Pressure Vessel Failure Mechanisms The following failure mechanism information may assist inspectors in identifying service-induced deterioration and failure modes encountered in pressure-retaining items. Fatigue Stress reversals (such as cyclic loading) in parts of equipment are common, particularly at points of high secondary stress. If stresses are high and reversals frequent, damage may occur because of fatigue. Fatigue damage in pressure vessels may also result from cyclic temperature and pressure changes. Locations where metals having different thermal coefficients of expansion are joined by welding maybe susceptible to thermal fatigue. Creep Creep damage may occur if equipment is subjected to temperatures above those for which the equipment is designed. Since metals become weaker at higher temperatures, such distortion may result in failure, particularly at points of stress concentration. If excessive temperatures are encountered, structural property and chemical changes in metals may also take place, which may permanently weaken equipment. Since creep is dependent on time, temperature and stress, the actual or estimated levels of these quantities should be used in any evaluations. Temperature Effects At subfreezing temperatures, water and some chemicals handled in pressure vessels may freeze and cause damage. Carbon and low-alloy steels may be susceptible to brittle failure at ambient temperatures. A number of failures have been attributed to brittle fracture of steels that were exposed to temperatures below their transition temperature and that were exposed to pressures greater than 20% of the required hydrostatic test pressure.However,most brittle fractures have occurred on the first application of a particular stress level (that is,the first hydrostatic test or overload).Special attention should be given to low-alloy steels because they are prone to temper embrittlement.Temper embrittlement is defined as a loss of ductility and notch toughness due to postweld heat treatment or high-temperature service,above700°F(371°C). Hydrogen Embrittlement a) The term hydrogen embrittlement (HE) refers to a loss of ductility and toughness in steels caused by atomic hydrogen dissolved in the steel. Hydrogen that is dissolved in carbon and low-alloy steels from steel making, welding, or from surface corrosion can cause either intergranular or transgranular cracking and “brittle” fracture behavior without warning. b) Hydrogen embrittlement typicallyoccursbelow200°F(93°C) because hydrogen remains dissolved with in the steel at or below this temperature. One example of hydrogen embrittlement is underbead cracking. The underbead cracks are caused by the absorption of hydrogen during the welding process in the hard, high-strength weld heat affected zone (HAZ). Use of low-hydrogen welding practices to minimize dissolved hydrogen and/or the use of high preheat, and/or postweld heat treatment can reduce susceptibility to cracking from hydrogen embrittlement.Thediffusivityofhydrogenissuchthatattemperaturesabove450°F (232°C),the hydrogen can be effectively removed,eliminating susceptibility to cracking.Thus,hydrogen embrittlement maybe reversible as long as no physical damage(e.g.,cracking or fissures)has occurred in the steel. c) Hydrogen Embrittlement Is a form of stress corrosion cracking (SCC). Three basic elements are needed to induce SCC: the first element is a susceptible material, the second element is environment, and the third element is stress (applied or residual). For hydrogen embrittlement to occur, the susceptible material is normally higher strength carbon or low alloy steels, the environment must contain atomic hydrogen, and the stress can be either service stress and/or residual stress from fabrication. If any of the three elements are eliminated, HE cracking is prevented. d) In environments where processes are conducted at elevated temperature, the reaction of hydrogen with sulfur in carbon and low-alloy reactor vessel steels can produce hydrogen sulfide stress corrosion(SSC), which is a form of hydrogen embrittlement.Susceptibility to sulfide stress corrosion cracking depends on the strength of the steel. Higher-strength steels are more susceptible. The strength level at which susceptibility increases depends on the severityof the environment.Hydrogen sulfide,hydrogen cyanide,and arsenic in aqueous solutions, all increase the severity of the environment towards hydrogen embrittlement by increasing the amount of hydrogen that can be absorbed by the steel during the corrosion reaction. In hydrogens ulfide environments, susceptibility to cracking can be reduced by using steels with a strength level below that equivalent