rate may be increased by utilising the accumulation capacity of the steam generator. For this combined steam blowing process the boiler shall be started up as cold start requirement. Upon start-up, the steam pressure shall slowly be raised during steam blowing by throttling via the valves installed in the steam blow circuit and subsequently be reduced to the initial pressure by opening these valves in due consideration of the maximum allowable pressure and temperature gradients. Steam blowing operation shall be performed 2 to 3 times to cover section by section. Then the steam generator shall be shut down and be cooled. To attain adequate cooling rates the aforementioned steam blowing operation should be performed only once or two times a day.



5.5 THE CLEANING EFFECT

The cleaning result of steam blowing is based on two effects.

Effect 1: Discharge of loose particles such as rust, scale, sand and, to a certain extent, also large foreign matter. To this end, high steam velocities are required. Effect 2: Thermal-shock induced exfoliation of adhesive deposits. Due to the cooling down of the system temperature differentials of up to 350 K will be achieved which additionally support the thermal-shock induced exfoliation of adhesive iron oxides, since the metal and the iron oxide show different strain behaviour. To achieve sufficient thermal-shock on components (e.g. headers and fittings), the metal temperature of these components should be reduced to approx. 150 °C (100 °C VGB recommendation). Experience has also shown that, at steam blowing temperatures < 300 °C, thermal-shock induced exfoliation has little effect.

5.6 CLEANING FORCE RATIO (CFR) As experience has shown, steam blowing will only be effective if it is done at higher steam velocities than those prevailing during full-load operation. In most cases, the condition of the steam generating plant will not permit, at the time of steam blowing, to establish the required design steam parameters. The steam velocities in the steam generator and associated pipework can be calculated from the mass to volume flow ratio. The mass to volume flow ratio is called the disturbance factor K. Experience gained over many years has shown that the optimum measuring location for determining the disturbance factor is at the steam generator outlet (final superheater outlet/beginning of steam line). At this location, disturbance factors of K = 1.2 to 1.7 shall be obtained. In the case of a steam range comprising several steam generators, each steam generator shall be blown through individually, then the steam range and finally the respective branch lines to the steam turbine. The location where the disturbance factor in the steam range is to be determined should be agreed between all parties involved at planning stage. The K factor is derived as follows:

;V

2V

B2

B

vmvm =K ⋅⋅

the following should be obtained: K = 1.2 to 1.7

where: mB = mass flow under steam blow conditions, [kg/s] mV = mass flow under full-load operating condition, [kg/s]

vB = specific volume of steam during steam blowing, [m3/kg] vV = specific volume of steam during full-load operation, [m3/kg]

The steam pressure during steam blowing should be selected in dependence of the steam blowing process, but shall not exceed the maximum allowable working pressure in the other individual sections. To avoid erosion damage caused by wet steam, the steam should show minimum superheating of 15 K during steam blowing, but shall not exceed the maximum operating temperature.



5.7 TARGET PLATE

5.7.1 DIMENSIONING AND SURFACE QUALITY

The result of steam blowing shall be checked by means of a target plate to consist of a holding fixture to which a mirror-finished impingement plate of carbon steel with a Brinell hardness between 140 and 140 HB at room temperature, e.g. S235JRG2 (St 37(3)) is attached. Should other materials than S235JRG2 (St 37) be used for the target plate, specific agreements shall be made regarding the assessment of steam purity (impact of debris). Example for target plate arrangement:

Plate 6 mm thickwith mirror-finish surface on both sides

Flow direction

Steam blowing conditions

Section A - A

A A

40 mm

0.85

x

d

d

d = diameter of the respective steam line to be blown through

5.7.2 LOCATION OF TARGET PLATE INSTALLATION

When selecting the location of installation, the inlet conditions should be chosen such that an as uniform as possible impact velocity on the target plate is ensured. Here, the specific local conditions shall be taken into account. The installation of target plates directly downstream of pipe bends shall be avoided. Not less than 5 times the nominal width of the line in which the target plate is installed is considered the guide value for an undisturbed inlet section upstream of the target plate. Principally two locations are possible: • installation of target plate in the inlet section of temporary pipework

• installation of target plate in the outlet section of the steam line.



