Maximum tolerable risk

Figure 3.1 Maximum tolerable risk, risk aversion, and the willingness to pay for safety...

Since we are dealing with a risk outside the plant, a target value for the maximum tolerable risk of 10 a (this is applicable to existing plants in the U.K., vid. Sect. 8.2) is stipulated. 

It is assumed that the plant causes a total of 10 equally large risks for the same group of population (e.g. tanker deliveries, other pipelines, site explosion), so that the system under consideration may only use up 1/10 of the maximum tolerable risk target. 

If a maximum tolerable risk of 10 a for up to three fatalities is assumed, the maximum tolerable frequency of system failure (the system is demanded more than once per year, vid. Table 11.2) is... 

Maximum tolerable risk for workers 10 per person-year... 

The scenario tolerable frequency associated to each consequence severity category indicates the frequency in which the organization tolerates an event with such impact. To provide an example, according to CCPS (2001), a maximum tolerable risk for workforce of 10 per year is a typical criteria used with LOPA, considering any one scenario affecting an individual (with potential to cause a fatality within a unit or local area). These values are used during LOPA to define how much risk reduction a scenario needs to achieve a tolerable frequency of occurrence. 

Here we address the WHOLE SYSTEM and set maximum tolerable risk targets and allocate failure rate targets to the various failure modes across file system. Effectively this defines what the safety function is by establishing what failures are protected against and how. Thus the safety functions are defined and EACH has its own SIL (see Chapter 2). 

The manufacturer and the user will be involved in far higher costs of retrospective redesign if subsequent changes are needed to meet the maximum tolerable risk. 

In order to set a quantified safety integrity target, a target Maximum Tolerable Risk is needed. It is therefore useful to be aware of the following rates ... 

Worst case Maximum Tolerable Risk in HSE R2P2 document 10 pa... 

It is important to note that the Individual Risk and the Societal Risk calculations are fundamentally different. Thus the starting points for Maximum Tolerable Risk, in the case of a single fatality, do not immediately coincide, which will be elaborated in Section 2.4. 

A typical assessment confined to employees on a site might use the recommended 10 pa Maximum Tolerable Risk (for 1—2 fatalities) but may address 10 sources of risk to an individual in a particular place. Thus, an average of 10 pa would be used as the Maximum Tolerable Risk across the 10 hazards and, therefore, for each of the 10 safety functions involved. By the same token, the Broadly Acceptable Risk would be factored from 10 pa to 10 pa. 

Despite the widely published figures for Maximum Tolerable Risk (e.g.. Table 2.2), the UK HSE sometimes press for a Maximum Tolerable Risk to be targeted at a lower level nearer to the Broadly Acceptable level (e.g., an order of magnitude). This, however, is a controversial area. In the authors opinion, whatever may the starting point be, the ALARP calculation will, in any case, cause the risk to be reduced to an appropriate level. 

In any event, the final choice of Maximum Tolerable Risk (in any scenario) forms part of the safety argument put forward by a system user. There are no absolute rules but the foregoing... 

This involves factoring the Maximum Tolerable Risk according to totally external levels of protection and to factors which limit the propagation to fatality of the event. Table 2.4 gives examples of the elements which might be considered. These are not necessarily limited to the items described below and the analyst(s) must be open ended in identifying and assessing the factors involved. 

The maximum tolerable failure rate is then targeted by taking the Maximum Tolerable Risk and factoring it according to the items assessed. Thus, for the examples given in Table 2.4 (assuming a 10 pa involuntary risk) ... 

From Table 2.2, the Maximum Tolerable Risk is 10 pa. Thus, the maximum tolerable failure rate (leading to the event) is calculated as ... 

Assume a Maximum Tolerable Risk target of 10 pa (public fatality). 

It should also he noted that the Maximum Tolerable Risk frequencies used are usually for ALL hazards. Thus where personnel are exposed to multiple simultaneous hazards, the Maximum Tolerable Risk frequency needs to be divided by the number of hazards. 

Maximum Tolerable Risk (/year) as specified in the user s procedure. 

Having established a SIL target it is insufficient merely to assess that the design will meet the Maximum Tolerable Risk target. It is necessary to establish whether further improvements are justified and thus the principle of ALARP (as low as reasonably practicable) is called for as good practice. In the UK this is also arguably necessary in order to meet safety legislation ( all that is reasonably practicable is called for in the Health Safety at Work Act 1974). 

Figure 2.4 shows the so-called ALARP triangle which also makes use of the idea of a Maximum Tolerable Risk. 

A Maximum Tolerable Risk target of 10 pa has been set for a particular hazard which is likely to cause three fatalities. The Broadly Acceptable Risk is 10 pa. 

Professional Paper on QRA and maximum tolerable risk comparisons (SaRS Journal) 2009... 

From Table 2.2 a voluntary (3 fatality) Maximum Tolerable Risk of 3 X 10 pa is chosen. 

SR/15 describes both quantitative and risk matrix approaches to establishing target SILs but a very strong preference for the quantitative approach is stressed. It addresses the setting of maximum tolerable risk targets (fatality rates). The tolerable risk targets were shown in Chapter 2 of this book. 

Since the risk is offsite, and a two-fatality scenario assumed, a target maximum tolerable risk... 

A Maximum Tolerable Risk target of 10 " pa which leads to a MAXIMUM TOLERABLE TARGET FAILURE RATE of 3 x 10 pa (see Section 12.2). 

IGEM SR/15 suggests target maximum tolerable risk criteria. These are, for individual risk ... 

The demonstration of ALARP is supported by calculating the Cost per Life Saved of the proposal. The process is described in Chapter 2. Successive improvements are considered in this fashion until the cost becomes disproportionate. The target of 3 x 10 pa corresponded to a maximum tolerable risk target of 10 " pa. The resulting 2 x 10 " pa corresponds to a risk of 6.6 X 10 pa. This individual risk is not as small as the Broadly Acceptable level and ALARP should be considered. 

The probability of an incident becoming fatal has been estimated, elsewhere, as 8.1%. The maximum tolerable risk has been set as 10 pa, thus the maximum tolerable incident rate is 10 /8.1% = 1.2 X 10 pa (Gate Gl). 

The LOPA Worksheets are presented below. Notice how the PFD, which determines the target SIL, is obtained, in each worksheet, from ratio of the Maximum tolerable risk to the column called Intermediate Event Likelihood (actually a frequency). 

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