Using Safety Integrity Levels - Part 2

Applying the safety integrity level (SIL) concept has been ongoing in process industries but is relatively new in production automation. SIL ratings (1, 2, 3, and 4) use the probability of a dangerous failure over time as a way to measure range of risk reduction afforded by a piece of equipment:

Part 1 discussed a general approach to the SIL selection process and described the four SIL probability ratings. Here's further guidance and description of the top-level risk assessment steps to use at any facility embarking on a risk reduction program.

Determine an acceptable level of risk - Select a SIL- (or category-level) rating that the assessment should meet.

Start by choosing who will look for hazards - Cross-functional teams can offer perspectives beyond one person's view. A control engineer, operator, maintenance technician, and custodian all may have insights into risks and risk avoidance in varied circumstances.

Look for areas to be assessed - This includes a device, a machine design, general area, or full manufacturing line or system.

List the hazards - Examine each situation from multiple perspectives and circumstances. Ensure clear delineations among commissioning, operations, and maintenance modes; look at each and transitions among them. Ensure all designed and installed safety equipment become a documented part of training for anyone with access to the area.

Analyze risks according to severity and probability - Evaluation and probabilistic analysis are based on failure rate and failure mode data. For any potential source of harm, figure out how bad risks are, how often they could occur, and combine the two. Typically for a protective function, a machine often will have more than one means of protection, perhaps a primary-perimeter guard or electronic guard, such as light curtains. How bad would it be if they fail; how often could they fail?

Mitigate risks, if needed -A shrink-wrap machine could present pinching or mechanical hazards. A guard or isolation with other protective device might stop injury. Over-temperature protection or a flame detector guard might be used if a flame or heater was involved. For some cases, design changes may be needed; there's not a piece of safety equipment that can mitigate every hazard.

Fortunately, most things in life require no safety function at the end of the discussion.