Automatic risk reduction systems

The definition tells us that a SIS may be responsible for shutdown functions, permissive functions, and even consequence reduction (mitigation) functions. AU these functions have a common attribute — they aU reduce risk. One common interpretation of a SIS definition is, therefore, "automatic risk reduction systems." In some cases the function is designed to reduce risk by decreasing the likelihood of a potential hazard. In some cases the function will decrease risk by reducing the magnitude of the consequence. 

Risk Reduction Factors Coutrol/ rator responses. Alarms, Control system response. Manual anti automatic ESD, Fire/gas detection system Sa/ety System Responses Relief valves. Depressurization system. Isolation systems, High reliability trips. Back-up systems... 

The qualitative assessment identified multiple opportunities for risk reduction in the feed purification and vaporization areas. These included providing gas detectors in the area, interlocked with automatic isolation valves. Upon detection of a gas release, the system would be shut down, significantly reducing the amount of hydrocarbon that could be released, thus reducing the likelihood of a damaging explosion. It was decided that, with the gas detectors and shutdown controls in place, the frequency of event Scenario 4 would be reduced from a "3" to a "2."... 

A hazard analysis may be performed to review reductions in the suggested separation distances. Increased risk can be mitigated by providing additional safeguards, such as fireproofing, automatic water-spray systems, emergency shutdown systems, or additional firefighting equipment. 

Verify that the reliability of the hardware of the safety-related system, taking into account any periodic proof testing and automatic diagnostics, is such that the required risk reduction or hazard rate, as appropriate, is achieved. 

This practical exercise is to eonstruct a fault tree diagram using the basic principles introduced in Chapter 3. It uses an example of a simple reactor with automatically controlled feeds that has the potential to cause a serious risk to plant personnel. Once the basic fault tree has been drawn, the model is to be adjusted to incorporate a safety-instrumented system and to demonstrate the resulting risk reduction. 

Having identified the risks to the organisation and its assets, the function of risk management is to determine the probability or likelihood of the risks arising and the severity of the outcome in each case. It could follow that the risk could be controlled, in the case of a fire risk, by the installation of a better fire protection system, e.g. sprinkler system, automatic fire alarm, etc. and improved safety precautions. The implementation of these measures frequently results in some form of savings in the long term, such as insurance premium reductions, cash grants and tax allowances. 

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