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Ndt in conventional power generation
 

Ndt in conventional power generation

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Boilers, Headers, Steam lines, Turbines, Feedwater Heaters and Condensers are the main components inspected in a non-nuclear power plant. ...

Boilers, Headers, Steam lines, Turbines, Feedwater Heaters and Condensers are the main components inspected in a non-nuclear power plant.
The reason for inspection depends on the component and its effect on plant operation:

• Boiler tubes and feedwater heater tubes are inspected to avoid forced outages.
• Turbine component inspections are done for safety and operational reasons.
• Steam lines are inspected for safety reasons.
• Condenser tube inspections are primarily done to assess condition for replacement decisions.

Appropriate selection and application of NDT techniques are key to the inspection of power plants.

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    Ndt in conventional power generation Ndt in conventional power generation Document Transcript

    • NDT in Conventional Power Generation: Steam Raising & Rotating PlantCompiled by Irvine Gilbert – October 2012 Page 1 of 7Boilers, Headers, Steam lines, Turbines, Feedwater Heaters and Condensers are the maincomponents inspected in a non-nuclear power plant.The reason for inspection depends on the component and its effect on plant operation: Boiler tubes and feedwater heater tubes are inspected to avoid forced outages. Turbine component inspections are done for safety and operational reasons. Steam lines are inspected for safety reasons. Condenser tube inspections are primarily done to assess condition for replacement decisions.Appropriate selection and application of NDT techniques are key to the inspection of power plants.Table of Contents1. STEAM RAISING PLANT ........................................................................................................................... 21.1 BOILERS .............................................................................................................................................. 21.1.1 OD EROSION, CORROSION AND OVERHEATING .......................................................................................... 21.1.2 HYDROGEN DAMAGE, CAUSTIC CORROSION, CHEMICAL ATTACK ................................................................. 21.1.3 CRACKING - CORROSION FATIGUE, STRESS CORROSION AND THERMAL FATIGUE ........................................... 21.1.4 CREEP - ID OXIDE SCALE ....................................................................................................................... 31.2 HEADERS............................................................................................................................................. 31.3 STEAM LINES........................................................................................................................................ 31.4 CONDENSERS....................................................................................................................................... 31.5 FEEDWATER HEATERS ........................................................................................................................... 42. ROTATING PLANT..................................................................................................................................... 52.1 ROTOR BORE ....................................................................................................................................... 52.2 SOLID ROTOR....................................................................................................................................... 52.3 DISK KEY WAY CRACKING...................................................................................................................... 52.4 DISK BLADE ATTACHMENT AREA ............................................................................................................. 62.5 DISK BLADES........................................................................................................................................ 62.6 BOLTS ................................................................................................................................................. 62.7 GENERATOR RETAINING RINGS ............................................................................................................... 73. CONCLUSIONS ......................................................................................................................................... 7
