Investigation of Radiant Superheater Crossover Pipe Weld Cracking at Big Cajun II Station

This paper describes the many features of a detailed investigation into the determination of a root cause for internal cracking found in the circumferential welds of radiant superheater crossover piping lines, in the Units No. 1 and 2 boilers at Big Cajun II Station in New Roads, Louisiana. The history of inside diameter, circumferential cracks dates back to 1992. The cracking had been recorded during several outages for both units. It was discovered by use of ultrasonic shear wave testing, and verified by ultrasonic time of flight diffraction methods. During each of the ensuing unit outages, the crack depths were recorded and mapped. Repairs were undertaken by machining out the complete girth weld followed by re-welding. During the interim years cracking did re-occur at many of the weld locations. In 2000, a detailed investigation into the cause of the cracking was initiated, which resulted in recommendations for resolving the ongoing problem. This detailed study included; nondestructive testing and metallurgy of removed metal samples, boiler performance testing and analysis and stress, fatigue and fracture mechanics evaluations. The detailed background, applications and results of the many and varied testing and analytical tasks are fully described herein. The main conclusion to the root cause of the cracking is identified as fatigue caused by the combined effects of thermal and pressure cycles. Recommendations are given which address the actions needed to limit or prevent re-occurrence of the cracking, including revised boiler operating procedures. In addition, a series of fatigue crack growth curves is presented, as a monitoring toot for evaluating existing cracks in the welds.

Case History of Solidification Cracks in 2-1/4Cr 1Mo Low Carbon Welds — Cholla Unit 2

An examination of the main steam girth welds at Cholla Unit 2 was performed during a scheduled outage in Spring 1999. The examination revealed two distinct types of cracks. Nine girth welds had extensive arrays of small discontinuous ultrasonic examination indications in the weld deposit near the weld roots. Two girth welds had cracks connected to the outside surface of the pipe. Justifications for removing and replacing 11 of the 35 examined main steam girth welds are presented in this paper. Nondestructive examinations revealed small discontinuous indications near the root of several welds and throughout the weld deposit. In the most severe cases, these weld metal indications extended all the way around the circumference of the pipe. A metallurgical evaluation of both shop and field welds determined that the indications were arrays of small solidification cracks, typically 1/16-inch high by 1/32-inch long. The solidification cracks were attributed to wide weave beads in combination with low carbon content consumables. There was also a concern that those weld deposits with very low carbon (less than 0.05 weight percent) may have significantly shorter creep lives. In addition to the fabrication-induced solidification cracks, two girth welds were identified with service-induced creep cracks. The first of these was detected during ultrasonic and fluorescent magnetic particle examinations of selected welds. The second was detected visually in an auxiliary steam piping weld connection that was identified as a high priority weld resulting from a life consumption evaluation of the piping system.

Fitness-for-Purpose Assessment of Encirclement Split-Tees

Hot tap tees of the full encirclement split tee design are currently used in the UK gas industry to provide connections onto existing high pressure gas transmission pipelines and above ground installations. The fitness-for-purpose of this type of tee is not covered by the UK design code for above ground gas installations, IGE/TD/9. A fitness-for-purpose methodology has been developed by Advantica Technologies to determine the integrity of the fitting and attachment welds. The fitness-for-purpose assessment addresses the following: • The compliance of the fitting to plastic collapse, shakedown and fatigue design criteria. • The integrity of the attachment welds onto the carrier pipe. • The use of Engineering Critical Assessments in conjunction with existing procedures to ensure overall integrity.

Limit Analysis for Anisotropic Solids Using Variational Principle and Repeated Elastic Finite Element Analyses

Many structural components, such as rolled sheets, directionally solidified superalloys and composites, are made of anisotropic materials. The knowledge of limit load is useful in the design and the sizing of these components and structures. This paper presents the extension of the modified mα-method to anisotropic materials. Mura’s variational principle is employed in conjunction with repeated elastic finite element analyses (FEA). The secant modulus of the discretized finite elements in the reference direction in successive elastic iterations is used to estimate the plastic flow parameter for the anisotropic components. The modified initial elastic properties are adopted to ensure the “elastic” stress fields satisfy the anisotropic yield surface. Using the notion of “leap-frogging” to limit state, improved lower-bound limit loads can be obtained. The formulation is applied to two anisotropic components, and the limit load estimates are compared with those using elastic compensation method and inelastic FEA.

Development of FFS Handbook for Refinery and Petrochemical Industries

Pressure equipment in refinery and petrochemical industries in Japan has been getting old, mostly more than 30 years in operation. Accordingly, there are many fitness for service evaluations required for pressure equipment. However, Japanese regulation still requires as the same quality level as the original fabrication for pressure equipment in service. In order to introduce the latest FFS methodologies to Japanese industries, a special task group was established at the High Pressure Institute of Japan and started study of API RP579 from its drafting stage as a member of TG579. And then, FFS handbook for refinery and petrochemical industries has been developed based on API RP579 with several modifications to meet Japanese pressure vessel regulations on April 2001. It is expected that FFS handbook will be introduced as an exemplified standard with Japanese regulations for practical maintenance.

