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Improving the Safety Instrumented System (SIS) Design Process with Graphic Diagrams

SIS Design Diagram - Logic Solver Diagram

During a Safety Instrumented System (SIS) implementation project at a plant site new to the ANSI/ ISA 84 process safety lifecycle world, we discovered the importance of utilizing graphic diagrams in the development of SIS‐related documentation to support the on‐site team meetings and document decisions. In a room full of plant operators and engineers accustomed to working “hands on” in the field, it was often far easier to keep the team on track when they were provided with a drawing to discuss, as opposed to having the team look at a screen full of text. The graphic diagrams also provided the design team with equal benefits as we received greater focused team member feedback, allowing for more efficient and thorough updates to documentation. This method of capturing team member input also enabled concise integration of the team input into various SIS‐related documents during and after the meetings.

Examples of these graphic diagrams included the following:


- A logic solver block diagram ‐ used to quickly identify which Logic Solver Safety PLCs, Independent Protection Layers (IPLs), Logic Narratives, and Equipment were related to each other.

- Logic flow diagrams for heaters and boilers ‐ used to visualize the order in which light off permissive

would be met, which statuses would cause a partial or complete trip, and related IPLs.

- SIF Diagrams ‐ used to depict complex SIF architecture to keep track of how a SIF would function.

The author will present examples of the different types of graphic diagrams, methods in which the diagrams were utilized, and the benefits that each provided in the implementation of certain phases of an ANSI/ ISA 84 SIS lifecycle project. These diagrams were considered to be valuable process safety information and part of the final SIS Front End Loading design.

PROCESS SAFETY KEYWORDS: Continuous Improvement, Integration, Safety Culture, Process Design, P&ID, Codes, Safety, SIS, Facility, Knowledge, Design Philosophy.

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