Tube Failure Class

Causes of Tube Failure1. Overheating.2. Erosion.3. Corrosion.4. Material Defects.5. Manufacturing Defects.

OverheatingOverheating can be localised, extensive, prolonged or of a short duration. Metallographic analysis indicates the approximate temp. to which the tube was subjected before failure occured. Observation of the grain growth & microstructure of the failed tube material also indicates if the overheating was of a prolonged or short duration.Water wall tube failure results in a burst with a fish mouth opening. Occasionally cracks will also appear up to length of 2 Mts. on either side of the burst.This may lead the power station authorities to apprehend that the tube is of the CRW type. Bursting occurs due to excessive reactive force cause by change of state from water to steam. In case of SH tube failure takes the form of a narrow opening with multiple stallite cracks. The reasons for overheating of water wall or SH tubes are, 1. Chocking with foreign material.2. Starvation due to,a) Improper circulation.b) Insufficient flow3. Flame impingement4. Secondary burning of fuel.5. Other causes.

Choking with foreign materialsForeign materials like mill scales, weld slag, sand, electrodes bits, rust products, chips, small tools, nuts etc. Which collect at the bends or weld joints where the internal cross section is restricted will cause choking of tubes. These materials enter the tube during various stages of manufacture, shipping and / or erection. Choking of tube with foreign materials will impede the flow fully or partially & cause overheating.Precautions can be taken to mitigate tube choking at various stages of manufacture, shipping, storage & erection. By using TIG root welding for all SH coils at shop & site, the problem of choking can be minimised by eliminating the construction at the weld joints due to excessive weld penetration to check the blocking of tube with foreign material an instrument called contract flow meter (developed by CE research lab UK, & manufactured by Land Pyrometers LTD, UK) can be used. This meter can be used during the commissioning of new boiler to ensure that there is not blockage of & in the case of operating boiler, it can be used during overheating.

Starvation due to improper circulation:-Insufficient circulation in the water walls may lead to departure from onset of nucleate boiling & may lead to overheating. Which in turn will result in tube failures. Where the failure is traced to improper circulation, the same can be improved in the region of water wall by providing additional downcorners / spider tubes to the existing downcomers.

Starvation due to insufficient flow:-Starvation can occurs in SH tubes due to an insufficient flow resulting in overheating. This is generally observed in the binder tubes of the platen SH. These binder coils have a number of bends & are longer in length than the other coils in the platen. The flow through these binder coils is, therefore, inadequate. The prolong overheating in such tubes results in creep failure. Such failures can be avoided by replacing the long binder tubes with shorter tubes, which in turn increases internal flow, & prevents overheating. Overheating can also be avoided by allowing cooler steam through the wrapper tubes of platen to better cooling of the tube materials.The materials of the bottom portion of the outermost coils of platen can also be replaced by stainless steel to enhance their life since the bottom most portion faces direct radiation from the furnace.

Flame ImpingementWater wall failures occur mostly near the burners. This is due to the flame impingement from burners, which get distorted in service. To avoid such failures new burner nozzles such as honeycomb types, which resist distortion, are now used.Additional peep holes can also be provided for better monitoring of the flame & observation of the burner tip.Arrangements could be made to supply mellowing air to bring down air temperature wherever necessary so that the combustion front can be kept away from the burner nozzles.

Secondary burning of fuel:-In certain cases oil from the oil gun may flash on to the tubes & then burning takes place which results in overheating the tubes. Even in coal fired boilers, the unburnt fuel particles may catch fire at the top of the furnace or in the second pass causing secondary combustion, explosion, or overheating of the tubes. This can be avoided by proper control of the atomisation of oil, coal particle size & the firing rate.

Excessive airExcess air plays an important role in the heat absorption pattern of various zones of the boiler. Too much of excess air leads to cooler furnace & higher heat absorption rates in convective paths. The too little amount of excess air leads to higher furnance temperature resulting in higher radiation, heat absorption & slagging problems.

In oil fired boilers too much of excess air is favourable to the formation of SO3 due to the increased availability of O2 thereby promoting a higher rate of low temp. corrosion. To avoid such failure O2 content in flue gases should be measured periodically during operation & adjustments made to achieve design values as closely as possible. Further, to avoid overheating the flue gas temperature in different zones should be closely monitored & kept within the design limits.

Internal DepositsInferior quality of feed water leads to internal deposits of salts & silica in the water wall tube. This internal deposit will cause overheating of water wall tubes leading to failure. To avoid this, the feed water of boiler water quality should be maintained within the allowable limits as per the international standards.