Probabilistic Integrity Assessment of Axial Flaw in CANDU Pressure Tube Due to the Diameter and Thickness Variation

In the CANDU nuclear reactor, pressure tubes of cold-worked Zr-2.5Nb material are used in the reactor core to contain the fuel bundles and heavy water coolant. The pressure tubes are installed horizontally inside the reactors and only selected samples are periodically examined during In-Service Inspection (ISI) due to numerous numbers of tubes. Also, these tubes gradually pick up deuterium, as such are susceptible to a crack initiation and propagation process called delayed hydride cracking (DHC). If undetected, such a cracking mechanism could lead to unstable rupture of the pressure tube. Up to this time, integrity evaluations are performed using conventional deterministic approaches. So it is expected that the results obtained are too conservative to perform a rational evaluation of lifetime. In this respect, a probabilistic safety assessment method is more appropriate for the assessment of overall pressure tube safety. This paper describes probabilistic fracture mechanics analyses of the pressure tubes in consideration of the diameter and thickness variation. Initial hydrogen concentration, the depth and aspect ratio of an initial surface crack, the DHC velocity and fracture toughness are considered as probabilistic variables. In all the analyses, failure probabilities are calculated using the Monte Carlo (MC) simulation. It is clearly demonstrated from these analyses that failure probabilities are somewhat sensitive in size change of the pressure tube and the hydride precipitation temperature.

Flaw Growth Prediction and Fitness-for-Service Assessment of a PWR Nuclear Plant Steam Generator U-Tube

Primary water stress corrosion cracking (PWSCC) continues to be a dominant degradation mechanism affecting the service life of steam generators in several operating pressurized water reactor (PWR) nuclear plants. Recently, in one operating nuclear plant, a steam generator U-tube ruptured catastrophically while the unit was on-line. Although the plant operators were able to shutdown the reactor without significant release of radiation, the Nuclear Regulatory Commission (NRC) and the owner utility launched a full-scale investigation of the incident. The owner utility requested that a crack growth analysis and engineering evaluation of the tube rupture be performed, as well as assess the fitness-for-service of the generator for continued operation. This paper presents a summary of elastic-plastic finite element and fracture mechanics analyses performed for a steam generator U-tube, subjected to crack initiation at the inner diameter of the tube in the apex region. Residual stresses were computed from a finite element model of the tube simulating the mechanical bending process with the use of an anvil. Fracture mechanics and crack growth evaluations were performed to predict the time required for a pre-existing flaw at the inside diameter of the tube to propagate through-wall. Additionally, a fitness-for service assessment was performed in order to permit a degraded tube to remain in service, given an initial flaw size as determined by nondestructive examination.

3D Finite Element Modeling of the Welding Process Using Element Birth and Element Movement Techniques

The modeling and simulation of the welding process has been of main concern for different fields of applications. Most of the modeling of such a problem has been mainly in 2D forms that may also include many sorts of approximation and assumptions. This is due to limitations in the computational facilities as the analysis of 3D problems consumes a lot of time. With the evolution of new finite element tools and fast computer systems, the analysis of such problems is becoming in hand. In this research, a simulation of the welding process with and without metal deposition is developed. Change of phase and variation of properties with temperature are taken into account. A new technique for metal deposition using element movement is introduced. It helps in performing full 3D analysis in a shorter time than other previously developed techniques such as the element birth.

Assessment of Local Wall Thinned Pipeline Under Combined Bending and Pressure

Failure of a pipeline due to local wall thinning is getting more attention in the nuclear power plant industry. Although guidelines such as ANSI/ASME B31G are still useful for assessing the integrity of a wall thinned pipeline, there are some limitations in these guidelines. For instance, these guidelines consider only pressure loading and thus neglect bending loading. However, most pipelines in nuclear power plants are subjected to internal pressure and bending moment due to dead-weight loads and seismic loads. Therefore, an assessment procedure for locally wall thinned pipeline subjected to combined loading is needed. In this paper, three-dimensional finite element (FE) analyses were performed to simulate full-scale pipe tests conducted for various shapes of wall thinned area under internal pressure and bending moment. Maximum moments based on ultimate stress (σu) were obtained from FE results to predict the failure of the pipe. These results were compared with test results, which showed good agreement. Additional finite element analyses were performed to investigate the effect of key parameters, such as wall thinned depth, wall thinned angle and wall thinned length, on maximum moment. Also, the effect of internal pressure on maximum moment was investigated. Change of internal pressure did not show significant effect oll the maximum moment.

Steam Piping Fitness-for-Service Evaluation

Recently, in the boiler room of a high-rise commercial building in New York City, a flash tank ruptured. As a result of dynamic loads generated during the explosion, steam piping in the boiler room was subjected to large displacements and severe vibrations. The building owner, concerned about the safety and integrity of the steam piping, requested an assessment of the piping and a fitness-for-service evaluation for continued operation. This paper presents a summary of the investigation performed including inspection of the piping, determination of operating and residual stresses, metallurgical evaluation and fitness-for-service assessment. A tolerable or critical pipe flaw size was established using the failure assessment diagram (FAD) method and, to the extent possible, the guidelines of API 579. Fracture mechanics evaluation and crack growth calculations were performed to establish re-inspection frequency for critical welds and pipe sections. Finally, nondestructive examinations were performed to determine which pipe sections and welds, if any, should be repaired or replaced.