5.7.3 STEAM BLOWING RESULTS, CONTROL, EVALUATION AND RECORDS

Proper steam blowing results can only be expected if the process and the steam parameters correspond to the statements made before. Apart from the persons and companies involved, all relevant data, such as: • disturbance factor, • steam pressure and temperature, • firing rate, • steam mass flow rate, • pressure reduction and number of bursts per steam blowing operation • cooling time and metal temperature of thick-walled components after the cooling phase prior to

repeated steam blowing,

shall be entered in tables and be ordered numerically and recorded with the number of steam blowing operation. Foreign matter entrained by the steam will impinge on the target plate surface and leave impacts. The number, size and edge shape of the impacts will be used to evaluate the results of steam blowing. Steam blowing can be completed if the blow criteria have been satisfied. The relative evaluation shall only be understood as auxiliary means to evaluate the result of steam blowing.

5.8 ABSOLUTE NUMBER OF IMPACT SIZE PER UNIT AREA For the purpose of evaluating the impacts, the total steam-blown target plate surface shall be assessed, and then a reference area, i.e. a square area of 40 x 40 mm, shall be selected to evaluate the steam blowing process within the area of the largest/most frequent impacts. At no point of the target plate reference area shall impacts be found with the following extension and number: • no impact with a size > 1 mm • less than 4 impacts with a size > 0.5 mm • less than 10 impacts with a size > 0.2 mm. This evaluation shall only apply to carbon steels, e.g. S235JR (St 37). For other materials specific agreements shall be made. In the case of new plants, the evaluation and acceptance of the steam blowing result should be made by representatives of the plant user and the manufacturers of the steam generator, pipework and steam turbine. For old plants special agreements shall be made. The positive completion of steam blowing operations shall be entered in a record to be signed by the plant user and the manufacturers of the steam generator, pipework and steam turbine. By signing the record the parties involved recognise the proper completion and positive results of steam blowing. The following should be added to the record: • the absolute number and size of impacts per unit area, ordered in the sequence of steam

blowing operations performed. • operational records made during steam blowing. • The target plates shall be continuously and durably marked (e.g. by chronological stamp

markings) with the number of the pertinent steam blow operation.



5.9 RELATIVE NUMBER OF IMPACTS

With each additional steam blowing operation (performed with same steam blowing parameters) the number of impacts on the target plate should be clearly reduced. For clarification the relative decrease of impact frequency may be illustrated by a diagram as follows:

5.10 CONTROL OF CLEANLINESS PRIOR TO AND AFTER STEAM BLOWING

Prior to steam blowing normally no tube samples need be taken. The result of steam blowing is evaluated by means of the target plates. Upon steam blowing the pipes shall be checked at the points of connection of the removed temporary pipework. Further internal inspections, where required, should depend on the results obtained. Where dirt traps are installed in the steam lines, they shall be controlled and be cleaned, where necessary. Main steam line runs forming dead pockets/legs (e.g. headers or tees) or which have not been flown through during steam blowing, should be checked for cleanliness when dismounting the temporary pipework. It is recommended to cut the caps of sand-blasted headers and to check the headers for freedom from blasting media.

5.11 NOISE REDUCTION MEASURES

Noise reduction during steam blowing of steam generating plants is gaining increased importance. The required noise reduction measures shall be agreed in time between manufacturer and plant user in which case the valid regulations shall be observed. Essential noise reduction measures are: • silencers • condenser • injection of water • discharging into waters.

No. of blowing operations [-]

No.

of r

elat

ive

impa

cts

[%]

100

20

0

AA A

A

A

A A

A =: Steam blowing operations with equal steam blow parameters

B =:Steam blowing operations with reduced steam blow parameters

B

Steam blow diagram for relative impact frequency as a function of the number of steam blowing operations



5.11.1 SILENCERS

During steam blowing the noise may be reduced by a silencer installed at the outlet of the temporary blow-through pipework.

