Application of non-destructive and fracture mechanics techniques for the condition assessment of naphtha hydrotreater furnace tubes in oil refinery

Furnace tubes of oil refineries undergo thermal, mechanical and environmental loadings during the lifetime of the plant. Apart from highly corrosive conditions, components are also under severe temperatures and pressures which lead to the material degradation and ultimately causing failure. Due to these loadings, the tubes have a finite life and it is important to monitor the condition of the tubes during inspection to avoid any rupture during the service.The aim of the present work is to evaluate the condition of the naphtha hydrotreater furnace tubes of an oil refinery. These tubes were manufactured of SA 312 P321 material and remained in service for about twenty-five years. Two different techniques were applied for the condition assessment of used and fresh tubes – non-destructive (ND) examination and fracture mechanics based fractographic evaluation. Based on the ND testing, significant deterioration of material was noted in the used specimens. Similarly, besides micro-structural defects, the used material also exhibited sufficient loss of hardness. On the other hand, mechanical properties obtained from standard tension, impact and bend tests also exhibited the poor condition of the tubes. A comparison of the used specimens with the un-used one shows that the condition of the furnace tubes has been considerably deteriorated and its further use for the long term service cannot be considered as satisfactory.     

Failure assessment of crude oil preheating tubes in mono ethylene glycol–water mixture solution

Aqueous solutions of glycol are commonly used in many applications as heat transfer media. In this study, failure analysis and the leakage of oil tubes in a crude oil pre-heater of a desalting plant were assessed. Crude oil pre-heater (furnace) is a type of heat exchanger in which heat is transferred to oil tubes via ethylene glycol solution in order to increase its temperature for further processing. With the increase of temperature, the viscosity of the oil plus water mixture inside the tubes is reduced and desalting is facilitated. Reports show that, these tubes facing external corrosion in the form of smooth cavities finally lead to leakage. These tubes are made from ASTM A106 Gr B steel. The failure assessment of tubes was carried out by performing various chemical and metallurgical tests. Corrosion coupons were also immersed in ethylene glycol fluid at different places inside the furnace. The investigation revealed that degradation of ethylene glycol forming organic acids (acetic acid, carbonic acid, and formic acid), as the consequent reduction of the pH, take place progressively, being responsible for carbon steel acidic corrosion and eventually to the failure of pre-heating tubes. An action was taken to examine different pH stabilizers for mono ethylene glycol solution. The results obtained by polarization curves showed that, by using stabilizer EN231 at pH = 10.5, corrosion rate reduces to a minimum.     

Investigations on the Failure of Economizer Tubes in a High-Pressure Boiler

This article describes the results of an investigation concerning the failure of economizer tubes of a high-pressure boiler in a dual-purpose power/water cogeneration plant. The failure was observed in the form of rupturing of one tube and a macrohole or pinhole in another tube. The boiler had an operating period of 116,123 h since its inception. For approximately the first 100,000 h, the fuel for the boiler was crude oil, which was replaced by Bunker C oil. The boiler tube is fabricated from carbon steel SA 210A1. The location of the failure was determined by on-site visual inspection of the boiler. Detailed macro- and microexaminations of inner and outer scales on the tube were begun to determine the cause of the rupture. The composition of the fire- and waterside scale and ash deposited on the outer surface of the tubes was analyzed by energy-dispersive x-ray (EDX) technique. The reduction percentage of wall thickness of the tube facing inside and outside the furnace was calculated. The cause of the failure of the economizer tube appears to be H₂SO₄ dew-point corrosion. The relatively low temperature of feedwater lowered the tube metal temperature and promoted the condensation of H₂SO₄. The external deposits on the tubes, as a result of bunker oil firing, further helped to lower the tube metal temperature, thus promoting H₂SO₄ condensation over the deposit and subsequent corrosion of the tube wall. Recommendations are given to prevent/minimize such failures.     

Failure of a transfer line on an ethane cracking furnace due to sulfidation

An Incoloy 800H (UNS N08810) transfer line on the outlet of an ethane-cracking furnace failed during decoking of the furnace tubes after 9 years in service. A metallographic examination using optical and scanning electron microscopy (SEM) as well as energy dispersive X-ray spectroscopy (EDS) revealed that the failure was due to sulfidation. The source of the sulfur in the furnace effluent was either dimethyl disulfide (DMDS), injected into the furnace feed to prevent coke formation and carburization of the furnace tubes, or contamination of the feed with sulfur bearing oil.     

