Failure analysis of superheater tube

The failure analysis of a ruptured superheater tube after 20 years service in the oil-fueled boiler, as the typical problems in power plants, was investigated. A thin-lipped rupture at failed region was observed in superheater tube. By measuring the tube’s wall thicknesses far from failed region, non-uniformity was seen. The suggested main root cause of failure was fireside corrosion of the tube during the service. Because of low grade of used fuel, sodium, sulfur, and vanadium elements were observed at the outer surface, which caused continuously scale formation and reduction of wall thickness, by metal consumption. In addition, it seems that it has been worsened by occurrence of long-term overheating. Coagulation of carbides at both outer and inner regions of tube was observed that could prove the occurrence of overheating during the service. In addition, the formation of sigma-phase particles was revealed because of being in the susceptible temperature after 20 years in service. At the end, in order to prevent or decelerate such failure, some recommended remedies were suggested.     

Metallographic investigations of the failed superheater tubes used in liquid hydrogen plant

2.25Cr–1Mo low alloy steel tubes of diameter 42.5 mm ID and 46.5 mm OD were used in superheater of liquid hydrogen plant. This superheater had 10 rows of tubes for carrying naphtha and steam and is heated by flue gas. The flue gas directly impinges on the first three tubes of the tube bank. A failure occurred in the first tube leading to the removal of a portion of the material causing to shut down of the plant.Detailed metallurgical investigations were carried out to understand the cause of failure. This paper brings out the metallographic investigations on the failed tubes and the necessary remedial actions thereon.     

Failure analysis and retrofitting of superheater tubes in utility boiler

The extreme spray water mass flow rate deviation was observed to occur in the middle temperature superheater of Sahand 2 × 325 MW Power Plant utility boiler, which severely affected its economic performance and safe operation. Boilers operating in these conditions led to failure in superheater tubes at the same place for two consecutive times in a three year span. Thus, the failure analysis of superheater tubes by investigating the visual inspection, chemical, scale and creep analysis was carried out. The brittle failure occurred in the superheater tubes after the fuel was changed from natural gas to heavy oil. Failure analysis showed that tubes were suffering from long term overheating which was instigated by high spray water flow rate. In order to rectify the boiler operating conditions, some modifications were applied in the boiler unit 1 and operating parameters on this boiler were compared with boiler unit 2. The results showed that the 8.33% reduction in heating surface area corresponds to 52.84 and 17.80% reduction in spray water mass flow rate for capacities equal to 300 and 260 MW, respectively.     

Analysis of a diesel generator cylinder failure

This paper analyses a catastrophic cylinder failure of a four stroke 14 V diesel engine of an electrical power plant when running to nominal speed of 600 rpm. The rated power of the engine was 7.5 MW and before failure had accumulated 80,000 h in service operating mainly at full load. As a result, the piston and liner of cylinder 4 were broken; the crankcase and main crankshaft bearings next to this cylinder were also damaged. The mechanical properties of the liner (grey cast iron) and piston body (aluminium alloy) including tensile properties and Brinell hardness were evaluated. No signs of fatigue failure were identified in liner and piston. A finite element model of the liner has shown that the most heavily loaded areas match the fractured zones.     

Failure analysis on welded joints of 347H austenitic boiler tubes

Microstructural and thermo-mechanical analyses using the finite element method (FEM) were performed on the welded joint of 347H boiler tubes in a coal-fired power plant after 3600 h of operation. The cracks in the failed tube started on the inner surface of the heat-affected zone (HAZ). This area had a high hardness value and volume fraction of strain-induced martensite. The plastic deformation around the crack was concentrated near the grain boundaries. This failure occurred as the stabilizing effect disappeared due to carbide dissolution in the heat-affected zone, and plastic deformation and tensile residual stress were formed at the inner side due to the solidification contraction of the outer bead.     

Failure of tube used in high pressure steam generator due to incrustation deposits

In this work a failure analysis was conducted to investigate the premature failure of ASTM 106-Gr.B seamless tube used in steam generators. The failure occurred after 3 years of operation, due to a macroscopic crack with approximately 50 mm in the longitudinal direction. Incrustration deposits found in the region of failure were identified as sodium aluminum silicates (albite and nepheline). Micro-hardness profile showed a sharp increase of hardness near the crack, which was associated to microstructural changes observed by optical microscopy. Local over-heating of the tube has occurred due to the incrustation deposits. As a consequence, creep damage was produced and caused the crack of the tube. Temperatures in the austenitic field were achieved, since martensitic, bainitic and Widmansttäten ferrite structures were observed near the crack. In the crack tip, at the moment of leaking, the austenite underwent rapid cooling from the high temperature, producing hard microstructures in the crack neighborhood. Correct water treatment is recommended to avoid future failures in this type of tube.     

