Preamble
Failure Investigation is a complex phenomenon that requires effective communication among equipment operators, maintenance personnel, plant management, equipment manufacturers, and technical experts in materials, chemistry, and mechanical engineering.
The focus of reliability improvement in existing plants will likely be on analysing the causes of component failures and then taking steps to minimise their effects or eliminate them. Historical operating data can be analysed to identify sources of unreliability and suggest technical means to reduce them.
An Investigation into a tube failure in an electric utility steam-generating boiler can help determine the root cause of that failure. Determining the root cause can lead to the implementation of corrective actions that could reduce or eliminate the likelihood of a similar failure occurring.
For an investigation to be successful, the following activities must be performed by plant personnel. Information and data concerning tube failure must be gathered quickly before repair activities begin.
Failure descriptions, operating conditions at the time of failure, historical records, and tube samples must be acquired and transferred to others who will investigate while repairs are being performed.
Case 1 – Failure Investigation of Convective Thermic Fluid Heater Coil Tubes
Background
Failure was noticed in Convective Thermic Fluid Heater Boiler Coils.
Material of Construction -SA106 Gr. B, Size – 42 mm OD
Observations
Thick Lip Violent Burst observed near Stub connections Inlet Header @ 200 mm from Stub joint at Convective Thermic Fluid Heater Coil No. 17 & 19 & at @1.5 meters from stub connections of both the Coils.
Fish Mouth Opening @ 200 mm. observed at both the failure locations of both coils. Severe bulging at the failure locations, i.e., an increase in OD from 42 mm to 54-62 mm, was observed.
On the inner surface, brownish color deposits & complete blockage of Coil tubes were also observed. The surrounding area shows a serration-like pattern due to high-temperature exposure.
The attached Photographs taken during our Site Visit at the failure locations illustrate all the above observations.
Root Cause Analysis
Stress Rupture due to Short-term Overheating is the Root Cause of the failure.
This type of Failure occurs when Oil circulation in the Tube is badly interrupted & causes Rapid overheating of the metal to a point where the Metal becomes highly Plastic & results into Violent Burst.
Overheating of Tubes is due to Partial or Full choking of tubes, resulting in Flow Starvation & due to Internal Deposits attributed to the Poor Quality of Oil.
When the Tube Metal Temperature rises, the Hoop Stress from the Internal Tube Pressure equals the Tensile Strength at Elevated Temperature & Rupture occurs.
Hoop Stress (Pd/2t) in this case increased due to deviation in diameter from 42 mm to 62 mm & reached the Tensile Strength Level.
When Circulation is affected, the Oil Blanket is formed & the Heat transfer through this Blanket is very poor & it acts like Insulator leads to a rapid increase in the Temp & results into failures very quickly.
The estimated Peak temperatures at the time of failure & Metallurgical Microstructural analysis confirm the root cause of Failure.
From failure investigation point of view the characterization of the failed Boiler Tubes including Chemical Composition, Microstructure, Phase Analysis & studying the effect of Temperature on the Phase transformation & relevant changes in the Microstructure examining the Morphology of various defects, second phase like Carbide Precipitation, Spheroidization level, hardening of Carbides, Coarsening of precipitates, Increase in Grain size, Reduction in no. of Grain boundaries, Orientation & Migration of Grain Boundaries, Voids in Grain boundaries are very crucial & important.
Generally, Ideal Microstructures consist of uniformly distributed grains of Ferrite & Pearlite.
Elongated but fine-Grained Ferrite Morphology with Pearlite orientation at Grain boundaries is an indication of Temperature Excursion.
Preventive & Corrective Action Plan
Material of the Failure Tube is Plain Carbon Steel, i.e., SA106 Gr. B.
Limitations of this Carbon Steel are Low Corrosion & Oxidation Resistance & Low Creep Strength at elevated temperatures.
Major Loss of Hardness during Thermal Stresses & increase in Strength at the Cost of Ductility & Toughness are the major limitations. So, the Feasibility of Upgradation of the Material to Low Alloy Steel is to be explored.
At the same time, Perfect Monitoring & Control & Necessary Implementation on the Improvement of Fluid Quality is to be achieved.