The carryover of salts by steam can cause deposits in the tubes with consequent overheating & failure. To avoid this, the salts, content in the drum water should be maintained as per the standard operation with high water levels in drum could lead to carryover of water drops & dissolved solids leading to internal tube deposits in SH tubes. These deposits hinder heat transfer & lead to increase in metal temperature & consequent tube failure. Hence it is necessary to restrict the drum WATER LEVEL TO THE prescribed limits under all operating conditions. Other causesMal-operation can sometimes leads to overheating & results in tube failure. When high-pressure heaters are out of service, the convection SH O/L temperature can shoot up leading to overheating of tubes. This can be avoided by suitable control of the excess air & the boiler load. During hot restart if the flow of auxiliary steam from the drum tap off point is high it will result in a reduced flow through the SH which in turn will lead to overheating. EROSIONErosion is a second major cause of tube failure. The tube wall thickness gets reduced due to erosion & when the thickness is not sufficient to withstand the operating pressure and temperature of the tube, the tube will fail.

Erosion of SH & economiser tube may be due to following reasonsi) Flue gas erosion.ii) Erosion due to steam or water.

Flue gas ErosionThe rate of erosion is proportional to the cube of velocity. The ash content of the Indian coals is of more abrasive in nature containing high silica & alumina. Due to the above, if the velocity of the flue gas at narrow gaps between coils & walls & SH coils and ash hoppers below them is high, then erosion may occurs in these zones. Therefore the boiler have to be designed with 15m/s velocities as for as possible.A typical ash analysis data is given below :Silica -- 55.5 to 56.5%Alumina -- 27.3 to 27.9%Unburnt carbon -- 4.4 to 6.2%FeO -- 5.3 to 6.7%Lime -- 1.37 to 2.16%Sulphates-- 0.5 to 0.68%

The flue gas erosion in the horizontal SH & economiser can be prevented by providing baffles. The flue gas erosion in the bands of the convection SH in the horizontal pass can be prevented by increasing the height of refractory lining of the ash hoppers in front of the coils.

Erosion due to steam or waterWhenever there is a tube failure the water or steam from the faulty tube escapes in the form of a high velocity jet & when it impinges on the adjacent tube they get eroded. If the boiler is not shutdown immediately after detection of the failure & allowed operating for a protected period the damage due to steam or water erosion will be considerable.

Additionally, sometimes leakage from the soot blowers or wall blowers causes erosion of water wall, SH or economiser tubes. In some boiler, vertical bar type soot blowers have caused tube failures in the horizontal SH & RH & to overcome this the sort blowers should be moved from the ceiling to the sidewalls.

Corrosion This can be mainly grouped into two types 1) External corrosion due to depositing of chemicals.2) Internal corrosion taking place inside the tube due to impurities in steam and water.

1) External corrosion due to depositing of chemicals carried by ash.On review of ash analysis detailed under flue errosion, it can be seen that ash consists of sulfate up to 0.5 to 0.681. This type of corrosion results mainly from the deposits of ash on tube surface. The deposits may be classified either as slagging or fouling.

SlaggingThis is the deposition of molten or partially fused particles of fuel constituents (non-combustible) on furnace tube surface. Though it is usually is associated on furnace tube surface, slagging can also occurs in screen tube and in the super heater when molten ash carried over into these solutions and is exposed to excessively high gas temperature.

FoulingFouling on other hand is the condensation of combustible constituents such as sodium sulphate on fly ash particles and on boiler tube in area of the unit where temperature are such that the constituents are remain in the liquid state. The combustibles, fly ash, and flue gas react chemically to form the deposit.

Phosphate depositThis is also similar to the sodium deposits fouling, which are initiated by the attack of acidic phosphorous compounds on the tube metal & the fly ash particles. Indian coals do not contain phosphorous as one of the major constituents and as such this type of corrosion is rarely expected.

Low Temperate Corrosion This is caused by sulphuric acid, & can occur in the economisers of some units if the feed water temp. is lower than about 150C. However the feed water temperature is most of the utility boilers & the sulphur content of the coal burnt are such that dew point problems already rarely encountered. Condensation problem further down stream where lower temperature exists may corrode air heater, precipitator, hoppers, fans, ducts & stacks. As the sulpher content is more in the fuel oil & also as the flue gas temperature will be low during starting usually the air preheaters will be experiencing the cold corrosion problems. To overcome this problem, steam coiled Air preheater should be kept in service till flue gas temperature rises above 300C.