Stress Assisted Corrosion of Waterwall Tubes in Recovery Boiler Tubes: Failure Analysis

Waterside cracking of carbon-steel boiler tubes is one of the major safety and efficiency concerns in kraft recovery boilers in the pulp and paper industry, because any water leak into the furnace could cause a smelt-water explosion in the boiler. Failed carbon-steel boiler tubes from different kraft recovery boilers were examined to understand the role of carbon-steel microstructure on crack initiation and crack morphology. A number of carbon-steel tubes showed a deep decarburized layer on the inner surface (water-touched) and also an unusually large grain size at the inner tube surface. In some boiler tubes, cracks were found to initiate in areas with large-grained-decarburized microstructure. However, tubes without such microstructure were also found to have stress assisted corrosion (SAC) cracks. It was found that the decarburization and large grained microstructure may facilitate initiation and growth, but it is not necessary for SAC of carbon-steel boiler tubes.     

Failure of hydrocracker heat exchanger tubes in an oil refinery by polythionic acid-stress corrosion cracking

Heat exchanger tubes of a hydrocracker unit in an oil refinery developed cracks at the bent sections after about 48 h of operation at 400 °C following a period of downtime. The tubes were fabricated from 321 stainless steel. Detailed analysis by various electron-optical techniques and X-ray diffraction showed that the tubes were cracked by stress corrosion cracking induced by polythionic acid. Due to the presence of H₂S in the environment, the inner surface of the tubes was converted into sulfur-bearing scale during operation. This could promote the formation of polythionic acid by aqueous condensates during downtime leading to cracking in the presence of residual internal stresses.     

Sensitization induced stress corrosion failure of AISI 347 stainless steel fractionator furnace tubes

Austenitic stainless steel tubes are used as furnace tubes in petrochemical industries mainly because of their corrosion resistance and mechanical strength. AISI 347 grade stainless steel is used as furnace heater tubes in the fractionator of hydrocracker unit. Even though this stainless steel is stabilized with the addition of niobium thus preventing sensitization related corrosion failures, operational and maintenance errors may result in premature failures if conditions prevail. The present work reports the premature failure of AISI 347 grade fractionator furnace tubes after nearly 8 years of service. The failure occurred after shutdown. Carbonaceous deposits were found on the inner walls of the tube and circumferential cracks were found beneath the deposit. The service exposed 347 SS alloy tube was in the sensitized condition as confirmed by microstructure and double loop electrochemical potentiodynamic reactivation test. The tube material got sensitized possibly by localized overheating at the carbon layer deposited site. During shutdown of hydrocracker unit polythionic acid formation occurred possibly due to errors in shutdown procedures. Sensitized alloy 347 tube undergone polythionic acid induced intergranular stress corrosion cracking (PASCC).     

Failure Investigation of Cooling Plates of a Blast Furnace

Premature failure of cooling plates of a blast furnace in service has been investigated. The cooling plates are water-cooled copper channels shielded by refractory lining. Industrial cooling water (ICW) flows at 5 bar pressure through the cooling plates. The investigation consisted of visual inspection, chemical analysis, microstructural examination by optical microscopy, energy-dispersive spectroscopy (EDS), and hardness measurement. Visual inspection revealed that the refractory lining on the component was damaged, thereby exposing the cooling plates to elevated temperatures of the furnace atmosphere. This led to overheating of the cooling plates. Chemical analysis revealed that the component was essentially pure copper (~99.24% purity). Microstructural analysis showed grain size variation across the section of the component. Also, annealing twins were revealed on optical examination. A through-thickness pit was identified on the failed component during the optical examination. The examination also revealed branched cracks associated with scale in the component. Energy-dispersive spectroscopy (EDS) confirmed the scale to be oxides indicating high-temperature oxidation of the component resulting in pitting. Analysis suggested overheating due to damage of the refractory lining as the prime cause of failure of the component.     

焊接对HK40高温炉管的蠕变损伤与断裂的影响

采用含有局部效应的蠕变损伤力学方法,计及材料不均匀性,建立母材和焊缝金属两材料有限元模型,分析了具有缺口的ＨＫ４０带焊缝试样以及实际ＨＫ４０炉焊接接头的蠕变损伤发展与断裂。蠕变损伤模拟结果表明,虽然焊缝金属为蠕变硬焊缝,但在服役寿命的后焊缝损伤较母材严重,出现损伤局部化特征,最终的失效发生在焊缝中。股役１０年的ＨＫ４０炉管的金相剖析也明显焊缝比母材具有较高的损伤级别,这预示着焊缝在长期股役过程中是最薄弱的环节之一。     

Failure analysis of TP304H tubes in the superheated steam section of a reformer furnace

The cracking failure of TP304H tubes in the superheated steam section of a reformer furnace was analyzed. Through the analysis of macro-appearance, micro-appearance of specimens with cracks, metallurgical structure of specimens from an intact pipe section and cracked pipe section, energy spectrum detection of fracture surface, residual stress measurement, and investigation of the service medium, the cracking mode was described as the stress corrosion cracking (SCC) of austenitic stainless steel. In this case, the materials in the heat affected zone were sensitized by inappropriate welding technology. This together with the higher pH value in the steam due to the failure of a gas-liquid separator led to the final cracking of the reformer furnace tube. So the inappropriate welding technology and the failure of the gas-liquid separator were the main factors in this fracture accident.     