to a hardness of 22 on the Rockwell C scale e)Other forms of hydrogen embrittlement are wet hydrogen sulfide(H2S) cracking, hydrogen stress cracking, hydrogen-induced cracking (HIC), and stress-oriented hydrogen-induced cracking (SOHIC). In each case, three basic elements are required for this damage mechanism — susceptible material, hydrogen generating environments, and stress (either residual or applied). Organic or inorganic coatings, alloy cladding or linings, are often used as a barrier to mitigate wet H2S corrosion and subsequent cracking. High-temperature Hydrogen attack Hydrogen attack is a concern primarily in refinery and petrochemical plant equipment handling hydrogen and hydrogen-hydrocarbon streams at temperatures above 450°F(232°C) and pressure above 100psi (700 kPa). A guideline for selection of steels to avoid hydrogen attack is given in API Publication 941, “Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petrochemical Refineries and Petrochemical Plants.” Also widely known as the “Nelson Curves,” API 941 shows that the severity of hydrogen attack depends on temperature, hydrogen partial pressure, exposure time, and steel composition. Additions of chromium and molybdenum increase resistance to hydrogen attack. It is important to understand that hydrogen attack is different from hydrogen embrittlement, which is discussed in NBIC Part Hydrogen attack occurs in a high-temperature, high-pressure hydrogen environment that can degrade the mechanical strength of carbon and low alloy steels. The damage is caused by hydrogen permeating into the steel and reacting with carbon to form methane. Since carbon is an element that strengthens steel, its removal by the high-temperature reaction with hydrogen causes the steel to lose strength. In addition, methane can become trapped within the steel at high pressures, eventually forming bubbles, fissures (cracks), and/or blisters. c) Damage caused by hydrogen attack is preceded by an incubation period with no noticeable change in properties.After the incubation period, decarburization and/or blistering and fissuring will occur.Thelength of the incubation period varies with service temperature, the partial pressure of hydrogen, and alloy content of the steel. Damage is reversible during the incubation period, during which no loss of mechanical properties will have occurred. Once permanent degradation begins, the damage is irreversible. Hydrogen Damage a) Hydrogen damage has been encountered in steam boilers that operate in the high-pressure range (1200 psi[8.27MPa]orhigher),with relatively high-purity boiler feed water.In boilers,the mechanism of hydrogen damage is initiated by under deposit corrosion on water-touched surfaces. During operation of the boiler, water wall tubing exposed to high heat flux can result in a departure from nucleate boiling (DNB) condition on the ID(waterside)surface duet o small flow disturbances.Because of the increased tube metal temperature, low levels of contaminants in the boiler feed water precipitate (e.g., plate out) on the hot tube surface.The intermittent wetting from flow,overtime,results in the accumulation of deposits. b) As the deposit begins to thicken, the tube metal beneath the deposit locally increases in temperature, causing oxidation of the tube metal. The oxidation/reduction corrosion mechanism creates atomic hydrogen, which permeates into the tube wall at boiler pressures greater than 1200 psig (8.27 MPa). c) The atomic hydrogen reacts with the carbon in the steel, forming methane gas that results in micro fissures at grain boundaries and decarburization. The combination of decarburization and micro cracks increases the susceptibility to brittle fracture in service. The typical appearance of hydrogen damage in boiler tubes is a thick-lipped, “window-type” blow out of tube metal. d) Hydrogen damage in copper and copper alloys has also been observed and is sometimes known as steam embrittlement. This type of damage commonly occurs when the copper contains oxygen. Hydrogen entering the metal reacts with the oxygen to form water. At certain combinations of pressures and temperatures steam forms and the pressure generated is sufficient to produce micro-cavity formation and cracking. Bulges and Blisters a) A bulge may be caused by overheating of the entire thickness of the metal, thereby lowering the strength of the metal which is then deformed by the pressure. Bulges may also be caused by creep or temperature gradients. b) A Pressure Vessel Failure Mechanisms The following failure mechanism information may assist inspectors in identifying service-induced deterioration and failure modes encountered in pressure-retaining items. Fatigue