TABLE OF CONTENTS 
In order to justify the need for a safety instrumented system (SIS), it's important to understand why such systems are installed in the first place. Safety systems are used in the process industries to lower the risk to personnel, the environment, loss of production, capital equipment and company image. In fact, it's not unusual to find that the safety integrity level (SIL) determined for some safety instrumented functions (SIF) are higher for economic or environmental reasons than for personnel protection.
The SIL determined for each SIF is usually based on practices developed by a company that follow industry standards (as covered in Chapter 6). The internal practices would normally be calibrated to reflect the amount of risk the company is willing to tolerate. The need to further justify a system may not be required as the justification process is an integral part of their internal practices.
Many companies install safety systems to address only the personnel protection issues and comply with regulatory requirements. In these instances, the SIL determined only reflects the "personnel SIL." With this approach, the assumption is that shutting down the process will address the safety concerns. There usually aren't any other needs for justification the safety requirements have to be satisfied for legal, moral, financial, and ethical reasons. Justifying a safety system from a purely personnel safety point of view is usually not an issue.
Some companies prefer to have an additional justification step in order to demonstrate that the cost of providing the...