Low Alloy Steel, in this case, is preferred as it has increased Hardenability, Improved Yield & Tensile Strength, & Enhanced Corrosion & Oxidation Resistance, & Improved Creep Strength.
After going through the Oil Analysis Report, it is observed that Insoluble Solids should be less than 125 mg per 100 grams, but the measured value is 278 mg per 100 grams. This level is elevated in the Warning Range.
The presence of Insoluble generally indicates the contamination of Dirt, Corrosion products, Severe Oxidation, or Severe Thermal Stressing tendencies, as in this case, Thermal Stress Rupture is the Root Cause of failure.
A decrease in Thermal Stability of the Oil indicates reduced ability to withstand higher temperatures. The concentration of Low Boiler is elevated above the action limit & should be removed from the system to ensure safe operation, typically through the venting of the Expansion Tank.
Prolonged Operation with High concentration of Low Boiler can lead to Significant Operational difficulties like Pump Cavitation & Excessive System Pressure. Thus, the Hoop Stress is also increased considerably (Pd/2t).
Advice from the Fluid Specialist for fluid conditioning is to be sought at the earliest.
Replacement of the Oil is the Most Appropriate Option in this case to prevent such a type of occurrences in the future.
Repeated Heating & Cooling of Oil results in Degradation of oil, leading to deviated Viscosity & Overheating.
These failures are always accompanied by the formation of Scale & Corrosion Product on the inner side of the tube.
Partial closing of the Valves, Partial passing of the drains, air locks, & Vapor locks are also to be monitored & controlled & should not be ignored.
Case 2 – Failure Investigation of Soot Blower Bends of Super Critical Boiler (660MW)
Background
Failure was noticed in the water wall tube at Soot Blower Bends at the Rear Water wall at the topmost row of Soot Blowers.
Material of Construction – SA 213 Gr T12 (Low Alloy Steel)
Tube Size – 28.8 x 7.1 mm. (Vertical Water wall Rifle tube)
Sulphidation
After a critical examination of this Ash deposition, it can be concluded that Sulphidation is the root cause of the deposition of such Hard Scale Matrix.
The temperature of the tube surface must be high enough so that mixtures of Na2S2O7 & K2S2O7 exist in the molten state. Carbon from unburned coal is unlikely to be sufficiently active to set up a reducing environment at the tube metal deposit interface.
The decomposition of CO would give a finer, more uniform, and perhaps a more active form of carbon within the deposit matrix.
Hydrogen Sulphide Corrosion
- Hydrogen sulfide is the main corrosion-active component in furnace gases.
- It proceeds via the following steps
- H2S +Fe =FeS +H2
- FeS +2O2 =FeSO4
The ferrous sulfate (FeSO4) layer produced on the tube wall surface flakes off due to erosion, exposing fresh tube metal to further corrosion.
Analysis of the Ash Characteristics is required to be carried out as a top priority to evaluate the following Slagging Indices. Presentation of the same has already been shared with the concerned staff.
- Base/ Acid Ratio.
- Sulphur Index.
- Iron- Calcium Ratio.
- Silica – alumina Ratio.
- Dolomite Percentage.
- Sodium Oxide.
- Sodium and Potassium Oxide.
- Iron Oxide.
After carrying out the Analysis of Ash characteristics of the samples of all possible Coal Collieries, the tendencies of severe High, Moderate & Low Clinker formation are to be displayed in the Control Room to take Preventive measures in line with the Frequent Inspection of Furnace Bottom Neck, increasing the frequency of Soot blowing & ash Evacuation.
Technologies Available for the Prevention of Deposition Problems in Boilers.
Ash Behaviour Prediction Tool
There are some ash behaviour prediction tools like Ash pro (SM) used to assess the slagging and fouling situations in coal-fired boilers, integrating boiler computational fluid dynamics (CFD) simulations with ash behaviour models, including ash formation, deposition, deposit growth, and strength development.
These prediction results are very important to assess the overall performance of power plants, including fuel quality, ash properties, fouling, slagging, etc.
The characterisation of ash is extremely important to predict the mineral transformation in coal particles during combustion, which controls the characteristics of resulting ash particles.
Computer Controlled Scanning Electron Microscopy (CCSEM) and Scanning Electron Microscopy Point Count (SEMPC) methods are available today to fully characterise ash with respect to the ash deposition process.
