Steam Boilers are the backbone for processing operations in industries. Plant managers take every measure to ensure the safety of the steam boiler and the operating personnel. One such measure includes maintaining the boiler tubes. Overlooking boiler tubes can lead to hazardous consequences like overheating and boiler failure. Examining the root cause of the boiler tube failures is crucial to reduce the possibility of future boiler issues. An overall assessment is required to examine the cause of boiler tube failure that is usually a cause of other boiler problems. Some of the most common boiler tubes that occur on steam boilers are as follows:
Waterside failure for boiler tube:
- Caustic Attack:
The caustic attack takes place due to excessive deposition on the boiler tube surfaces, leading to reducing the cool water flow in contact with the tube. As a result, it causes local under-deposit boiling and boiler water chemicals concentration. It leads to a caustic condition that corrosively attacks and breaks down protective magnetite if blended with boiler water chemistry with high pH.
It is identified by localized wall loss on the surface of the boiler tube that increases the stress and strain in the tube wall and impacts furnace wall tubes or any inclined tube. Preventing caustic attacks requires avoiding the accumulation of excessive deposits and maintaining boiler water chemistry for preventing the local formation of caustic in areas with chemicals concentrated. In some cases of caustic attack, the top of a sloped tube is related to the steam-water separation that is avoided by using ribbed tubes. Maintaining boiler water chemistry is ensured by appropriate feed water chemistry with phosphate boiler water treatments.
- Acid Attack:
The acid attack occurs due to poor process control of boiler chemical cleanings or inadequate cleaning of residual acid. The tube metal surfaces result in an irregular pitted appearance on the boiler tube and affect water wall tubes.
It is prevented by maintaining water chemistry, proper controls for boiler chemical cleanings, and assessing the chemical hideout.
- Hydrogen Damage:
Hydrogen damage in the steam boiler is the result of excessive deposition on boiler tube surfaces and boiler water with less pH excursion. Inappropriate water chemistry from condenser leaks with saltwater cooling medium leads to acidic contaminants that are concentrated in the deposit and causes under-deposit corrosion releasing atomic hydrogen that passes to the tube wall metal and reacts with carbon in the steel. It causes intergranular separation.
Hydrogen damage is detected by intergranular micro-cracking, ductility loss, and boiler tube material embrittlement. It leads to brittle catastrophic rupture. It usually takes place in regions of high heat flux and can be prevented by maintaining water chemistry and avoiding scaling on the waterside of the boiler tube.
- Oxygen Pitting:
Oxygen pitting occurs by the excess oxygen in boiler water that takes place during operation due to in-leakage of air at pumps or failure in boiler water treatment. It also occurs during extended out-of-service periods of steam boilers. Aggressive localized boiler tube corrosion and loss in tube walls are signs of oxygen pitting.
Proper layup procedures during steam boiler outages, enhanced oxygen control during steam boiler startups, and operations can prevent oxygen pitting.
- Stress Corrosion Cracking:
Stress corrosion cracking occurs due to the combination of high-tensile stresses and the presence of corrosive fluid. It results from cracks that propagate from the boiler tube. The corrosive fluid results from the carryover into the superheater from the steam drum.
Stress corrosion cracking is identified by a thick wall, brittle-type crack at locations of higher external stresses. Stress corrosion cracks are usually related to various secondary cracks branched with the main ruptured area. The ideal solution for stress corrosion cracking includes preventing water carryover control and flush after cleaning.
- Waterside Corrosion Fatigue:
Waterside corrosion fatigue occurs due to thermal fatigue and corrosion. Corrosion fatigue depends on steam boiler design, water chemistry, boiler water oxygen, and steam boiler operation, leading to the breakdown of the protective layer on the surface of the boiler tube that prevents the boiler tube from corrosion.
Waterside corrosion fatigue can be detected by wide transgranular cracks taking place adjacent to external attachments. It is prevented by minimizing the operational cycles, constraints on tubes, and reducing dissolved oxygen on start-up.
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Steam boilers are an integral asset in the process and manufacturing industries. Boiler tube failures are one of the major concerns of the process plant managers as they can cause severe steam boiler issues if overlooked. In such situations, it is advisable to contact an expert boiler manufacturer to seek guidance and immediate action to prevent boiler tube failure. Usually, a boiler tube failure causes other boiler problems. Therefore, it is important to perform a careful examination to understand the cause of the boiler tube failure, neglecting which may lead to the steam boiler failure and related expenses for the same.
