Boiler Tube Leakage Analysis Of Maithan Power Limited

ACKNOWLEDGEMENT

I would like to thank all the competent authority at Maithan Power Limited, without whose help this vocational training would not ever have happened. I would like to give our special thanks to Mr. Arnab Paul, Mr. Anirban Pal and Mr. Tapas Mahato who guided me throughout this training and gave their significant contribution in the making of this project.I would like to thank the Head of OPERATION, Head of Mechanical Maintenance and the Human Resource Department for their guidance and support. They made it possible in every way for me to understand the PROCESS FLOW OF THE THERMAL POWER PLANT. They have given a lot of time from their busy schedule. I would also thank all the Senior Managers for making my learning process easy and referring to suitable person. I give my sincere thanks to all the personnel at Maithan Power Limited for all their teaching, co-operation and help. A special vote of thanks to Mr. Arnab Paul, who chose such a wonderful project for me to work upon and assisted in every possible way for its successful completion. I would also like to thank the authority of National Power Training Institute(ER), Durgapur for arranging this vocational training .

Table of ContentsTypes of Tube Failures3Boiler Chemistry9Condensate and Feedwater Parameters and Treatment Cycle12The Metallurgy of Power Boilers..14Acoustic Sound Level Detection 18Boiler Tube Leakage incidents at Maithan and their Analysis .22Report of failure on 14th December, 2011 22LTSH failure report on 4th may, 2012 24Boiler operation to minimize tube leakages .26Boiler Startup/Shutdown ..27

TYPES OF TUBE FAILURES

CAUSTIC ATTACKSYMPTOMS: Localized wall loss on the inside diameter (ID) surface of the tube, resulting in increased stress and strain in the tube wall.CAUSES: Concentration of caustic can occur as a result of steam blanketing (layer of steam between the tube ID and the boiler water), which allow salts to concentrate on metal surface (due to quick vaporisation on the metal surface at point of localized overheating thus leaving behind conc. caustic solution) which causes the dissolution of the protective magnetite layer causing loss of the base metal and eventual failure. The metal must be stressed and a least trace of silica must be present. Caustic attack can be controlled by the phosphate/pH control method. Phosphate buffers the boiler water, reducing the chance of large pH changes due to the formation of caustic.Na2HPO4 + NaOH Na3PO4 + H2OOXYGEN PITTINGSYMPTOMS: Aggressive localized corrosion and loss of tube wall, most prevalent near economizer feedwater inlet on operating boilers. Flooded or non-drainable surfaces are most susceptible during outage periods.CAUSES: Oxygen pitting occurs with the presence of excessive oxygen in boiler water. It can occur during operation as a result of in-leakage of air at pumps, or failure in operation of preboiler water treatment equipment. This also may occur during extended out-of-service periods, such as outages and storage, if proper procedures are not followed in lay-up. Non-drainable locations of boiler circuits, such as superheater loops, sagging horizontal superheater and reheater tubes, and supply lines, are especially susceptible. More generalized oxidation of tubes during idle periods is sometimes referred to as out-of-service corrosion. Wetted surfaces are subject to oxidation as the water reacts with the iron to form iron oxide. When corrosive ash is present, moisture on tube surfaces from condensation or water washing can react with elements in the ash to form acids that lead to a much more aggressive attack on metal surfaces.HYDROGEN DAMAGESYMPTOMS: Intergranular micro-cracking. Loss of ductility or embrittlement of the tube material leading to brittle catastrophic rupture.CAUSES: in high pressure boilers, contaminants due to the condenser leakages can lower the pH to a significant amount so that the acid reacts with the steels producing hydrogen. This occurs under hard, porous, adherent deposits. The hydrogen pressure at these points can build up to such high levels that the hydrogen penetrates the metal tubing. This hydrogen reacts with the carbon present in the steel to form methane. Methane being larger in size cannot penetrate the metal and soon the pressure build up gets too high, causing the metal to rupture along the grain boundaries where methane has formed. The cracking that is formed is primarily intercrystalline or intergranular and decarburization occurs at the point of rupture.

ACID ATTACK

SYMPTOMS: Corrosive attack of the internal tube metal surfaces, resulting in an irregular pitted or, in extreme cases, a Swiss cheese appearance of the tube ID.

CAUSES: Acid attack most commonly is associated with poor control of process during boiler chemical cleanings and/or inadequate post-cleaning passivation of residual acid.

STRESS CORROSION CRACKING (SCC)

SYMPTOMS: Failures from SCC are characterized by a thick wall, brittle-type crack. May be found at locations of higher external stresses, such as near attachments.

CAUSES: SCC most commonly is associated with austenitic (stainless steel) superheater materials and can lead to either transgranular or intergranular crack propagation in the tube wall. It occurs where a combination of high-tensile stresses and a corrosive fluid are present. Cold deformation, welding, heat treatment, machining and grinding can induce residual stresses. These residual stresses can approach the yield stress of the material. The damage results from cracks that propagate from the ID. The source of corrosive fluid may be carryover into the superheater from the steam drum or from contamination during boiler acid cleaning if the superheater is not properly protected.

