Safety Valve Positioners and Common Cause Failure Questions

If you work with pumps in your facility, you may be familiar with the Empowering Pumps site, a wealth of information to help you with these important assets in your plant. Founder Charli K. Matthews has launched something similar for valves, Empowering Valves.

I had the opportunity to contribute a guest post, Common Cause Failures in Safety Valve Positioners? It was based on a question I recently received. The question:

It’s unclear to me whether position feedback from a smart positioner is truly independent of the reference signal from the control system, as the positioner ostensibly uses that same information as a measurement in its own local position feedback loop (for which the reference signal is the setpoint). I’m guessing it’s not in most cases (and note that this trait is probably not unique to Emerson devices).

If you’re driving the valve to a certain position with the reference, and then using the position feedback to verify that the valve is actually at the position you drove it to, there is a potential common-cause failure in the position sensing and processing. For independence I’d think you would have to either use other means to drive the valve (e.g., a dump solenoid valve), or have position sensing distinct from that used by the positioner.

Riyaz Ali

Riyaz Ali

Emerson’s Riyaz Ali responded:

Common cause factor is a key concern when using a position transmitter within a safety valve positioner as is typically done.

Fisher FIELDVUE DVC6200 SIS digital valve controller

Fisher FIELDVUE DVC6200 SIS digital valve controller

In the case of a valve positioning transmitter designed for process safety applications, it is designed to isolate the positioning function. This design makes it completely independent of the positioner, should input signal or power to positioner fail, or any issue related to positioner cease functioning. The position transmitter continues to function to provide the valve’s position.

As part of the certification process for use in safety instrumented functions up to safety integrity level 2 (SIL 2), the position transmitter function is certified separately from the positioner.

Process manufacturers managing the safety lifecycle for their plants follow the IEC 61511 standard. They rely on the suppliers to provide technologies including safety shutdown valves, actuators, positioners, and positioning transmitters suitable for application in level of risk they are mitigating.

Read the full post over at Empowering Valves and consider joining their community of valve professionals.