Investigation into effect of rubber bushing on stress distribution and fatigue behaviour of anti-roll bar

Anti-roll bars used in ground vehicle to reduce body roll by resisting any uneven vertical motion between the pair of wheels suffer from fatigue failure. In this study, several structural analyses of an anti-roll bar made of SAE 9262 were carried out by means of finite element (FE) technique to determine stress distributions. The result of FE analyses indicated that equivalent stress in the inner surface of the corner bend was the maximum; wherein the shear stress dominates. Fatigue tests that were carried out under 7° rotational loading also confirmed the failure location. In addition, the effects of hardness and wall thickness of rubber used in bushing on stress distribution were investigated for various polyurethane rubbers (75, 85, 90 and 95 Shore A) and bush wall thickness (5.25, 7 and 8.75 mm), systematically. It was found that both soft rubber and thick wall thickness tend to reduce stress in the critical region. Based on the results of FE analyses, several proof tests of anti-roll bars with specific bushing were planned. It was concluded that the reduction of equivalent stress in anti-roll bar accomplished by modifying the bushing provided a significant improvement in the fatigue life. Approximately 9% improvement in the fatigue life with respect to base bushing could be obtained by selecting relatively soft rubber materials. For the rubber hardness 75 Shore A, changing in wall thickness from 7 mm to 8.75 provided 11% improvement. Total improvement from the proof tests reached up to approximately 21% in the fatigue life.     

The dynamic creep rupture of a secondary superheater tube in a 43 MW coal-fired boiler by the decarburization and multilayer oxide scale buildup on both sides

A dynamic creep mechanism has been proposed and verified through a case study. A secondary superheater tube burst occurred in a 43 MW coal-fired boiler. Microstructural examination indicates that the overheating temperatures reached 900 °C (above Ac3). The overheating duration was estimated to be 3 h by calculating with LMP formula. The 710 μm steam-side scale and 960 μm fireside scale built-up in the short time of overheating. The burst scenario was a short-term severe overheating on the basis of the long-term creep. The multilayer oxide scales on both sides have been studied with ESEM/EDS, indicating FeO. At 900 °C, full decarburization had gone throughout the tube. As the strength reduced due to the decarburization, the creep mechanism transformed from long-term intergranular creep to short-term transgranular rupture. The two types of dimples on the fractograph and two types of cracks in the microstructures confirmed the mechanism transformation. The overheating, the scale buildup and the decarburization constituted the full picture of the dynamic creep rupture.     

Failure analysis of Grade-80 alloy steel towing chain links

An experimental investigation of the causes of failure of chain links that occurred during towing operation of heavy-weight army vehicles is reported in this study. All failures took place at the weld area of the links after a short service life. Tensile tests on both base metal and the weld samples indicated high tensile strength of 800 MPa for both materials. However, the weld exhibited brittle fracture at relatively small strains of ε ≈ 0.05, while the base metal failed at strains ε ≈ 0.20, indicating moderate ductility. Optical metallography and scanning electron microscopy (SEM) analysis revealed that fatigue was initiated due to inherited cracks at the outer circumference of the weld. Fatigue crack propagation was evident by progressive marks and intervening striations. Their distribution was not rotationally symmetric, indicating a possibility of combined cyclic loading on the links. The large area of final rupture indicated a ductile rupture in the weld center and brittle fracture in the outer region of the weld due to overloading. The results suggest that the major causes of chain failure are as follows: high cyclic loading, weld defects, improper post-weld heat treatment, and decrease in material hardness and corrosion resistance due to insufficiency of some alloying elements.     

Failure analysis on high temperature superheater Inconel® 800 tube

This paper presents failure analysis on a super alloy Inconel^® 800 superheater tube in Kapar Power Station Malaysia. Visual inspection, microscopic examinations and creep analysis utilizing available related data are carried out to evaluate the failure mechanism and its root cause. The failed high temperature superheater (HTSH) tube was found snapped into two parts, heavily distorted shape and bent at several points. Microstructures of the failed tube showed that creep crack initiated at both external and internal surfaces of the tube and propagated as grain boundary creep cavities coalesced to form intergranular cracks. The severe geometry of tube causing steam flow starvation is identified to have caused increasing tube metal temperature resulting in overheating of the failed tube. Creep rupture is revealed as the cause of failure of the superheater tube.     