WATERSIDE CORROSION FATIGUE

SYMPTOMS: ID initiated, wide transgranular cracks which typically occuradjacent to external attachments.

CAUSES: Tube damage occurs due to the combination of thermal fatigue and corrosion. Corrosion fatigue is influenced by boiler design, water chemistry,boiler water oxygen content and boiler operation. A combination of these effects leads to the breakdown of the protective magnetite on the ID surface of the boiler tube. The loss of this protective scale exposes tube to corrosion.The locations of attachments and external weldments, such as buckstay attachments, seal plates and scallop bars, are most susceptible. The problem is most likely to progress during boiler start-up cycles.

SUPERHEATER FIRESIDE ASH CORROSION

SYMPTOMS: External tube wall loss and increasing tube strain. Tubes commonly have a pock-marked appearance when scale and corrosion products are removed.

CAUSES: Fireside ash corrosion is a function of the ash characteristics of the fuel and boiler design. It usually is associated with coal firing, but also can occur for certain types of oil firing. Ash characteristics are considered in the boiler design when establishing the size, geometry and materialsused in the boiler. Combustion gas and metal temperatures in the convection passes are important considerations. Damage occurs when certain coal ash constituents remain in a molten state on the superheater tube surfaces. This molten ash can be highly corrosive.

WATERWALL FIRESIDE CORROSION

SYMPTOMS: External tube metal loss (wastage) leading to thinning and increasing tube strain.

CAUSES: Corrosion occurs on external surfaces of waterwall tubes when the combustion process produces a reducing atmosphere (substoichiometric). This is common in the lower furnace of process recovery boilers in the pulp and paper industry. For conventional fossil fuel boilers, corrosion in the burner zone usually is associated with coal firing. Boilers having maladjusted burners or operating with staged air zones to control combustion can be more susceptible to larger local regions possessing a reducing atmosphere, resulting in increased corrosion rates.

FIRESIDE CORROSION FATIGUE

SYMPTOMS: Tubes develop a series of cracks that initiate on the outside diameter (OD) surface and propagate into the tube wall. Since the damage develops over longer periods, tube surfaces tend to develop appearances described as elephant hide, alligator hide or craze cracking. Most commonly seen as a series of circumferential cracks. Usually found on furnace wall tubes of coal-fired once through boiler designs, but also has occurred on tubes in drum-type boilers.

CAUSES: Damage initiation and propagation result from corrosion in combination with thermal fatigue. Tube OD surfaces experience thermal fatigue stress cycles which can occur from normal shedding of slag, sootblowing or from cyclic operation of the boiler. Thermal cycling, in addition to subjecting the material to cyclic stress, can initiate cracking of the less elastic external tube scales and expose the tube base material to repeated corrosion.

SHORT-TERM OVERHEAT

SYMPTOMS: Failure results in a ductile rupture of the tube metal and is normally characterized by the classic fish mouth opening in the tube where the fracture surface is a thin edge.

CAUSES: Short-term overheat failures are most common during boiler start up. Failures result when the tube metal temperature is extremely elevated from a lack of cooling steam or water flow. A typical example is when superheater tubes have not cleared of condensation during boiler start-up, obstructing steam flow. Tube metal temperatures reach combustion gas temperatures of 1600F or greater which lead to tube failure.

LONG-TERM OVERHEAT

SYMPTOMS: The failed tube has minimal swelling and a longitudinal split that is narrow when compared to short-term overheat. Tube metal often has heavy external scale build-up and secondary cracking.

CAUSES: Long-term overheat occurs over a period of months or years. Superheater and reheat superheater tubes commonly fail after many years of service, as a result of creep. During normal operation, alloy superheater tubes will experience increasing temperature and strain over the life of the tube until the creep life is expended. Furnace water wall tubes also can fail from long-term overheat. In the case of water wall tubes, the tube temperature increases abnormally, most commonly from waterside problems such as deposits, scale or restricted flow. In the case of either superheater or water wall tubes, eventual failure is by creep rupture.

GRAPHITIZATION

SYMPTOMS: Failure is brittle with a thick edge fracture.

CAUSES: Long-term operation at relatively high metal temperatures can result in damage in carbon steels of higher carbon content, or carbon-molybdenum steel, and result in a unique degradation of the material in a manner referred to as graphitization. These materials, if exposed to excessive temperature, will experience dissolution of the iron carbide in the steel and formation of graphite nodules, resulting in a loss of strength and eventual failure.

DISSIMILAR METAL WELD (DMW) FAILURE

SYMPTOMS: Failure is preceded by little or no warning of tube degradation. Material fails at the ferritic side of the weld, along the weld fusion line. A failure tends to be catastrophic in that the entire tube will fail across the circumference of the tube section.

