Causes basic

The failure rate changes over the lifetime of a population of devices. An example of a failure-rate vs product-life curve is shown in Figure 9 where only three basic causes of failure are present. The quaUty-, stress-, and wearout-related failure rates sum to produce the overall failure rate over product life. The initial decreasing failure rate is termed infant mortaUty and is due to the early failure of substandard products. Latent material defects, poor assembly methods, and poor quaUty control can contribute to an initial high failure rate. A short period of in-plant product testing, termed bum-in, is used by manufacturers to eliminate these early failures from the consumer market. 

ETA breaks down an accident iato its contributing equipment failures and human errors (70). The method therefore is a reverse-thinking technique, ie, the analyst begias with an accident or undesirable event that is to be avoided and identifies the immediate cause of that event. Each of the immediate causes is examined ia turn until the analyst has identified the basic causes of each event. The fault tree is a diagram that displays the logical iaterrelationships between these basic causes and the accident. 

The immediate causes of the TOP event are shown in the fault tree with thek relationship to the TOP event. If any one of the immediate causes results dkecdy in the TOP event, the causes are connected to the TOP event with an OR logic gate. If all the immediate causes are requked for TOP event occurrence, then the causes are connected to the TOP event with an AND logic gate. Each of the immediate causes is then treated in the same manner as the TOP event, and its immediate, necessary, and sufficient causes are determined and shown on the fault tree with the appropriate logic gate. This development continues until all intermediate fault events have been developed into thek basic causes. 

Once the fault tree is constructed, quantitative failure rate and probability data must be obtained for all basic causes. A number of equipment failure rate databases are available for general use. However, specific equipment failure rate data is generally lacking and. 

Critical factors. The basic cause of incomplete fusion is failure to elevate the temperature of the base metal, or of the previously deposited weld metal, to the melting point. In addition, failure to flux metal oxides or other foreign substances adhering to metal surfaces properly may interfere with proper fusion. 

During the reaction the alumina usually attains a pink color which is due to some decomposition of p-tolylsulfonyldiazomethane. However, the colored decomposition products adhere strongly to the alumina and will therefore not contaminate the final product. If the alumina becomes reddish rather than pink, the type of the alumina in use may be too basic, causing more extensive decomposition of the -tolylsulfonyl-diazomethane the reaction time should then be reduced as much as possible to prevent a considerable decrease in yield. 

With these definitions in mind, a systematic classification has been made. The various types of embrittlement found in refineries and petrochemical plant equipment, susceptible steels, basic causes, and common remedies are listed in the accompanying table. 

Types of Embrittlement Temperature Range Manifestations TVpical Service, Equipment Structures Involved Susceptible Steels Basic Cause(s) Common Remedies... 

In a more quantitative sense, cause-consequence analysis may be viewed as a blend of fault tree end event tree analysis (discussed in tlie two preceding cliapters) for evaluating potential accidents. A major strengtli of cause-consequence analysis is its use as a communication tool. For example, a cause-consequence diagram displays the interrelationships between tlie accident outcomes (consequences) and Uieir basic causes. The method can be used to quantify the expected frequency of occurrence of the consequences if the appropriate chita are available. 

As can be noted in Figure 21.7.2, steam and ediane are mi.xed before entering die reactor tubes where pyrolysis reacdons take place. All feed and product lines must be equipped with appropriate control devices to ensure safe operation. The FTA flow chart breaks down a TOP event (see descripdon of fault tree in Unit II) into all possible basic causes. Aldiough, diis mediod is more structured than a PHA, it addresses only one individual event at a dine. To use an FTA for a complete liazard analysis, all possible TOP events must be identified and investigated this would be extremely time consuming and perhaps urmecessary in a preliminary design. 

Consider the frequent issue of oxygen corrosion that may affect a (pre-boiler section) economizer and also various downstream (boiler section) boiler surfaces. In most cases, the basic cause, common to both resulting problems, is inadequate or inefficient FW deaeration. 

In LP steam boiler systems, this problem of uncontrolled or excessive water or condensate return loss also occurs. It may be uncontrolled perhaps because of leaking steam traps or excessive as a result of too frequent or prolonged BD. Irrespective of the basic cause, it is necessary to obtain an accurate assessment of materials balance as a first step in understanding the magnitude of the problem and providing resolution. 

Almost all boiler section scale, fouling, deposit, or corrosion problems, however, tend to be associated with additional (and often very similar) problems both upstream and downstream of the boiler itself. Therefore, the presence of, for example, boiler deposits or localized corrosion issues should never be viewed in isolation. Rather, they should be investigated in the light of a likely chain of cause and effect, and the basic causes of the problems should be eliminated wherever possible, rather than simply the noticeable effects. 

Proliferation concerns have been and continue to be the basic cause ofthe official US. opposition to reprocessing and plutonium recycle, and have thus led to the official U.S. categorization of spent fuel as nuclear waste which should be permanently buried in geologic repositories. 

Steric interactions between bulky substituents such as t-Bu, leading to larger C-E-C bond angles, obviously affect the Lewis basicity caused by the increased -character of the electron lone pair. However, the strength of the Lewis acid-base interaction within an adduct as expressed by its dissociation enthalpy does not necessarily reflect the Lewis acidity and basicity of the pure fragments, because steric (repulsive) interactions between the substituents bound to both central elements may play a contradictory role. In particular, adducts containing small group 13/15 elements are very sensitive to such interactions as was shown for amine-borane and -alane adducts... 

FTA is a deductive method that uses Boolean logic symbols (i.e., AND gates, OR gates) to break down the causes of the top event into basic equipment failures and human errors. The analysts begin with the top event and identify the causes and the logical relationships between the causes and the top event. Each of the causes, called intermediate events, is examined in the same manner until the basic causes for every intermediate event have been identified. 

As this list reveals, the complexity of the phenomenon is considerable, and there is still a degree of uncertainty about the exact roles of the various defect species involved. However, the basic cause of the effect is believed to involve the following steps ... 

Modelling is an art but also a very important learning process. In addition to a mastery of the relevant theory, considerable insight into the actual functioning of the process is required. One of the most important factors in modelling is to understand the basic cause and effect sequence of individual processes. 

These three features are proposed to be basic causes for pore formation and will be illustrated by a few examples. 

Before discussing the problem of self-selection of foods and its relationship to individuality in nutrition, we should emphasize at this point that individual differences in nutritional needs may have many basic causes. 

The investigator then follows the Human Performance Difficulty category through a series of questions (or subcategories). These questions help the investigator identify which of several human performance related branches (sometimes known as basic causes) to investigate further. (Some predefined trees use statements rather than questions, but the selection process is similar.)... 

Figure 9-34 illustrates one of these branches. Human Engineering, showing three levels of the tree, designated as basic cause, near-root cause, and root cause. (Note that other trees may use different terminology for these levels, although root cause is a common term.)... 

The ability of the crown ether ligand, 12-crown-4, to separate the lithium cation from the organic moiety, thus stabilizing the SSIP structure, was also observed in the study dealing with cyclopentadienyllithium systems . As described in Section II.C.3, SSIP structures of these compounds are characterized by low x( Li) values. However, it is important to reahze that the variation found for x( Li) is basically caused by the local symmetry around the lithium cation and other highly symmetric situations will also lead to small x( Li) values. Examples are the sandwich compounds mentioned in Section n.C.3. It is thus necessary to consider also / Li and C chemical shift data in order to classify a certain complex as SSIP or CIP. 

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