Condenser Since here by-pass operation is concerned and this is a quasi-operating condition, the noise emitted to the neighbourhood is within routine plant operation so that no further noise reduction measures are required.

5.11.2 SILENCING THROUGH WATER INJECTION

This process means the injection of water at a suitable location which leads to a reduction of the exit velocity and of noise development. In industry, this process is known under several trademarks (e.g. silent steam blowing, dBmin or low-noise blowing). In sliding-pressure steam blowing operation the noise can be reduced by this method instead of using a conventional silencer. In this process the velocity of the steam exhausted into the atmosphere via the temporary steam blow piping is reduced by spray attemperation such that the use of an additional silencer is not required. Normally, sound pressure levels of less than 90 dBA at a distance of 10 metres are obtained. Among others, this process has the following characteristic features: • low steam-blow exhaust pressure and extremely reduced steam temperature in the temporary

pipework downstream of the point of injection. • low steam-blow exhaust pressure ensures a major cleaning effect and a high disturbance

factor K. • extreme reduction of sound pressure level (< 90 dBA at a distance of 10 m) • little reaction forces in the temporary pipework downstream of the point of injection. • higher water consumption. • large nominal size of temporary pipework.

Sample sketch for steam blowing equipment as used for Kostolac B2



6. COMBINATION OF PRE-OPERATIONAL CLEANING AND STEAM BLOWING 6.1 GENERAL

In practice it will happen that regarding internal cleaning prior to first putting into operation, the plant is to be subjected to pre-operational cleaning and subsequent steam blowing. Here, it is important to consider the type of cleaning procedure with which the various plant sections can be cleaned. Pre-operational cleaning Pre-operational cleaning can be applied to the total steam-water circuit and, when correctly planned and performed (see Chapter 6 "Pre-operational Cleaning), in connection with subsequent by-pass operation will also be suited as single cleaning procedure for the plant. Steam blowing Steam blowing can only be applied to the steam section of a plant. When correctly planned and performed (see chapter 7 "Steam Blowing") steam blowing may be used as single cleaning procedure for the steam section.

6.2 COMBINATION OF CHEMICAL CLEANING AND STEAM BLOWING Where a combination of the two cleaning methods is performed, several items should be taken into account to avoid unnecessary loss of time and thus related costs. Weak points of the individual methods may be identified by a general analysis of the system and be compensated.

6.2.1 WEAK POINTS OF THE INDIVIDUAL METHODS

In the following the weak points will be analysed individually and be divided into two main groups where planning is considered first since the planning stage provides the greatest optimisation potential. • Planning

As a rule, the two cleaning procedures are planned and performed by people from differing areas of responsibility of the plant user and the manufacturer. Due to this fact planning is often done twice and the installation of temporary pipework is envisaged separately by the two parties, which leads to unnecessary costs and especially will be time-consuming with respect to keeping the times scheduled for putting into operation.

• Cleaning procedures Steam blowing o can only be applied to the steam section. o for the water section an additional cleaning procedure is required. o the period of time for steam blowing directly depends on the plant fouling condition and

therefore cannot be predicted exactly. o the necessary temperature cycles are time-consuming and will lead to additional loading of

material. o is always time-consuming with respect to keeping scheduled commissioning times.



Pre-operational cleaning (1) as a rule, the water velocities obtainable during flushing will be limited. For this reason,

discharge forces are often relatively low so that there is great risk of insoluble residual matter remaining in the system.

6.2.2 ADVANTAGES OF COMBINING PRE-OPERATIONAL CLEANING AND STEAM BLOWING

Where the two procedures are combined, the process step "thermal-shock induced exfoliation of deposits" required during steam blowing can be waived since such deposits will have been completely removed before by the preceding pre-operational cleaning method. Therefore if the plant steam section has already been subjected to pre-operational cleaning, the second step is unnecessary.

