Nozzle Weld Research Articles

Application of phased array in inspection of nozzle welds of high thickness reactors manufactured by GE oil & gas Massa plant

Application of phased array in inspection of nozzle welds of high thickness reactors manufactured by GE oil & gas Massa plant

Comparative Study for Stresses in Nozzle and Flange Welds Generated During Conventional Pressure Testing and Local Pressure Testing Using Bolted Devices

Weld repairs and alterations of pressure vessels and piping built to ASME codes may require pressure testing to prove the integrity of the weld and/or design. Conventional hydrostatic pressure testing requires filling an entire vessel or piping system with water and pressurizing it to the test pressure. In recent years, several designs were developed to employ bolted devices to perform local pressure testing of flange-to-nozzle, flange-to-pipe, and nozzle-to-shell attachment welds. Due to the cost and equipment downtime associated with performing a full conventional pressure test and the desire to reduce repair costs, several petrochemical companies adopted the use of such devices. The purpose of this paper is to compare the stress values and stress distribution associated with conventional and local pressure testing techniques. The advantages and disadvantages of both approaches are discussed and the conclusions are supported by a practical example.

Bottom nozzle failure mechanism of water reactor pressure vessel under severe accident conditions

Most of past studies devoted to the creep rupture of a nuclear reactor pressure vessel (RPV) lower head under severe accident conditions, have focused on global deformation and rupture modes. Limited efforts were made on local failure modes associated with penetration nozzles as a part of TMI-2 vessel investigation project (TMI-2 VIP) in 1990s. However, it was based on an excessively simplified shear deformation model. In the present study, the mode of nozzle failure has been investigated using data and nozzle materials from Sandia National Laboratory's lower head failure experiment (SNL-LHF). Crack-like separations were revealed at the nozzle weld metal to RPV interfaces indicating the importance of normal stress component rather than the shear stress in the creep rupture. Creep rupture tests were conducted for nozzle and weld metal materials, respectively, at various temperature and stress levels. Stress distribution in the nozzle region is calculated using elastic–viscoplastic finite element analysis (FEA) using the measured properties. Calculation results are compared with earlier results based on the pure shear model of TMI-2 VIP. It is concluded from both LHF-4 nozzle examination and FEA that normal stress at the nozzle/lower head interface is the dominant driving force for the local failure. From the FEA for the nozzle weld attached in RPV, it is shown that nozzle welds failure occur by displacement controlled fracture of nozzle hole not by load controlled fracture of internal pressure. Considering these characteristics of nozzle weld failure, new concept of nozzle failure time prediction is proposed.

Bottom-Mounted Nozzle Failure Modes of a Nuclear Reactor Pressure Vessel under Severe Accident Conditions

Local failure modes associated with bottom-mounted penetration nozzles are examined as a part of research on sever accident management. Conventional creep rupture studies on reactor vessel lower head during a meltdown accident were based on an excessively simplified shear deformation model. In the present study, the mode of nozzle failures is investigated using data and nozzle materials from Sandia National Laboratory’s Lower Head Failure Experiment (SNL-LHF). Crack-like separations were revealed at the nozzle weld metal to RPV interfaces indicating the importance of normal stress component rather than the shear stress in the creep rupture. Creep rupture tests were conducted for nozzle and weld metal materials, respectively, at various temperature and stress levels. Stress distribution in the nozzle region is calculated using elastic-viscoplastic finite element analysis (FEA) using the measured properties. Calculation results are compared with earlier results based on the pure shear model of TMI-2 VIP. It has been concluded from both LHF-4 nozzle examination and FEA that normal stress at the nozzle/lower head interface is the dominant driving force for the local failure with its likelihood significantly greater than previously assumed.

Improved plant availability by advanced condition based inspections

An industrial plant has to operate safely, reliably and efficiently at the lowest possible cost. Plant availability plays an important role regarding economic life optimisation. Industrial installations that are under pressure and are operating at high temperatures have a limited life due to creep and fatigue. It is, therefore, of critical importance to know the location of any possible weak spots in the installation. To avoid safety risks, unplanned plant shutdown and, as a consequence, high costs for unavailability, cycling and repair, periodic inspections and strain measurements are recommended. A Speckle Image Correlation Analysis (SPICA) system enables on-stream measurement of deformation due to creep in critical areas like the heat-affected zone in welds. Plant management and operators use the strain measurements to take action when necessary and, consequently, prevent failures. In those plants that have been provided with SPICA-technology for some years plant availability has improved significantly as a result. Another important development for yielding improved availability concerns steam drums. During some 20 years, KEMA has been performing automated ultrasonic steam drum inspections from outside. The Dutch authorities accepted this methodology in this period as an alternative (rather than an addition) after several pilot projects. An advantage of this inspection methodology is the possibility to record of the inspection results and possibility of thus trending these data. The resulting reduction of through time appeared a major benefit for plant owners. Since the authorities adopted the RBI approach during the last 10 years, another advantage of the inspection methodology became apparent: complete scanning and recording of the inspection data of circumferential and longitudinal (butt and fillet) welds, inspection of nozzle welds and inner radius as well as corrosion mapping has been covering all higher risk areas in these drums. This enhanced inspection programme together with improved monitoring of operation parameters during operation has resulted in dramatic inspection interval extension. This attractive method for inspection interval extension has been applied at several Dutch power plants.