Since it is vital to prevent severe damage in the process plant and the operating personnel, here are the listed causes and solutions of boiler tube failures in fireside of the steam boiler:
- Fuel Ash Corrosion:
Fuel ash corrosion results from the ash characteristics of the fuel and the design of the steam boiler. It occurs usually in coal-fired boilers but also takes place in oil-fired boilers. The ash is considered in the steam boiler design while determining the size and materials to be used in the steam boiler. Additionally, the combustion gas and temperature of the metal in the convection passes are essential considerations. Harmful consequences take place with coal ash constituents remaining in a molten state on the superheater tube surfaces, leading it to be highly corrosive.
It is characterized by loss in external tube walls and increasing boiler tube strain, with an appearance usually seen during the removal of scale and corrosion products. It affects the superheater in a steam boiler. Ash corrosion can be reduced significantly by using materials with higher chromium concentrations. Installing stainless steel boiler tube shields reduces fuel ash corrosion in steam boiler components with very corrosive conditions. Introducing calcium and magnesium to the fuel can also help lower fuel ash corrosion.
- Fireside Corrosion Fatigue:
Fireside corrosion fatigue is caused by corrosion and thermal fatigue. Boiler tube surfaces face thermal fatigue stress cycles that take place due to soot blowing, slag, or cyclic operation of the steam boiler. Additionally, thermal cycling can cause cracking of the less lubricated external boiler tube scales, exposing the base to repeated corrosion.
Boiler tubes develop multiple cracks on the outer surface and spread into the tube wall. It is usually found on furnace wall tubes of coal-fired boilers but also takes place on tubes in drum-type boilers. The solution to prevent fireside corrosion fatigue is to lower the ramp rates during start-up and shut down for reducing the thermal stress. Additionally, optimizing soot-blowing operations can minimize thermal stresses.
- Waterwall Fireside Corrosion:
Waterwall fireside corrosion is found on external surfaces of water wall tubes as the combustion process produces a reduced atmosphere. Coal-fired boilers with maladjusted burners or staged firing are more likely susceptible to localized regions with a reducing atmosphere, resulting in increased corrosion rates. It is indicated by external tube metal loss leading to thinning and increasing boiler tube strain and usually affects water wall tubes.
The ideal method to prevent water wall fireside corrosion is by using a weld of high nickel overlaid on boiler tubes with the worst corrosion. Corrosion-resistant thermal sprays are also an ideal solution for this issue.
Erosion of boiler tube surfaces occurs from impingement on the external surfaces. Firing fuels with high ash leads to increasing erosion, slagging, and fouling issues in a steam boiler. The erosion occurs along with thermal fatigue in cases of soot blower steam. It is indicated by the boiler tube experiencing metal loss. Ultimately, failure is caused by the rupture due to increasing strain as boiler tube material erodes.
Erosion is commonly found near soot blowers on the economizers, superheaters. It can be prevented by distributing flow evenly in the steam boiler and considering a lower ash fuel.
- Fatigue Failure:
Fatigue failure is the result of cyclical stress in the steam boiler. Contrary to thermal fatigue, damage of fatigue failure is caused by external stress through vibration by flue gas flow, soot blowers, or boiler cycling.
It is characterized by the failures localized to the area of high stress and impact tube penetrations, welds, or supports. Fatigue failure is prevented by detecting and minimizing the source of stress.
General Failure of Boiler Tubes:
- Short-Term Overheat:
Short-term overheat failures occur during boiler start-up and fail with the increasing temperature of tube metal due to lack of cooling steam or water flow and impacts furnace wall tubes and superheaters. It is prevented by avoiding blockages within boiler tubes and bends and following standard procedures for shut-down and start-up.
- Long-Term Overheat:
Long-term overheat takes place over months or years as superheater tubes fail after many years of service due to creep. Furnace water wall tubes also fail due to long-term overheating. It is prevented by cleaning tubes with chemicals for better heat transfer and by balancing the furnace/flue gas temperatures with circulation to reduce the tube temperatures.
Rakhoh Boilers has been a leading boiler manufacturer since its inception in 1983 with world-class manufactured steam boilers, waste heat recovery boilers, thermic fluid heaters, and boiler accessories. We deliver the best boiler services like annual boiler maintenance, boiler automation, energy audit, steam trap assessment, fuel conversion, etc.
For more details on our products and services, visit www.rakhoh.com