Radiation doses and hazards from processing of crude oil at the Tema oil refinery in Ghana

Processing of crude oil has been carried out in Ghana for more than four decades without measures to assess the hazards associated with the naturally occurring radionuclides in the raw and processed materials. This study investigates the exposure of the public to (226)Ra, (232)Th and (40)K in crude oil, petroleum products and wastes at the Tema oil refinery in Ghana using gamma-ray spectrometry. The study shows higher activity concentrations of the natural radionuclides in the wastes than the crude oil and the products with estimated hazard indices less than unity. The values obtained in the study are within recommended limits for public exposure indicating that radiation exposure from processing of the crude oil at the refinery does not pose any significant radiological hazard but may require monitoring to establish long-term effect on both public and workers.     

Analysis of the fatigue failure of a mountain bike front shock

We present an analysis of a mountain bike front shock failure. The failure of the 1-year-old shock occurred catastrophically as the bike was ridden off of a 1-m drop. The failure was the result of fast fracture through both shock tubes at the location where the tubes were press fit into the shock upper crown. Examination of the fracture surfaces of the tubes revealed regions of fatigue crack growth that nearly penetrated the entire thickness of both tubes. An estimate of the forces during use, coupled with stress analysis, revealed three stresses near the fracture site—axial compression, bending, and hoop stresses. During operation, the axial compressive stress is negligible while the hoop and bending stresses are significant. Based on fracture mechanics, and an estimate of the bending stress from a 1-m drop, it is confirmed that the fatigue cracks present on the fracture surface were large enough to induce fast fracture. Prior to the existence of the fatigue cracks, the stresses were magnified locally near the fracture site by a significant stress concentration caused by the sharp transition from the shock tube to the crown. The fatigue cracks initiated at a circumferential location in the tube commensurate with high tensile bending stress and the stiffest region of the crown (highest stress concentration). Based on the evidence, the most probable cause of the bike shock fatigue failure was the shock design, which facilitated high local stresses during use.     

Determination of global failure pressure for tubes with two parallel cracks

The 40% of wall thickness criterion, which has been used as a plugging rule is applicable only to a single cracked steam generator tubes. In the previous studies performed by the authors, several failure prediction models were introduced to estimate the plastic collapse pressures of steam generator tubes containing two adjacent parallel axial through‐wall cracks. These models were applied preliminarily for thin plates with two parallel through‐wall cracks and the crack opening displacement (COD) base model was selected as the optimum one. The objective of this study is to verify the applicability of the proposed failure prediction model for real steam generator tubes with two parallel axial through‐wall cracks. For this purpose, a series of plastic collapse tests and finite element analyses have been carried out for the steam generator tubes with two machined parallel axial through‐wall cracks. Thereby, it was proven that the proposed failure prediction model can be used for estimating the global failure pressure quite well. Also, interaction effects between two adjacent cracks were assessed through additional finite element analyses to investigate the effect on the global failure behaviour.     

高炉钛渣对废润滑油的吸附性能

为研究高炉钛渣的高附加值利用,采用静态吸附法研究了高炉钛渣对废润滑油的吸附性能,考察了高炉钛渣投加量、搅拌转速、吸附温度以及吸附时间对吸附效果的影响,并通过正交实验探讨了高炉钛渣吸附废润滑油的工艺优化条件,在此基础上进行了高炉钛渣与活性白土吸附废润滑油的对比实验。实验结果表明:高炉钛渣对废润滑油有着较好的吸附效果,且吸附效果与活性白土相当,可作为吸附剂用于废润滑油的吸附再生。     

缓冲器轴套夹布胶木失效分析

起落架缓冲器轴套在使用过程中发生开裂或断裂.通过外观检查、断口观察、性能测试等方法,结合缓冲器轴套夹布胶木的装配与受力情况,分析了缓冲器轴套断裂与裂纹产生的原因.实验结果表明:夹布胶木裂纹为疲劳裂纹,夹布胶木断裂也是疲劳开裂所致.疲劳裂纹与断裂的主要原因是轴套夹布胶木与铜套之间为过渡配合,二者之间存在间隙,油进入间隙后...     