Metallurgical analysis of cracking in weldment of propellant tank

Medium strength Al–Zn–Mg aluminium alloy is extensively used in propellant tanks of satellite launch vehicles. The cylindrical tanks are fabricated by TIG welding of sheets and rings into larger diameter pressure vessels. End domes are fabricated by welding petals, and nozzle openings are provided by shrink fit nozzles, welded to the petals. A crack, partly on the overlap region was noticed on one such nozzle weld. A detailed analysis revealed that the crack initiated from the root of pits, propagated under the high longitudinal restraint at the weld stop area, through solute enriched dendrites at the weld fusion line, resulting in an intergranular mode of failure. This is typical of stress corrosion cracking (SCC).

Examination of Ginna pressure vessel nozzle welds using focussed probes : Nuclear plant journal, vol. 8, no. 6, pp. 64, 66–67 (Nov.–Dec. 1990)

Examination of Ginna pressure vessel nozzle welds using focussed probes : Nuclear plant journal, vol. 8, no. 6, pp. 64, 66–67 (Nov.–Dec. 1990)

Ultrasonic examination of Ginna reactor pressure vessel nozzle welds using focussed probes : Proceedings of the 10th International Conference on NDE in the Nuclear and Pressure Vessel Industries, Glasgow (Scotland), 11–14 Jun. 1990. pp. 133–137. Edited by M.J. Whittle, J.E. Doherty and K. Iida. ASM International, (1990)

Ultrasonic examination of Ginna reactor pressure vessel nozzle welds using focussed probes : Proceedings of the 10th International Conference on NDE in the Nuclear and Pressure Vessel Industries, Glasgow (Scotland), 11–14 Jun. 1990. pp. 133–137. Edited by M.J. Whittle, J.E. Doherty and K. Iida. ASM International, (1990)

Validation of Inelastic Analysis by Full-Scale Component Testing

This paper compares theoretical and experimental results for full-scale, prototypical components tested at elevated-temperatures to provide validation for inelastic analysis methods, material models, and design limits. Results are discussed for piping elbow plastic and creep buckling, creep ratcheting, and creep relaxation; nozzle creep ratcheting and weld cracking; and thermal striping fatigue. Comparisons between theory and test confirm the adequacy of components to meet design requirements, but identify specific areas where life prediction methods could be made more precise.

厚肉圧力容器ノズル部の溶接残留応力に関する研究

In order to make sound welds free from defects like weld cracking, characteristics of residual stresses around nozzle welds were experimentally investigated in an as welded condition for a thick-wall reactor vessel and an accompanying heat exchanger utilized in a petroleum refining process or a coal liquefaction process.The residual stresses were measured throughout the thickness of the weldments by applying the measuring methods that had been newly developed for axisymmetrical stress distributions by the authors.The main results obtained for set-in type nozzle, set-on type nozzle and simple butt welds of pipes are summarized as follows.(1) Stress parallel to the weld line-The stress indicates the same distribution in the thickness direction regardless of the joints. That is, the maximum value appears just behind the final layer of welds and with descending to the initial layer it decreases. The stress gradient, however, differs from one joint to another, depending on the self-restraint intensity of the joint. Moreover, the peak value of simple butt welds of pipes is about 100 MPa smaller than the others.(2) Stress perpendicular to the weld line-It indicates the similar distribution as the stress parallel to the weld line. The maximum value in the set-in type nozzle changes from 400 MPa to 600 MPa with the decrease in the diameter of welds from 600 mm to 300 mm. As for the set-on type nozzle, it reaches almost the same level due to the existence of channel as a stress raiser. The stress for the simple butt welds of pipes, however, indicates much smaller value of about 245 MPa and further the same order of tensile stress occurs in the inner surface as well as just behind the final layer of welds in the outer side.(3) The stress in the thikness direction shows the distribution where the maximum value appears in the center portion of the thickness and it is a low stress level of about 150 to 200 MPa, regardless of the joints.