I had the chance to catch up with Emerson’s Riyaz Ali at the Emerson Exchange last month in Nashville. You may recall Riyaz from many process safety-related posts. In his conversations with process automation and safety professionals, he has been encountering some misconceptions he hoped we could clear up in this post. These misconceptions surround the use of the DVC6000 SIS digital valve controller in safety instrumented functions (SIFs).
One point he wanted to clarify was a reference to the now obsolete DIN 19250 standard, which was replaced by the globally adopted IEC 61508 functional safety standard, an umbrella standard for safety instrumented systems. Safety instrumented function loops consists of logic solvers, sensors, and final control elements. Suppliers of these components need to certify them for use in safety applications of a certain safety integrity level—typically SIL 1, SIL 2, or SIL 3 in the process industries.
Another misconception is that the DVC6000 is not failsafe. Riyaz notes that the failure mode in basic process control systems (BPCS) and safety instrumented systems (SIS) are different. In a BPCS, the valve is dynamic and constantly being adjusted by the digital valve controller and its control loop based on the desired control output. For a safety instrumented function (a.k.a. safety loop), the safety valve remains dormant until a safety demand requires its movement to safe state. In BPCS applications, the DVC6000 has been in use for more than 16 years. In the case of a power or electronics failure, it drives the valve to a safe state. These 16 years of field-tested performance provide the data for its functional safety assessment and SIL 3 suitability.
Another misconception is that the scope of the certification for use in up to SIL 3 applications is limited to the pneumatic portion of the positioner. Here is the IEC 61508 Functional Safety Assessment for the DVC6000 SIS Digital Valve Controller in DETT [de-energize to trip] Applications. The digital valve controller has been evaluated and SIL rated when operated by 0-24VDC / 0-20mA as well as 4-20mA. The report includes the Failure Modes, Effects, and Diagnostic Analysis (FMEDA) of the mechanical, electronics, sensor, and related components of the digital valve controller.
A final misconception Riyaz wanted to address was the SIL level of solenoid valves (SOVs). Riyaz notes that Emerson’s ASCO brand has solenoids rated for use in up to SIL 3 applications. In an earlier post, Testing the Safety Valve in a Safety Instrumented Function, we highlighted how the health of the SOV can be monitored by the DVC6000 (see slide 24):
Riyaz gave this presentation, SIS “Final Element” Diagnostics Including The SOV, Using A Digital Valve Controller, at the 2010 Emerson Exchange in San Antonio. Hopefully these clarifications and the embedded presentation will help to address any misconceptions you have based on what you may have heard.