6.2.3 CONCLUSIONS FROM THE COMBINATION OF PRE-OPERATIONAL CLEANING AND STEAM BLOWING

Combining pre-operational cleaning and steam blowing following could be schieved:

Advantages • maximum obtainable cleaning result and consequently a minimum risk of damage. • more effective keeping to scheduled set of dates is possible. Disadvantages • higher costs since more temporary equipment, especially for the high-temperature range

> 450 °C. • under certain circumstances, require additional project lead times.

7. METHOD APPLIED AT TPP KOSTOLAC B2

As presented above, the most conveninat method of combined pre-operational chemical cleaning and steam blowing had been applied at boiler and main pipelines preparation for operation. Combining pre-operational chemical cleaning and steam blowing by using the same temporary connections between RA-RC pipelines and release from RB outlet an optimal structure of temporaqry piping has been obtained. By avoinding installation of separate temporary piping for each of cleaning steps an economi of time along the criticval pat had been obtained. As addition a cost saving for temporary piping installation has been achieved.

7.1 CHEMICAL CLEANING

7.1.1 REMOVED COPPER / IRON OXIDE

The table below shows the amount of dissolved and removed iron oxide during the chemical cleaning of Kostolac B2.

DISSOLVED IRON [kg/m³]

VOLUME [m³]

TOTAL OXIDE [Fe3O4] kg

HP, IP/RH cleaning 7.3 770 23,300(1)

The table below shows the amount of dissolved and removed copper during the cleaning of Kostolac B2.

DISSOLVED COPPER [kg/m³]

VOLUME [m³]

TOTAL OXIDE [CU] kg

HP, IP/RH cleaning 0.303 770 233(1) (1) The given value is a calculated value resulting of the measured dissolved iron and copper value. To this value the amount of iron and copper removed during

flushing process has to be added. As this cannot be clearly evaluated it will be only mentioned in this foot note.



7.1.2 SAMPLE TUBES / PIECES

The table below show the results of the measured base metal removal rate of the sample tubes/pieces. The results indicate the good protection due to the inhibitor during the cleaning. Material loss

Da

ring surfaceDi

L

Material Loss I

Do Di L Ai Ao AR Atot W1 W2 ∆m mML

[mm] [mm] [mm] [mm²] [mm²] [mm²] [mm²] [g] [g] [g] [g/m²]

Pregrejac 1; 2.5 16 Mo3 46,800 41,700 52,80 6917 7763 709 15389 148,0593 147,9582 0,101 6,6Pregrejac 1; 2.6.3 13CrMo45 44,500 39,600 48,00 5972 6710 647 13329 121,2392 121,1519 0,087 6,5Pregrejac 3; 2.8.1 X20CrMoV121 36,300 31,900 65,00 6514 7413 471 14398 119,3452 119,2495 0,096 6,6Medju-Pregrejac 2; 2.9 10CrMo910 59,100 55,400 40,10 6979 7445 665 15090 105,1202 105,0189 0,101 6,7

Do outer diameter Ao outer surface W1 weight before acid testDi inner diameter Ai inner surface W2 weight after acid testL total length AR ring surface ∆m metal weight loss

Atot total surface mML specific metal loss

Sample Material

Table: During acid cleaning removed base material of the Test coupons (limit as per VGB guideline R 513: 20 g/m²).

7.1.3 EXAMINATION OF CHEMICAL CLEANING RESULT

These pictures of tubes cut in half show the successful chemical cleaning process of Kostlac B2.



7.1.4 TREATMENT OF THE WASTE

The waste water stored in a mobile temporary tank (900 m³) was neutralised by use of lime together with demin water to cover the increase of temperature due to thermal reaction. Therefore two trucks with lime were connected (one after another) to the disposal line and during draining of the acid solution the lime was injected. After the lime dosing additional demin water was dosed (approx. 100 t/h) using the existing discharge line coming from the boiler. Before the inlet into the concrete ash and bottom ash basin, the pH was measured. According to the result the flow of acid was than adjusted.