    • NDT in Conventional Power Generation: Steam Raising & Rotating PlantCompiled by Irvine Gilbert – October 2012 Page 2 of 71. Steam Raising Plant1.1 BoilersBoiler tubes are usually the number one cause of forced outagesin a thermal power plant. There are a total of twenty-two failuremechanisms in boiler tube failures (Lamping 1985). Thesemechanisms are directly responsible for failure of boiler tubes.1.1.1 OD Erosion, corrosion and overheatingOuter Diameter (OD) wall loss in a boiler is caused by erosion, fireside corrosion and short termoverheating. Outer diameter erosion is measured by ultrasonic thickness measurement. Wallthickness measurements are performed with commercially available ultrasonic digital gauges orportable ultrasonic pulser-receivers using a dual transducer. Calibration is performed on a curvedcalibration plate to simulate actual boiler tube geometry. In addition, the alignment of the dualtransducer is maintained the same on both the boiler tube and the calibration block.1.1.2 Hydrogen Damage, Caustic Corrosion, Chemical AttackInner diameter (ID) pitting in boiler tubes may be caused by hydrogen damage, caustic corrosion,chemical attack, etc. Because this type of pitting is usually isolated, a careful examination of the boilertube length is required. Digital gauges are severely limited when measuring tubes with ID pitting.Ultrasonic scattering from ID pits will produce an undefined back surface reflection signal and impairthickness measurement. When measuring the thickness of a tube with ID surface corrosion, aninstrument with a CRT screen display is recommended. The screen presentation will identify the backwall reflection for reliable thickness measurement.Hydrogen damage is one of the mechanisms that produces ID corrosion. This damage is produced inthe water wall tubes from imbalance in water chemistry (Partridge, 1963). Tube bends, circumferentialwelds and tube lengths across the burners are most susceptible locations for such damage. Hydrogendamage is of serious concern because it not only results in ID wall loss but also a zone ofdecarburized material under the corroded area. Ultrasonic thickness scanning is the first step towardsdetection of corrosion caused by hydrogen damage. Since ID corrosion can be caused by othermechanisms, hydrogen damage should be verified by NDT methods. Decarburization caused byhydrogen damage reduces the ultrasonic velocity. Velocity measurement technique should thereforebe applied for verification of such damage (Birring, 1989 ).1.1.3 Cracking - Corrosion Fatigue, Stress Corrosion and thermal FatigueOuter diameter (OD) cracking in a boiler tube can be produced through thermal fatigue, corrosionfatigue, etc. Visual Testing (VT), Magnetic Particle Testing (MT), Penetrant Testing (PT) andRadiography Testing (RT) are commonly applied for detection of OD cracking. The depth sizing ofsuch cracks can be performed visually from the crack length and width or by eddy current testing.Special send-receive eddy current surface probes are recommended for crack sizing.Inner diameter cracks with axial orientation may be caused by stress corrosion and corrosion fatiguemechanisms. Refracted shear waves are used to detect these cracks. Inspection is performed byplacing the transducer on the tubes OD surface with the beam directed towards the area beinginspected. A refracted angle that maximizes reflectivity from the crack should be selected. Maximumreflectivity from the crack is produced when the incident angle on the crack is 45 degrees. Thisincident angle should then be used to calculate the transducers wedge refracted angle. Thecalculated refracted angle is always less than the incident angle.Dissimilar metal weld (DMW) cracking occurs in welds that join the low alloy steels with the stainlesssteel. These welds are present in high temperature sections of the boiler, including the superheaterand reheater sections. The DMW cracking occurs along or near the fusion line between the low alloysteel and the weld. In addition to the crack, there can be presence of an oxide notch that is commonlyfound on the OD surface of the DMW. An oxide notch is initiated because of differences in the creepstrength between a weld metal and the low alloy steel heat affected zone (HAZ). The presence of
    • NDT in Conventional Power Generation: Steam Raising & Rotating PlantCompiled by Irvine Gilbert – October 2012 Page 3 of 7oxide notch is not an indication of crack. Ultrasonics and radiography are two methods to inspectDMWs. When properly applied, these methods can resolve DMW cracking from the oxide notch.1.1.4 Creep - ID oxide ScaleID oxide scale can be produced when tubes in the reheater and superheater have experienced hightemperatures for extended periods of time. The formation of ID scale reduces heat transfer and resultsin a further increase of tube metal temperature. The increase in ID scale and the associated tubemetal temperature promotes creep in the tube metal. Formation of creep results in a loss of strength athigh temperature. The final outcome of excessive scale is a thick lipped, long term overheat failure.Scale thickness measurements should be taken just upstream of material upgrade and thicknessupgrade locations. A history of prior long term overheat failures should also be used to select tubes foroxide scale inspection. The ultrasonic method for measuring scale thickness is based on transmitting awave through the tube thickness. The thickness is calculated by measuring the time differencebetween the signals reflected from the steel/scale interface and the tube ID surface. Because of theextremely small time difference, the application requires the use of high frequency transducers in the15 to 30 MHz range.1.2 HeadersHeaders are inspected for cracking in the welds and ligaments.Weld cracking is inspected by using