The Final Control Element’s Role in the Safety Loop

I used to be one of those who thought of the logic solver piece of a safety loop as being the “safety system.” In reality it’s the sensors, logic solvers, and final control elements which make up the safety loop, or safety instrumented function (SIF) in safety-speak.

Tom Jeansonne, a regional sales manager in Emerson’s Valve Automation division, presents a paper at next week’s Emerson Exchange entitled Safety Instrumented Systems, The Role Of The Final Control Element. Tom sets the tone for the importance of the final control element in the safety loop. It exists for the purpose of taking the process to a “safe” state when predetermined conditions are violated.

The final control element or actuated valve typically remains energized for long periods of time in a fixed position. According to Offshore Reliability Data (OREDA), when failures do occur in the safety loop, it happens in the final control element 50-60% of the time. The key is to reduce the Probability of Failure on Demand (PFD) to meet the appropriate safety integrity level (SIL). The standards IEC 61511 and ANSI/ISA S84 defined this risk-based approach to safety.

Tom describes a way to reduce PFD for final control elements through partial stroke testing. As the name implies, the actuator and valve are periodically operated a partial amount to help ensure the valve will perform on demand. This testing process also increases the diagnostic coverage on the final control element while allowing the normal process operations to continue. In turn this can extend the time between scheduled plant shutdowns reducing operating costs and increasing efficiency while maintaining SIL ratings.

With digital controllers like the Fisher Fieldvue DVC6000 and smart logic solvers like the DeltaV SIS system, these partial stroke tests can be automatically performed and data compared and retained. These tests can identify issues like broken valve stems, torque degradation, stick slip, friction degradation, and pneumatic path leakages. Any anomalies can be sent to the operations and maintenance staff as valve stuck alerts, travel/pressure/deviation alerts, and supply pressure alerts. These diagnostics help identify any issues before the final control element is requested by the logic solver to take the process to a safe state.

Tom sums up his presentation with how the Valve Automation group has packaged the DVC6000 with several different types of Bettis, FieldQ, Hytork, and El-O-Matic actuators, and solenoids into a SIL-PAC TÜV and Exida-certified final control element solution.

Posted Wednesday, September 27th, 2006 under Safety.

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