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NDT in Non-Nuclear Power Generation: Pressure Vessels, Piping, Turbines by Anmol S. Birring 12/25/2010 10:16:40 AM 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. Inspection of turbine components is done for safety and operational reasons. Steam lines are inspected for safety reasons. The inspection of condenser tubes is primarily done to assess condition for replacement decisions. Appropriate selection and application of NDT techniques are key to the inspection of non-nuclear power plants.
NDT in Non-Nuclear Power Generation: Pressure Vessels, Piping, Turbines by Anmol S. Birring Terms of Use
Published in The Encyclopedia of Materials: Science and Technology, Elsevier Science Limited, UK, 2001  

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. Inspection of turbine components is done for safety and operational reasons. Steam lines are inspected for safety reasons. The inspection of condenser tubes is primarily done to assess condition for replacement decisions. Appropriate selection and application of NDT techniques are key to the inspection of non-nuclear power plants.

1. Boilers

Boiler tubes are usually the number one cause of forced outages in a thermal power plant. There are a total of twenty-two failure mechanisms in boiler tube failures (Lamping 1985). These mechanisms are directly responsible for failure of boiler tubes.

1.1 OD Erosion, corrosion and overheating.

Outer Diameter (OD) wall loss in a boiler is caused by erosion, fireside corrosion and short term overheating. Outer diameter erosion is measured by ultrasonic thickness measurement. Wall thickness measurements are performed with commercially available ultrasonic digital gauges or portable ultrasonic pulser-receivers using a dual transducer. Calibration is performed on a curved calibration plate to simulate actual boiler tube geometry. In addition, the alignment of the dual transducer is maintained the same on both the boiler tube and the calibration block.

1.2 Hydrogen Damage, Caustic Corrosion, Chemical Attack

Inner 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 boiler tube 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 impair thickness measurement. When measuring the thickness of a tube with ID surface corrosion, an instrument with a CRT screen display is recommended. The screen presentation will identify the back wall reflection for reliable thickness measurement.

Hydrogen damage is one of the mechanisms that produces ID corrosion. This damage is produced in the water wall tubes from imbalance in water chemistry (Partridge, 1963). Tube bends, circumferential welds and tube lengths across the burners are most susceptible locations for such damage. Hydrogen damage is of serious concern because it not only results in ID wall loss but also a zone of decarburized material under the corroded area. Ultrasonic thickness scanning is the first step towards detection of corrosion caused by hydrogen damage. Since ID corrosion can be caused by other mechanisms, hydrogen damage should be verified by NDT methods. Decarburization caused by hydrogen damage reduces the ultrasonic velocity. Velocity measurement technique should therefore be applied for verification of such damage(Birring, 1989 ).

1.3 Cracking - Corrosion Fatigue, Stress Corrosion and thermal Fatigue.

Outer diameter (OD) cracking in a boiler tube can be produced through thermal fatigue, corrosion fatigue, etc. Visual Testing (VT), Magnetic Particle Testing (MT), Penetrant Testing (PT) and Radiography Testing (RT) are commonly applied for detection of OD cracking. The depth sizing of such 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 fatigue mechanisms. Refracted shear waves are used to detect these cracks. Inspection is performed by placing the transducer on the tube's OD surface with the beam directed towards the area being inspected. A refracted angle that maximizes reflectivity from the crack should be selected. Maximum reflectivity from the crack is produced when the incident angle on the crack is 45 degrees. This incident angle should then be used to calculate the transducer's wedge refracted angle. The calculated 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 stainless steel. These welds are present in high temperature sections of the boiler, including the superheater and reheater sections. The DMW cracking occurs along or near the fusion line between the low alloy steel and the weld. In addition to the crack, there can be presence of an oxide notch that is commonly found on the OD surface of the DMW. An oxide notch is initiated because of differences in the creep strength between a weld metal and the low alloy steel heat affected zone (HAZ). The presence of oxide notch is not an indication of crack. Ultrasonics and radiography are two methods to inspect DMWs. When properly applied, these methods can resolve DMW cracking from the oxide notch.

1.4 Creep - ID oxide Scale

ID oxide scale can be produced when tubes in the reheater and superheater have experienced high temperatures for extended periods of time. The formation of ID scale reduces heat transfer and results in a further increase of tube metal temperature. The increase in ID scale and the associated tube metal temperature promotes creep in the tube metal. Formation of creep results in a loss of strength at high 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 thickness upgrade locations. A history of prior long term overheat failures should also be used to select tubes for oxide scale inspection. The ultrasonic method for measuring scale thickness is based on transmitting a wave through the tube thickness. The thickness is calculated by measuring the time difference between the signals reflected from the steel/scale interface and the tube ID surface. Because of the extremely small time difference, the application requires the use of high frequency transducers in the 15 to 30 MHz range.