Injection of lime during acid neutralisation Mobile waste water tank 900 m³



7.2 STEAM BLOW

7.2.1 DPR-FACTOR (DISTURBANCE FACTOR)

The K-Factors during steam blow of the different parts of the system were equal or higher than 1.2.

Steam -blow of HP system (RA)

steam flowG in [kg/s] G in [t/h]

pressurep in [bar]

temperature

spez. volumev in [m³/kg] G² x v CFR

100% load 277,78 1000,00 182,4 543 0,018167 1401,8 in out1 03.12.2012 77,0 277,2 12,0 465 0,280773 1665 1,2 18:20 18:402 77,0 277,2 12,0 444 0,272419 1615 1,2 18:40 19:003 77,0 277,2 12,0 460 0,278787 1653 1,2 19:00 19:204 0,05 04.12.2012 0,06 74,8 269,4 11,0 457 0,303126 1697 1,27 74,8 269,3 11,0 454 0,301828 1689 1,28 74,8 269,4 11,0 456 0,302693 1695 1,29 0,010 0,0

the cleaning force ration (CFR) is calculated according following formula

Stea

m b

low

Kos

tola

c B

2 R

A

Target

1002

100

2

vGvG

CFR aa

••

=

Steam -blow of HP system (RB)

steam flowG in [kg/s] G in [t/h]

pressurep in [bar]

temperature

spez. volumev in [m³/kg] G² x v CFR

100% load 254,00 914,40 43,8 540 0,083312 5375,0 in out1 80,0 288,0 450,0 3 1,113868 7129 1,32 80,5 289,8 450,0 3 1,113868 7218 1,33 80,0 288,0 458,0 3 1,126278 7208 1,34 0,05 04.12.2012 0,06 77,6 279,4 445,0 3 1,106110 6661 1,2 11:50 12:107 77,6 279,4 445,0 3 1,106110 6661 1,2 12:10 12:308 77,6 279,2 445,0 3 1,106110 6653 1,2 12:30 12:509 0,010 0,0

the cleaning force ration (CFR) is calculated according following formula

Stea

m b

low

Kos

tola

c B

2 R

B Target

1002

100

2

vGvG

CFR aa

••

=



7.2.2 IMPACT INVESTIGATION

Figure 1: RA 10 line Figure 2: RA 20 line

Figure 3: RB13 line Figure 4: RB 23 line



Investegation was done with mobile microsope MMX 200.

Figure 5: 4 impacts 0.3, 0.1 mm Figure 6: Impact 0.8 mm

7.2.3 TARGET RESULTS

The targets have been in accordance with VGB R513 Guideline. At no point of the target plates HP and IP sites on an area of 40 mm x 40 mm impacts were found with the above mentioned extension and number and therefore steam blowing was performed successfully.

8. BIBLIOGRAPHY (1) VGB PowerTech e.V. Guideline for "Internal Cleaning of Water-Tube Steam Generating Plants and associated

Pipework" [VGB-R 513 e, 2nd edition 2006].

(2) Die Verwendung von Flußsäure bei der chemischen Reinigung von Anlagen

[Use of hydrofluoric acid for chemical cleaning of plants], by Bieler und Borchardt (Therm-Service GmbH), VGB

Kraftwerkstechnik, 1978 No. 12, pp 927 930.

(3) Studija “Mere i postupci za pouzdan i efikasan sistem kontrole korozionog stanja vodeno parnog ciklusa TE i TE-TO

EPS-a i preporuke za primenu novih tehnologija”, Beograd 2004, by Prof.-Dr. Ljubinka Rajaković.

(4) Engineering for chemical cleaning and continous steam blow for Unit 2 TPP Kostolac B Project in Serbia [Revision

3, September 2012], by Martin Herberg (Therm-Service GmbH).

(5) Reports of chemical cleaning and continous steam blow for Unit 2 TPP Kostolac B Project in Serbia [December

2012], by M. Herberg/P. Pias (Therm-Service GmbH).



Documents

Operational Cleaning and Steam Blow of Large Thermal Power Plant Boilers (Kostolac b2 Experience) (1)