Figure 7-4: Typical Desuperheater Details
Superheater and desuperheater liner inspections should be conducted every three to five years.
This inspection is accomplished with a boroscope. The boroscope is inserted through the spray water nozzle opening after removing the spray water control valve or via a hand hole plate.
Examine the liner for any gross deformations. Examine the spray nozzles for any enlargement of the nozzle holes. If extensive wastage is found, replace the spray nozzle.
7.6 TUBE FAILURE ANALYSIS (Short Term Overheating)
For a specific tube material, there is a maximum allowable stress at a particular temperature. If the tube metal temperature increases beyond this point, creep will occur and the tube will eventually fail by stress rupture.
Superheaters and reheaters can experience interruptions and/or reductions in steam flow that can increase tube metal temperatures that lead to stress rupture failures.
With ferritic steel, a "fish mouth" or longitudinal rupture, with a thin edge fracture is most likely.
With other tube materials, still other appearances are possible. The causes for this type of failure are the following:
• Abnormal coolant flow from a blockage in the tube
• Blockage due to debris in the tube
• Blockage due to scale in the tube
• Blockage due to condensate in the tube following an incomplete boilout
• Excessive combustion gas temperatures
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Figure 7-5: Short Term Overheating Appearance
High Temperature Creep
A small fracture may be associated with a blister, while a large fracture could have a thick edged, “fish mouth”, longitudinal crack. The area around the fracture may have an alligator hide appearance, with significant oxide scale penetration. The root causes for high temperature, longer term failure such as these are the following:
• High heat flux into a section of the boiler that could have used a higher grade of steel
• Excessive hot gas flow through an area that is plugged
• Excessive heat absorption from an adjacent lug, or other welded attachment
• Partial pluggage from blockage or internal scale
Figure 7-6: High Temperature Creep
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Dissimilar Metal Welds
The weld failures will normally have one side of the weld that responds to a magnet, while the other does not. The weld crack will be circumferential at the weld; over on the side that responds to the magnet; the ferritic side. The cause of failure relate the stress of the two metals expanding differently plus:
• Stress from internal steam pressure
• Stress from the vertical weight on the weld
• Stress from the constraints of how the tube is supported or attached
• Internal thermal gradients, which add up to the total stress. The higher the value, the sooner the weld fails.
Figure 7-7: Dissimilar Metal Welds
Appearance Welding Defects
If the defect is most notable on the inside, it can become a failure from an internal scale build-up, and resultant corrosion, or corrosion fatigue failure. If the defect is with the integrity of the weld itself, the failures often appear as a brittle failure, where stress is concentrated in a small area. Causes again relate to quality control:
• The procedure.
• Weld material used.
• Preparation of the tube ends before the first pass.
Pitting - Localized Corrosion (Water-Side Corrosion)
Water containing dissolved oxygen is highly corrosive to many metals; therefore everything must be done to minimize the introduction of oxygenated water into the boiler and pre-boiler systems. Oxygen corrosion can dramatically affect various components in operating and non-operating boilers. Much of the suspended crud that enters an non-operating boiler is the direct result of oxygen attack of components in the pre-boiler system.
Localized pitting is found where oxygen is allowed to come in contact with the inside of the tubes, which is just about anywhere. It appears as a steep edged crater with red iron oxide surrounding the pit. The tube surface near the pit may show little or no attack. Sometimes there is a series of smaller pits. The typical cause starts with:
• High levels of oxygen in the feedwater, i.e., poor deaeration at start-up
• Filling of condensate in low point, such as bends, when the steam cools
• Outages where air gets inside the assembly from adjacent repairs, or vents being left open as the steam condenses
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Figure 7-8: Localized Pitting Appearance
Stress Corrosion Cracking (Water-Side Corrosion)
These thick-edged fractures can be either circumferential or longitudinal, depending on how the stress is oriented. Typically the chemical attack is on the inside of the tube and works its way out through the growing crack. Far less commonly, the chemical attack exists on the outside (gas side) and works its way inward. The root cause is the coupling of more than one factor working on the same location:
From the chemistry side are the contaminants of chlorides, sulfates, or hydroxides on either the inside (common) or outside (less common)
• Contaminants can come from boiler steam drum carry over
• Contaminants can come from contamination in the desuperheater spray
• External contaminants come from acidic components to the fuel
• Additionally there must be a stress possibly from a bend in the tube
• Weld attachments from initial assembly
• Or possibly from cyclic unit operation
Figure 7-9: Stress Corrosion Appearance Low Temperature Corrosion (Gas Side)
External surfaces of economizer tubes that are exposed to a moist environment containing flue gases can experience acid corrosion. Certain acidic salts (ferrous sulfate for example) can hydrolyze in moist environments to produce low pH conditions that will attack carbon steel.
Sulfur trioxide (SO3), present in the cooler flue gas areas, and can react with water vapor to produce sulfuric acid. If the temperature is below the dew point, sulfuric acid condenses along metal surfaces and corrodes the metal. Water washing can also produce acid attack.
A gouged exterior and a thin ductile failure characterize this form of failure. When the pressure becomes too great, the pressure inside blows out a hole. The root cause for low temperature failures are:
• The presence of sulfur in the oil, which has an opportunity to condense on the last rows of economizer tubes.
• The condensing of sulfur and ash when the exit gas temperature is low.
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Figure 7-10: Low Temperature Corrosion Appearance
Vibration Fatigue
In locations where boiler tubes are welded to support lugs, a thick edge failure can form at the toe of the weld. This fracture is circumferential, running at right angles to the weld.
The root cause is:
• The vibration of the tube, caused by the steady flow of exhaust gases.
• Along with a lug location that induces a rigid point that will concentrate the force into a short distance.
Figure 7-11: Vibration Fatigue Appearance
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Corrosion Fatigue
Like the previous fatigue mechanism, cyclic stresses produce a series of parallel surface cracks, however this time the corrosive environment adds to the deterioration by forcing the oxide wedge into the cracks, further leveraging the fracture. The thick edge fracture will be coated with an oxide layer. Pits can often be found on the inside surface of the cracks. The causes have two key ingredients which are Corrosion and Stress. There is either induced stress from the way the tube connects to another pressure part or there is induced stress from the way the tube is tied to a structural support.
• There is residual stress left over from fabrication.
• Internal pits from dissolved oxygen or acidic corrosion from the pre-boiler circuit aggravate the cracking process in the water cooled tubes.
• External corrosion in steam cooled units aggravates the cyclic flexing where the tube enters the header.
Figure 7-12: Corrosion Fatigue Appearance
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ALSTOM Power Revision: 0 8-1 SECTION 8: HRSG START UP CURVES
8.1 LEARNING OBJECTIVES
Understand HRSG operational phenomena under various conditions
8.2 DESCRIPTION OF CURVES