Safety Systems Definitions

28 Dec 2019 at 21:03 // under Simple Saturdays

For my final Simple Saturday post of the year/decade I will cover terms and acronyms used as part of shutdown systems.

ESD: Emergency ShutDown. This is the generic name given to a system that trips the plant if it detects a problem. This generic name could refer to simple logic that is part of the basic control system, a hard wired trip or a fully modern system.

SIS: Safety Instrumented System. This is the more modern term and is generally used to refer to shutdown systems that meets the current standards. Note that it just refers to keeping things safe, not necessarily shutting them down.

SIF: Safety Instrumented Function. This is talking about a specific task that the SIS has to perform. This involves measuring the process (furnace outlet temperature), performing some logical check (must remain within the safe limits), and taking an action (if not safe, close the fuel gas valve). There will usually be many SIFs within the SIS.

SIL: Safety Integrity Level. This is how critical it is that this function works. SIL1 will only reduce the chance of failure by 10 to 100 times. SIL3 will reduce it 1000 to 10,000. The level you need is based on various analysis or risk assessments.

The only other thing that I am going to add is a comment from a seasoned consultant in this area. He told us¹ not to worry too much about the numbers in the calculations. From a compliance perspective it is important to get them right, but it is not the numbers that kill people. In almost every case when an accident happens, there is something else that contributes to it. Is the system fully effective during the startup/shutdown? Does the system cause spurious trips that might encourage overrides to be applied? Are you measuring the correct thing?

Systematic errors tend to be where the real problems occur that can lead to major accidents. These are the areas you should put focus your energy on.

1. Those of us on a 'familiarisation' course ↩

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