CAUSES: DMW describes the butt weld where an autenitic (stainless steel) material joins a ferritic alloy, such as SA213T22, material. Failures at DMW locations occur on the ferritic side of the butt weld. These failures are attributed to several factors: high stresses at the austenitic to ferritic interfacedue to differences in expansion properties of the two materials, excessive external loading stresses and thermal cycling, and creep of the ferritic material. As a consequence, failures are a function of operating temperatures and unit design.

EROSION

SYMPTOMS: Tube experiences metal loss from the OD of the tube. Damage will be oriented on the impact side of the tube. Ultimate failure results from rupture due to increasing strain as tube material erodes away.

CAUSES: Erosion of tube surfaces occurs from impingement on the external surfaces. The erosion medium can be any abrasive in the combustion gas flow stream, but most commonly is associated with impingement of fly ash or soot blowing steam. In cases where soot blower steam is the primary cause, the erosion may be accompanied by thermal fatigue.

MECHANICAL FATIGUE

SYMPTOMS: Damage most often results in an OD initiated crack. Tends to be localized to the area of high stress or constraint.

CAUSES: Fatigue is the result of cyclical stresses in the component. Distinct from thermal fatigue effects, mechanical fatigue damage is associated with externally applied stresses. Stresses may be associated with vibration due to flue gas flow or sootblowers (high-frequency low-amplitude stresses),or they may be associated with boiler cycling (low-frequency high-amplitude stress mechanism). Fatigue failure most often occurs at areas of constraint, such as tube penetrations, welds, attachments or supports.

Types of Tube Leakages

BOILER CHEMISTRYIt is very important to maintain proper feed water quality for the trouble free operation of the steam boilers and turbines. The feed water required for the High Pressure boiler parts should be demineralized to meet the stringent quality standards of the operating fluid. Without proper water quality it is not possible to obtain the optimal quality of steam for operations of superheaters and reheater tubes and turbines. Also, it may lead to deposits and corrosion in the boiler tubes and failures.In spite of the demineralization process, the tubes may become deposited over a long period of operation and proper cleaning process has to be carried out whose frequency is determined by the cleanliness of the boiler and operational requirements. The cleaning has to be carried out in such a way that the integrity of the tube is maintained. During shutdown processes, the boiler tubes have to be properly laid up, so that the tubes does not get damaged by oxidation. The lay-up is done by keeping the tubes filled with treated water and nitrogen blanketing.PARAMETERS TO BE MAINTAINEDIONIZED SALTS: The ionized salts have to be removed for the elimination of boiler tube scaling, turbine blade fouling and corrosion. The demineralization plant deals with this problem to a considerable extent by removing of the ionizing salts.DISSOLVED OXYGEN: Dissolved oxygen must be reduced to about 10-20 ppb at the economizer inlet to prevent the oxidation of the HP components. Generally, the deaerator considerably reduced the oxygen content to about 10 ppb. Hydrazine is added for further scavenging of the dissolved oxygen. Further the hydrazine forms a protective layer on the magnetite layer of the tube ID thus protecting it from erosion by fluid drag.SILICA: Demineralizers can reduce silica conc. to about 20 ppb. Silica conc. has to be maintained according to the working pressure as the amount of dissolved silica in the saturated steam increases with the operating pressure. Silica carryover to the turbine blading is abrasive and deposits are difficult to remove. Further silica can react with calcium and aluminium salts in boiler water to form hard scales which are detrimental to heat transfer properties of the tube material.During the initial commissioning of the boiler, it is likely to get high silica content. This silica has to be brought into control by proper blowdown. The pressure should not be raised until the silica conc. has been brought to the specified limits. If proper boil out operations have been carried out, the silica levels is brought down to the specified limits within some days of operations. During normal operation, the main source of silica contamination is condenser leakages and demineralizers. The specified limits for operation at specified pressures is shown below to minimize the carryover of silica to 20 ppb:

OTHER SALTS : The intent of providing phosphate in the boiler water is to provide conditions conducive to the precipitation of calcium and magnesium salts as Calcium Hydroxyapalite 3Ca3(PO4)2Ca(OH)2 and Serpenline 3MgO 2SiO2.2H2O. The addition of Phosphate is also done in a co-ordinated Phosphate-pH method as stated below to maintain the pH of the system as alkaline, absorbing any caustic which in the free form is harmful to the system causing deposition known as caustic gouging. This leads to material erosion leading to tube failures.pH : Demineralized is often called hungry water as it is highly corrosive in nature. Further a low pH means higher H+ ion conc. which is reactive with the tube materials (decarburization)leading to strength loss. Thus an alkaline pH has to be maintained to avoid this. This is generally done with the help of ammonia dosing. TSP(Tri sodium phosphate) used for sludge formation in the Drum also has basic nature and is also judiciously used for maintain an alkaline environment. The recommended phosphate levels for pH control is shown:

