Independent protection levels

Once the severity and the probability corresponding to a scenario are estimated, that is, the risk is assessed, a decision can be made on the nature of the protection system to be implemented. If a safety instrumented system (SIS) is to be used, consisting of one or more independent protection levels (IPL), the required reliability of the protection system, constituting a so-called Safety Integrated Level (SIL) can be determined by using this risk assessment, respective of the required risk reduction. 

Independent Protection Layer (IPL) - Protection measures that reduce the level of risk of a serious event by 100 times, which have a high degree of availability (greater than 0.99) or have specificity, independence, dependability and auditability. 

Where k is a multiplier chosen from a table of normal quantiles to give the desired protection level and s is the square root of the mean square error from the regression. The choice of k value allows the control of the confidence level and thus the rate of false alarms. This approach depends on the data being normally distributed and independent and is applicable only to data with a common linear slope for all batches. 

LOPA can also be used to develop the quantified specifications for Safety Instrumented Levels (SILs), for SISs—a necessary step in complying with ANSI requirements). The technique is also effective at identifying safeguards— including those to do with human performance. From a regulatory point of view LOPA can provide a basis for specification of Independent Protection Layers (IPLs) and can help address the requirements of standards such as OSHA s 29 CFR 1910.119 and the Seveso II directives. 

HSL Recommendation 3 - Provide an absolutely independent high level overfill protection system completely independent from the routine gauging system. Install a separate and independent high integrity overfill prevention system on all storage tanks at Buncefield-type sites to prevent storage tank overflow of highly flammable liquids. 

This practice is built around the idea that, if hydrocarbons can remain contained in the system of pipes and vessels, then a serious process-related accident is unlikely to occur. This goal is achieved by identifying those process hazards that could lead to a release, and then installing two independent protective devices for each detectable event. The two levels of protection should be independent of, and in addition to, the control devices used in normal process operation. In general, the two levels should be provided by functionally different types of safety devices for a wider spectrum of coverage. Two identical devices would have the same characteristics and might have the same inherent weaknesses. 

Layer of protection analysis (LOPA) LOPA is a systematic and structured way of quantification of risk reduction and safety integrity level (SIL) determination. Usually, it starts its work on the data developed in HAZOP analysis. For each documented undesired event with an initiating cause, it provides an independent protection layer (IPL), which will mitigate or prevent the hazard. Then, the total amount of risk can be determined. If safety instrumented function is necessary, LOPA methodology can be used to determine SIL also. From ISA 84 transaction it is found that LOPA is a simplified risk assignment tool used to evaluate the effectiveness of IPLs that are designed to reduce the likelihood or severity of an undesirable event. Quantitative PHA LOPA deals with single cause consequence pairs. Detailed documentation is possible and can be applied for continuous process. 

Conceptually, it analyzes the hazard events, then offers a number of independent protection layers (see Fig. 11/4.3.7-1) so that risk frequency is reduced and system is acceptable. In LOPA, each of the independent protection layers (IPLs) are tested and analyzed individually to see how it provides prevention/mitigation of hazards for which they are used. Thus IPLs could be preventive as well as mitigating types. LOPA is mainly concerned with mitigating type protection layers. In a later part of this clause, it will be discussed that, in LOPA each of the protection layers are analyzed for their probability of failure on demand (PFD), which is very relevant in the context of safety integrity level (SIL) and associated international standards like International Society of Automation (ISA) 84 and lEC 61508/61511. In fact, it was initially... 

However, in a prospective study, atherosclerosis risk in communities (ARIC), there was doubt about a relation between CHD and Hey and no association between CHD and C677T mutation of methylenetetrahydro-folate reductase gene or three mutations of cystathionine P-synthase gene. However, it was possible that vitamin Bg offered an independent protection (143), and recently a review concluded that there is insufficient evidence to regard Hey levels as a causative factor in atherosclerosis (144). 

Independent protection layers (IPLs) the level of protection provided by each IPL is quantified by the probability that it will fail to perform its function on demand. The smaller the value of the PFD, the larger the risk reduction factor that is applied to the calculated initiating likelihood, hence where no IPL is claimed, a 1 is inserted into the LOPA worksheet. 

If severity increases to, say, complete failure with probable severe injury and/or loss of life S = 9), the designer should reduce occurrence below the level afforded by two independent characteristics protecting against the fault. If each is designed to, then this implies a failure rate of ... 

Napoli et al. [286] found that the nifedipine treatment of stroke-prone spontaneously hypertensive rats (SPSHR) suppressed the plasma and LDL oxidation and the formation of oxidation-specific epitopes and increased the survival of rats independently of blood pressure modification. Their results suggest that the protective effects of calcium blockers of dihydro-pyridine-type on cerebral ischemia and stroke may, at least in part, depend on their antioxidant activity. In vivo antioxidant effect of nilvadipine on LDL oxidation has been studied in hypertensive patients with high risk of atherosclerosis [287], It was found that there was a significant decrease in the level of LDL cholesterol oxidation in patients after nilvadipine treatment. 

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