Pertti Auerkari

Accuracy requirements for life assessment

Abstract Several methods can be regarded as routine tools for creep life assessment of power plant components. The selection and extent of applying a technique, as well as interpretation of the results, varies, but the main challenges in reliable predictions remain quite similar. In general, apart from small tubes inside the boiler, the final failure is rarely expected in components and locations where destructive sampling would be easy. Representative sampling, e.g. for creep testing of welded joints of heavy components, is particularly problematic, although miniature specimens can be helpful. However, the methods based on creep testing appear to have potential for long-term life prediction. Nondestructive metallographic inspection with replicas combined with other NDT is suitable for welds, but the present judgement on safe life is limited to short- or medium-term predictions. Improved monitoring of the service history in terms of mechanical, thermal and environmental loadings would deserve more attention. To fully profit from this requires more sophisticated instrumentation than the present average, and a systematic approach in dealing with the accumulating service records. This is likely to be justified in terms of the life cycle costs of most old plants.

Effect of short-term data on predicted creep rupture life – pivoting effect and optimized censoring

Abstract Fitting data to classical creep rupture models can result in unrealistically high extrapolated long-term strength. As a consequence, the standard strength values for new steel grades have frequently needed downward correction after obtaining more long-term test data. The reasons for non-conservative extrapolation include the influence of short-term data, which are easiest to produce but tend to pivot upwards the extrapolated values of creep rupture strength. Improvement in extrapolation could be expected by reducing this effect through model rigidity correction and censoring of very short-term data, but it may not be immediately clear how to justify the correction of particular models or censoring. Analogously to the instability parameter in the minimum commitment model for creep rupture, a rigidity parameter correction (RPC) is introduced to assess the pivoting effect of creep rupture models for the purpose of reducing potential to non-conservativeness in extrapolation. The RPC approach can be used with any creep rupture model for comparing the model rigidity and the potential benefit from censoring short-term data. The correction itself will never introduce non-conservatism, regardless of the model. The RPC approach is demonstrated by analyzing an ECCC data set for cross-welded 9%Cr steel (E911).

Emerging risk of autoignition and fire in underground coal storage

The risk and prevention of autoignition in underground coal storage facilities are reviewed in the light of the recent incidents of smouldering fires. Also, the opportunities are considered on the efficiency of the alternatives to prevent and extinguish closed-space fires. The complexities in avoiding and extinguishing underground fires are highlighted in the case example, describing the observations and outcome of a smouldering coal fire in the storage. The principles of self-heating and most critical factors in spontaneous combustion such as the condition and quality of coal are fairly well known, but usually only provide partial help in fire prevention. The documented cases and the case example suggest that nitrogen injection can be useful for extinguishing controllable fires. Three-phase foams and oxygen-displacing exhaust gases appear preferable against uncontrolled fires, particularly if access to the fire area is limited or impossible. Otherwise, efficient fire extinction during power plant operation can be challenging, as any air ingress tends to feed the fire and results in losses of the extinguishing agent and the heating value of coal. Methods and indicators for detecting and predicting the ignition are discussed, and improvements are suggested to enhance the storage and plant availability.

Creep Damage, Ductility and Expected Life for Materials With Defects

Defects can pre-exist and grow by creep in structures subjected to loading at high temperatures. As structural integrity is not necessarily conveniently predicted and managed by applying design and life assessment techniques intended for nominally defect-free material, it is important that methods are available for quantified and safe assessment of defects. In addition to the assessment methods, also materials behaviour will affect the likely outcome. In particular, ductility of the materials is important, and unfortunately ductility tends to decrease when shifting from short-term testing to long term creep conditions. In this paper, two examples are shown of materials with such ductility effects when combined with defects. The first example involves 316H stainless steel subjected to creep loading with an extensive crack-like defect, resulting in a transformation from microscopically ductile to brittle intergranular cracking within a relatively modest time span. The second example will demonstrate a corresponding shift in OFP copper that shows a radical ductility and life reduction in creep when including so small weld defects that they would be undetectable in conventional NDT.Copyright

Life extension of hot steam piping after 200 000 h of service

Abstract The units 2 and 3 of Naantali power plant have been successfully operated for >200 000 h, and it is of interest to ensure the eligibility of the units for continued production. For this purpose and to assist in the related life assessments, the condition of the high temperature piping has been subjected to long term inspection and monitoring programs. Inspections have been concentrated on sites of earliest indications of creep damage or highest rates of damage accumulation as observed from monitoring and repeated measurements. Particular locations of interest are found at the welded branches of the high pressure steam mixing headers. With these example components, two traditional solutions of directly welded and wrought branch connections are compared in terms of observed creep damage and expected life.