CONDENSATE AND FEEDWATER PARAMETERS AND TREATMENT CYCLE

STANDBY PROTECTION The boiler tubes are susceptible to atmospheric corrosion during the down time in the presence of moisture and oxygen. Thus proper laying up of the boiler becomes an absolute necessity. Keeping the high capacity boiler absolutely dry is a very difficult task owing to the complex nature of the construction. The tubes can be dried to a considerable extent by evacuating at a hot state. But the non drainable portions are not drained. Thus condensation can take place. Thus in most of the high capacity boilers, wet lay up is carried out. During the down times, the tubes are filled with demineralized water dozed with ppm of ammonia and 200 ppm of Hydrazine. This is further blanketed with a nitrogen gas environment at about 5 psi(g).BLOW DOWNBlow Down must also be conducted on regular basis (When Boiler is in Low Steaming Stage) from side wall, rear & front wall headers; so that sludge accumulation in these headers may be avoided; which otherwise would rise in furnace tubes creating conditions for circulation restrictions/ blockage, thus overheating at local spots.

THE METALLURGY OF POWER BOILERS

Steels are alloys of iron and carbon, usually with one or more alloying elements added to improve some properties of the material (strength, high-temperature strength, oxidation or corrosion resistance, for example). By definition, steels contain at least 50% iron. For welded construction, theASME Boiler and Pressure Vessel Codelimits the carbon content to less than 0.35%. Thus, virtually all of the materials used in the construction and repair of pressure parts of boilers fall into this classification. Some high-temperature, corrosion-resistant alloys of nickel and chromium with less than 50% iron are not, strictly speaking, steels, but are still occasionally used. Further, steels are divided into two subcategories: ferritic steels and austenitic steels, depending on the arrangement of atoms within the solid.NOMINAL COMPOSITION (max.%)SPECIFICATION NO.GRADE DESIGNATIONPRODUCT FORMUSEFUL TEMP.

C- 0.35, Mn - 0.29-1.06, Cr-0.40, Mo-0.15SA 106CPipe, Hot finished seamless455

Carbon steelSA 210CTube, seamless455

C-0.5, Mn-0.30-0.61, Mo-0.44-0.65,Cr- 0.80-1.25SA 335 P12Pipe, Hot rolled seamless435

C-0.05-0.15, Mn-0.30-0.61, Mo-0.44-0.65, Cr-1.90-2.6SA335P22Pipe, Hot rolled seamless580

C steel, Cr-1.25, Mo- 0.5,Mn- 0.30-0.60, Ni- 0.5-1.0SA 213T11Tube, seamless550

C steel, Cr-2.25, Mo- 1.05SA 213T22Tube, seamless580

Austenitic steel, Cr- 17, Ni-9, Columbium & Tantalum- 1SA 213TP347HTube, seamless815

C steel, Cr-9, Mo-1, V-0.25, Mn-0.3-0.6, Ni-0.25-1.0SA 213T91Tube, hot rolled seamless541

C steel, Cr-9, Mo-1SA335P91Pipe, seamless420

C steelSA 299------plates369

Steels are used in boiler construction because they are inexpensive, readily available, easily formed and welded to the desired shape and, within the broad limits, are oxidation- and corrosion-resistant enough to provide satisfactory service for many years. The table lists the used steels in the BHEL boiler at MPL, tubing specifications and the maximum recommended service temperatures.

The maximum useful temperature is determined either by corrosion or oxidation concerns that limit the useful life before premature failure or changes within the microstructure occur that weaken the steel too much for elevated-temperature service.Based on the crystal lattice, the steels can also be classified as Ferritic or Austenitic. Ferritic steels are those category of steels which have a Body Centred Cubic crystal lattice. The Austenite phase (FCC)converts to the two phases of Ferrite(BCC) and cementite(Hcp) mainly in the pearlitic phase(Alternate bands of Ferrite and Cementite). The temperature of this transformation depends upon the composition but is about 727oC(called the eutectoid temperature) for a plain-carbon steel similar to the SA178 or SA210 grades. Austenitic stainless steels are a class of alloys with a face-centered-cubic lattice structure of austenite over the whole temperature range from room temperature (and below) to the melting point. When 18% chromium and 8% nickel are added, the crystal structure of austenite remains stable over all temperatures. The nickel-based alloys with 35-70% nickel and 20-30% chromium, while not strictly steels (a steel must have at least 50% iron), do have the face-centered-cubic lattice arrangement and are also called austenitic materials.EFFECTS OF ALLOYING ELEMENTS ON STEELELEMENT&SYMBOLINFLUENCEUPON FERRITEINFLUENCE UPONAUSTENITE(HARDENENABILITY)PRINCIPAL FUNCTION OF THE ELEMENT.

Chromium(Cr)Hardens slightly;increasedcorrosion resistance

Increaseshardenabilitymoderately, similarto manganesea. Increases corrosion and oxidation resistance.b. Increases hardenability.c. Increases strength at high temperature.d. With high C resists wear and abrasion.

Manganese(Mn)Hardens, ductilitysomewhat reducedSimilar to Nia. Counteracts effect of brittleness fromsulphur.b. Increases hardenability inexpensively.c. High Mn. high C produces steels resistant towear and abrasion.

