5.1 Introduction

Once the need for a safety instrumented system (SIS) is identified and the target safety integrity levels (SIL) have been established for each safety instrumented function (SIF), the safety requirement specification (SRS) needs to be developed. The SRS can either be a single document or a series of documents. The SRS lists the requirements for all functions to be performed by the system. This should also include the application software requirements. The SRS consists of two main parts:

1. The functional requirement specifications

2. The integrity requirement specifications

The functional requirements describe the logic of the system. In other words, it defines what each safety function should do. For example, low pressure in vessel A will cause valve B to close. The integrity requirements describe the performance needed for each function. In other words, it defines the capability or how well each safety function should perform. For example, the probability or likelihood that valve B will close when the pressure in vessel A is low needs to be greater than 99%. When the term "specification" is used hereafter in this chapter, it refers to both the functional requirement and the integrity requirement specifications.

It's best to design plants that are inherently safe, ones where hazards won't even exist. Unfortunately, this is not always possible. There are times when the only way to prevent a hazard is to add instrumentation that will monitor the...