Introduction

Many SIF designs have been done using the BPCS to implement the safety functions over the years. This practice is now banned in some functional safety standards and strongly discouraged in other standards. The ANSI/ ISA-84.00.01-2004 (IEC 61511 Mod) standard (Ref. ^[1]) does however allow the use of combined BPCS and SIS designs under certain conditions and when justified by detailed analysis. In general, the analysis work can be extensive and usually results in complete separation between the BPCS and the SIS. However, when there is a strong justification, the analysis may be worth the effort in the quest for an optimal design.

Analysis Tasks

Any engineer who wants to attempt combining control and safety must go through a series of analysis and classification tasks that should spell out the potential problems and identify any design flaws. The steps include:

1. Classification of all equipment used in the SIS,

2. Justification of all equipment used in the SIS,

3. Analysis of possible "initiating events" caused by failure of equipment used in the combined SIS and control system and comparison of resulting hazard rate to tolerable limits,

4. Practicality analysis of resulting operation and maintenance procedures.

When these steps are done, alternative designs that meet the requirements of the standard can be compared to determine the optimal solution.