Creep Life Evaluations of ASME B31.1 Allowance for Variation From Normal Operations: 11 Materials

The ASME B31.1-2018 Power Piping Code (Code) paras. 102.2.4, 102.3.3, and 104.8.2 provide an allowance regarding operating above the design temperature and design pressure for short time periods. The concept of allowing occasional operation for short periods of time at higher than the design pressure or design temperature has been in the Code since 1967. These 1967 Code para. 102.2.4 limitations were based on engineering judgment that can now be quantitatively evaluated for the additional creep life consumption (creep rupture damage accumulation). This study primarily is a quantitative estimate of the permitted increased life consumption, considering minimum creep rupture properties, associated with the 2018 Code operating allowances for piping materials operating in the creep range. Eleven base metal materials are considered in this paper — low carbon steel, 1.25Cr 0.5Mo, 2.25Cr 1Mo, 9Cr 1 Mo V, Type 304 SS, Type 316 SS, Type 316L SS, Type 321 SS, Type 321H SS, Type 347 SS, and Type 347H SS. Results of this evaluation may be used to improve the ASME B31.1 Code, including a technical basis for a possible revision to para. 102.2.4. Previous studies have revealed that Grade P22 base metal creep damage is slightly more sensitive to stress than Grade P11 material creep rupture damage, and Grade P91 base metal creep damage is substantially more sensitive to stress than Grade P22 material creep rupture damage. Therefore, the allowable pressure and temperature variations result in a range of increased creep life consumption for different materials. The intent of this study was to modify the two Code allowance criteria so that the permitted increased creep life consumption (considering the minimum creep rupture properties of the material) of Allowance B is about the same amount as the increased creep life consumption result of Allowance A for the same material. Consequently, this study was performed to realign the allowable increased creep rupture life consumption of Allowance B to be approximately equivalent to the allowable increased creep life consumption of Allowance A. If the Allowance B event duration is increased from 80 hours per year to 400 hours per year, the Allowance B increased creep life consumption is slightly less than the Allowance A life consumption for each of these materials.

Creep Life Evaluations of ASME B31.1 Allowance for Variation From Normal Operation

The ASME B31.1-2012 Power Piping Code (Code) paras. 102.2.4, 102.3.3, and 104.8.2 provide an allowance regarding operating above the design temperature and internal pressure for short time periods. This study is a quantitative evaluation of the permitted increased life consumption associated with the 2012 Code operating allowances for piping materials operating in the creep range. Three base metal materials are considered in this paper—ASTM A335 Grades P11, P22, and P91. Results of this evaluation could be used to improve the ASME B31.1 Code, including a technical basis for a recommended revision. The para. 102.2.4 allowables were evaluated: (A) 15% stress increase for 10% of the operating hours and (B) 20% stress increase for 1% of the operating hours. It was determined that these allowances increased the base metal creep rupture life consumption of Grade P11 material up to 8%, Grade P22 material up to 14%, and Grade P91 material up to 25%. Allowance A results in permitting significantly more creep life consumption than Allowance B. An evaluation was performed to realign the increased creep life consumption of Allowance B to be approximately equivalent to the increased creep life consumption of Allowance A. If Allowance B event duration is increased from 80 hrs per year to 400 hrs per year (from 1% to 5% of the operating hours per year), Allowance B increased the creep life consumption which is slightly less than Allowance A life consumption for Grades P11, P22, and P91 materials. Main steam (MS) and hot reheat (HRH) piping system typical operating temperatures and stresses were evaluated for these variation allowances. This study revealed that Grade P22 base metal creep damage is slightly more sensitive to stress than Grade P11 material creep rupture damage, and Grade P91 base metal creep damage is substantially more sensitive to stress than Grade P22 material creep rupture damage.