Molybdenum(Mo)Age-hardeningsystem in highMo-Fe alloys.Increaseshardenabilitystronglya. Raises grain coarsening temperature ofaustenite.b. Increases depth of hardening.c. Raises hot and creep strength promotes redhardness.d. Enhances corrosion resistance in stainlesssteels.e. Forms abrasion resistant particles

Nickel (N)Strengthens andtoughens by solidsolutionIncreaseshardenabilityslightly, austeniteretention with highercarbona. Strengthens unquenched or annealed steels.b. Toughens pearlitic-ferritic steels (especiallylow temperatures).c. Renders high Cr/Fe alloys austenitic.

Vanadium (V)Hardens moderatelyin solid solutions.Increaseshardenability verystrongly as dissolveda. Promotes fine grain-elevates coarseningtemperature of austenite.b. Increases hardenability when dissolved.c. Resists tempering and causes markedsecondary hardening

Apart from this, other special alloying elements are added for specific property rendering in the steels. The Austenitic grades having high chromium content has a tendency to sensitize, that is, form chromium carbides along the austenite grain boundaries. The formation of these carbides reduces the chromium content of the austenite grains at the boundary, and, therefore, reduces the local corrosion resistance along the grain boundaries.To prevent sensitization, additions of columbium and tantalum to form 347 were invented. If these alloys are given a second heat treatment, called a stabilization anneal, at 1600-1650oF after the solution anneal, titanium carbide or columbium-tantalum carbide will form preferentially to chromium carbide. With all of the carbon removed as innocuous carbides, no chromium carbide can form. There is no loss of chromium at the grain boundaries, and no loss of corrosion resistance, and thus no sensitization. However, in boiler applications, these grades are not given stabilization anneal.One other microstructural constituent will form at elevated temperatures, and that is a chromium-iron intermetallic called "sigma phase."Both the sensitization and the formation of sigma phase occur over long periods at ill-defined temperatures. Both will occur at temperatures beginning at about l,000oF and will form more rapidly at slightly higher temperatures. Since the formation of chromium carbide and sigma phase are governed by the ability of individual atoms to move or diffuse through the lattice, these atomic movements will occur more rapidly at higher temperatures. As the temperature is increased above 1200oF, however, chromium carbide begins to redissolve in the austenite; thus the rate of carbide formation and growth decreases. By about 1600oF, chromium carbide is completely gone from the microstructure. Sigma phase is unstable and redissolves above a temperature of about 1600oF; the exact temperature depends on the composition.Unfortunately, from an estimation of operating-temperature perspective, all of these changes within the microstructure of austenitic stainless steel occur over a range of temperatures and over a range of times. There are no discrete temperatures that indicate with any degree of precision the peak failure or operating temperature. Thus there are only estimates of operating temperature and not an accurate "calling card" within the microstructure as there are in the ferritic steels.The 18 chromium-8 nickel austenitic stainless steels have been used for several decades in high-temperature applications within a steam generator. They have excellent high-temperature tensile and creep strengths and excellent corrosion resistance. The microstructural changes during long-term operation are more subtle than in the ferritic steels. In addition to this, the ductile-brittle transition temperature for these steels are at very low temperatures compared to the operating temperatures and hence recommended.

ACOUSTIC SOUND LEVEL DETECTIONEarly detection of steam leakage in a boiler due to tube leakage in steam coils will avoid secondary damage to great extent. Tube failure in a boiler is considered to be one of the major reasons for boiler shutdown. This problem is more pronounced in pulverized coal fired boilers, due to ash erosion and combustion being vigorous. Tube failures are bound to increase in present day boilers, where steam generation at high temperatures is attempted to maximize combustion efficiency, outstripping the metallurgical developments. Sonic tube leak detection system address this issue.TECHNICAL DISCUSSION

OVERVIEW

As a leak develops in a pressurized system, turbulence created by escaping fluid generates pressure waves within the contained fluid itself, throughout the low pressuremedium (usually a gas) into which the fluid is escaping, and within the container structure. These are commonly referred to as fluid-borne, airborne, and structure-borne acoustic waves, respectively. To detect leaks, the energy associated with these mechanical waves can be converted into electrical signals with a variety of dynamic pressure transducers (sensors) that are in contact with the medium of interest. Several methods of signal processing are available that allow the voltages generated by these sensors to be evaluated for the presence of a leak. As mentioned above, leaks in a pressurized system generate sound waves in three media. The decision regarding which types of acoustic waves are most reliably detected is important from both functional and economical considerations. This decision, in some cases, is not simple. Factors such as background noise level, sound attenuation within the medium, signal processing strategy, and installation costs play a role.

FLUID-BORNE LEAK DETECTION

The most well known use of fluid-borne leak detection is in the earlier work on feedwater heater applications. Itwas ultimately found, however, to be inadequate due to widely fluctuating background noise levels during loadchanges. In other applications, this method is rarely used because the sensor must be in contact with the contained fluid, which requires mounting it through the wall of the pressurized container.