RemedyAll the deposits, which cause corrosion of the above types, are easily soluble in water & will be loose also. This deposit can be cleaned by normal operation of soot blowers. When this method is not totally effective, water washing during outage is recommended. It is very important to schedule water washing so that the tube surfaces can be dried out immediately after cleaning, as otherwise corrosion will occur. A good approach is to water just before returning a boiler to service. If this is not possible, fire at a low rate until tubes are dry.Design ImprovementsIn coal fired boilers most major corrosion problems are caused by coal ash with in a specific temp. range certain coal produce liquid ash compounds that are very corrosive to all conventional boiler materials. This temp. range normally extend from about 1000F to 1200F essentially restricts attack to the SH & RH .Engineers weigh carefully the four major factors that influence the severity of coal ash corrosion viz. Ash properties, ash deposition rate, tube external temperature, & tube chromium content, before finalizing the design of SH & RH surfaces. If the high temp. Corrosion occurs inspite of design efforts, the option for correcting the problems are limited; for example the following remedial measures may be resorted to. 1) Replace damaged tubes with one of higher Chromium Content.2) Switch to a fuel with more favourable ash characteristics. 3) Install stainless steel tube shields. These shields will effectively keep the liquid ash from the tube surface & operate at temperature above that at which liquid phase can exist. Extensive shielding however, inhibits heat transfer performance.

Internal corrosion due to impurities in water & steamInternal corrosion is mainly due to improper feed water treatment. The most prevalent forms of waterside attack in the drumtype utility boiler are, i) Hydrogen damage.ii) Bulks under deposit corrosion. iii) Corrosion fatigue.iv) Stress corrosion. v) Steam blanketting. vi) Oxidation. vii) Pitting. viii) Galvanic attack.ix) Caustic embrittlement.

i) Hydrogen Damage This induces brittle fracture & will occur beneath a relatively dense deposit when boiler water pH is too low. The accepted thereby of this type of attacks is that the hydrogen atoms are produced between the deposit and the tube surface. They in turn react with cementite a hard brittle iron compound at the grain boundaries of the tube material to form molecular methane gas, which removes carbon from metal weakening it by creating fissures in its grain structure.The pressure of the gas that is formed literally blows the material apart. This damage is most common where condenser leakage occurs in units cooled by sea water. Some metal loss may be caused by corrosion mechanism, but the tube which failure would occur.ii) Bulk Under deposit Corrosion This is caused by the concentration of traces amounts so soluble corrosive compounds usually strong alkalies, such as sodium hydroxide between the tube walls & a relatively porous deposit. (60-90 % porosity compared to theoretically dense magnetite). The term caustic gouging is sometimes used to describe this form of corrosion, which is characterised by rapid attack & subsequent tube failure.iii) Corrosion Fatigue Material that undergoes cyclic strain may suffer fatigue failure. The strain can be mechanical in nature such as vibration or thermal such as soot blower condense quenching, corrosion or oxidation can accelerate failure in other words, failure may occur after fewer cycles at a lower level or strain in a corrosive environment.iv) Stress Corrosion CrackingPortion of austenitic stainless steel SH elements containing residual stress such as stress supports & ring welds are susceptible to cracking in high temp. water containing chloride or hydroxide compounds & oxygen. Though such condition are relatively uncommon, they do occurs after because of operator error. Stress corrosion attack on the secondary SH tube was caused by inadvertent use of a boil out cleaning solution containing caustic. The boiler was fired only a few hours, but the damage was significant, and tube replacement was required.v) Steam Blanketting: -Steam blanketting phenomenon occurs in tubes, which are slightly inclined, for example flow started slag screen tubes since the steam flow in some what restricted resulting in obstruction for heat transfer. In such environment if the feed water possesses caustic soda, it may give rise to general wasting of the crown of the tube or the formation of grooves at the water line.vi) Oxidation Oxidation of low alloy ferritic steels operating at temp. above about 450C is a natural phenomenon in the boiler water side environment. All materials used in high temp. SH & RH tubing are subjected to oxidation, although at different rates. Problems arise when oxide scale on the tubes internal surface become so thick that differential expansion between the oxide and the parent metal results in spalling of the oxide from the metal surface a process called exfoliation. The loose flakes are hard & brittle & generally range from the size of a match HEAD to that of a quarter. Loose scale can clog tubes at bends causing their failure by overheating.PittingThis is mainly due to the difference in effective electrode potential between adjacent areas of the metal surface. This can result due to the following reasons.1. Differential heat treatment resulting in localised difference in stress concentration.2. Surface irregularities such as scratches & cuts developing during manufacturing.3. Different concentration in dissolved compounds. The effect of individual factor depends upon their relative magnitude & most of them are independent.Galvanic Attack The corrosion of the less noble member of a pair of metal which are joined together is called Galvanic corrosion or Dissimilar metal corrosion. The effects of galvanic corrosion are often serious especially in sea water power station as feed water conditions are conductive, since at temperature that occurs in the high pressure boiler tubing, are very high Galvanic attack will be more severe in these areas.Caustic embrittlementThis is due to the caustic ALKALINITY formed in the drum & the scale of this caustic soda reacts with the metal & may result in cracking at welding joints where the welding is weakMaterial DefectsSome of the tube failures in the boiler may be due to the usage of the defective raw materials. Though different quality control measurement are adopted in various stages of the manufacturing, defective materials may find there way in rare cases and cause failure. High quality of the materials can be ensured by selecting tubes, which have undergone ultrasonic tests, & by resorting to strict quality control inspection procedure. Successful waterside corrosion control requires careful selection of the material used throughout the steam cycle including the feed water heaters & condensers.Manufacturing defects Tube failure may occur due to the defect in the manufacturing process, such as weld defect, & improper heat treatment. Mix up of material sometimes lead to failure because of the wrong usage of materials.Creep and fatigue are very important factors while designing the thermal power plant equipment. Metals undergoing high temperature will also be subject both creep and fatigue.