2. Headers

Headers are inspected for cracking in the welds and ligaments. Weld cracking is inspected by using ultrasonics and Wet Fluoroscent Magnetic Particle Testing (WFMT). WFMT is used for OD cracking while UT is used for ID or midwall cracks.

Leaks in Headers can be caused by ligament cracking. Ligament cracking is produced in the bore holes and the stub tube ID. The cracking occurs due to cyclic events such as startup and shutdowns, transients and thermal shocks. The hottest areas are the most susceptible to ligament cracking, however, there can be exceptions.

The most reliable method for detection of ligament cracking is to first remove the stub tube and then perform penetrant inspection. ID grinding is first done to remove the oxide scale. This is followed by the wet fluoroscent penetrant method. The inspection determines the length of the crack in the bore hole region. The crack length information is the used to make decision on the disposition of the header.

3. Steam Lines

Inspection of steam lines is done to detect cracking in the welds. Bending loads can produce OD cracking in the circumferential welds. WFMT is the recommended approach for such an inspection. The inspection should also be done on the hanger welds to determine their integrity.

High temperature creep can cause midwall cracking or ID connected cracking in seam welds. Over a long period of time, creep voids can grow to microcracks interlink and cause failure of a long seam weld (Viswanathan, 1989). The inspection of long seam 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 failure depends 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 with refracted shear waves. Because of the pipe curvature, proper selection of refracted angles is key to this inspection. The refracted angle is always higher than the incident angle at the crack.

4. Turbines

There are several components that are inspected in a turbine. These include bore, disk keyway, disk blade attachment area, blades, nozzles, casing, bolts, etc.

4.1 Bore

The 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 around 1000°F. Fatigue is more prevalent in LP rotors that operate at lower temperatures. Sensitivity of the bore 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 under the control stage. Sensitivity of examination should therefore be highest in the HP rotors at the ID bore surface under the HP disks.

Three methods are commonly used for bore inspection: magnetic particle testing, eddy current and ultrasonics. The first two methods are limited to surface cracking. Magnetic particle is performed by applying a circumferential 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 is the only method that can perform a complete volumetric examination. A combination of transducer angles is used to perform the inspection. The transducers are installed on a scanner and the data is recorded on an ultrasonic imaging system. The detection sensitivity is controlled by adjusting the scan step interval.

4.2 Solid Rotor

The main advantage of a boreless rotor is its lower level of stress compared to the bored rotor. The lower 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 is performed by using a combination of L-wave transducers and S-wave transducers. The L-wave transducers can detect flaws directly below the transducer. However, rotor sections directly below the disks 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 complete volumetric examination. Inspection of boreless rotors requires that the selected angles be able to inspect 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 the rotor OD surface. The depth of the crack can be measured using the ultrasonic tip diffraction method.

4.3 Disk keyway Cracking

The primary cause of disk keyway cracking is stress corrosion. High stress concentration in the keyway region promotes growth of this cracking. Because of the SCC mechanism, keyway cracking is mostly observed past the Wilson Line. In some cases stress corrosion cracking may be found before the Wilson Line if condensation occurred during standby. Inspection of keyway cracking is performed using a range of ultrasonic refracted angles. A combination of transducer angles for each disk is selected so that the entire length of the keyway can be inspected.

Both pulse-echo and pitch-catch modes are used during inspection. The pulse-echo mode is preferred as it is easier to apply and interpret. Normally, the ends of the disks are inspected in this mode. The middle section of the disk cannot be inspected with the pulse echo mode. This area of the keyway is inspected in the pitch-catch mode. In this mode, a transducer is placed on each side of the turbine disk; one transducer transmitting and the other receiving ultrasound. Alignment of the transducers, in the pitch-catch mode, is very critical to assure a reliable inspection.

4.4 Disk - Blade Attachment Area

The mechanism of crack initiation and growth in turbine disk blade attachment (steeples) depends on three variables: the operating temperature, stresses and environment. Creep is the primary mechanism in HP and IP rotors. Stress corrosion cracking(SCC), combined with fatigue, is the primary mechanism for LP rotors. Initially, cracking in an LP rotor grows slowly by SCC. When the stress intensity KI exceeds Kth, crack growth is predominantly due to fatigue. Crack growth rates in this 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 access on 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 disks can be inspected by eddy current testing and magnetic particle testing. However, ultrasonics is the only 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 is performed with a yoke on each steeple individually. The process is slow, but results in a highly sensitive inspection. It can also get evidence of cracking at early stages.