AIRBORNE LEAK DETECTION

Since 1974, airborne leak detection has been predominantly used in large commercial boilers. Airborne methodsare well established and have detected leaks as much as a week before any other means available. In airborne applications, microphones or low frequency resonant piezoelectric transducers are coupled by hollow waveguides to the gaseous furnace medium. The waveguides are usually attached through penetrations in inspection doors, unused sootblower ports, or the casing. The airborne waveguide, shown in Figure 1, serves three purposes. It couples the sound waves from the furnace interior to the transducer face, protects the transducer from excessive heat, and allows easy access to the transducer for inspection or replacement.

STRUCTURE-BORNE LEAK DETECTION

The structure-borne method of leak detection has found applications in valves and pressurized pipelines. Under a recent Electric Power Research Institute (EPRI) sponsored project, a high frequency structure-borne approach was found to be the best method for detecting leaks in feedwater heaters. The structure-borne technique uses piezoelectric transducers coupled to acoustic emission type waveguides which are weld-attached as shown inFigure 2. In this application, a single structure-borne sensor mounted to the outlet side of the tube sheet will fully cover a high pressure feedwater heater. Low pressure feedwater heaters require an additional structure-borne sensor mounted to the inlet side of the tube sheet.

BACKGROUND NOISE

In leak detection applications, the most important factor to consider is background noise within the propagationmedium of interest. Almost all background noise can be characterized as white noise combined with discrete frequency noise. White noise can be defined as containing components at all frequencies within a range or band of interest. Both normal boiler noise and leak noise are considered to be white noise. Boiler noise is best described as low frequency white noise (rumbling) while leak noise is best described as higher frequency white noise (hissing). Discrete frequency noise is usually composed of a fundamental single frequency (tone) and several associated harmonic frequencies. These sounds are best described as whistling or humming noises.

SCHEMATIC OF THE ACOUSTIC SOUND LEVEL DETECTION SYSTEM

Sonic tube and sensor assembly -The sensor assembly consists of a sonic transducer which converts acoustic sound to electrical signal and preamplified in the preamplifier board housed in the assembly. It also consists of a test sound source. The whole assembly is mounted on a sonic tube which is mounted at an angle of 45 to the walls of the boiler, 1.5 m away from the wall. It houses the transducer in a Teflon protective coupling. Purging can be carried out on timely basis to avoid the deposition of ash and soot particles.

FIELD AMPLIFIER UNIT The field amplifier unit further amplifies the signal from the sensor and filters the low frequency combustion noise. The filtered signal is converted into 0-20mA Ac signal for transmission up to the control room. This also consists of a test signal push button to quickly check the functioning of the unit.

SIGNAL PROCESSOR MODULE

The signal from the Field Amplifier Unit is converted into AC voltage signal and is passed through a Band Pass Filter whose mid frequency is tuned for characteristic steam leakage frequency. The band pass filter output is precision rectified and a dc voltage proportional to the sound level in the required band is generated. This is converted into decibel in a dB converter circuit. The background noise is subtracted from the decibel output and the signal above the background noise is further taken for processing alarm generation.During soot blower operation, the channels near by will pick up sound from soot blower steam spray and show sound level in dB. In case, this sound exceeds the alarm set value for the duration of alarm delay, the contact output will be initiated, leading to alarm. To avoid this, a ANY SOOT BLOWER ON input is hardwired in to the logic circuitry, which inhibits any alarm during soot blowing. Alarm set valuecan be adjusted from 0 40 dB. Recommended set value is 20 dB. Alarm delay range is 1 min(nominal) to 10 min(nominal).

BOILER TUBE LEAKAGE INCIDENTS AT MAITHAN AND THEIR ANALYSISREPORT OF FAILURE ON 14TH DECEMBER, 2011INTRODUCTIONThe Maithan Thermal Power Station has BHEL make 2X525 MW coal fired sub critical units. On 14.12.2011 MTPS reported tube leaks in the first unit boiler. The leak occurred at 60 mt elevation in 2nd pass Screen Tube(Water Cooled) located near LRSB 132L accompanied by Pin hole leak observed from the Extended water wall at same location.Six tubes (one tube of 63.5 OD and five tubes of 51 OD), were received for metallurgical failure investigation at CTDS, Mumbai in Jan 2012. Of these, three tubes were with evidence of rupture/leaks while erosion marks arising out of the primary leak were noticed on all tubes other than the 63.5 OD sample.Description- The failed tubes are of 63.5 mm OD X 7.1 mm thick and 51 OD X 5 thk. The 51 OD tubes are all from the back pass of the Superheated Steam cooled Front Screen Spacer Tubes and the 63.5 OD tube is from the Screen tube section.The design pressure and temperature of these tubes are 202.1 ksc, 363 deg C(51 OD) and 214.4 ksc and 397 deg C (63.5 OD) respectively. All the tubes are rifled and to ASME SA 210 gr C specifications.Preliminary examination of the tubes revealed:1. No marks of corrosion or pitting or deposits in the tube ID of any of the samples.2. One 51 OD tube has a through crack starting at the end of the support plate weld and the support weld has a weld defect of non uniform weld throat thickness and thinning due to the weld run deposit. This crack does not exhibit any deformation to indicate that the failure has initiated from the steam side.(uniform diameter around the crack)3. 63.5 OD tube has a large window like opening showing no erosion marks around. The tube has a diameter change in one direction. No ballooning, swelling, overheating type of wall thinning is observed. The 30 X 20 mm opening appears like a small portion of the tube failed and got separated.Physical tests, chemical analysis and metallographic tests is carried out:Table 1 : Tensile test resultsValues identificationSpecimen sectionYield strengthN/sq mmUTSN/sq mmElongation%

