Using Safety Integrity Levels - Part 1

Applying the SIL (safety integrity level) concept has been ongoing in process industries but is relatively new in production automaton. It offers different, perhaps more flexible, means of assessing risk and augmenting safety at discrete manufacturing sites. SIL uses statistics to represent the reliability of a safety-instrumented system for a process that occurs through the requirement of the system.

SIL ratings (1, 2, 3, and 4) offer probability of a dangerous failure over time and can be used to measure range of risk reduction afforded by a piece of equipment. SIL selection determines how much risk reduction is needed. Risk reduction with SIL safety measures is:

Level 1—between 10 and 100 times;

Level 2—between 100 and 1,000 times;

Level 3—achieves a risk reduction of 1,000 to 10,000 times; and

Level 4—achieves a risk reduction of >10,000 times.

SIL 3 is considered the highest risk reduction level achievable using one programmable electronic system. Need for SIL 4-rated applications is rare-for example, in some areas of a nuclear-power generating plant. Standards caution that one programmable safety system shouldn't be used to meet SIL 4 requirements.

Here is a suggested general approach to the SIL selection process, leading to required level of risk reduction with cost-effectiveness:

- Estimate the consequence—analyze all outcomes of an unwanted event;

- Estimate the likelihood—combine initiating events and failure of devices designed to prevent accidents;

- Integrate the risk—determine impacts on people, property, profits, and the environment; and

- Select the required risk reduction—this is the difference between baseline risk and what's considered tolerable.

Additional guidance and insight into SIL selection process will be provided in next week's Tech Tip.