Stress reversals (such as cyclic loading) in parts of equipment are common, particularly at points of high secondary stress. If stresses are high and reversals frequent, damage may occur because of fatigue. Fatigue damage in pressure vessels may also result from cyclic temperature and pressure changes. Locations where metals having different thermal coefficients of expansion are joined by welding maybe susceptible to thermal fatigue. Creep Creep damage may occur if equipment is subjected to temperatures above those for which the equipment is designed. Since metals become weaker at higher temperatures, such distortion may result in failure, particularly at points of stress concentration. If excessive temperatures are encountered, structural property and chemical changes in metals may also take place, which may permanently weaken equipment. Since creep is dependent on time, temperature and stress, the actual or estimated levels of these quantities should be used in any evaluations. Temperature Effects At subfreezing temperatures, water and some chemicals handled in pressure vessels may freeze and cause damage. Carbon and low-alloy steels may be susceptible to brittle failure at ambient temperatures. A number of failures have been attributed to brittle fracture of steels that were exposed to temperatures below their transition temperature and that were exposed to pressures greater than 20% of the required hydrostatic test pressure.However,most brittle fractures have occurred on the first application of a particular stress level(that is,the first hydrostatic test or overload).Special attention should be given to low-alloy steels because they are prone to temper embrittlement.Temper embrittlement is defined as a loss of ductility and notch toughness due to postweld heat treatment or high-temperature service,above700°F(371°C). Hydrogen Embrittlement a) The term hydrogen embrittlement (HE) refers to a loss of ductility and toughness in steels caused by atomic hydrogen dissolved in the steel. Hydrogen that is dissolved in carbon and low-alloy steels from steel making, welding, or from surface corrosion can cause either intergranular or transgranular cracking and “brittle” fracture behavior without warning. b) Hydrogen embrittlement typicallyoccursbelow200°F(93°C) because hydrogen remains dissolved within the steel at or below this temperature. One example of hydrogen embrittlement is underbead cracking. The underbead cracks are caused by the absorption of hydrogen during the welding process in the hard, high-strength weld heat affected zone (HAZ). Use of low-hydrogen welding practices to minimize dissolved hydrogen and/or the use of high preheat, and/or postweld heat treatment can reduce susceptibility to cracking from hydrogen embrittlement.Thediffusivityofhydrogenissuchthatattemperaturesabove450°F (232°C),the hydrogen can be effectively removed,eliminating susceptibility to cracking.Thus,hydrogen embrittlement maybe reversible as long as no physical damage(e.g.,cracking or fissures) has occurred in the steel. c) Hydrogen Embrittlement Is a form of stress corrosion cracking (SCC). Three basic elements are needed to induce SCC: the first element is a susceptible material, the second element is environment, and the third element is stress (applied or residual). For hydrogen embrittlement to occur, the susceptible material is normally higher strength carbon or low alloy steels, the environment must contain atomic hydrogen, and the stress can be either service stress and/or residual stress from fabrication. If any of the three elements are eliminated, HE cracking is prevented. d) In environments where processes are conducted at elevated temperature, the reaction of hydrogen with sulfurincarbonandlow-alloyreactorvesselsteelscanproducehydrogensulfidestresscorrosion(SSC), whichisaformofhydrogenembrittlement.Susceptibilitytosulfidestresscorrosioncrackingdependson the strength of the steel. Higher-strength steels are more susceptible. The strength level at which susceptibilityincreasesdependsontheseverityoftheenvironment.Hydrogensulfide,hydrogencyanide,and arsenic in aqueous solutions, all increase the severity of the environment towards hydrogen embrittlement by increasing the amount of hydrogen that can be absorbed by the steel during the corrosion reaction. In hydrogens ulfide environments, susceptibility to cracking can be reduced by using steels with a strength level below that equivalent to a hardness of 22 on the Rockwell C scale e)Other forms of hydrogen embrittlement are wet hydrogen sulfide(H2S) cracking, hydrogen stress cracking, hydrogen-induced cracking (HIC), and stress-oriented hydrogen-induced cracking (SOHIC). In each