4.5 Blades

The failure mechanism of turbine blades is dependent on their temperature, environment and stress state. 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 the following three locations: blade attachments, airfoil and tenon. The inspection methods chosen for each of these locations depend on whether the inspection is performed with the blade removed from the disk or not.

Eddy current and magnetic particle are the two methods used to inspect the blade attachment areas of side entry blades. Eddy Current inspection is an attractive method since it can be performed without removal of turbine from the casing. During examination, port holes in the turbine casing are used to gain access to the blades. An eddy current probe along with a fiber optic probe are held on the end of a 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 directly accessible 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 AC coils or a yoke.

WFMT is the most commonly used method for inspecting blade length. The inspection is performed by magnetizing the blades with AC coils. The AC magnetization allows a highly sensitive examination on the surface, while leaving minimal residual magnetism in the blades. Blade tenons are located at the tip of the blades and hold the shroud. Cracking and failure of the tenons may release the shroud and cause mechanical damage to other blades. The only method available to inspect blade tenons is ultrasonics. An ultrasonic transducer is placed on the tenon. A flat surface on the tenon is required so that a contact with a transducer can be achieved. Tenons without a flat face can not be inspected unless they are ground flat.

4.6 Bolts

Creep-rupture and brittle fracture are two primary reasons for bolt failures. The low toughness that leads to brittle fracture is due to the inherent high strength of bolts. The failures are usually initiation controlled. Hence, the failure time is very short after the crack initiation.

Ultrasonic 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 is performed when the top surface of the bolt is flat, since it allows placement of a normal beam transducer. But when the top face of the bolt is not flat, an angle beam exam is performed through the heater holes.

Cracking in bolts occurs only in the threads next to the joint. These threads experience the highest level of stress. The stress on the last thread at the end of the bolt is almost zero. Therefore, the inspector should carefully investigate threads right next to the joint.

4.7 Retaining Rings

The susceptibility of 18 Mn 5 Cr steels to SCC produces cracking in retaining rings. Initiation of cracks in the retaining rings occurs when moisture enters and settles on the inner diameter (ID) surface. The initiation time of the cracks is quite long. Nevertheless, once the crack has initiated, crack growth can be quite rapid. The high crack growth rates limit the application of NDE for crack detection. No effort is made to size the cracks once they are 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 testing, 2) fluorescent penetrant testing, 3) eddy current, and 4) 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 ultrasonic attenuation 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 is accomplished 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 are performed using the fluorescent penetrant method. High sensitivity Lipophilic emulsifiers are used for these inspections.

5. Condensers

The main reason for inspection of condenser tubes is to determine the condition of the tubes. The information 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 tubing. OD erosion/corrosion is very common in brass tubing. Tubes in the top row are susceptible to OD erosion.

The inspection techniques for condenser tubes depend on the material. Conventional eddy current is applied 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 thin

ferritic 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 pusher pullers.

6. Feedwater Heaters

Tube failures in feedwater heaters are one of the major causes of forced outages in a fossil power plant. Inspection of HP feedwater Heaters produces one of the highest cost benefits of any NDT inspection in a power plant.

The most common tube materials in feedwater heaters are carbon steel, stainless steel, brass and copper-nickel alloys. OD erosion is the most common type of damage in carbon steel tubes. The locations most susceptible to OD wear are the drain cooler 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 regular inspection 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 and impeding tube failures. During such an emergency inspection, tubes around the leaking tubes should be tested. Tubes with damage above certain level should be immediately plugged. Such an approach results in effective plugging and avoids future forced outages.

Conventional eddy current is applied for non-ferromagnetic materials. Remote Field Eddy current is quite effective for inspection of carbon steel tubing. Unlike conventional eddy current, this technique is only sensitive to wall loss and not pitting. However, pitting is not a problem in carbon steel tubing. In addition to remote field, ultrasonic IRIS technique can also be applied for inspection while the IRIS technique is more accurate, it is slow compared to remote field eddy current technique. In general Remote field is used for the normal inspection and IRIS can be used for verification

7. Conclusions

There are a variety of components in non-nuclear power plants. For each of these components, there can be different types of flaws and damage. This may include cracks, pitting, material degradation, etc. Because of this combination of component types and defect types, several types of NDT methods have to be implemented. A careful selection of NDT methods is necessary for effective NDT of non-nuclear power plants.


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