The Quantification of Risk in System Design

Many manufacturers in the U.K. are becoming more aware of the benefits achieved by safety and reliability studies on new systems and products, particularly those involved in high risk situations or operating in hazardous areas. The basic criteria of merit for plants, systems, and equipment are: safety, economy of operation, and availability. These criteria are interdependent. Safety criteria can be most effective and incorporated easily in the reliability targets if all possible risks were identified and quantified, and system design will then lead to the specification of corresponding quality requirements for components and systems. This paper introduces new extensions to modern safety and reliability techniques for the combination of hardware failures and human errors in the same study for the evaluation of risk in quantitative terms. An assumed design of a semi-automatic boiler control is studied purely for the purpose of this analysis. Shell boilers in the U.K. are designed and operated to a high standard set by the Health and Safety Executive and Engineering Insurance Companies. The example was deliberately simplified to illustrate the methodology, applicability, advantages, and limitations of each reliability technique.