How Can We Reach SIL 2 With Manual Intervention?

Gary Law, Product Manager

Emerson Process Management, www.easydeltav.com

The first step would be to identify the SIL requirement for the natural gas compression station. There are several means to do this including cause and effect matrices, standards such as EN 1050 (Safety of Machinery-Principles for Risk Assessment), and ANSI B11.TR3 (Risk Assessment and Reduction). This involves analyzing the hazard using risk parameters that include extent of damage or consequences, which ranges from minor injury to fatalities. Then the probability of the hazard occurring is evaluated. This could involve elements such as exposure time, occurrence probability, and hazard avoidance/mitigation. Once the appropriate SIL level is determined, the appropriate SIL reduction measures can be applied.

...Many companies perform a Levels of Protection Analysis (LOPA) to determine what abatement measures are possible. IEC 61511-1, paragraph 9.4, provides examples of typical risk reduction measures and methods. These protection levels include the basic process control system (BPCS), prevention measures such as alarms with operator interaction, and SISs. An approach could be to claim a risk reduction for the BPCS, another for the SIS and operator corrective action.

...Each of these measures needs to be independent. The SIS, for example, would have its own logic solver, sensors, and valves and would be programmed to automatically perform an emergency shutdown when required. Rockwell Automation's ControlLogix has been certified for use up to SIL 2 as an SIS for emergency shutdown.