Creep When metal stressed at sufficiently high temperature it will continue to deform with time although at a relatively low rate. This process is known as creep and is of major importance in selecting metals for service in power plant. This is because at temperature much above 350C (660F) steels are subjected to this phenomenon. The figure illustrated that the creep process can be viewed in three stages. The primary stage in which the initially high rate of strain remain constant over a period of time, the second stage in which strain rate remain constant over a period of time, and the tertiary stage in which strain rate increases continuously and which culminates in rupture. This behaviour may be explained in terms of a balance between the effect of straining which are to be strengthen the material and increase its resistance to further deformation and effect of heating which are to be soften the material and decrease resistance to deformation. Thus strain hardening predominates initially until the strain rate falls to level at which the opposing influence are in balance accompanied by continuous deformation until the tertiary stage is reached, where loss of strength leads to fracture.Fatigue Metals undergoing high temperature service may also be subjected to fatigue. This process is one in which failure may arise exposer to many cycle of alternating stress, with or without super imposition of mean stress. This type of failure is comparatively rare in power plant. The predominant failure is creep not fatigue.In power plant, it is possible to encounter situation that are classified as thermal fatigue. In these frequency of straining is given by the number of stops and starts endured the full life of plant (say 5000 to 10,000). The level of strain is enhanced by the creation of thermal gradient during operation, and/or by geometric strain concentration. This problem is believed to involve both creep and fatigue process.Factors affecting fatigue lifeThe rate of cycle loading has only small effect on fatigue strength. Fatigue strength increase with increasing rate of cycling probably because of the increased strain rate.i) The form of the stress cycle such as square, triangular or sinusoidal wave has no effect on the fatigue life.ii) The environment in which component undergoes stress reversal has marked effects on fatigue life. The fatigue life in vacuum is about 10 times more than that in moist air. This clearly indicates that the environment has a corrosive effect and reduce the fatigue life. Fatigue occurring under the specific corrosive environmental is caused corrosion fatigue.iii) Size of component has no effect on fatigue life if component is identical one another in all respects such as defect microstructure, inclusion, etc. except size. However in certain cases it has been observed that larger size less is a fatigue life. The decrease fatigue strength is more for high strength material. This is due to more chance of imperfection or stress rises in large size components.iv) Stress gradient has strong effect on fatigue life. Higher stress gradient, lesser is fatigue life.v) An increase in temperature above room temperature decrease the fatigue life to increase in the crack growth rate. Decrease in temperature below room temperature increase the fatigue life.

