Boiler is a pressure vessel in which steam is generated by the application of heat resulting from the combustion of fuel , ( Solid, Liquid or Gaseous ) is classed as fired steam boiler . There are also unfired boilers where sensible heat emanating from some primary process is further utilized to generate steam , installed at down stream . Such boilers are commonly known as Heat recovery steam generators( HRSG/ boilers.)
The water tube boilers ( where water to steam flow is within tubes and combustion of fuel is out side, in a furnace), are top supported units, suspended from a self supporting steel structure , arranged to provide total freedom of expansion for all parts of the boiler in down ward direction. All the parts meet the design codes, quality standards, material testing procedure and hydraulic test code laid by Indian Boiler Regulation ,ASME standards or code of practices of respective country.
Components of all types of boilers are exposed to adverse operative conditions of high temperature, stress and cyclic loading eventually leading to failures. These failures can be grouped under following major causes.
- Stress Rupture
- Water Side Corrosion
- Fire Side Corrosion ( High Temperature Corrosion)
- Lack Of Quality Control ( Shop Weld Failures Or Mixing Of Carbon Steel With A Cr. Mo . Alloys)
The material of different components of boiler are selected to last for at least 100,000 operating hours with a reasonable factor of safety , however ,actual operating conditions like variation in fuel quality, increase in metal temperature, uneven flow of fluid inside the tubes ,uneven flow of gases over the tube surface , uneven ash/slag coverage and number of cold or hot starts govern the actual life of components.
In certain cases , it is experienced that , failures start within few hundred hours of operation due to design mismatch or deficit in quality control during manufacture. However, seven to ten years of operational period is commonly observed when tube failures start due to one or combination of reasons already mentioned earlier . It is experienced, while few boilers encounter failures during early stage of commissioning , there are boilers in active operation for more than 35-40 years without major problem.
Not with standing to above , designers and owners of boilers endeavor to achieve a maximum life cycle from their units. Extension of boiler life cycle is possible by periodic identification of vulnerable components and timely action for their repair or replacement . These actions not only extend the life span of boilers but ensure safety of personnel and prevent the surroundings of boiler plant from consequential damages. To this affect, Indian Boiler Regulation has been amended under section 391A for conducting.
Inspections by qualified agencies authorized by Central boiler Board to carry out inspection and conduct set of defined non destructive and destructive tests in order to assess remnant life of boilers.
IRC Engineering services India Pvt. Limited (IRC) is one of the Central Boiler Board accredited organizations, having all the requisite facilities and resources to undertake the responsibility of comprehensive inspection of boilers and prediction of their remnant life.
In addition to Remnant Life Assessment Service , IRC provides specialized mechanical testing and failure analysis service for high temperature application components like boiler tubes, steam and gas turbine components. The captive test facilities of IRC laboratory ,are also accredited by the Central Boiler Board to serve following objectives.
- The designed component must serve its purpose with efficiency and performance availability.
- To establish life management system of critical and vital components to organize timely repair or replacement.
During the mandatory inspection activities, termed as REMNANT LIFE ASSESSMENT STUDY ( RLA Study) , following major activities are undertaken;
- Study of operation and maintenance (O&M) records.
- Visual inspection
- Structural Integrity (By NDT & dimensional measurement)
- Verification of mechanical properties and analysis of results.
- Material degradation due to transformation of micro structure.
- Impact of Chemical regime during operation.
To accomplish , Remnant life assessment study , a multiple disciplinary team needs to be deployed to collect (O&M) data , NDT results , destructive test results from laboratory and analyze them to offer conclusive recommendations .
Study of operation and maintenance records: This is an activity which provides sufficient input to focus on the intent of the study. During the review of operating parameters, there is possibility of getting feedback about the excursion of temperature and pressure to assess the magnitude and duration of stress. This is an important input to identify vulnerable locations. Similarly, tube failure data and their replacement data can provide information about the problem areas , vulnerability of components and existing status of the boiler unit.
Data about pH value ,conductivity, of variation of silica in water & steam etc. along with quantity of feed water makeup may indicate about the chemical regime of boiler during operation and probability of corrosion.
