Quantitative Hazard Analysis

The fault trees for even a simple process unit will be complex, with many branches. Fault trees are used to make a quantitative assessment of the likelihood of failure of a system, using data on the reliability of the individual components of the system. 

When a fault tree has been constructed, it can be used to estimate the probability of the system failing if the probabilities of the events in the fault tree can be estimated. In most cases, this requires a good understanding of the reliability of instruments, alarms, and safety devices, since these devices would be expected to maintain the process in a safe condition. 

If the failure rate. A, is the number of occasions per year that a protective system develops a fault (yr ) and the time interval between tests of the device is t years, then intuitively the device on average fails halfway between tests. The probability that the device is inactive (or fractional dead time) is then approximately 

If the demand rate, 8, is the number of occasions per year that the protective device is actuated, then the hazard rate, or number of occasions per year when a hazardous condition exists, rj, is 

The intuitive result in equation 9.2 is true if and only if S, At, and St are all C 1. For a more rigorous analysis of the probability of failure, see Chapter 13 of Lees (2004). 

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The data on probabilities given in this example are for illustration only, and do not represent actual data for these components. Some quantitive data on the reliability of instruments and control systems is given by Lees (1976). Examples of the application of quantitive hazard analysis techniques in chemical plant design are given by Wells (1996) and Prugh (1980). Much of the work on the development of hazard analysis techniques, and the reliability of equipment, has been done in connection with the development of the nuclear energy programmes in the USA (USAEC, 1975) and the UK. 

Formal examination of engineering diagrams with quantitative hazard analysis to ensure the above measures have been implemented. Systematic check that hardware (as constructed) and operating instructions are as intended in the design. 

KAZAN is a quantitative hazard analysis approach that can be used to estimate the probability of occurrence of a hazard given the probability of failure of various control systems and the probability of occurrence of various events that can lead to the hazard. The FMEA and KAZAN approaches are similar. The following example explains the principle behind KAZAN Consider a packed bed, countercurrent absorber used to absorb chlorine gas from an exit stream using alkali solution in a countercurrent manner. Chlorine enters the bottom of the bed and alkali is sprayed from the top. Chlorine could escape from the top exit of this absorber for several reasons a decrease in alkali concentration, insufficient alkali, excess chlorine in the inlet, leakage in the chlorine pipeline, and failure of the absorber column. If the probabilities of these possible failures are known, one could estimate the overall probability of chlorine leak. 

Normally, quantitative/semi-quantitative hazard analysis tool. 

Generally, this analysis requires follow-up analysis because many of the documents are not available at the time, therefore after further information is generated, follow-up analysis is very likely. Also for large/complex systems, the output of PrHA could be the basis for further quantitative analysis or for quantitative hazard analysis. 

A system is a part of the universe within a certain domain in space and time. What is an environment Outside the frontier of the system is the environment [1], Here, system shall have an identity, that is, deterministic. There shall be an external boundary to the system. An external boundary is determined by what aspect of system performance is of concern. This is stated here because for quantitative hazard analysis, boundary definition is extremely important. Also, the interface part needs to be considered (See Fig. V/3.0-l). The process definition for qualitative risk analysis is Qualitative Risk Analysis assesses the priority of identified risks using their probability of occurring, the corresponding impact [...] as well as other factors such as the time frame and risk tolerance [..On the contrary, quantitative risk analysis (QRA) as per DNV is Typically, a QRA can be defined as the formal and systematic approach of identifying potentially hazardous events, estimating the likelihood and consequences of those events, and expressing the results as risk to people, the environment or the husiness. ... 

LOPA is based on the assessment of a single event, and the associated consequence scenarios. LOPA is a rather simplified form of quantitative hazard analysis technique normally applied for the cases where the system is too complex or the consequence too severe to be handled by HAZOP. As a matter of fact, it is a quantitative analysis technique, but lying almost at the lower edge of quantity analysis technological scale. It is in between HAZOP and QRA. This is shown in Fig. V/4.0-1B. At times, it takes input from HAZOP and its output could be used as input to QRA. 

Layer of protection analysis concept. (A) LOPA onion, (B) position of LOPA between qualitative quantitative hazard analysis. 

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