Failure Investigation is a complex phenomenon requiring effective communications between 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 probably be on analyzing the causes of component failure and then taking steps to minimize their effects or eliminate them. Historical operating data can be analyzed 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 has the potential to determine the root cause of that failure. Determination of the root cause can lead to implementation of corrective actions, which could reduce or eliminate the likelihood that a similar type of failure will occur.
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.
Immediate corrective actions based on the initial results of the investigation must be approved and implemented before repairs are completed.
Follow-up corrective actions based on the complete results of the investigation must be planned and implemented before additional failures are experienced.
Case 1: Failure Investigation of LP SH Header Stub Joints of HRSG Boiler.
Background:
LP SH stub joints failed during the hydraulic test while water filling at @ 50% of working pressure.
Observations:
Failed stub connections were thoroughly examined & observed with Black-Reddish color at internal surfaces & the branched cracks propagated from inside surfaces to outside diameter, as clearly seen from the photograph attached below.
Another failed stub joint, revealing the transverse cracks with a wide opening seen adjacent & very close to the header surface.
In short, the inner surface of the failed stubs showed black adherent scale & showed branched type cracking on the ID surface.
Thorough inspection at the site for LP SH bottom & top headers was carried out.
The middle header center portion near the stub was found to have a few corrosion pits & the inside surfaces of failed stubs also had corrosion pits throughout the length, which can be seen from the photographs depicted below.
Root Cause Analysis:
Stress Corrosion Cracking is the Root Cause of the failure involving both corrosion & straining of a Metal due to Residual or applied stresses.
Stress Corrosion cracking is an insidious, i.e., the damage is not detectable on casual or routine inspection & often leads to catastrophic failure.
Stress corrosion cracking is viewed as the result of synergistic interaction between static tensile stress & a corrosive medium to which a metal is exposed.
The tensile stress leading to SCC is either acting, i.e., the applied stress arising out of pressure exerted by internal fluid or the residual stress developed during operation.
Failure due to SCC occurs when an aggressive environment & sufficient tensile stress, either acting or residual, are present simultaneously.
Though the stub failures have not occurred during the running condition of the boiler, it can be concluded that minor hairline cracks have developed at the protective Magnetite layer by carefully examining the appearance & nature of the crack.
Corrosion starts with the initiation of the crack in the protective magnetic scale during the first phase of additional stress loading. These stresses resulting expansion of the tube & undergoes cracking.
Once the crack is formed, further damage occurs as a result of fresh attack by corrosive medium, such that the exposed metal at the root of the crack is oxidized. The oxide layer so formed at the crack root breaks due to the effect of the stresses.
This cyclic process of formation & breaking of oxide layer continues until a blunt & wedge-shaped crack propagates through the entire thickness of the tube wall & final rupture occurs on the tube wall.
These cracks are typically straight, branched, wide, blunt, wedge shaped & the trans granular (propagates through the grains & not through the grain boundaries)
These cracks are found close to the regions of physical restraint, such as stub connections to Inlet & outlet headers, toe of weld, weld undercut, corrosion pits & notches.
Boilers that are operated cyclically or discontinuously have a high probability of Stress corrosion cracking.
Typical Nature of Failure:
The failures of the stub joints have not occurred when the Boiler was running & noticed during the Hydro Test, that too at @ 50% of working pressure.
During Boiler operation, the tubes & headers are subjected to Thermal stresses due to Temp gradients. These stresses caused the tubes to expand & contract, which helps to relieve some of the stresses in stub joints.
During Hydro Test in cold conditions, the tubes & headers are not subjected to the same level of Thermal stress as they are subjected to Mechanical stress due to the pressure of fluid. These stresses caused Stub joints to experience increased loads, particularly if the joints are not perfectly aligned.
At lower temperatures, the material properties of tubes & headers changed & become more brittle & less ductile. This made the material more susceptible to failures under stress.
At lower temperature the Yield Strength (stress at which the plastic deformation takes place, i.e. material will not come to its original position when the stress is removed) may be higher, but the material is prone to failure.
The stub joints have residual stress introduced due to the operation. These stresses can add to the stresses caused by the hydraulic test & the Geometry of the Stub joint can create the Stress concentration.
During operation, headers & coils experience different temperatures, leading to varying expansion rates. The Headers typically being thicker & more massive, tend to expand less than the tubes, which are thinner & more flexible.
The difference in expansion rates creates relative displacements between the header & coil. Therefore, the guiding, aligning expansion arrangements & proper relative expansion play a very critical role as far as Stress concentration is concerned.
The tube supports maintain Structure Integrity, but if not properly maintained, tube supports can restrict the tube’s ability to expand freely. The Stresses concentrated at specific points, such as the tube to header joint area.
The relative expansion between the Header & Coil caused the tubes to bend or deform, leading to the increased stress concentration.
Corrosion Issues Attributed to Stub Failure:
- Pitting at internal surfaces & Reddish Black appearance confirms the severe corrosion.
- Reason for this Corrosion in this case is dissolved gases such as Oxygen, Carbon di oxide & Ammonia.
- In Boiler Feed Water Oxygen is most important from a corrosion point of view.
- Dissolved Oxygen is the main cause of Corrosion by Oxygen Pitting.
- Once the dissolved Oxygen is removed from the feed water, the potential for corrosion drastically decreases.
- Additional Protection against Corrosion is ensured by injecting Chemicals such as Hydrazine, known as an oxygen scavenger, into the deaerator.
- This Corrosion is more prevalent in the case of Idle Boilers & malfunctioning of the Deaerator & improper Oxygen Scavenging treatment matters a lot.
- Pitting frequently occurs in shutdowns mainly at bends, stubs, headers & non-drainable locations.
- Pits seen are pinhole-sized craters that are Hemispherical in their Morphology.
- Pit tubercles are covered with Corrosion products (FeO), having a reddish appearance.
- Black Iron Oxide is observed in the Pit.
- Pits act as Stress Risers, leading to subsequent failure of stubs under the corrosion failure mode.
Anodic Reaction: Fe –> Fe++ + 2e-
Cathodic Reaction: ½ O2 + H2O + 2e- -> 2OH –
Overall Reaction: Fe + ½ O2 + H2O –> Fe (OH)2 - The oxygen-rich area acts as Cathode whereas the surrounding area, which is low in Oxygen acts as the anode, leading to the formation of a galvanic cell.
A ring of corrosion product is formed at the anodic area, isolating it from the surrounding area. Gradually, as the cap of corrosion product thickens, corrosion proceeds by a different corrosion mechanism.
Criticality of Dissolved Oxygen:
- Dissolved Oxygen level of water typically should be less than 0.1 ppm, but, in any case, should not be more than 2.0 ppm.
- Dissolved Oxygen level is @ 9 ppm in Cold water & make-up water.
- In feed water, the dissolved oxygen level is brought down to 2 ppm by heating the water to 900 C.
- This can be further reduced to 5 to 7 ppb by using Oxygen Scavengers such as Hydrazine, Hydroquinone & Hydroxyl Amines.
Stress Concentration Attributed to Stub Failure:
- Temperature differences between inside & outside of the tubes caused Thermal Stresses on the stubs & led to cracking.
- Stub connectors expand & contract due to temperature changes, causing stresses at the tube to header joints or other constrained areas & these are enhanced due to the hurdle in expansion.
- The fluctuating pressure of the steam inside the tube caused uneven Hoop stresses & led to deformation or failure.
- The stubs have experienced stresses due to external load, such as uneven Thermal expansion.
- Corrosion-related stresses of the stubs weakened the Tube material & reduced its strength.
- Localized Corrosion at the stub created Stress concentrations.
- Stubs have experienced vibration fatigue due to fluid flow or external factors.
- The Design of the Boiler, Operating Pressure & Temperature for long duration also have an impact on the stresses on the stubs.
- Degraded strength, Ductility & Corrosion resistance have affected its sturdiness under stress.
- The Chemistry of Water or Steam has influenced the Risk of Corrosion & the Stress corrosion cracking.
Short Term Action Plan:
Adequate expansion gaps of a minimum of 10 mm were maintained at tube supports located at intervals of every 3 meters between the bottom and top headers. Non-destructive testing, including Ultrasonic Testing (UT), Magnetic Particle Inspection (MPI), and Dye Penetrant Testing (DPT), was carried out on the stub joints of the failed tubes. UT inspections were conducted on the middle and LHS header stubs, while MPI was performed on the RHS header stubs.
In-situ metallography and plastica replica examinations were performed at multiple locations, six spots on the middle header and six spots on the LHS header, to evaluate the residual life of the material.
After completion of all NDT inspections, stress relieving of the entire header length was carried out following the agreed stress-relieving cycle. Subsequently, crack arresting was performed by drilling approximately 6 mm at the crack tips, followed by grinding until the cracks were removed, and the affected regions were repaired through welding.
Once crack repairs were completed, the header was restored by careful metal filling around the stub holes to protect the surrounding affected areas. After all weld repairs, post-weld heat treatment (PWHT) was carried out on the stub joints and tube welds following the specified cycle.
Proper expansion clearances were ensured to allow free expansion and contraction of the tubes during operation.
The failed stubs were also sent for microstructural analysis to examine changes in material morphology. Energy Dispersive Spectroscopy (EDS) was conducted to detect the presence of chlorides, which increase susceptibility to stress corrosion cracking, and a Strain-Induced Martensite (SIM) test was performed to assess internal stresses.
Additionally, precautions were implemented to control chloride and hydroxide ion concentrations, and proper dosing of oxygen scavengers was strictly maintained to prevent corrosion.
Long Term Action Plan:
Depending upon the results of In situ Metallography & Plastica Replica, LP SH Header replacement is to be planned with duly provided stub connections.
Proper expansion clearances are to be maintained & monitored. Stringent Water Chemistry control is to be achieved.
Case 2: Failure Investigation of Platen Water Wall Tube of Sub-Critical Boiler.
Background:
Failure was noticed in the Platen WW area at the outermost circuit of Coil Assembly.
Material of Construction:
The damage is in the form of grooving covering approx. 80% length of the circuit.
Conclusion:
As the above failure is due to reduced atmosphere corrosion & therefore combustion optimization of fuel is required to enhance the Combustion Efficiency & the Fuel Characteristics are required to be controlled & monitored regularly.
The main reason is an inadequate amount of oxygen in the burner zone. Under such circumstances, volatile components of Sulphur Na2S2O7 (Sodium Pyrosulphate) & K2S2O7 (Potassium Pyrosulphate) with low melting point are formed.
These components volatilize at high temp & condense as a liquid on relatively cooler surfaces of WW tubes. These tubes are required to be replaced on priority because the reaction between Pyrosulphate compounds & protective Magnetic layer damages the protective layer & result into the loss of the underlying metal & finally leads to failure.
The bends of the external circuit assemblies are also to be checked for Oxygen attack/ Oxygen pitting. As these bends are exposed to an excessive amount of dissolved O2, it frequently leads to pitting occurring in shutdowns, mainly at non-drainable locations such as bends or elbows, where water is not easily drained out.
These pits are pinhole-sized craters that are hemispherical in their morphology. These pits act as stress risers leads to subsequent failure of tubes.
Author:
Pramod Kate
Boiler Technical Consultant
Vyankatesha Engineers & Consultants