Experience and the unexpected: risk and mitigation issues for operating underground storage silos for coal-fired power plant

Observed autoignition events and extinguishing the resulting smouldering fires in an underground storage system of a coal-fired power plant have provided insight into the array of contributing variables, and some experience on quantifying the risk with alternative scenarios of event initiation, progress and potential mitigation. Although the first attempts to quantify the risk suggest high sensitivity to the sequence of action taken after fire alarm, and no similar storage sites really exist, some recommended preventive, corrective and other mitigating activities can be at least partly defined and improved by using the cumulative experience and parallel efforts in other closed or underground storage sites. However, there are also so-called black (or at least grey) swans: unexpected events for which the facility may be poorly prepared for. In the case of the underground storage silos, such an event was experienced when incoming cold coal during a harsh winter season froze the sewer system that normally protects the stored coal from seepage water. With blocked normal bypass, the seepage water found its way to the coal silos and created large clumps of icy coal that blocked the coal conveyors. Although freezing weather is not unusual at high-latitude power plants, the common methods to combat freezing of coal are mainly useful for open storage sites and above-ground transport. Options for mitigation are discussed, as well as the event chain leading to an event that had never previously occurred. The case is discussed from the point of view of options to prepare for rare or unforeseen events.

Observations on the Smith-Glasser index for self-heating of bituminous coal

Self-heating of coal depends partly on intrinsic, coal-related factors. This article aims to discuss the Smith–Glasser index as a potential indicator of the self-heating propensity of a large set of bituminous coals with similar heating value but different origins. For this purpose, recorded properties and experience were reviewed for 7.5 million tonnes of coal delivered to the same power plant operator. The results suggest that the propensity to self-heat can be conveniently indicated by Smith–Glasser index that only involves routinely measured moisture and volatile contents of coal. The results appear consistent with the observed incidences of self-heating in storage.

Predicted life of P91 steel for cyclic high temperature service

Abstract The P91 steel is widely used in high temperature components of power plants, and it is a candidate material for Gen-IV reactors. The P91 steel has relatively attractive mechanical and physical properties combined with resistance to stress corrosion cracking in water–steam environments. This study aimed to explore the combined cyclic, creep and relaxation behaviour of P91 material. Uniaxial specimens were subjected to cyclic loadings with periodic forward creep or relaxation at peak stress. The results indicate that prior creep or intermediate relaxation periods up to 72 h will influence the subsequent cyclic softening of P91, but do not significantly reduce the cyclic life. In contrast, prior cycling has a detrimental effect on the subsequent creep life. A simplified creep-fatigue model is shown to predict life better than usual code-based approaches. Improved verification of all models would benefit from the availability of more extensive long-term data on P91 steel.

Supercritical Oxidation of Boiler Tube Materials

The advantage of using supercritical water systems for power generation is based on the increased thermodynamic efficiency when operating at higher temperature and pressure. High efficiency in power generation is not only desirable because of economical reasons but also for enhanced environmental performance. Steam oxidation has become an important issue for steam power plants as operating temperatures increase from current to 650#xb0;C and even higher. To achieve these higher steam values new materials are needed. This paper presents results of the oxidation performance of potential new alloys (FeCrAlY, NiCrAl and Sanicro 25) in a supercritical water autoclave environment at 650#xb0;C/250 bar.

Early warning indicators for challenges in underground coal storage

Early warning or leading indicators are discussed for unexpected incidences in case of large-scale underground coal storage at a power plant. The experience is compared with above-ground stockpiles for which established procedures are available but where access for prevention and mitigation are much easier. It is suggested that while the explicit organization, procedures, and the general safety systems aim to provide the targeted levels of performance for the storage, representing new technology without much precedence elsewhere in the world, the extensive experience and tacit knowledge from above-ground open and closed storage systems can help to prepare for and to prevent unwanted incidents in the underground storage. This kind of experience has been also found useful for developing the leading or early warning indicators for underground storage. Examples are given on observed autoignition and freezing of coal in the storage silos, and on occupational hazards. Selection of the leading indicators needs to consider the specific features of the unique underground facility.