case, three basic elements are required for this damage mechanism — susceptible material, hydrogen generating environments, and stress (either residual or applied). Organic or inorganic coatings, alloy cladding or linings, are often used as a barrier to mitigate wet H2S corrosion and subsequent cracking. High-temperature Hydrogen attack Hydrogen attack is a concern primarily in refinery and petrochemical plant equipment handling hydrogen and hydrogen-hydrocarbon streams at temperatures above 450°F(232°C) and pressure above 100psi (700 kPa). A guideline for selection of steels to avoid hydrogen attack is given in API Publication 941, “SteelsforHydrogenServiceatElevatedTemperaturesandPressuresinPetrochemicalRefineriesand Petrochemical Plants.” Also widely known as the “Nelson Curves,” API 941 shows that the severity of hydrogen attack depends on temperature, hydrogen partial pressure, exposure time, and steel composition. Additions of chromium and molybdenum increase resistance to hydrogen attack. It is important to understand that hydrogen attack is different from hydrogen embrittlement, which is discussed in NBIC Part Hydrogen attack occurs in a high-temperature, high-pressure hydrogen environment that can degrade the mechanical strength of carbon and low alloy steels. The damage is caused by hydrogen permeating into the steel and reacting with carbon to form methane. Since carbon is an element that strengthens steel, its removal by the high-temperature reaction with hydrogen causes the steel to lose strength. In addition, methane can become trapped within the steel at high pressures, eventually forming bubbles, fissures (cracks), and/or blisters. c) Damage caused by hydrogen attack is preceded by an incubation period with no noticeable change in properties.After the incubation period, decarburization and/or blistering and fissuring will occur.The length of the incubation period varies with service temperature, the partial pressure of hydrogen, and alloy content of the steel. Damage is reversible during the incubation period, during which no loss of mechanical properties will have occurred. Once permanent degradation begins, the damage is irreversible. Hydrogen Damage a) Hydrogen damage has been encountered in steam boilers that operate in the high-pressure range (1200 psi[8.27MPa]or higher),with relatively high-purity boiler feed water.In boilers,the mechanism of hydrogen damage is initiated by under deposit corrosion on water-touched surfaces. During operation of the boiler, water wall tubing exposed to high heat flux can result in a departure from nucleate boiling (DNB) condition on the ID(waterside)surface due to small flow disturbances.Because of the increased tube metal temperature, low levels of contaminants in the boiler feed water precipitate (e.g., plate out) on the hot tube surface.The intermittent wetting from flow,overtime,results in the accumulation of deposits. b) As the deposit begins to thicken, the tube metal beneath the deposit locally increases in temperature, causing oxidation of the tube metal. The oxidation/reduction corrosion mechanism creates atomic hydrogen, which permeates into the tube wall at boiler pressures greater than 1200 psig (8.27 MPa). c) The atomic hydrogen reacts with the carbon in the steel, forming methane gas that results in micro fissures at grain boundaries and decarburization. The combination of decarburization and micro cracks increases the susceptibility to brittle fracture in service. The typical appearance of hydrogen damage in boiler tubes is a thick-lipped, “window-type” blow out of tube metal. d) Hydrogen damage in copper and copper alloys has also been observed and is sometimes known as steam embrittlement. This type of damage commonly occurs when the copper contains oxygen. Hydrogen entering the metal reacts with the oxygen to form water. At certain combinations of pressures and temperatures steam forms and the pressure generated is sufficient to produce micro-cavity formation and cracking. Bulges and Blisters a) A bulge may be caused by overheating of the entire thickness of the metal, thereby lowering the strength of the metal which is then deformed by the pressure. Bulges may also be caused by creep or temperature gradients. b) A blister maybe caused by a defect in the metal,such as a lamination,where the side exposed to the fire overheats but the other side retains its strength due to cooling effect of water or other medium. Blisters may also be caused by a hydrogen environment OverHeating a) Overheating is one of the most serious causes of deterioration. Deformation and possible rupture of pressure parts may result. b) Attention should be given to surfaces that have