Tube size51 OD X 5 thk6.55 thk X 12.61 wide421.6548.9125.94

Tube size51 OD X 5 thk7.04 thk X 12.60 wide402.1533.3729.18

Tube size63.5 X 7.5 thk7.72 thk X 12.66 wide369.40546.1426

ASME SA 210 gr C specs.-27548530

Table 2 : Chemical Analysis ResultsElementsIdentificationCarbon %Silicon %Manganese%Sulphur%Phosphorus%

Tube 51 OD0.230.250.70.0150.02

Tube 63.5 OD0.240.20.70.0230.02

ASME SA 210 gr C0.350.100.29-1.060.0350.035

Hardness :51 OD (tube site sample no 7) BHN 180-18463.5 OD tube BHN 215-217DISCUSSION : the chemical analysis, flattening, and hardness test results conforms to the ASME SA210 gr C grade. The tensile test results except the elongation are in order. The percent elongation against 30% is less by 4%. This deviation in elongation is not a significant factor for further use.On the 51 OD tubes across the openings/failure are taken for metallography.The following is seen fron the microstructure of the affected portions:Microstructure reveals ferrite and banded pearlite. Some grain flow is seen due to deformation and grain drooping is also observed at ID. In some locations voids can be seen along with the lamellar pearlite. Other tube samples mostlt showed equiaxed grains of pearlite and ferrite.On the 63.5 OD tube, Microstructure mostly revealed equiaxed grains of pearlite and ferrite.CONCLUSIONThe visual examination of the intenal surface revealed that the tubes are devoid of any sign corrosion products, pits, wastage; and the external surface is free from any defects such as cracks, weld defects, etc. there is no evidence of overheating on the primary failure (63.5 OD tube). Banding indicates yielding. There is no phase transformation indicating severe overheating.The 63.5 OD tube has a 30 mm X 20 mm opening. This opening has no wall thinning or ballooning or swelling. Except for a slight diameter change there is no distress. This indicates that this could have been caused by the presence of an oxide or non metallic layer which has got removed after a few hours of loading and resistance.The leak apparently has started from this opening and the release steam has damaged the nearby tubes, its portions and supports. While the external wall thinning was sustained by the other tubes until their internal pressure and temperature load crossed the threshold. The yielding and thinning after this threshold has led to the other leaks.

LTSH FAILURE REPORT ON 4TH MAY, 2012INTRODUCTIONMaithan Power Limited reported a failure in U #1 LTSH area on 4.5.2012. The failed tubes were received at CTDS for Component Damage Analysis on 22.05.12.The particulars of the tubes are: Material to SA 213 gr T11.Design Pressure 105.3 Bar; Design Temperature 464 deg CSize (specified) is 47.63 OD X 5.6 mm thk.Corrective Action carried out by the station:a) Inspection of the area around the leakage and the neighbouring area.b) Replacement of the portions of tubes in the affected area and its immediate vicinity.ROOT CAUSE ANALYSIS:The sound portion of the sample tubes received fron Maithan was subjected to tensile tesr and the results are found to conform to the ordered ASME SA213 gr T11 specifications:As per SpecsUTS (415)YS (205)% elongation (30)

Actual473.48379.7931.20

The spectro chemical analysis was carried out on the two of the failed tubes and the welds and the result conforms to ASME SA 213 gr T11.The tube has failed due to overheating caused by an excess penetration in the adjacent butt joint. The tube is at the beginning of the bank of tubes close to the LTSH inlet header which could be the source of debris which got entangled in the sharp internal defect of the weld. The primary failure has resulted in external corrosion of the neighbouring tubes, the weld quality apparently harbouring foreign material from the header is the root cause.PREVENTIVE ACTIONS:A. It is suggested at the next opportunity to carry out radiography test or UT on all such joints to identify similar defects, repair the welds which have these defects and inspect close to th headersany OD ballooning and attend the same.B. Photograph of the location of failure from site suggest that the freedom for movement of the tubes in that area is to be restricted by the use of standard fixtures to permit the tubes only for thermal expansion in the necessary direction. In case site engineers agree that the supports are not adequate then at the next opportunity the design drawing for supports may be verified and the supports put in place.