Assessment of plastic collapse behavior for tubes with collinear cracks

The 40% of wall thickness criterion has been used as a plugging rule of steam generator tubes but it can be applicable just to a single-cracked tubes. In the previous studies preformed by the authors, a total of 10 local failure prediction models were introduced to estimate the coalescence load of two adjacent collinear through-wall cracks existing in thin plates, and the reaction force model and plastic zone contact model were selected as optimum models among them. The objective of this study is to verify the applicability of the proposed optimum local failure prediction models to the tubes with two collinear through-wall cracks. For this, a series of plastic collapse tests and finite element analyses were carried out using the tubes containing two collinear through-wall cracks. It has been shown that the proposed optimum failure models can predict the local failure behavior of two collinear through-wall cracks existing in tubes well. And a coalescence evaluation diagram was developed which can be used to determine whether the adjacent cracks detected by NDE coalesce or not.     

Determination of failure pressure for tubes with two non-aligned axial through-wall cracks

The 40% of wall thickness criterion which has been used as a plugging rule is applicable only to a single cracked steam generator tubes. In the previous studies performed by authors, several failure prediction models were introduced to estimate the plastic collapse pressures of steam generator tubes containing two adjacent collinear or parallel axial through-wall cracks. The objective of this study is to examine the failure prediction models and propose optimum ones for two non-aligned axial through-wall cracks in steam generator tubes. In order to determine the optimum ones, a series of plastic collapse tests and finite element analyses were carried out for steam generator tubes with two machined non-aligned axial through-wall cracks. Thereby, either the plastic zone contact model or COD based model was selected as the optimum one according to axial distance between two cracks. Finally, the optimum failure prediction model was used to demonstrate the conservatism of flaw characterization rules for multiple cracks having equal lengths according to ASME code.     

An Investigation on the Causes of Failure in A210 Carbon Steel Boiler Tubes in an Oil Refinery

As boilers are subjected to corrosive streams and harsh operating conditions, their tubes are susceptible to corrosion and failure by different damage mechanisms. In the current study, the causes of cracking and failure of connection tubes of a boiler header are studied. These tubes were connected between the primary and secondary headers of the boiler and according to the design information were constructed of ASTM A210-A1 carbon steel. There were two cracked tubes wherein one of them had a crack in a weld HAZ region and another had a crack in the base material. To investigate the failure, different microscopical and analytical techniques such as SEM and mechanical tests were used. The results of analysis showed that the material of construction was in accordance with the design specification. Considering the cracks dagger-shaped appearance, it was concluded that thermal fatigue due to the operation conditions and utilization of water spray cooling technique was the main cause of the failure.     

原油输送管道内腐蚀原因分析

为明确原油输送管道内腐蚀的原因,通过实际腐蚀管道失效分析,对原油中水溶性腐蚀性组分进行了分析测试,利用扫描电镜(SEM)、能谱(EDS)等分析了腐蚀产物的形貌及成分,结合管道中油水分布多相流模拟,指出:原油管道发生内腐蚀破坏的必要条件是管道内壁与管输物料之间形成可发生腐蚀的原电池。评价管输原油是否会造成管道腐蚀,首先应当明确水能否从原油中析出,析出的水能否直接接触管道表面,析出的水会在管道什么部位聚集。通过原油中水的稳定性测试,可以确定水能否从原油中析出,原油在管道表面的润湿性测试表征管道表面能否接触到腐蚀性水,多相流分析可以明确管道中聚集水的部位。     

Fatigue Failure of 321 Stainless Steel Superheater Tubes from a Thermal Incinerator

Several boiler superheater tubes showed circumferential cracking at weld seams after 2 years in noncontinuous service (several shutdowns). On-site inspection revealed that several tubes were cracked and leaked; while many others were cracked, however, the severity was less pronounced. Two types of superheater tubes samples were collected: one with butt-welded tubes and the other with fillet-welded sleeve. The latter was found to be out of the boiler fireplace, and the sleeve was used as tubing support to the boiler shell. Detailed investigation showed that the butt-welded tubes contained circumferential fatigue cracks that initiated from the internal surface. The cracks initiated in the heat-affected zone and propagated as a result of tube vibration. The variations in the tube internal diameter and tube wall thickness are expected to play a role in tube fatigue failure. On the other hand, tubes with fillet-weld sleeve showed circumferential cracking as a result of fatigue crack initiation from weld defects on the tube external surface. The high vibration during several unscheduled shutdowns in addition to several other factors such as variations in tube inside diameter, wall thickness, and weld defects resulted in the initiation and propagation of fatigue cracks and premature failure. White deposits, similar to those observed when boiler tubes failed by caustic exposure, were seen in the vicinity of the tube cracks. Therefore, it was difficult to confirm whether the boiler tubes failed because of the fatigue cracks or because of the caustic salts (pH control chemical).