Failure-rate reduction

Failure-rate reduction—This principle uses components and design arrangements to produce expected lifetimes far beyond the proposed periods of use. This reduces the probabilities of failure during operation. 

Low Flow Operation. The optimum operation of a pump is near the best efficiency point. Some manufacturers curves indicate the minimum allowable continuous stable flow (MCSF) limits for every pump (43). In the 1980s, the processing industry experienced a reduction in flow requirement as a result of business downturn and installation capacity downsizing. The pumping equipment, however, was generally not replaced by smaller pumps, but was forced to operate at reduced flow rates, often below allowable MCSF. This has resulted in increased failure rates and reduced pump component life. 

It should be noted that the data collection and conversion effort is not trivial, it is company and plant-specific and requires substantial effort and coordination between intracompany groups. No statistical treatment can make up for inaccurate or incomplete raw data. The keys to valid, high-quality data are thoroughness and quality of personnel training comprehensive procedures for data collection, reduction, handling and protection (from raw records to final failure rates) and the ability to audit and trace the origins of finished data. Finally, the system must be structured and the data must be coded so that they can be located within a well-designed failure rate taxonomy. When done properly, valuable and uniquely applicable failure rate data and equipment reliability information can be obtained. 

In 1991, the EORTC reported significantly improved 3-yr survival with concurrent reduced-dose cisplatin (either 30 mg/m2 weekly or 6 mg/m2 5 d per week) and split course RT (30 Gy/2 wk + 25 Gy/2 wk) compared to the same RT alone (26% vs 13%) (66). In contrast to the sequential chemoradiation strategy, failure analysis showed a reduced risk of in-field failure with the concurrent regimen (70% vs 81%). A smaller European phase III trial using twice daily RT with and without concurrent cisplatin and etoposide also reported more favorable outcome with the concurrent chemoradiation regimen and a similar reduction of local failure rates (67). 

Mulder P, Barbier S, Chagraoui A, et al. Long-term heart rate reduction induced by the selective 1(f) current inhibitor ivabradine improves left ventricular function and intrinsic myocardial structure in congestive heart failure. Circulation 1674 109 1674-9. 

Power-traosistors that are commonly used in electronic devices consume large amount of electric power. The failure rate of electronic components increases almost exponentially with operating temperature, As a rule of thumb, the failure rate-lof electronic components is halved for each 10°C reduction in the junction operating temperature. Therefore, the operating temperature of electronic components is kept below a safe level to minimize the risk of failure. 

Temperature can ultimately be viewed as a thermal stress — one that causes an increase in failure rate (and life if applicable). But how severe a stress really is, must naturally be judged relative to the ratings of the device. For example, most semiconductors are rated for a maximum junction temperature of 150°C. Therefore, keeping the junction no higher than 105°C in a given application represents a stress reduction factor, or alternately — a temperature derating factor equal to 105/150 = 70%. 

Lifetime is affected by the junction temperamre of the diode. The graph in Figure 5.25 refers to a 50 % failure rate and a lumen output maintenance of 70 %, the threshold at which the eye begins to detect a reduction in fight output at various continuous forward currents. As junction temperature increases, lifetime decreases as indicated by a reduction in light output. 

The protection provided by the IPL reduces the identified risk by a known and specified amount. This does not mean that the IPL will always work— particularly with regard to human error, but an estimate as to its failure rate is available. A general rule is that the frequency reduction for an IPL should be two orders of magnitude, i.e., it has an availability of 99%. Operator response to an alarm can be one order of magnimde. 

It must be emphasized that a component whose lifetime is exponentially distributed cannot be improved by maintenance. For an improvement would imply a reduction of its failure rate. In the present model it is ensured that the unavailability is equal to zero after every functional test. This is achieved by determining in the first place whether it is still capable of functioning or has failed. In the latter case the component is either repaired or replaced. If it is still capable of functioning it is as good as new because components with a constant failure rate do not age by definition. If it has to be repaired, as good as new is a hypothesis usually corroborated in plants with a good safety culture. 

First of all, these models have been applied to monotonously increasing failure rates this leads to a reduction of the value of X but never to postponing the start of its increase at the end of the useful period, despite maintenance is also - and first - carried out with the latter objective. 

In the lEC 61508 the determination of the SILs is related to the necessary risk reduction for the system. However, the SILs themselves are defined quantitatively by numerical failure rates. Even the rather qualitative risk graph described in the lEC 61508 for the determination of SILs seems to be more specific. It needs four parameters consequence, frequency and exposure time, possibility of avoiding hazard and probability of unwanted occurrence. Each parameter is described in the lEC 615 08 and there is not much room for interpretation. We conclude that the lEC 61508 is somewhat more restrictive with respect to precision than the AOP 52 concerning the determination of SIL and SSCI, respectively. 

One of the approved experimental methodologies of debugging of prototype is to use Highly Accelerated Life Tests (HALT). The experience shows that HALT method obtains be st re suits with a minimum number of samples to test (Hobbs 1996). Moreover, HALT makes possible to highlight significant improvement of reliability by a reduction of early failures as well as a lengthening of the duration of the maturity period of the system i.e. with a constant failure rate period (see Figure 1). 

Vacuum Failures Pressure reduction in a container can damage it. For example, a tank can collapse while being drained if there is nothing to replace the removed contents. Even if there is venting, the vent line must provide adequate flow rates to prevent formation of a vacuum. 

Given a frequency of 1 in 10 years for destructive storms, the necessary risk reduction would be assessed using the low demand figures in Table 1 above. If 90% to 99% risk reduction is sufficient, SILl will suffice. Typically, a Commercial-off-the-shelf (COTS) application provides 95% success (5% failure). The exact figure would depend on evidence as to failure rate assumptions for that particular item. 

A concern regarding the probabiiistic approach used in iEC 61508 and ANSI/ISA-84.00.01-2004-1 to determine the adequacy of the SiF design is that owners/operators could mistakenly assume unrealistically low failure rates for the SIF devices. The resulting erroneously low PFDavg could potentially lead to inadequate risk reduction. There are many sources of failure rate data, and sometimes it is difficult to decide what number best represents the device in the field application. ISA-TR84.00.02 provides more information on device failure rates, including a sampling of data from five owners/operators. 

For field devices, extreme caution should be used in reducing the minimum fault tolerance from ANSI/ISA-84.00.01-2004-1, Clause 11.4.4, Table 6. Understanding the dangerous failure modes of the process interfaces is not trivial, and misapplication could result in the SIF being under-designed for the SIL target. Owners/operators should be cautious when using manufacturer data to support reduction in the fault tolerance and should review the assumptions, boundaries, and sources for the data. The methods used by manufacturers to calculate the device failure rate vary considerably. 

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