TABLE OF CONTENTS Numerous case histories of process incidents are provided in Appendix A1-A6. The most effective way to prevent these incidents is use of inherently safer design to eliminate or reduce the potential hazard. Chapter 11.7 provides a discussion of inherently safer design principles. In addition, the Center for Chemical Process Safety book Inherently Safer Chemical Processes: A Life Cycle Approach (CCPS, 1996) provides an excellent source of information on reducing the inherent risk associated with process operation.
In many processes, technical or manufacturing issues limit the engineer s capability to design an inherently safer process. Further, there is generally a point where the required capital investment is disproportional to the additional risk reduction provided by the process modification. In other words, the derived safety benefit is too low relative to the economic investment.
When this occurs, protection layers or safeguards must be provided to prevent or mitigate the process risk. A safety instrumented system (SIS) is a protection layer, which shuts down the plant, or part of it, if a hazardous condition is detected. Accounts of SISs are given in Reliability Technology (A.E. Green and Bourne, 1972) and by Hensley (1968), R.M. Stewart (1971), Kletz (1972a), de Heer (1974), Lawley and Kletz (1975), Wells (1980), Barclay (1988), Rushton (1991a, b) and Englund and Grinwis (1992).
Throughout the years, SISs have also been known as emergency shut-down systems (ESDs, ESSs), safety shutdown systems (SSDs), safety interlock systems (SISs), safety critical systems (SCSs), safety protection systems...