either been exposed to fire or to operating temperatures that exceed their design limit. It should be observed whether any part has become deformed due to bulging or blistering. If a bulge or blister reduces the integrity of the component or when evidence of leakage is noted coming from those defects, proper repairs must be made. CracKs Cracks a) May result from flaws existing in material or excessive cyclic stresses. Cracking can because by fatigue of the metal due to continual flexing and maybe accelerated by corrosion. Fire cracks are caused by the thermal differential when the cooling effect of the water is not adequate to transfer the heat from the metal surfaces exposed to the fire. Some cracks result from a combination of all these causes mentioned. b) Cracks noted in shell plates and fire cracks that run from the edge of the plate in to the rivet holes of girth seams should be repaired. Thermal fatigue cracks determined by engineering evaluation to be self arresting may be left in place. c) Areas where cracks are most likely to appear should be examined. This includes the ligaments between tube holes, from and between rivet holes, any flange where there maybe repeated flexing of the plate during operation, and around welded connections. d) Lap joints are subject to cracking where the plates lap in the longitudinal seam. If there is any evidence of leakage or other distress at this point, the Inspector shall thoroughly examine the area and, if necessary, have the plate notched or slotted in order to determine whether cracks exist in the seam. Repairs of lap joint cracks on longitudinal seams are prohibited. e) Where cracks are suspected, it may be necessary to subject the pressure-retaining item to a pressure test or a nondestructive examination to determine their presence and location. For additional information regarding a crack or determining extent of a possible defect, a pressure test may be performed f) Cracks shall either be repaired or formally evaluated by Crack Propagation Analysis to quantify their existing mechanical integrity. blister maybe caused by a defect in the metal ,such as a lamination, where the side exposed to the fire overheats but the other side retains its strength due to cooling effect of water or other medium. Blisters may also be caused by a hydrogen environment OverHeating a) Overheating is one of the most serious causes of deterioration. Deformation and possible rupture of pressure parts may result. b) Attention should be given to surfaces that have either been exposed to fire or to operating temperatures that exceed their design limit. It should be observed whether any part has become deformed due to bulging or blistering. If a bulge or blister reduces the integrity of the component or when evidence of leakage is noted coming from those defects, proper repairs must be made. CracKs Cracks a) May result from flaws existing in material or excessive cyclic stresses. Cracking can because by fatigue of the metal due to continual flexing and maybe accelerated by corrosion. Fire cracks are caused by the thermal differential when the cooling effect of the water is not adequate to transfer the heat from the metal surfaces exposed to the fire. Some cracks result from a combination of all these causes mentioned. b) Cracks noted in shell plates and fire cracks that run from the edge of the plate in to the rivet holes of girth seams should be repaired. Thermal fatigue cracks determined by engineering evaluation to be self arresting may be left in place. c) Areas where cracks are most likely to appear should be examined. This includes the ligaments between tube holes, from and between rivet holes, any flange where there maybe repeated flexing of the plate during operation, and around welded connections. d) Lap joints are subject to cracking where the plates lap in the longitudinal seam. If there is any evidence of leakage or other distress at this point, the Inspector shall thoroughly examine the area and, if necessary, have the plate notched or slotted in order to determine whether cracks exist in the seam. Repairs of lap joint cracks on longitudinal seams are prohibited. e) Where cracks are suspected, it may be necessary to subject the pressure-retaining item to a pressure test or a nondestructive examination to determine their presence and location. For additional information regarding a crack or determining extent of a possible defect, a pressure test may be performed f) Cracks shall either be repaired or formally evaluated by Crack Propagation Analysis to quantify their existing mechanical integrity.
Posted on: Wed, 17 Dec 2014 22:03:54 +0000
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