Chang-Yu Zhou

Creep Stress Analyses Affected by Defect Geometries on P91 Pipe With Local Wall Thinning Under High Temperature

P91 heat-resistant steel pipes are widely used at high temperature in power plants and nuclear power plants. The service condition and manufacturing process may produce defects of local wall thinning, which may result in stress redistribution of the pipes during the service process at elevated temperature. For the purpose of understanding the creep stress and strain accumulation affected by local wall thinning geometries under creep condition, three groups of models were calculated, using three-dimensional models based on finite element analyses (FEA) codes ABAQUS. In this study, monotonic internal pressure was conducted on P91 full-scale steel pipes at 625°C, with local wall thinning located at the inner surface. Then, the creep strain and stress of pipes after 100,000h could be obtained corresponding to different models. Based on the analysis, the figures of creep stress and strain varying with defect geometries were plotted. Then, the stress and strain of pipes with local wall thinning affected by defect geometries, including different defect depths, different defect axial lengths and different defect hoop angles, were discussed. The results indicate that creep stress and creep strain increase with defect geometries. The variation laws have been summarized. The research results can provide the possibility on safety assessment and structure integrity analysis of the pipe with local wall thinning at high temperature effectively.Copyright

Interaction and Assessment of Multiple Local Wall Thinning Defects

The safety assessment of pipes with multiple local wall thinning defects (LWTs) is systematically investigated by finite element analyses (FEA) with special attention to the interaction of multiple LWTs. It is interesting that the arrangement of LWTs (axial arrangement and circumferential arrangement), the load condition (pure pressure, pure bending and complicated load) and the relative depth play important roles in the interaction of multiple LWTs. The effective stress area and critical stress can be used to explain their influences. Moreover, existing assessment methods of multiple LWTs in some defect assessment standards such as API 579/ASME FFS, ASME B31G, BS 7910 and GB 19624 are reviewed. It’s noticed that the influences of arrangement, load condition and relative depth are ignored in existing standard methods, but they can influence the assessment results significantly. In order to consider these issues, an improved assessment of multiple LWTs based on API 579/ASME FFS is proposed. This improved assessment method has considered the influences of arrangement, load condition and relative depth, and can give better results.Copyright

The Limit Load and Safety Assessment of Pressure Pipe With an External Pit Defect at High Temperature

Local pit is the common volume defect in high temperature pressure pipe which is widely used in the fields of electric plant, nuclear power station, petrochemical plant and so on. In this paper finite element analysis code ABAQUS was used to simulate limit load of high temperature pressure pipe with an external pit defect which is in service for 105 hours. Four-point bending loading model was applied to calculate the limit load of the pipe. There are three dimensionless factors: relative depth, relative gradient and relative length which characterized the shape of an external pit defect. Orthogonal test of three factors at four different levels was carried out to analyze the sequence of the influence of these three parameters. In present paper when the maximum principal strain reaches 2%, the corresponding load is selected as the limit load. According to this strain criterion and isochronous stress strain data of P91 steel, limit load of high temperature pressure pipe with an external pit was determined by using ABAQUS. Firstly, isochronous stress strain data was generated and was inputted into ABAQUS as equivalent elastic-plastic constitutive relation. Then, sustained load versus cumulative strain curves at high temperature during service was obtained after the simulation. At last, limit loads of high temperature pipe during service time was determined based on 2% total strain criterion. In order to obtain the safety assessment curve of high temperature pipe, five types of limit loads for pressure pipe with an external pit were needed: ultimate limit bending moment, limit internal pressure, limit bending moments at the pressure of 0.25PL ,0.5PL and 0.75PL individually. 16 sets of data formed 16 groups of curves which expressed the relationship between the ratio of limit pressure and the ratio of limit bending moment for defective pipe and non-defective pipe. Based on the calculation results of limit load for pipe with 16 kinds of defects, a set of limit load formulae were established through multiple nonlinear regression of relative depth, relative gradient and relative length. So the equations of limit load and safety assessment for pressure pipe with an external pit under combined loading of pressure and bending moment were obtained. The results could provide a reference for safety assessment of high temperature creep pressure pipe with local pit defect.Copyright