ultrasonics and WetFluorescent Magnetic Particle Testing (WFMT). WFMT is usedfor OD cracking while UT is used for ID or midwall cracks.Leaks in Headers can be caused by ligament cracking. Ligamentcracking is produced in the bore holes and the stub tube ID. Thecracking occurs due to cyclic events such as startup andshutdowns, transients and thermal shocks. The hottest areas arethe most susceptible to ligament cracking, however, there can beexceptions.The most reliable method for detection of ligament cracking is to first remove the stub tube and thenperform penetrant inspection. ID grinding is first done to remove the oxide scale. This is followed bythe wet fluorescent penetrant method. The inspection determines the length of the crack in the borehole region. The crack length information is the used to make decision on the disposition of theheader.1.3 Steam LinesInspection of steam lines is done to detect cracking in the welds.Bending loads can produce OD cracking in the circumferentialwelds. WFMT is the recommended approach for such aninspection. The inspection should also be done on the hangerwelds to determine their integrity.High temperature creep can cause midwall cracking or IDconnected cracking in seam welds. Over a long period of time,creep voids can grow to microcracks interlink and cause failureof a long seam weld (Viswanathan, 1989). The inspection of longseam welds gained significant coverage in technical literature after their failures caused loss of life.The failure of a steam line can either be a "leak before failure" or a rupture. The type of failuredepends on the length of the crack. Cracks longer than the critical length can result in rupture.Ultrasonic testing is the recommended approach for such inspection. The inspection is done withrefracted shear waves. Because of the pipe curvature, proper selection of refracted angles is key tothis inspection. The refracted angle is always higher than the incident angle at the crack.1.4 CondensersThe main reason for inspection of condenser tubes is to determine the condition of the tubes. Theinformation from the inspection can then be used to make decisions on replacement of all the tubes.The inspection of condensers is normally limited to a 5 to 10 percent random sample of tubes.The most common tube materials in a feedwater heater are copper-nickel alloys, brass, titanium,stainless steel and ferritic stainless steel. Pitting is the most common form of damage in condenser
    • NDT in Conventional Power Generation: Steam Raising & Rotating PlantCompiled by Irvine Gilbert – October 2012 Page 4 of 7tubing. OD erosion/corrosion is very common in brass tubing. Tubes in the top row are susceptible toOD erosion.The inspection techniques for condenser tubes depend on the material. Conventional eddy current isapplied for non-ferromagnetic materials such as:copper-nickel alloys, brass, titanium and stainless steel tubing. Conventional eddy current can,however, not be used on ferromagnetic materials such as thinferritic stainless steel tubing. For such materials full saturation eddy current technique is applied.Because of the long length of the tubing, inspection of condenser tubing is done at high speed pusherpullers.1.5 Feedwater HeatersTube failures in feedwater heaters are one of the major causesof forced outages in a fossil power plant. Inspection of HPfeedwater Heaters produces one of the highest cost benefits ofany NDT inspection in a power plant.The most common tube materials in feedwater heaters arecarbon steel, stainless steel, brass and copper-nickel alloys. ODerosion is the most common type of damage in carbon steeltubes. The locations most susceptible to OD wear are the draincooler section and the desuperheating zone.Pitting can occur in tubes made out of stainless steel and copper alloys.The tubing in the feedwater heaters should be periodically inspected to determine its condition.Depending on the rate of degradation, an inspection interval of 3 to 6 years is recommended.Selection of tubes for inspection is key to an effective feedwater heater inspection. For a regularinspection the plan should include tubes in the drain cooler section, tubes in the desuperheating zone,tubes around previously plugged tubes and some tubes at random. In addition to regular inspections,inspection after a tube failure is highly recommended. A tube failure is an indication of damage andimpeding tube failures. During such an emergency inspection, tubes around the leaking tubes shouldbe tested. Tubes with damage above certain level should be immediately plugged. Such an approachresults in effective plugging and avoids future forced outages.Conventional eddy current is applied for non-ferromagnetic materials. Remote Field Eddy current isquite effective for inspection of carbon steel tubing. Unlike conventional eddy current, this technique isonly sensitive to wall loss and not pitting. However, pitting is not a problem in carbon steel tubing. Inaddition to remote field, ultrasonic IRIS technique can also be applied for inspection while the IRIStechnique is more accurate, it is slow compared to remote field eddy current technique. In generalRemote field is used for the normal inspection and IRIS can be used for verification.