Failure analysis of cupronickel evaporator tubes of a chilling plant

Pin-hole leakage was observed in a number of 90/10 cupronickel evaporator tubes of a chilling plant which was in service for about 6 years. Chilled water is cooled from 12 °C to 7 °C which flows inside the evaporator tubes, while the OD side has LiBr environment. The failure investigation of the failed tubes revealed greenish/blackish uniform deposits together with some spots where localized thick deposits on the ID surfaces were present. Underdeposit corrosion beneath these thick deposits was the primary reason for failure of the tubes. Intergranular corrosion was also observed underneath the thick deposits. The ID surface of the failed tubes confirmed the presence of wide variety of deposits, e.g. carbonates, silicates, phosphates, and sulphur-bearing compounds. The stagnant conditions under the thick deposits alter the chemistry inside the crevices thus formed. The altered chemistry conditions caused rapid corrosion (and intergranular attack) on the cupronickel tubes. Remedial measures that may be undertaken to prevent future occurrence of the problem are addressed.     

Failure investigation of a truck diesel engine gear train consisting of crankshaft and camshaft gears

A failure investigation has been conducted on a diesel engine gear train consisting of a drive crankshaft and a driven camshaft gears that were used in a truck. The gears are made from a nitrided 42CrMo steel. Adjacent teeth fracture regions appeared on the gears after a service of 4.2 × 10⁴ km. Fractographic features indicate that multiple origins fatigue fracture was the dominant failure mechanism for the gear teeth. The crankshaft gear fracture first, followed by the camshaft gear. Low hardness in subsurface and core region of the nitrided crankshaft gear makes it difficult for the matrix to support the load by the engaged camshaft gear to lead to initiating the fatigue crack at the root fillet bearing the maximum tensile stress. The crankshaft gear is the component causing trouble for the failed gears train.     

Failure analysis of heat exchanger tubes

Primary waste heat exchanger tubes of material ASTM A213 grade T11 failed after operation of only three and a half months. The heat exchanger was of the bayonet type with boiler water inside the tubes and secondary reformer outlet process gas at the shell side. The heat exchanger environment was rich in hydrogen, carbon monoxide and nitrogen. The temperature of the process gases was 960 °C and the heat exchanger was producing steam at a temperature of 306 °C and a pressure of 1500 psig. The failed, used and new heat exchanger tubes were subjected to stereo/optical microscopy, chemical analysis and hardness testing. The cause of the failure was thoroughly investigated using optical/scanning electron microscope equipped with energy dispersive spectrometer. The study revealed that the material was exposed to thermal cycling and excessive local heating. The same was also confirmed by simulated experimentation. These conditions lead to thermal fatigue of the material with consequent failure.     

Cracking of radiant tube of heater coil carring heavy oil in a hydrocracking unit

Failure investigation was made on a cracked radiant tube made of 9Cr–1Mo (wt.%) alloy steel (ASTM A 213-T9). The failed tube was removed from a heater coil that carried crude vacuum residue and on external fuel gas firing in a hydrocracking unit operating at high pressure. The failed tube had been in service for about 15 years. The tube visually exhibited cracks on the external surface in the longitudinal axis direction. Cross-sectional examinations showed a straight and un-branched crack that reaches up to 70% of the tube’s wall thickness. Examination of the internal surface along the cracked area showed a surface of wavy topography. The cracking of the tube was attributed to long-term service at high temperature in sulphur bearing crude. High temperature sulphidic corrosion at the internal tube surface is believed to have aided the dissociation of carbides, resulting in the diffusion of carbon towards the hot external surface where fresh carbides precipitated and accumulated during shut-downs. Areas of carbide accumulation exhibited high hardness and were brittle, facilitating cracking at the external surface during start up operations.     

Optimization of gas tungsten arc welding process by response surface methodology

Gas tungsten arc welding is widely used for connecting of boiler parts made of A516-Gr70 carbon steel. In this study important process parameters namely current, welding speed and shielding gas flow rate were optimized using response surface methodology (RSM). The simultaneous effects of these parameters on tensile strength and hardness were also evaluated. Applying RSM, simultaneous effects of welding parameters on tensile strength and hardness were obtained through two separate equations. Moreover, optimized values of welding process parameters to achieve desired mechanical properties were evaluated. Desired tensile strength and hardness were achieved at optimum current of 130 A, welding speed of 9.4 cm/min and gas flow rate of 15.1 l/min.     