Ultrasonic Inspection for Metal Creep Based on Flight-Time-Attenuation

Attenuation coefficient is frequently-used for ultrasonic creep inspection in metals. However, in practical inspecting, e.g. thick wall weldment in high temperature pipings, it is usually difficult to measure the accurate propagating distance of ultrasound, which results in an inaccuracy of attenuation coefficient calculation. Therefore, a modified method for attenuation coefficient calculating was proposed here, in which the flight-time of ultrasound was substituted for propagating distance. The new calculated attenuation coefficient was named as flight-time-attenuation parameter, and had a unit as dB/us. A uniaxial tension creep experiment with pure lead was conducted to verify the detecting ability of this new parameter. Two piezoelectric wafers were pasted on the pure lead specimen, and ultrasonic testing was carried out simultaneously without interrupting the tension procedure. Attenuation coefficient and flight-time-attenuation parameter were calculated with inspecting signal acquired from creep specimen, and compared with each other. The results showed that two ultrasonic parameters had the same variation tendency along with creep procedure, and flight-time-attenuation parameter had a better sensitivity. Accordingly, the flight-time-attenuation parameter could be used for ultrasonic creep inspection in metals, and worked as a replacement of attenuation coefficient calculated with propagating distance. Another creep inspecting experiment with P91 steel weldments was conducted to verify the practicability of the new parameter, in which the new flight-time-attenuation parameter showed a good distinguishing ability for different creep status.

Creep Life Evaluations of ASME B31.1 Allowance for Variation From Normal Operation

The ASME B31.1-2012 Power Piping Code (Code) paras. 102.2.4, 102.3.3, and 104.8.2 provide an allowance regarding operating above the design temperature and internal pressure for short time periods. This study is an evaluation of the permitted increased life consumption associated with the above Code operating allowances for piping materials operating in the creep range. Three base metal materials are considered in this study, ASTM A335 Grades P11, P22, and P91. Two case studies were evaluated, A) 15% stress increase for 10% of the operating hours, and B) 20% stress increase for 1% of the operating hours. It was determined that these allowances increased the base metal creep rupture life consumption of Grade P11 material up to 8%, Grade P22 material up to 14%, and Grade P91 material up to 25%. Allowance A results in permitting significantly more creep damage life consumption than Allowance B. Main steam and hot reheat piping system typical operating temperatures and stresses were evaluated for these variation allowances. This study revealed that Grade P22 base metal creep damage is slightly more sensitive to stress than Grade P11 material creep rupture damage, and Grade P91 base metal creep damage is substantially more sensitive to stress than Grade P22 material creep rupture damage.

Microstructure and Creep Property of Long-Term Serviced Mod.9Cr-1Mo Steels After Repair Welding

The microstructure and creep properties of the Mod.9Cr-1Mo steel of repair welds after long-term service for about 80,000 hours at around 600°C were investigated. Two types of heat affected zone (HAZ) were formed after repair welding performed at the interface between base metal and original weld metal. The first type was a HAZ formed in an ex-service original weld metal and another type was formed in an ex-service base metal. Creep rupture life of HAZ formed in an ex-service original weld metal was extremely short and it was about one fourth of that of HAZ formed in an ex-service base metal. From microstructural observation, many fine ferrite grains surrounded by carbides were present on grain boundaries and, therefore, duplex structure of ferrite and martensite was observed in the fine grain HAZ region formed in an ex-service original weld metal. These fine ferrite grains were considered to be formed by diffusion transformation of gamma phase to alpha phase instead of martensitic transformation during cooling of thermal history due to repair welding. It has been concluded that the creep strength of an ex-service weld metal is remarkably reduced by the formation of many fine ferrite grains due to repair welding.

Finite Element Analysis on Seal Quality of the Tank Engine’s Common Rail System

As the oil pressure of tank engine’s common-rail system is as high as 180MPa, it could bring a grate challenge to the oil con-rail’s seal quality. Especially in the condition of high oil pressure system, pulse load, metal creep deformation as well as machining error could low the reliability of con-rail’s. In tradition, shrinking the part’s tolerance is the main measure to ensure seal reliability .The measure eventually cause low efficiency, unnecessary waste, and high product cost in mechanical process. In this analysis, ABAQUS is exploited to discuss the influence that caused by various machining error on seal quality .Finally , the least taper angle error (critical point) lead to leakage is found. Besides the efficiency would be improved, the process cost also could be cut down a lot by using this data to constitute optimal technical files.