Chemical Technology July 2015

PLANT MAINTENANCE, SAFETY, HEALTH & QUALITY

Layer of protection analysis (LOPA) Layer of protection analysis is a simplified form of risk assessment that evaluates the risk of an individual hazard scenario. It only considers safeguards which are independent layers of protection. The purpose of this is to assist in avoiding the addition of excessive protection incurring high costs, ie, over protection. It uses an order of magnitude technique to evaluate the adequacy of existing or proposed layers of protection against known hazards. In order to carry out a LOPA the following information must be available: • A hazardous event must have been identified, eg, fire, explosion, toxic release, pollution, etc. • The causes of the event. • The consequences of an event, eg, fatalities, injuries, damage, spoilt environment, etc. • Existing safeguards. The above information is often readily available from a Hazard and Operability study (Hazop) and, therefore, a LOPA can be done as part of such a study. Alternatively, a separate LOPA can be done where the above informa- tion is generated.

Setting tolerance criteria Some criteria or target of tolerance or acceptability is required to enable one to decide how much protection is enough. In the following simple example a typical scale of risk has been established. Risks can be classed as follows: • Class I - Intolerable risk, not acceptable • Class II - Undesirable risk, which is tolerable only if risk reduction is impractical or the cost is disproportionate to the improvement gained • Class III - Tolerable risk if the costs of reduction exceeds It is normal to design hazardous processes or operations to meet a Class III risk. A target risk table may be drawn up for a particular industry or organisation. A hypothetical example is shown below in Table 1. Note this matrix is not an industry standard, see HSE (2001). For example, if a particular hazardous event, eg, over- filling of a tank, could lead to an irreversible health effect, long term environment damage or a R50-m financial loss, then this should not happen more than once in a 1 000 years, to meet the Class III risk criteria, that would make the risk tolerable. Alternatively, this can be expressed as the improvement gained • Class IV - Negligible risk.

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Chemical Technology • July 2015