Review and discuss the paper, in the context of the approach to risk management outlined in Australian Standard AS 4360:1999 – Risk Management.

After reading the paper and the Australian Standard, what is your definition of the term ‘safe’, when applied to workplace hazardous substances, and processes and plant involving their use?

In your opinion, is the approach to risk control outlined in the paper ‘Protective Systems for Hazardous Processes’, appropriate for our industrial situation in Victoria? How could it be improved?

Your answer should consist of a two or three page typed essay.

Introduction

Many plants and processes have an element of hazard associated with them. There is often the risk that system or equipment failure may lead to a local hazard such as an explosion, fire, release of toxic gas or leak of radioactive material. There is a probability that this limited hazard could injure or kill people working in the vicinity, as well as destroying part of the process and causing loss of production. It is established practice that these plants and processes, are fitted with automatic protection systems. These systems detect the movement of the process towards a hazardous situation and shut all or part of it down quickly and automatically, thus making it safe without the need for intervention by the process operator.

There are, however, a number of ‘super’ hazardous plants, which by their very nature present a far greater and wider potential hazard than the ‘conventionally’ hazardous plants mentioned above. These are often very large, contain large amounts of explosive, poisonous or radioactive material and are operated at high temperatures and pressures. When a fault does occur, then a large amount of energy and/or dangerous material can be released over a wide area with potential loss of life.

There have been several occurrences during the last two decades. At Flixborough, UK in 1974 a vapour cloud explosion at a chemical plant caused 29 deaths. At Ixhuatepic, Mexico in 1984 a liquified petroleum gas (LPG) explosion caused around 500 deaths. A release of toxic gas (MIC) at a plant at Bhopal, India, also in 1984, caused 2500 deaths. Finally a nuclear reactor fire at Chernobyl, USSR in 1986 caused 31 immediate deaths due to radiation sickness, together with many hundreds of delayed deaths due to cancer and leaukaemia.

Society can only allow such ‘super’ hazardous plants to be built and operated, if the hazard associated with these processes is greatly reduced to around that of the ‘conventionally’ hazardous type. In order to achieve this reduction, the ‘super’ hazardous plant must be protected by a ‘super’ automatic protective system, which is far more comprehensive and reliable than conventional systems. These super systems are referred to as ‘high integrity protective systems (HIPS).

Target hazard rate for the protective system

Having established that the process is of the ‘super’ hazardous type, it will need to be protected by a high integrity protective system. The first stage in the design of any system or item of equipment is to establish a design specification; the design specification for the HIPS is the numerical value for the target hazard rate. To calculate this value the background risk must first be established; this is the mean fatal accident rate (FAR) on all of the chemical plants operated by the company concerned.

For one company he FAR was found to be 3.5 deaths in 100 million exposed hours. To put this figure in perspective, the following table gives some corresponding FAR figures for different industries and occupations in the United Kingdom and some non-industrial activities.

Assuming continuous operation of the plants in the company for 8700 hours in any year, then the mean probability of a fatality in any one of the company’s plants is 0.0003 per year. This is the background risk for any employee on any of the company’s plants.

If the company expects the employee to work on a ‘super’ hazardous plant, then the additional risk should be no more than one tenth of the background risk i.e 0.00003 per year.

Since the high integrity protective system is required to keep the additional risk to 0.00003 per year, this is the numerical value for the target hazard rate.

Reference: An Introduction to Reliability & Quality Engineering , John P. Bentley, Longman Scientific & Technical , John Wiley & Sons, Inc., New York.