Visual Inspection of boiler :
This is an activity where , physical deviations of the critical components can be examined . Distortion & bowing of super heater and re- heater coils, displacement of headers , Drum suspension arrangement, expansion of boiler, condition of seismic supports, buck stays, expansion joints of air and gas ducts , condition of hangers and supports of critical pipes, condition of main structure may help to identify spots of restraints and over all integrity of carcass where entire weight of boiler- steel is suspended . Erosion of second pass components , soot blowing zone of water wall tubes and ‘S’- panels are the locations where visual assessment is possible . Subsequently, extent of erosion can be quantified by other mean to calculate if thickness of the tube can sustain the hoop stress , applied to particular component. The bowing of super heater coils, swell & blistering of headers is a result of over heating or reduction in creep strength at a higher temperature or overheating. The hot spots due to burner flame impingement on water wall tubes can also be located to undertake corrective action.
Dimensional measurements of tubes , headers & drum :
The erosion is generally observed in economizer coils low temperature super-heater coils and water wall tubes. If thickness of tubes is reduced below certain limit ,they cannot sustain the operating pressure of the component at the prevailing metal temperature.
The headers are checked for ovality, straightness, blistering and localized swell to notice the status of creep . Probability and status of creep can further be verified by optical signatures.
Boiler drum is another critical area where symptoms of corrosion , pitting , presence of copper, condition of magnetite layer, and possibility of silica slippage into steam can be judged from the condition of drum internals and drum surface. Samples of deposits from drum are also collected for chemical analysis and conclusions.
For boiler drum, Indian Boiler Regulation stipulates that, difference between internal diameter measured at any cross section and the nominal diameter should not exceed 1 % . The deviation from straight line shall not be more than 0.3 % of the cylindrical length.
NDT Tests conducted at site:
Besides, visual inspection and dimensional measurements, non destructive tests like dye penetrate test, ultrasonic test, magnetic particle inspection and hardness profile of drum surface, headers, main steam pipe , reheat pipe , dissimilar weld joints and other critical components like start up valve, safety valves and main stop valve are conducted to evaluate soundness or structural integrity of respective component.
For the inspection of internal surface of headers and other narrow passages like valves and drains ,fibroscopy camera with extended probe is used to inspect, likely crack, corrosion and deposits of chemical scales.
While measurement of oxide scale and raising of replica films of micro structure are non invasive tests, evaluation of their results are conducted at laboratory by relevant experts. Tube samples from various locations, stipulated in Indian boiler regulation , are collected for destructive tests and evaluation of REMNANT LIFE ,described as below.
- Evaluation of oxide scale data
- Evaluation of Mechanical Properties.
- Evaluation by creep rupture Test
- Evaluation of micro structures.
- Chemical analysis of different deposits.
In addition to above inputs for remnant life computations, measurement of accurate tube thickness is also an important and time consuming activity. A minimum remaining thickness of tube is important , to sustain the maximum permissible stress at actual operating conditions of pressure and temperature. Economizer coils, Low temperature super heaters area around soot blowers and burners are the vulnerable locations where extra care for accuracy of measurement is taken. Suitability of their lowest available thickness is worked out in accordance with the following IBR provisions. Tubes which can not sustain the prevailing stress ,are recommended for replacement.
IBR under section 338 (a) stipulates:
T = WP x D + C
2f + WP
WP = Working pressure in the drum
T = Minimum required thickness in mm
D = Outer diameter of tube in mm.
f = Allowable stress for the tube at design temperature.
C = 0.75 up to 70 kg/cm2 ; 0.00 above W.P.
Oxide Scale Measurement:
This is an indirect method of evaluating the status of creep damage in lieu of accelerated creep rupture test which needs an elaborate testing facilities and time. The measurement of oxide scale beyond 500 μs. are examined by using following relationship popularly known as Larson Miller parameter.
Log x = 0.00022 P-7.5
Where, P = T( 20 + log t )
X = Thickness of oxide scale in mils. ( 1 mil = 25.4 microns)
T = Maximum temperature that metal has seen.
( 0F + 460 )
t = Time to rupture.
Evaluation of mechanical properties:
Irrespective of mode of life reduction mechanisms present in a particular boiler, results of mechanical tests like ultimate tensile strength, percentage of elongation and hardness etc. reveal the condition of component.
The value derived from tensile test also used to arrive at the value of maximum permissible stress to decide ,whether particular component can sustain the operating parameters or will take some more time to fail. Accordingly, owners are advised to plan replacement .
Accelerated creep rupture Test:
During 1950, F.R. Larson and J. Miller, employed by General Electric while performing research on turbine blade used a model for experimental tests so that results at certain temperature and stress could predict rupture lives. The relationship was broadly based on physical justification and interpretation of equation by a Swedish Chemist Swante Arrehenius in 1889 .
