Catastrophic-type failures

Uniform microstractuie is cracial to the superior performance of advanced ceramics. In a cerantic material, atoms are held in place by strong chentical bonds that ate impervious to attack by corrosive materials or heat. At the same time, these bonds are not capable of much "give." When a ceramic material is subjected to mechanical stresses, these stresses concentrate at minute imperfections in the microstmcture, initiating a crack. The stresses at the top of the crack exceed the threshold for breaking the adjacent atomic bonds, and the crack propagates throughout the material causing a catastrophic brittle failure of the ceramic body. The rehability of a ceramic component is directly related to the number and type of imperfections in its microstmcture. 

Fiber failure is usually of the catastrophic type where the failure is sudden. This is typical in polymeric materials where the material is broken at the weak-link and where the strength-related property is related to a combination of individual links (chains). 

It is certainly my experience that the most common and catastrophic accidents in process units are related to corrosion-type failures. I cannot actually bring to mind any equipment vessels which were overpressured and failed because a relief valve did not open. 

A common problem in boilers is the occurrence of calcium oxide build-up on the heating elements. This is not a corrosion problem in itself, because it is caused by a chemical reaction in the water at high temperatures. However, a scale deposit present on a metal surface may cause corrosion under the deposit. This type of underdeposit corrosion can be aggravated when corrosive species such as sulfides and/or chlorides are present in the water. While scale deposits reduce the thermal conductivity of the steel, and thereby increase energy costs, corrosion of the heating element can lead to a catastrophic tubing failure, which requires costly repairs. 

In complete service failure, the entire system is typically unavailable, a situation which is often called an outage. This type of unavailability may of course be either planned or unplanned and may be expected or unexpected by the individual user. Service failures are often due to catastrophic hardware failure where insufficient redundancy has been employed. The clinical risk is determined by a number of factors ... 

Brittle Fracture. In this type of failure the part fractures extensively without yielding. A catastrophic mechanical failure such as the one in the case of general-purpose polystyrene is observed. 

Figure 3.14 illustrates the appearance of seals manufactured with honeycombs of Aluchrom YHf, PM2Hf and Haynes 214 after exposure in test 2 (with target seal surface temperatures of 1100°C). The sample with the PM2Hf honeycomb failed after 90 cycles, that with the Aluchrom YHf honeycomb after 190 cycles, and the Haynes 214 was removed after 300 cycles. Both the PM2Hf and Aluchrom honeycomb samples show the same type of catastrophic oxidation failure just above the honeycomb/braze joint line. In contrast, the sample with the Haynes 214 honeycomb shows only a little surface oxidation and some internal damage on the braze. 

Risk-Based Inspection. Inspection programs developed using risk analysis methods are becoming increasingly popular (15,16) (see Hazard ANALYSIS AND RISK ASSESSMENT). In this approach, the frequency and type of in-service inspection (IS I) is determined by the probabiUstic risk assessment (PRA) of the inspection results. Here, the results might be a false acceptance of a part that will fail as well as the false rejection of a part that will not fail. Whether a plant or a consumer product, false acceptance of a defective part could lead to catastrophic failure and considerable cost. Also, the false rejection of parts may lead to unjustified, and sometimes exorbitant, costs of operation (2). Risk is defined as follows ... 

The accuracy of absolute risk results depends on (1) whether all the significant contributors to risk have been analyzed, (2) the realism of the mathematical models used to predict failure characteristics and accident phenomena, and (3) the statistical uncertainty associated with the various input data. The achievable accuracy of absolute risk results is very dependent on the type of hazard being analyzed. In studies where the dominant risk contributors can be calibrated with ample historical data (e.g., the risk of an engine failure causing an airplane crash), the uncertainty can be reduced to a few percent. However, many authors of published studies and other expert practitioners have recognized that uncertainties can be greater than 1 to 2 orders of magnitude in studies whose major contributors are rare, catastrophic events. 

The previous problems are some of the more common types encountered on a gas turbine train. Regular and preventive maintenance is the key to a successful operation. Problems will arise, but by proper monitoring of the aerothermal and mechanical problems, preventive maintenance can often avert major or catastrophic failures. 

For compressors in general and for some types in particular, the cleanliness of the gas stream is the key factor in a reliable operation. Moisture or liquids in various forms may be the cause of an early failure or in some-cases a catastrophic failure. Corrosive gases require material considerations and yet even this may not entirely solve the loss of material issue that can certainly cause early shutdowns or failures and high maintenance cost. Fouling due to contaminants or reactions taking place internal to the ( i-pressor can cause capacity loss and the need for frequent shutdowns. 

Both time-related failure rates and demand-related failure rates can apply to and be reported for many pieces of equipment. Both types of rates are included in some of the data tables in Chapter 5. If a piece of equipment is in continuous service, such as a transformer, the failure rate is dominated by time-related stresses compared to demand-related stresses. Other failure rates may be dominated by demands. Take a piece of wire and repeatedly bend it. With each bend its probability of catastrophic failure increases. In a relatively short time, if the bending is continued, the wire will fail. On the other hand, the same wire could be installed in a manner that would prevent mechanical bending demands. In this case, the occurrence of catastrophic wire breakage would be remote. In the first instance, the failure rate is dominated by demand stresses and in the second by time-related stresses, such as corrosion. 

The IEEE Std 500 document is based on a hierarchical structure of component types set down in the manual s table of contents. The preface for each subsection (defined by a component type) provides a tree diagram that clearly shows the way the component classes have been subdivided to determine "data cells". The failure modes for each component class are also hierarchically organized according to failure severity catastrophic, degraded, or incipient. Rates per hour and demand rates (per cycle) are both included, as well as upper and lower bounds. 

For catastrophic demand-related pump failures, the variability is explained by the following factors listed in their order of importance system application, pump driver, operating mode, reactor type, pump type, and unidentified plant-specific influences. Quantitative failure rate adjustments are provided for the effects of these factors. In the case of catastrophic time-dependent pump failures, the failure rate variability is explained by three factors reactor type, pump driver, and unidentified plant-specific Influences. Point and confidence interval failure rate estimates are provided for each selected pump by considering the influential factors. Both types of estimates represent an improvement over the estimates computed exclusively from the data on each pump. The coded IPRDS data used in the analysis is provided in an appendix. A similar treatment applies to the valve data. 

For mechanical explosions a reaction does not occur and the energy is obtained from the energy content of the contained substance. If this energy is released rapidly, an explosion may result. Examples of this type of explosion are the sudden failure of a tire full of compressed air and the sudden catastrophic rupture of a compressed gas tank. 

Physical property data and sometimes reaction rate characteristics are required for making relief sizing calculations. Data estimated using engineering assumptions are almost always acceptable when designing unit operations because the only result is poorer yields or poorer quality. In the relief design, however, these types of assumptions are not acceptable because an error may result in catastrophic and hazardous failures. 

BLEVE types of incidents arise from the reduction in yield stress of a vessel or pipe wall to the point that it cannot contain the imposed stresses by the design and construction of the container and are also influenced by the relief valve set point. This results in a sudden catastrophic failure of the containment causing the violent discharge of the contents and producing a large intense fireball. 

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