The world of functional safety has its own tribal language and acronyms, e.g. safety instrumented systems (SISs), safety instrumented functions (SIFs), logic solvers, final elements (FEs), safety integrity level (SIL), probability of failure on demand (PFD), etc.
A BCPS, consisting of a transmitter, controller and control valve, operates under dynamic conditions, with outputs constantly being adjusted for process control. In terms of FCE [final control element] use, it is a high-demand system.
In contrast, a SIS is typically passive, with low demand, and takes action only when a dangerous condition is detected. It consists of a sensor, logic solver and FE…
Riyaz described the challenge with a valve being the low-demand, final element in the safety instrumented function, a.k.a. safety loop:
Without any mechanical movement, unreliability inherently increases, and SIS valves are prone to sticking due to long static dormant status.
This sticking or other issues affecting the valve’s ability to change states from open-to-close or close-to-open is a failure on demand. The safety integrity level:
…specifies the safety integrity requirements of the SIF and is a quantifiable measurement of risk, used as a way to establish safety performance targets of SIS systems.
…can be expressed in terms of probability of failure on demand (PFD), a value that indicates the probability of a system failing to respond to a demand.
Riyaz highlights two types of failures—physical and functional. Physical failures can be predicted based on operating histories and experiences. Functional failures such as programming bugs cannot be accurately predicted.
The IEC 61511 functional safety lifecycle standard [hyperlink added for additional information]:
…insists upon validation and verification, so these valves are proof-tested at regular intervals. A partial stroke test (PST) is performed, decreasing the pressure to move the valve from 1 to 30%.
These tests improve:
…the PFDAVG of the valve by helping diagnose possible valve failures before they occur, moving the SIS valve into the realm of predictive maintenance.
Read the article for a greater understanding of these concepts and additional ones such as mean time to fail (MTTF) and how it’s used in the determination of suitability for the risk mitigation required by the safety instrumented function.
Visit the Valves, Actuators & Regulators section and Safety Consulting Services section on Emerson.com for more on the final elements and solutions to apply to meet the risk mitigation requirements of your safety applications.