    • NDT in Conventional Power Generation: Steam Raising & Rotating PlantCompiled by Irvine Gilbert – October 2012 Page 5 of 72. Rotating PlantThere are several components that are inspected in a turbine.These include bore, disk keyway, disk blade attachment area,blades, nozzles, casing, bolts, etc.2.1 Rotor BoreThe mechanism of crack growth in a rotor bore is due to the combined action of creep and fatigue(Viswanathan, 1989). Creep is more prevalent in HP rotors that operate at temperatures around1000°F. Fatigue is more prevalent in LP rotors that operate at lower temperatures. Sensitivity of thebore examination depends on locations that experience the highest level of stress and temperature.The hoop stress is higher under the disks because of mass loading. The temperature is highest underthe control stage. Sensitivity of examination should therefore be highest in the HP rotors at the ID boresurface under the HP disks.Three methods are commonly used for bore inspection:1. Magnetic particle testing,2. Eddy current and3. Ultrasonics.The first two methods are limited to surface cracking. Magnetic particle is performed by applying acircumferential magnetic field at the bore ID.The circumferential field detects axial cracks on the bore surface.The second approach for surface crack detection is the eddy current method. Ultrasonic inspection isthe only method that can perform a complete volumetric examination.A combination of transducer angles is used to perform the inspection. The transducers are installed ona scanner and the data is recorded on an ultrasonic imaging system. The detection sensitivity iscontrolled by adjusting the scan step interval.2.2 Solid RotorThe main advantage of a boreless rotor is its lower level of stress compared to the bored rotor. Thelower level of stress makes the boreless rotor tolerant to larger flaws. Because of this reason,inspection procedures for boreless rotors are less stringent. The inspection of a solid rotor isperformed by using a combination of L-wave transducers and S-wave transducers. The L-wavetransducers can detect flaws directly below the transducer. However, rotor sections directly below thedisks cannot be inspected with the 0° transducers. These locations are inspected using refracted S-wave transducers. A range of refracted angles, between 40° to 70°, is used to assure a completevolumetric examination. Inspection of boreless rotors requires that the selected angles be able toinspect the entire material volume of interest.Transverse cracking in LP rotors initiates from corrosion pits can grow during service by corrosion-fatigue. Transverse cracking is easily detected by application of magnetic particle testing (MT) on therotor OD surface. The depth of the crack can be measured using the ultrasonic tip diffraction method.2.3 Disk Key Way CrackingThe primary cause of disk keyway cracking is stress corrosion. High stress concentration in thekeyway region promotes growth of this cracking. Because of the SCC mechanism, keyway cracking ismostly observed past the Wilson Line. In some cases stress corrosion cracking may be found beforethe Wilson Line if condensation occurred during standby. Inspection of keyway cracking is performedusing a range of ultrasonic refracted angles. A combination of transducer angles for each disk isselected so that the entire length of the keyway can be inspected.
    • NDT in Conventional Power Generation: Steam Raising & Rotating PlantCompiled by Irvine Gilbert – October 2012 Page 6 of 7Both pulse-echo and pitch-catch modes are used during inspection. The pulse-echo mode is preferredas it is easier to apply and interpret. Normally, the ends of the disks are inspected in this mode. Themiddle section of the disk cannot be inspected with the pulse echo mode. This area of the keyway isinspected in the pitch-catch mode. In this mode, a transducer is placed on each side of the turbinedisk; one transducer transmitting and the other receiving ultrasound. Alignment of the transducers, inthe pitch-catch mode, is very critical to assure a reliable inspection.2.4 Disk Blade Attachment AreaThe mechanism of crack initiation and growth in turbine diskblade attachment (steeples) depends on three variables: theoperating temperature, stresses and environment. Creep is theprimary mechanism in HP and IP rotors. Stress CorrosionCracking (SCC), combined with fatigue, is the primarymechanism for LP rotors. Initially, cracking in an LP rotor growsslowly by SCC. When the stress intensity KI exceeds Kth, crackgrowth is predominantly due to fatigue. Crack growth rates inthis mode are significantly high because of vibratory loads.Generally, failure can be imminent when the threshold for fatigue crack growth Kth is reached.Therefore it is important that NDE inspections detect cracks before their stress intensity reaches Kth.The inspection methods applied to detect steeple cracking depends on the geometry. Dovetail design(GE turbines) can be inspected only by ultrasonics (Bentzel, 1993). This design does not allow accesson the surface for eddy current or magnetic particle testing. On the contrary, side entry steeples(Westinghouse turbines) allow access to the side surface. In addition to ultrasonic testing, these diskscan be inspected by eddy current testing and magnetic particle testing. However, ultrasonics is theonly method that is capable of