In most fossil plants, boiler tube failures are usually the number one cause of forced outages. There are a total of 22 mechanisms in boiler tube failures. These mechanisms are directly responsible for failure of boiler tubes.
- 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.
- 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. 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.
- Cracking: Corrosion/Thermal Fatigue & Stress Corrosion
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. Most recently advancements in Computed Radiography has gained wide spread acceptance as the inspection of choice to identify corrosion fatigue cracking.
Dissimilar metal weld (DMW) cracking occurs in welds that join the low alloy steels with the stainless steel. These welds are present in high Ultrasonics and radiography are two methods to inspect DMWs. When properly applied, these methods can resolve DMW cracking from the oxide notch.
- Creep-ID Oxide Scale
Creep-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. Ultrasonics thickness measurements should be taken to detect ID Scale.