The material of boiler components like super heaters and re-heaters are exposed to very high temperature in the range of 600-700 0C and prone to creep failures and fatigue. Creep is a time dependent deformation of material while an applied load remains below its yield strength.
In order to quantify remnant life of components operating at high temperature an accelerated creep rupture test is conducted at the laboratory. Accelerated aging is a process that USES AGGRAVATED CONDITIONS OF HEAT AND STRESS TO STEP UP THE AGING PROCESS OF THE COMPONENT DRAWN FROM THE BOILER UNDER STUDY.THE TEST PROCEDURE CONFORMING TO ASME -139 or BS 3500, are USED TO DETERMINE LONG TERM EFFECTS OF ACTUAL OPERATING CONDITIONS IN TERMS OF STRESS LEVEL.
The tests can be conducted by keeping the stress constant and raising the temperature ( ISO-stress ) or by keeping the temperature constant and increasing the level of stress ( ISO thermal). A LMP verses stress points are plotted on logarithmic graph and time to rupture can be obtained in hours. Mathematically, following equation is used to compute the remnant life of boiler components. The value of P ( LMP ) is calculated by following equation:
P = T ( 20 + log t )10-3
Where, T = Temperature in Kelvin.
t = Expected time to rupture.
i.e. tr = log-1 [P x 103]]
[ T- 20]
Evaluation of micro structure:
The combination of accelerated creep rupture test results and evaluation of micro structure are the best available tools to assess the realistic determination of remnant life of boilers.
For this purpose optical signatures are drawn from all the vulnerable locations to understand diffusional decomposition of metals to figure out its grain growth due to;
Spheherodization of pear lite / Bainite Break up of pear lite / bainite with precipitation at grain boundaries.
Coursing of carbide phase:
On these objectives , two German Scientists , Neuber & Wedel have provided following classifications depending upon the status of micro structure.
Stage -1 :- No remedial action required.
Stage-2:- Re-inspection required in 3 years.
Stage-3:- Repair or replace with in 6 months.
Stage- 4:- Immediate repair or replacement required.
Based on the above classifications the remnant life of boiler component is worked out with following relation.
trem = t exp[tr / texp -1]
trem = Residual life
texp = Expended life
tr = Rupture life
Evaluation of Chemical deposits:
Analysis of Chemical deposits from boiler drum, internal surface of water walls and external deposits provide valuable information about the trend of aging due to corrosion and identification of its source.
The scale formation varies greatly and can be sub divided into following groups.
- Alkali-Earth Scales: Consisting of Calcium And Magnesium compounds like, CaCo3 , CaSo4 , CaSio3, Ca3(PO4)2 ,MgO, Mg(OH)2 Mg(PO4) 2Depending upon which of these compound prevails. Composite scales are also possible where many compounds are present in small quantities.
- Iron Oxide Scales: Deposits of this kind may contain iron silicates, ferrous phosphates, sodium ferro phosphates , sodium pherrophosphate and Iron oxides in the form of Fe2O3 and Fe3O4.
- Copper Scales: Copper scales containing considerable amount of copper.
- Silicate Scales: These may contain various combination of compositions.
Most important property of silicate scales, is their low thermal conductivity, which may vary with their structure as well as porosity of depositions from 0.12 to 1.2W/MK.
THE FORMATION OF SCALE IMMEDIATELY RESULTS IN INCREASED TEMPERATURE SPECIFIED BY FORMULA :
tw = tm + dsc /sc .q + q/2
Where, tw = Wall temperature
tm = Temperature of media(Water).
dsc = Thickness of scale
sc = Thermal Conductivity of scale
2 = Coefficient of heat transfer from tube wall to the medium(water)
Experience and calculations indicate that with high heat flux in the boiler the metal of boiler tube is over heated dangerously even at a scale thickness of o.1 to 0.2 mm .
The internal scaling have detrimental effect on the remnant life of boilers and owners are advised to go for acid cleaning or other suitable corrective actions in water treatment and chemical dozing. Presence of copper beyond an acceptable limit may initiate galvanic corrosion.
IRC Engineering Services Pvt. Ltd. With an experience of Study of about 150 boilers & 30 turbines , is well equipped in technology and resources to examine all aspects of REMNANT LIFE ASSESSMENT ( RLA)OF BOILERS , STEAM TURBINES & GAS TURBINES and can mobilize service resources ,with in a very short response time .