inspecting the entire length of the side entry steeple under the blade.Once the blades are removed, WFMT is the preferred method for inspecting steeples. WFMT isperformed with a yoke on each steeple individually. The process is slow, but results in a highlysensitive inspection. It can also get evidence of cracking at early stages.2.5 Disk BladesThe failure mechanism of turbine blades is dependent on their temperature, environment and stressstate. Corrosion fatigue is the major failure mechanism of blades in the next-to-last stage of the low-pressure turbine. Creep blade failures are limited to HP turbines. Cracking of blades occurs at thefollowing three locations: blade attachments, airfoil and tenon. The inspection methods chosen foreach of these locations depend on whether the inspection is performed with the blade removed fromthe disk or not.Eddy current and magnetic particle are the two methods used to inspect the blade attachment areas ofside entry blades. Eddy Current inspection is an attractive method since it can be performed withoutremoval of turbine from the casing. During examination, port holes in the turbine casing are used togain access to the blades. An eddy current probe along with a fiber optic probe are held on the end ofa rod which is inserted in the casing through a port-hole for inspection.Once the turbine has been removed from the casing, the blade attachments become directlyaccessible for inspection. Either eddy current or magnetic particle testing may be used at this stage.Magnetic particle testing is the preferred method because it is faster. WFMT is performed with ACcoils or a yoke.WFMT is the most commonly used method for inspecting blade length. The inspection is performed bymagnetizing the blades with AC coils. The AC magnetization allows a highly sensitive examination onthe surface, while leaving minimal residual magnetism in the blades. Blade tenons are located at thetip of the blades and hold the shroud. Cracking and failure of the tenons may release the shroud andcause mechanical damage to other blades. The only method available to inspect blade tenons isultrasonics. An ultrasonic transducer is placed on the tenon. A flat surface on the tenon is required sothat a contact with a transducer can be achieved. Tenons without a flat face cannot be inspectedunless they are ground flat.2.6 BoltsCreep-rupture and brittle fracture are two primary reasons for bolt failures. The low toughness thatleads to brittle fracture is due to the inherent high strength of bolts. The failures are usually initiationcontrolled. Hence, the failure time is very short after the crack initiation.
    • NDT in Conventional Power Generation: Steam Raising & Rotating PlantCompiled by Irvine Gilbert – October 2012 Page 7 of 7Ultrasonic testing is the only method that can inspect bolts without removing them from the casing.Two different ultrasonic approaches are used for this inspection. A zero degree examination isperformed when the top surface of the bolt is flat, since it allows placement of a normal beamtransducer. But when the top face of the bolt is not flat, an angle beam exam is performed through theheater holes.Cracking in bolts occurs only in the threads next to the joint. These threads experience the highestlevel of stress. The stress on the last thread at the end of the bolt is almost zero. Therefore, theinspector should carefully investigate threads right next to the joint.2.7 Generator Retaining RingsThe susceptibility of 18 Mn 5 Cr steels to SCC producescracking in retaining rings. Initiation of cracks in the retainingrings occurs when moisture enters and settles on the innerdiameter (ID) surface.The initiation time of the cracks is quite long. Nevertheless, oncethe crack has initiated, crack growth can be quite rapid.The high crack growth rates limit the application of NDE forcrack detection. No effort is made to size the cracks once theyare detected. Repair or replacement actions are initiated once a crack is positively detected.Four methods are generally used when inspecting the retaining rings:1. Visual testing2. Fluorescent penetrant testing3. Eddy current4. Ultrasonic testing.Ultrasonic testing (UT) is the only method that may be applied without removal of the retaining ring,however, its detection sensitivity is limited. A combination of adverse factors, such as high ultrasonicattenuation and spurious geometrical reflection, result in the low UT detection sensitivity.Visual, eddy current and fluorescent methods can be applied after ring removal. Visual inspection isaccomplished using borescopes that detect moisture in accessible areas of the ID surface.Evidence of moisture is an indication of possible crack initiation. The penetrant examinations areperformed using the fluorescent penetrant method. High sensitivity Lipophilic emulsifiers are used forthese inspections.3. ConclusionsNDE is necessary if we are to gain essential knowledge of safety critical plant.There are a variety of components in power plants and for each of these components, there can bedifferent types of flaws and damage mechanisms.This depends on many factors around in-service operation and may include cracks, pitting, materialdegradation, etc.Several types of NDT methods have to be implemented, and a careful selection of NDT methods isnecessary in order to provide effective NDT of power plants.