Procedure For Failure Investigation & Collection Of Failed SamplesThe causes for failures are evaluated by removing carefully the failed material (eg. Tube) along with deposits if present. It is preferable to pack them with polythene wrappers & box, such that no corrosion & mechanical damage occur during transits. If the deposits are loose, water side & fire side deposit are collected in separate polythene bags with rigid tags. The flame cut region should be at least 200mm away from the region of failure since heat produced during flame cutting will change the microstructure, if the cut region is closed to fail region. For comparison, it is preferable to have good portion (about 300mm) of the tube (along with the deposit if it is present) which is considerably away from the failed region. The samples of material, which failed due to brittle fracture, should be taken out (if it is possible) & using rust preventive coatings should protect fractured facets. In some cases in site micro examination is carried out when specimen could not be cut or removed. This technique is also used for fracture analysis. In certain cases it becomes essential for the metallurgist or chemist to visit the site & have first hand information regarding the location and overall nature of failed tubes or any other components. These have to watch the performance under the existing condition at site. This will help in the interpretation of complex failures.

1. Corrosion Due to corrosion the normal structure of the material which consists of ferrite & pearlite will change from ferrite to ferritic & spheroidisition of carbides occur along the grain boundaries.2. Erosion Due to erosion the normal structure will change from pearlite to pearritic & spheroidisition of carbides occur along the grain boundaries.3. CreepsDue to creep the grain growth occurs along the boundaries which weakness the material strength & due to which the voids are formed along the boundaries & in case of prolonged period these voids combine & fracture takes place.Remedial MeasuresThe corrosion in general is resulting due to the oxygen present in feed water & the pH value of feed water. Oxygen may be carried over into boiler through make up water leaks, etc. Even the minute quantity of O2 is capable of causing sever corrosion in the boiler working at high pressure. Since the O2 content carried over the steam will go on increasing as the pressure of the boiler increases.The table below shows how the O2 content carried over to steam increases as the pressure increases.Boiler pressure kg/cm2O2 in feed water: O2 in steam

12.65000:1

425000:3.2

705000:5.3

1405000:10

Corrosion related problems could generally be avoided if,1. Recommended water treatment controls are followed.2. Corrosion products formed in the feed water system are kept within specific limit.3. Feed water O2 concentration properly controlled.4. Proper precautions are taken during chemical cleaning operation to prevent metal attack.5. Drum internal & drum level controls are maintained in good condition.6. Silica concentration in the boiler is controlled within limits7. Corrosion products formed in the feed water system should be minimised by proper phosphate dozing & hydrazine dosing.8. Feed water O2 concentration should be controlled by proper deaeration employing effective deaerator.In modern boiler hydrazine dosing is adopted to effectively remove the oxygen.N2H4 + O2 N2 + 2H2OThe possible measure to minimise fire side corrosion/erosion 1) Modifying the physical or chemical characteristics of deposits with the addition of chemicals. They may be introduced either by adding with fuel or injecting into the furnace or sprayed to external surfaces of the tubes. Addition such as silica based compound, lime, magnesia etc. have been reported to be useful by way of either raising ash softening temperature or fixing SO2 from high sulphur coal.2) Use of coating to improve corrosion or erosion resistance.3) Periodic removal of deposits by blowing of compressed air or steam.4) Design modification such as avoidance of sharp bends in the path of flue gas to avoid local high velocity, decreasing the flue gas velocity to an optimum level improving combustion condition etc.5) Reducing the ash content of the coal by using washed or blended coal.6) Use of thicker tubes or faceted tubes in the corrosion zone.7) Replacement of tubing with more corrosion resistant tubing.8) Resistance to erosion should require a hard brittle tube material, which would be unsuitable for pressure part use. For this reason the pressure part are required to be designed to allow for a rate of material removal by erosion within their design life.

Conclusion It is observed from the various failures at various power stations. Research Institute suggests them to follow the following practices during operation to avoid frequent failure of boiler tubes & thus prevents the frequency of outages.1. Proper operation of boiler whithin the permissible limits of various parameters.2. Shrouding, shielding the areas, which are prone to erosion & proper inspection of tube elements during overhaul, may help to reduce the outages.3. Particular care for operation of oil guns, proper distribution of secondary air and total airflow & proper soot blower operation may help to reduce outages.4. Soot blowers operation, proper care to be taken from retraction or leaky poppet valve to avoid erosion of nearby tubes may help to reduce tube failure considerably. Wall soot blowers should be properly aligned for hot operation of the blower considering the expansion of furnace & structure.5. Sudden variation of loads & abnormal operation beyond permissible limits should be avoided.6. Proper purging of boiler should be carried out during start up & after every shutdowns.7. Feed water quality should be maintained within permissible limit.8. Start up fuel automisation should be checked for its proper operation.