Safety integrity level (SIL) is “…a relative level of risk-reduction provided by a safety function, or to specify a target level of risk reduction. In simple terms, SIL is a measurement of performance required for a safety instrumented function (SIF).”
At the 2017 Emerson Exchange conference, Emerson’s Carsten Thøgersen posed the question if a valve alone can be SIL rated for use in a safety instrumented system (SIS). He described the challenge users face in selecting a valve that is suitable for use in a SIS, meets SIL targets with an overall SIL verification, and operates as required. Selecting the right valve for the application requires and understanding of SIL ratings, SIL certifications and SIL capabilities as well as other considerations required to properly select a valve engineered and built for use in an SIS.
Carsten shared that a common perception is that you buy a SIL 1, 2 or 3 valve on the shelf. SIL is not a rating of the individual elements in a SIF—the final element, sensor or logic solver—but rather the loop itself.
The final element is composed of the valve, control components and the actuator. Each component has its own SIL certificate which must be factored into the calculation for the SIF. The approach is to look at operating data on the components in the loop, looking for where failures are most common. From OREDA, the final element is typically 50% of the failures and would use half the budget in the SIL calculations.
In the Analyze phase of the safety lifecycle of the IEC 61511 global safety standard, a clear safety requirements specification (SRS) is required. Considerations for the calculations include both systematic and random integrity factors. If the safety valve has diagnostics in the valve controller, this diagnostic coverage can be factored into the calculations.
Response time is often the specification of emergency shutdown valves with 1-2 seconds for gas and longer for liquids and slurries. The verify a SIL of a SIF you need to consider the systematic integrity, architectural constraints (hardware fault tolerance) and route, and random integrity as defined by the average probability of failure on demand, PFDavg.
It’s important to make sure verification goes hand in hand with testing. It’s a good functional safety practice to increase diagnostic coverage provided by digital valve controllers, volume boosters, quick exhaust valves and one-out-of-two solenoid in series configuration.
Simplifying the assessment process for SIL suitability of the valve is have a pre-engineered solution with the valve, valve controller and actuator. Pre-engineering can minimize the systematic failures compared with picking components individually for the final element.
What is your process for calculating the suitability of your final control elements in safety instrumented functions?
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