Experimental investigation of two-phase flow characteristics of HFC-134a through short-tube orifices

The two-phase flow characteristics of HFC-134a, including flow pattern, mass flow rate, pressure distribution and temperature distribution through short-tube orifices are experimentally investigated. Short tube diameters ranging between 0.605 and 1.2 mm with length-to-diameter ratios ranging between 8.3 and 33 are used in the experiments. The test runs are performed at upstream pressure ranging between 900 and 1300 kPa, downstream pressure ranging between 300 and 400 kPa, and degree of subcooling ranging between 1 and 12 °C. Two groups of short-tube orifices are used in the experiment. The first is used to visualise the flow pattern. The second is used to measure temperature and pressure distributions along the tube. The results from the present experiment show that metastable flow and choked flow phenomena exist inside the short-tube orifices over the whole range of experimental conditions. The metastable liquid flow region increases with increasing degree of subcooling and upstream pressure. The mass flow rate is directly proportional to upstream pressure and degree of subcooling. The results of pressure distribution inside the short-tube orifices indicate that accelerational pressure drop at the inlet and outlet has a significant effect on the total pressure drop across the short-tube orifice.     

Improvement of magnetic properties of nanostructured Ni79Fe16Mo5 alloyed powders by a suitable heat treatment

The supermalloy (Ni₇₉Fe₁₆Mo₅) nanostructured powder with average crystallite size of about 8 nm was prepared by high energy milling. The obtained powders were characterized by X-ray diffraction, scanning electron microscopy, differential scanning calorimetry and vibration sample magnetometer. The results showed that the coercivity and the saturation magnetization reach about 8 Oe and 75 emu/g at 96 h and become approximately 1 Oe and 85 emu/g after a suitable heat treatment, respectively. The magnetic measurements confirmed that the supermalloy soft magnetic nanostructure powder was produced by mechanical alloying followed by a post heat treatment. The results revealed that a small amount of Mo element remain in the system up to 96 h due to (i) high fusion temperature, T_f = 2893 K, (ii) high mechanical hardness, (iii) low solubility of Mo into Ni at low temperatures in mechanical alloying conditions.     

Corrosion failure analysis of spent caustic drainage line pipe

A mid country oil refinery requested for the analysis of spent caustic 4 in. damaged line pipe leading to spent caustic storage tank which failed after installation by localize corrosion attack within 8 months. The refinery also reported that the failures were frequent and were occurring at same region between continuous flow line (CFL) and intermittent flow line (IFL) at 6 O’ Clock position. The line duty cycle was half filled used as and when required basis. The chemistry of spent caustic solution, operating conditions, metallurgy and electrochemical behavior of line pipe in spent caustic was carefully reviewed. The design flow was also examined and certain flaws in design were identified. It was concluded that by modifying the diameter of 4 in. diameter existing line pipe in the design, the problem was solved. The original suggestions have been validated after more than 3 years of satisfactory service of said line pipe.     

Failure analysis of superheater tube supports of the primary reformer in a fertilizer factory

A failure analysis of superheater tube supports of the primary reformer in a local fertilizer factory is presented. A number of tube supports failed at approximately half of their designed service life. Following the failure, the factory was visited, and relevant information and samples were collected. The samples were investigated in the laboratory by chemical analysis, macro- and microhardness measurements, macro-and micrometallographic examinations, and X-ray diffractometry. The analysis showed the supports were fabricated from HH-type heat-resisting alloy and that the failure mode was high-temperature creep. The microstructure of the alloy showed the presence of massive intergranular as well as intragranular σ-phase and intragranular needle-shaped M₂₃C₆ carbides. It was also concluded from the formation of massive σ-phase in the tube that the failure was hastened because the supports were operating at approximately 800 °C. The alloy composition led to the formation of σ-phase under the operating conditions of the reformer, and the use of an alloy with a higher concentration of austenite-stabilizing element(s) could have avoided the failure.     

Pressure drop correlation for oil–refrigerant R134a mixture flashing flow in a small diameter tube

This paper presents an experimental investigation of the ester oil ISO VG10/refrigerant R134a mixture flashing flow in a 6.0 m long, 3.22 mm ID tube, which is one of the primary steps towards the construction of a methodology for the study of the lubrication and gas leakage in refrigeration compressors. The phase change starts with solubility reduction of the refrigerant in the oil as the pressure decreases due to the friction forces. In this flashing flow the foam pattern is observed at the end of the tube as vapor quality reaches high values, and this is a particular phenomenon of this kind of mixture flow. In order to study this pressure drop, an experimental apparatus was designed to allow the measurement of both pressure and temperature profiles along the tube as well as the visualization of the flow patterns. Pressure and temperature distribution along the flow were measured for saturation pressure ranging from 450 to 650 kPa, mass flux ranging from about 2000 to 3000 kg/(m²s), temperatures around 303 K, and inlet refrigerant concentration varying between 0.2 and 0.4 kg ref/kg mixt. An available correlation proposed to predict the frictional pressure drop for a mixture composed by the mineral oil SUNISO 1GS and refrigerant R12 flowing in small diameter tubes yielded large deviations in predicting the ester oil and refrigerant R134a mixture flow. A new correlation has been proposed that fitted the experimental data with rms deviations of 24%.