Non-Probabilistic Failure Assessment Methodology About Titanium Pipes With Cracks

In this paper, the non-probabilistic failure assessment method for a pipe with crack was proposed, and interval model was applied in this assessment method. Compared with the deterministic approach, the assessment parameters were regarded as interval parameters in the non-probabilistic method, which could reflect the uncertainty of data and avoid the unsafety of results caused by the scatter of these parameters. On the other hand, compared with traditional probabilistic method, the non-probabilistic method only needs the range or bound of the parameters rather than detailed statistics information of the parameters which may be uneasy to be obtained from practical engineering. So the new assessment method reduces the request for detailed statistics properties of parameters. In this paper, load, material mechanical properties and the size of crack were treated as interval parameters, and the non-probabilistic failure assessment method was established based on the level 2 normal assessment of the BS 7910:2005. In this case, the assessment point (Lr , Kr ) is not a deterministic value, Lr and Kr would vary in a certain interval. As a consequence, the assessment point was changed into a rectangle zone, and the non-probabilistic failure measure for pipe with crack could be obtained by the variation of the rectangle. The non-probabilistic failure measure can be defined as the ratio of the area of failure region to the total area of the rectangle. At last, the non-probabilistic failure assessment method was used to evaluate a titanium pipe, and the analysis results were compared with the results of deterministic method, probabilistic method according to Monte-Carlo and partial safety factors method, respectively. The analysis results sufficiently demonstrated that the new non-probabilistic failure assessment method proposed in this paper was feasible and available.Copyright

Study on Limit Load of Double Pipe Bend Combined Fittings

Pipe bends and double pipe bends combined fittings including Back-to-back pipe combination (BB) and Face-to-face pipe combination (FF) play an important role in pressure pipeline system. Their carrying capacity will affect the integrity and safe operation of whole piping system directly. In this paper plastic limit loads for double pipe bends combined fittings (BB and FF) are analyzed by finite element method with elastic-perfect-plastic constitutive relation and the corresponding limit load equations are proposed which extends the range of pipe geometry λ Results show that the limit load equations available in references fail to be used for double pipe bends combined fittings and are unsafe, especially for FF combination. The limit loads increase with pipe thickness and bend radius in the meantime. The value of r/t is the main factor affecting limit load solutions. Detailed analysis also demonstrates that with applied load increasing, initial yielding appears in the FF combination firstly and finally overall yielding will happen. While the stress situation of BB combination is close to that of a single pipe bend. Eventually these proposed finite element based results are validated with experiment data and shows that finite element based solutions are believable.Copyright

Comparison of the Limit Loads Between Defected Pressure Pipes With an Internal Pit and an External Pit at High Temperature

High temperature pressure pipes are widely used in power stations, nuclear power plants, and petroleum refinery, which always bear combined effects of high temperature, high pressure, and corrosive media, so the local pits are the most common volume defects in pressure pipe. Due to various reasons, the defects usually appear on the internal or external wall of pipe. In this paper, the dimensions of a defect were characterized as three dimensionless factors: relative depth, relative gradient and relative length. The main objects of study were the pipe with an internal pit and pipe with an external pit. Orthogonal array testing of three factors at four different levels was applied to analyze the sequence of the influence of three parameters. In present study, when the maximum principal strain nearby the location of the defects reaches 2%, the corresponding load is defined as the limit load, which is classified as two kinds of load type: limit pressure and limit bending moment. According to this strain criterion and isochronous stress strain data of P91 steel, the limit load of high temperature pipe with a local pit was determined by using ABAQUS. And in the same load condition of the pipe with the same dimensionless factors, the limit load of the internal defected pipe was compared with that of the external defected pipe. The results of this study can provide a reference for safety assessment and structural integrity analysis of high temperature creep pressure pipe with pit defects.Copyright