The adoption of the standard ANSI/ISA.84.01.1996, "Application of Safety Instrumented Systems for the Process Industries," and the coming IEC.61511-based ANSI/ISA.84.00.01.2004, "Functional safety: Safety Instrumented Systems for the process industry sector" have added number of "S" words to the industry's vocabulary. Examples of these words include safety instrumented systems (SIS), safety instrument function (SIF), safety integrity level (SIL), safety life cycle (SLC) and the safety requirements specification (SRS).

The SLC in the new standard ISA.84.00.01.2004 (draft), the development of the safety requirements specification comes after Step 2: "Allocation of safety functions to protection layers." In the ANSI/ISA S84.01.1996 standard, the SRS development comes just after the target SIL selection step shown in the Figure.

Why an SRS?

For new systems that are to conform to S84, the SRS is part of the conformance. For a new safety system, SRS has four primary purposes:

1. The SRS is the specification that defines the functionality and safety integrity requirements of the safety instrumented functions in a SIS and provides the required information to design, implement and operate the system.
2. The SRS is the document that the SIF/SIS design is verified against during the design phase.
3. The SRS is the specification that the safety system is validated against for the pre-startup acceptance test (PSAT), sometimes called a site acceptance test (SAT), prior to turning the system over to Operations.
4. Once the SIS is operational, the SRS is a living document used to document the safety system’s current specifications during its lifetime.

For an existing SIS, the SRS is a document that defines the current as-built state of the SIS. The SRS can be used as part of the process of upgrading safety systems to meet S84, or as part of the grandfathering process. As with the new system, the existing system SRS is a living document used to document the safety system specifications of the system during its lifetime. The SRS is modified and kept current when changes are made to the system, and as such, it is a primary controlling document when the SIS is undergoing the management of change (MOC) process. The SRS is also used as the benchmark when the as-built system is periodically reviewed/audited.

By the Book

The SIS standards are performance based so the SRS’s format and style is left up to the individual company. The standards do not dictate exactly where the information needs to reside or how the information must be organized. Some companies put all their information in a single book, while other companies use a more distributed form utilizing their standard engineering documentation system, or some combination in between. In any case, the SRS should be part of the facilities’ formal engineering documentation system and placed under a formal revision process.

While there are obviously some advantages to using existing engineering documentation formats, keeping the SRS in one book provides significant advantages. The one-book format provides organization, which can help minimize errors and provide efficient usage for design, modification, review and auditing purposes.