Failure severity categories

In the analysis of pumps, IPRDS failure data for 60 selected pumps at four nuclear power plants were statistically analyzed using FRAC. The data cover 23 functionally different pumps, respectively, for two BWRs. Catastrophic, degraded, and incipient failure severity categories were considered for both demand-related and time-dependent failures. 

When different failure severity categories are used, every failure has to be classified to fit into either of the categories. Reliability estimations can be performed for each category or for all failures. For example, it is possible to derive a reliability measure in general or for critical failures in particular. 

Hypertension is the most common cardiovascular disease in fact, nearly 25% of adults in the U.S. are considered hypertensive. Hypertension is defined as a consistent elevation in blood pressure such that systolic/diastolic pressures are >140/90 mmHg. Over time, chronic hypertension can cause pathological changes in the vasculature and in the heart. As a result, hypertensive patients are at increased risk for atherosclerosis, aneurysm, stroke, myocardial infarction, heart failure, and kidney failure. There are several categories of antihypertensive agents ... 

The certification of the content of a substance of interest must follow strict, transparent and reliable procedures. The reputation of the certifying body will depend on this procedure and the absence of failures. Several valid procedures for the certification of material properties exist. They can be grouped into two categories ... 

Acute renal failure associated with NSAID falls into several categories (1) acute interstitial nephritis with glomerular abnormalities, (2) functional acute renal failure, probably haemodynamic in nature, (3) papillary necrosis, and (4) phenylbutazone anuria. 

Instrumentation equipment can fail in different ways. We call these "failure modes." Consider a two-wire pressure transmitter. This instrument is designed to provide a 4 - 20 milliamp signal in proportion to the pressure input. Detailed failure modes, effects, and diagnostic analyses of several of these devices reveal a number of failure modes frozen output, current to upper limit, current to lower limit, diagnostic failure, communications failure, and drifting/erratic output among perhaps others. These instrument failures can be classified into failure mode categories when the application is known. 

These eHects can then be compared to the severity category defined in the applicable safety criteria. Table 3.2 shows the goal/failure-based safety criteria typically used by EASA for large civil transport aircraft. 

Using the failure mode severity categories in Table 3.2, allocate allowable system failure probability targets using Table 3.3. The SSWG may elect to capture this activity as shown in the columns 5 and 6 of Table 7.3. 

Partitioning is a technique for providing isolation between functionally independent software components to contain and/ or isolate faults and potentially reduce the effort of the software verification process. If protection by partitioning is provided, the software level for each partitioned component may be determined using the most severe failure condition category associated with that component [ABDOIOO.1.3 Issue E para 2.1.5]. 

Failure rate X//(i) for failures of category 11 must be lower than 10 to one shot. Also this value of failure rate ensures that failures with severe impact on... 

From these two data categories a criticality matrix as detailed in Chapter I (in the name of risk matrix) is formed. In many cases, both probability of occurrences and severity categories are numbered in the scale of 1—10 and a criticality matrix can be formed for the same purpose. As these are already covered in Chapter I they are not repeated here. Analysts need to understand that these are for reference only. Analysts use their judgment of failure mode frequency for each specific application. The analyst should tailor the analysis to focus on significant components or subassemblies where failures will result in undesirable system-level effects. Based on these judgments, analysts develop risk categories for the specific application. 

A CSI is essentially the same as an SCI except that systems required to identify CSIs have additional statutory and regulatory requirements that the contractor must meet in supplying those CSIs to the government. For systems required to have a CSI list, HA and mishap risk assessment is used to develop that list. The determining factor in CSIs is the consequence of failure, not the probability that the failure or consequence would occur. CSIs include items determined to be life-limited, fracture critical, fatigue-sensitive, and so on. Unsafe conditions relate to hazard severity categories I and II of MIL-STD-882. A CSI is also identified as a part, subassembly, assembly, subsystem, installation equipment, or support equipment for a system that contains a characteristic, failure mode, malfunction, or absence of which could result in a Class A or Class B accident as defined by DoDINST 6055.7. 

Each identified hazard is allocated severity classification according to the defined safety criteria. Accident severity categories are defined to provide a qualitative measure of the consequences resulting from personnel error, environmental conditions, design inadequacies, procedural deficiencies or system, sub-system or component failures. The severity is the worst credible consequence of a hazard (i.e. the worst accident) and is independent of random or systemic failure modes. 

Failures affecting airworthiness can be defined according to the severity categories in Table B.l. 

Mishap severity categories are defined to provide qualitative measures of the worst credible mishap (i.e. accident) resulting from personnel error environmental conditions design inadequacies procedural deficiencies or system, subsystem or component failure/malfunction as shown in Table B.13. 

Software level. Safety monitoring software is assigned the software level associated with the most severe failure condition category for the monitored function. 

Heat for soldering is usually obtained from torches. The high conductivity of copper makes it necessary to use large flames for the larger sizes, and for this reason the location in which the joint will be made must be carefully considered. Soldered joints are most widely used in sizes 2 in and smaller for which heat requirements are less burdensome. Soldered joints should not be used in areas where plant fires are hkely because exposure to fires resiilts in rapid and complete failure of the joints. Properly made, the joints are completely impervious. The code permits the use of soldered joints only for Category D fluid service and then only if the system is not subject to severe cychc condions. 

Continuous monitoring is necessary on critical machines where problems can develop rapidly and have severe financial consequences. Typical machines in this category are unspared process compressors. Remotely located machinery such as pipeline gas compressors also require continuous monitoring. Also, continuous monitoring may be dictated by safety considerations. Even though the cost of a failure is small, machines should be continuously monitored if a failure will result in hazards to personnel. Figure 8-38 depicts a typical continuous monitoring system. 

Failure modes vary in degree of magnitude, for example, a pump may have no output or may have its output restricted. Consequently, failure modes have been divided into three categories of severity, which are defined as follows ... 

Both carvedilol and labetalol are contraindicated in patients with hypersensitivity to the drag, bronchial asthma, decompensated heart failure, and severe bradycardia The drugs are used cautiously in patients with drag-controlled congestive heart failure, chronic bronchitis, impaired hepatic or cardiac function, in those with diabetes, and during pregnancy (Category C) and lactation. 

Plasma proteins are contraindicated in those with a history of allergic reactions to albumin, severe anemia, or cardiac failure in the presence of normal or increased intravascular volume and in patients on cardiopulmonary bypass. Plasma protein fractions are used cautiously in patients who are in shock or dehydrated and in those with congestive cardiac failure or hepatic or renal failure. These solutions are Pregnancy Category C drugp and are used cautiously during pregnancy and lactation. 

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