BOILER OPERATION TO MINIMIZE TUBE LEAKAGESEfficient operation of the boiler, during startup, normal operation and shut down is an essential pre-requisite for minimizing boiler tube leakages. Just after erection, certain milestones have to be achieved in order to declare the boiler as suitable for operation.After erection, the following procedure is carried out:1. HYDROTEST : the first hydrotest that is carried out on the boiler is done at a pressure of about 315 ksc(1.5 times the design pressure of the boiler design pressure) by a hydrotest pump maintained by the station. The hydrotest is carried out on the non drainable tubes first and then on the drainable and non drainable tubes together. This hydrotest is primary importance as it points out the maximum number of weld defects and tube leakages (during pre-commissioning works) which cannot be determined by Radiography test.2. BOILER LIGHT UP3. CHEMICAL CLEANINGAfter lighting up the boiler, the boiler tube must be made free from the debris lodged during commissioning for facilitating efficient heat transfer, maintaining steam purity and preventing local heating which may be an important cause of boiler tube leakages. Cleaning process can be broadly classified as:

a. Alkali Boil out : carried out to remove oil, grease and rust. Normally soda ash and TSP is used. Sodium sulphite is used to reduce oxygen corrosion and sodium nitrite reduces caustic embrittlement.The boil out operation is carried out at about 0.2 times the rated operating pressure of the boiler. The boiler drum is filled with water upto the normal operating level and thw chemicals are introduced in the syste via the normal dosing inlets. Since this is the first time the boiler is being lighted, to facilitate checking up free and uniform expansion of the unit while raising pressure, the firing is to be maintained at a minimum. The silica and oil content is monitored till oil content is below 1ppm. Then the boiler is boxed up and allowed to slow cool. Hot dm water is flushed into the boiler followed by cold rinsing.b. Acid cleaning : An inhibited acid solution of EDTA (Ethylene Diamine Tetra Acetic acid) is circulated for 4-5 hours by acid circulation pumps. This cleaning is carried out to clear the tubs of any scale deposits as well as for chelation, which forms the protective magnetite film by reacting with the base metal on the tube ID. Further, the iron iron content, silica etc are to be dissolved to a considerable extent. After hot and cold rinsing, neutralisation may be doenby soda ash. After passivation, hydrazine and ammonia is charged and the boiler is laid up for a predetermined time.4. THERMAL FLOW TEST : The thermal flow test is carried out to check the chocking of the superheaters, reheaters, waterwalls, etc. by a thermal flow meter.

BOILER STARTUP/ SHUTDOWNThe processes involved in boiler startup and shut down is a major cause in boiler tube leakages as these processes involve boiler pressure parts thermal stresses caused by varying temperatures. The following are the important points to be kept in mind while Boiler start up and shut down : Purge the boiler for at least 5 min before start up and shut down with adequate air flow to dilute the combustible matter in the furnace to avoid explosion. Before introducing any fuel into the furnace, ensure sufficient ignition energy. Whenever fuel intake is increased or reduced, flame stability has to be kept in mind. While increasing fuel input, load the burners to their rated capacity before cutting in the other burners. While reducing fuel input, the load on the adjacent burners must be kept above 50% before cutting the remote burners. The varying of temperature in the boiler causes stresses in the boiler pressure parts. The rate of temperature variation is restricted. The cooling range is more stringent than the heating rate because the temperature stresses are acting in the same direction as the pressure stresses while cooling while it is opposing at the time of heating. Also, during heating the temperature is controlled by adequate draft. During startup, until adequate steam flow is established in the reheater and superheater coils, the furnace exit temperature has to be maintained below 540C. as soon as the furnace exit temperature reaches close to the metallurgical limit, the steam has to be bypassed in the reheaters to allow for cooling of the tubes. Maintain proper boiler chemical regime. Deaeration has to be ensured to remove oxygen to avoid corrosion of the tubes. Oxygen scavengers are further added to bring the level to satisfactory levels. The boiler drum chemistry has to be maintained by adequate blowdown quantity. During startup, high silica content can be encountered and boiler drum pressure has to be restricted to avoid excess carryover. Whenever boiler is operated at lower pressures, the waterwalls has to be protected from the excessive partial pressure operation losses by cutting on the boiler output. The condensed water in the undrained sections of the tube boils and avoids the flow of steam. This may cause local overheating and the metal temperature at these points have to be monitored by thermocouples located in the superheaters and reheaters. Low temperature corrosion can occur in the cold end of the APH due to lower temperature during startup operations. To avoid this SCAPH is employed which maintains the temperature of the flue gas above dew point of the gas. Economizer recirculation must be employed to prevent starvation of the economizer tubes at startup loads. The boiler drum has to be maintained and monitored continuously, since there is swelling at this time. Boiler drum blowdown has to be employed, as well as emergency blowdown has to be done to maintain the drum level.

The Hot, Warm and Cold Start Up characteristics are given as below: