Failure Investigation of Superheater Through Investigate the Nearest Component

. The failure of the superheater (SH) tube can cause the power plant to stop operating. A study was conducted to detect the cause of tube leak at the failure of the superheater in HRSG. This study investigated the mechanism of degradation and leak of SH HRSG by examining the SH tube adjacent to the failed SH tube. Because the failed sample was not found, this investigation was essential for the failure prevention of the recurring problem. This problem was analyzed through metallography examination, hardness test, Finite Element Method (FEM) simulation, SEM/EDS review, and tensile testing. The analysis showed that the cause of the superheater tube bending was the presence of a hotspot, which was assumed to happen when the lower flue gas flap was opened for a long time while the fluid circulation system in the superheater tube was not functioning perfectly. As a result, the thermal stress that occurs exceeds the yield strength.


Introduction
Boiler tube failure is the leading cause of power plant outages, which further affects the reliability, availability, and safety of the unit.So boiler tube failure analysis is critical to predicting the root cause of failure and the steps taken for future corrective action to prevent failure shortly.This paper investigates the possible causes of the Heat Recovery Steam Generated (HRSG) superheater tube failure of a thermal power plant.[1,2] Many pressurized HRSG components fail due to unpredictable working conditions, for instance, overheating, corrosion, and erosion.Most HRSG superheater tube pressure components operate under predictable conditions.The main design parameters that determine the resistance of the pressure part are temperature and pressure.Unbalance load caused by errors during maintenance and simplification during the design process often cause local stress concentration.The only boiler element that undergoes all combinations is the steam superheater.These superheater (SH) elements are characterized by work in high pressure and temperature parameters.[3,4,5,6] *Corresponding author: vitaastini25@gmail.comPower plant with practical solutions to HRSG thermal integrity and performance problems since 1993.Examiner results of hundreds of HRSGs of all types and sizes from almost every HRSG manufacturer and conducted numerous failure analyses on HRSG components, as shown in Figure 1.The operation of the HRSG superheater in power plants with high temperature and pressure conditions causes damage to the superheater tube frequently.High-temperature creep damage and overheating occur in the superheater tube, generating significant cost losses.Therefore, efforts to identify the cause of this failure and its maintenance are critical.Superheater tube failures have been damaged in several similar power-generating units in the same area [7][8][9].The superheater tube leaked and was partially bent during the maintenance shutdown, as shown in the picture below.The HRSG paper used a sample SH tube adjacent to the damaged tube.The damaged tube is hard to find because it was already damaged when cutting for tube replacement.Therefore, samples of SH that did not leak were taken adjacent to the leaking tube to investigate the form of the degradation mechanism of the tube.
In this study, only some information stated that at start-up, the lower flue gas flap opened first, and after some time, the upper flue gas flap was followed.The condition of superheater tube is bent, and the fastening belt is broken (Figure 2).According to T. superheater tube is angled, the trapped droplets will be centrifuged against the wall, where they will be evaporated rapidly [10].
Meanwhile, the leak occurred in the connection between the superheater tube and its header (no leaked superheater tube samples were obtained).In comparison, the HRSG superheater tube sample that did not experience a leak is shown in Figure 3.The specifications of the High-Pressure Superheater tube installed on HRSG is in Table 1.Outlet 520 This study aimed to investigate the mechanism of degradation and leakage of the SH HRSG tube through inspection of the SH tube adjacent to the damaged SH tube.This inspection is to determine preventive measures so that the failure does not repeat itself and examine the mechanism of the occurrence of bent superheater tubes following the break of the tube row fastening belt.

Experimental Procedure
This problem was analyzed using three methods to determine the cause of the damage to the Superheater Boiler tube.The procedures were as follows.
2. Finite Element Method (FEM) simulation to investigate the bending mechanism of the superheater tube.
3. SEM/EDS examination, tensile testing at room temperature by using an approach that was by referring to the results of the previous HRSG tube examination and to the ASTM A-370 standard [11] or ASME SA 370 (during the examination, the laboratory was closed due to covid 19 and the examination was not conducted) This method proved that there had been a temperature difference between the tubes, which was the cause of failure in the critical area [12][13][14].The difference in length between the tubes was the reason for the temperature rise in this case.Then, three solution methods were presented, namely (1) replacing the tube material, (2) balancing the tube length and redesigning the plate superheater, and (3) replacing the damaged tube with a new one.

Result and Discussion
The results of the metallographic examination revealed that the presence of a thin layer of scale (micron order) was followed by local corrosion and de-carburization processes and initiation of thermal fatigue.The hardness area in Figure 4(1) is HV 177.5 -HV 179.5 and the microstructure is ferrite-bainitic.The hardness Figure 4(2) is HV 179.5-183.The microstructure of bainitic ferrite (Figure 4(1)) shows local corrosion at the center of the superheater tube cross-section.Figure 4(2) shows microstructure in ferrite-bainitic condition with hardness: HV 179.5. Figure 4(3) shows a weak crust layer (micron order) found on the outer surface of the tube.Figure 4(4) shows the fin welds from the cross-section of the superheater tube.
The welds are perfectly integrated with the tube material.There are no welding defects; the ferrite-bainitic microstructure is covered with a thin crust (micron order).General corrosion has started to occur but is thin, still in the micron order, so it is not significant.Its degradation Hardness: HV 178 -179.5.In Figure 5, thin slag is seen in Figure 5 (1), and there is a minor defect in the center of the tube (Figure 5( 2)).Also visible is a thin layer of scale (micron order) followed by local corrosion and de-carburization (Figure 5(3)).The Microstructure in Figure 5 is Ferritebainitic.It is covered with a thin crust (micron order), and it looks like general corrosion is starting to occur, but it is thin, which is still in the micron order, so it is not significant.It is seen that there is a material defect in the form of small voids, and when it is formed into a tube, the void turns into an elongated one.This defect is relatively small, and its position is in the middle so that it does not significantly reduce the strength of the superheater tube when operated Figure 5 (2).The operating parameters are related to the mode of failure.Since the leaking superheater tube sample is unavailable, the damage analysis is based on the condition of the existing superheater row tube, which is deformed to be bent, as shown in Figure 2, and is based on inspection data carried out 2 (two) years earlier.Meanwhile, from statistics in developed countries, the location of the failure in the Superheater is significant due to the high operating pressure and temperature.
In Indonesia, there are no publications related to failure location statistics.However, the literature provides an overview, as shown in Figure 6.Experts have studied the HRSG superheater tube failure diagnostics with higher pressure and temperature operating conditions in the tube.As a result, creep damage, corrosion fatigue, and stress damage resulting from thermal expansion pose a greater risk of damage.The literature showing over the last 15 years has confirmed mainly that prediction [26][27][28][29][30][31][32].

Damage Location
Figure 7 indicates that the failure mode is thermal over stress.There is no detailed data regarding thermal overstress that occurred in the SH tube row, for example, when it happened, data on skin temperature in the SH tube row and the temperature of flue gas from the turbine, how long it took, and so on.
The inspection results indicate tensile overload in the leaky superheater tube position.It is suspected that thermal overstress also occurred, considering that the critical area at the connection of the superheater tube with the header often leaks.Boiler manufacturers have carried out stress analysis for these vital areas as given in Figure 7.It is essential to control the operating parameters so that the stress level does not exceed the yield strength of the superheater tube material (causing deformation), nor does it exceed the ultimate strength of the superheater tube material.The tube outside the chamber shows significant deflection due to the influence of external forces on the superheater tube.Therefore, at the design stage, it is necessary to consider all additional loads that can cause creep intensification.Also, during operation, the working condition of the pipes should be checked to avoid any other tension in the superheater area.Neglecting this work can result in additional on the coils and headers, often more significant than in the design phase.One indication of this tensile overload occurring in the superheater tube in 2018 is shown as follows: When the generator is operating, the temperature distribution of the components is in Figure 9. Verbal information also stated that the bottom flapping door position had been opened first at start-up.This condition requires careful control regarding the length of the lower flapping door opening and fluid flow in the circulation system, including the superheater row tube.Suppose there is an out-of-sync at the start-up.In that case, there can be an uneven heat distribution and cause thermal overstress [33,34,35], which exceeds the yield strength of the superheater row tube, resulting in permanent bowing [36,37] on the superheater row tube; even steel straps can break.Temperature sensors that monitor thermal distribution must be sufficiently representative in number, in good condition, and calibrated.
Simulation has been carried out using FEM analysis for the hot spot condition in the middle of the superheater row tube with the simulation results of the Flue Gas temperature of 424 O C producing a maximum tube stress of 218 Mpa (at the Yield Strength material tube limit).The classification determines that Level A has no spheroidization, Level B has mild spheroidization, Level C has moderate spheroidization, Level D has complete spheroidization, and Level E has serious spheroidization.From Level A to Level E, the lamellar structure in pearlite changes to a particle structure.The pearlite structure disappears at Levels D and E, resulting in decreased mechanical properties.[38] , 01 Generator temperature profile and suggested pinch and approach points.[39] If the pinch point temperature does not meet the technical specifications.Manually, the operation becomes uneconomical.Meanwhile, steaming can occur in the economizer if the approach point temperature is too small.This causes operational problems such as vibration, water hammer, and the possibility of salt deposition, reducing boiler performance [40][41][42][43].When steam loses heat, it will turn into condensate, where the specific volume is more than 1000 times smaller than when it was still steam.Then the steam in contact with the cooler condensate will condense, and its volume will be drastically reduced.[45][46][47].During the condensation process, the space initially occupied by steam will become a vacuum, and the condensate in the tube will experience surging to flow very quickly to fill the vacuum.[48][49][50] At that time, the surging condensate flow causes a collision of condensate waves between the two sides of the vacuum area in the tube.The result is a "steam-induced" water hammer which damages the tubing.A water hammer can also occur when the condensate valve closes suddenly for some reason, for example, during a power outage that activates the valve closing quickly or during operation.In steam operation, it is essential to note that the condensate is wholly drained when the steam flows.[51][52]

Conclusion
1.The cause of the bending of the superheater tube at the height of about 4.2 meters from the floor is a hotspot which is thought to have occurred when the lower flue gas flap was open for a long time while the fluid circulation system in the superheater tube was not functioning perfectly.As a result, the thermal stress will exceed the yield strength.

Figure 1 .
Figure 1.Percentage of damage to boiler tube components [*] *Modelling of thermal behaviour of iron oxide layers on boiler tubes, DOI:10.1088/1742-6596/721/1/012002Theoperation of the HRSG superheater in power plants with high temperature and pressure conditions causes damage to the superheater tube frequently.High-temperature creep damage and overheating occur in the superheater tube, generating significant cost losses.Therefore, efforts to identify the cause of this failure and its maintenance are critical.Superheater tube failures have been damaged in several similar power-generating units in the same area[7][8][9].The superheater tube leaked and was partially bent during the maintenance shutdown, as shown in the picture below.

Figure 2 .
Figure 2. The condition of the HRSG superheater tube, which is bent, and the fastening belt is broken

Figure 4 .
Figure 4. Microstructure of the cross-section of SH

Figure 5 .
Figure 5. Longitudinal section of the tube superheater.

Figure 7 .
Figure 7. Critical locations where thermal overstress occurs (red color) when the flue gas operating temperature rises beyond the specifications in the boiler operating manual.

Figure 9 .
Figure 9. Simulation results of CFD analysis conducted by Marco Torresi to describe the static temperature contour.[32]

Gambar 10 .
Gambar 10.Simulation results using FEM analysis for hot spot conditions in the middle of the row superheater tubeThe results of the simulation are to verify the condition of the superheater tube that is bowing, as shown in Figure10.The condition of the existing superheater tube visually does not show any significant defects or thinning.The operating parameters should also be considered: The microstructure is a normal ferrite-bainite phase; the spheroidization classification is still at to C, as shown in Figure11.

Figure 12 .
Figure 12.Generator temperature profile and suggested pinch and approach points.[39]

2 .
It is necessary to synchronize the boiler start-up process.Moreover, there is a direct operation plan to open the total flapping door in front of the superheater tube and a planned operating pattern from based to flickered load.3. The number and location of temperature sensors, flue gas flow, steam flow, etc., must be ensured to properly monitor operating patterns and parameters.

Table 1 .
Specifications of High-Pressure Superheater tube installed on HRSG.