As a result, direct comparison can be made between coal mineralogy and that observed in the ash sample.
This is of fundamental importance as it provides the database from which the relevant ash-deposition models can be constructed.
Thus, it is possible to establish the effect of coal preparation (grinding, pulverising, and cleaning) and combustion conditions on the size and mineralogy of ash particles.
Heat Flux Meters
Heat flux meters can be used to study thermal absorption diagnostics in boilers as part of the boiler monitoring systems.
These sensors can be installed at power plants by a supplier of a soot blowing system, though it is difficult to quantify the energy saving obtained from the development and application of such a sensor.
Continuous fouling formation on heat transfer surfaces is a great problem in existing conventional coal-fired utility boilers.
A cost-effective way to minimise this difficulty is the continuous monitoring of fouling tendencies in the boiler as a tool of operation.
Internal Cameras
Internal cameras (3.9 micrometre band) can be used to monitor the fouling problem. Special infrared cameras for this purpose are designed to scan the fouling by filtering the light from the boiler. This allows the camera to see through the flames to the walls by blocking the appropriate wavelengths.
These types of cameras are handheld; they must be used from various viewpoints.
Internal cameras can be installed in areas that are prone to ash buildup or can be installed in adequate numbers to cover all relevant areas of the boiler.
If rotatable cameras are used, they will also be able to see multiple places in the boiler.
Micro Beam Technology
Microbeam Technologies Inc (MTI) provides advanced quantitative information on the impurities in fuels, ash behaviour, and determines and predicts the effect of ash on power system performance using computer-controlled scanning electron microscopy (CCSEM).
MTI processes also identify problematic deposits, slagging, and fouling. This predicted information is very useful in the reduction of abrasion & erosion, slag flow problems, convective pass fouling, ash resistivity, and deposit strength.
Targeted In Furnace Injection (TIFI) Technology
Chemicals are injected into the flue gas system after mixing with water and air. Magnesium hydroxide slurry diluted with water and then atomised with air is the most common application of TIFI technology.
This mixture is sprayed into the furnace at computer-determined ports that allow for complete coverage of problem areas.
These cause a chemical reaction with existing deposits and affect their physical crystal characteristics.
This chemical treatment program is successfully applied for the inhibition and reduction of slag formation in the superheater, reheater, and furnace wall sections.
This technology is used in a western USA coal-fired utility boiler to control slagging. Fouling and tube cracking. This technology involves different forms of fluid dynamics modelling.
Recommendations
- The extent of High Temperature Excursion of the failed tube is to be assessed by carrying out X-Ray Diffraction & Microstructure Analysis.
- Sudden pulsating Load variations need to be avoided.
- Prolonged exposure of Water wall tubes to High temp needs to be avoided.
- Repeated Stress cycles attributed to fatigue need to be minimised.
- Slagging Indices mentioned above of the Coal of all the collieries supplying Coal are to be calculated.
- These slagging Indices are to be displayed in the Control Room serial as per Severe Tendency, Moderate Tendency, Low Tendency & No Tendency of Slagging.
- The Ash Behaviour Prediction Tool must be utilised.
- Internal Cameras are to be installed.
- Heat Flux Meters are to be installed at Soot Blowers for studying Heat absorption Logistics.
- Targeted in Fuel Injection is to be followed strictly after confirmation of determined Ports instead of injecting the Chemicals Arbitrarily.
- Suitable additives, only after studying the Coal & Ash Characteristics & using the Techniques CCSEM (Computer Controlled Scanning Electron Microscopy) & SEMPC (Scanning Electron Microscopy Point Count), are required to be injected.
Author:
Pramod Kate
Consultant
M/S Vyankatesha Engineers & Consultants
FAQs
- Upgrading to low alloy steels for better high-temperature strength and corrosion resistance
- Maintaining fluid quality and removing degraded oil components
- Monitoring ash behavior and slagging tendencies in coal-fired systems
- Installing heat flux meters and internal cameras for real-time diagnostics
- Using predictive tools like ash behavior models and applying targeted chemical treatments
- Avoiding load fluctuations and minimizing thermal stress cycles
