Risk prediction in the chemical industries can be presented in a way it allows easy judgemental of the acceptability of risks. This helps to reduce hazards in chemical process plant operations. Risk can be defined as the likelihood of an accident or unwanted event occurring at a particular period of time. Hazard can be a situation that can cause damage or injury. As it is known it is not possible to have ZERO risk in any operation. So it is highly important risk are identified, controlled if possible and therefore reduced.
Risk Assessment is defined as the process of examining the chances of a hazard in a workplace. For example the hazards in a flare gas recovery system can be analysed for ...view middle of the document...
Its aim is to ensure benefits of risk reduction measure are not disproportionate to cost. The concept of ALARP, As Low As Reasonable Possible is usually used for this.
The use of fault tree analysis enables prediction of risk which allows the estimation of the consequence and probability of the risk. Due to the inability to avoid hazard in chemical process industries, it is very imperative hazards are identified so it can be stopped or reduced. Risk prediction presentation can be categorised into two namely: Occupational risk (risk to workforce) and risk to public. The usual form of presenting occupational risk is the use of fatal accident rate (FAR). It is the number of fatalities which can be expected for every 100 million (108) exposed hours. This is almost the same as saying the number of death in a workforce of 1000 people. The table below shows some of the some FAR and risks to life in some selected workplace compared to chemical process industries.
Workplace | FAR | Risk per person per year |
Health and safety executive tolerable limit | 50 | 100 x 10-5 |
Offshore oil and gas | 62 | 125 x 10-5 |
Coal mining | 7.3 | 14.5 x 10-5 |
Construction | 5 | 10 x 10-5 |
Agriculture | 3.7 | 7.4 x 10-5 |
Chemical and allied industries | 1.2 | 2.4 x 10-5 |
Vehicle manufacture | 0.6 | 1.2 x 10-5 |
All manufacturing industries | 1.2 | 2.3 x 10-5 |
Clothing | 0.05 | 0.1 x 10-5 |
Data obtained from HAZOP and HAZAN by Trevor Kletz (p. 88)
Risk prediction can be further broken down into two: Individual risk assessment and Societal risk assessment ‘Individual risk is the frequency at which an individual may be expected to sustain a given level of harm from realisation of specified hazards.’ (Skelton b. 1997) ‘Societal risk is the relationship between the frequency and the number of people suffering from a specified level of harm in a given population from the realisation of specified hazards.’ (Skelton b. 1997) Individual risk is usually presented using a risk contour and may be used to represent individual fatality or individual risk. The risk contour can be used to show just one hazard or the combined results of many hazards. It is useful in showing how separated the hazardous item is to the places at most at risk from it.
Individual risk can be estimated for individual present in a location every day. This can be referred to as ‘Location-Specific Individual Risk’ which is not realistic. This is because the individual is not in the same location and not exposed to the same risk every time. Individual risk can also be estimated for an individual present at different location different time (Individual-Specific Individual Risk). There is also the ‘average individual risk’ usually calculated from historical data. It is the number of fatalities per year divided by the number of people.
A simple calculation can be used to give an estimation of individual risk. This includes the probability of being killed and being present.