A reader of this blog came upon the post, When is a SIL Suitability Rating Required for Final Control Elements? The reader sent an email to the blog post author, Emerson’s Riyaz Ali, asking (of which I’ll paraphrase):
When is it possible to use control valves like final elements for SIS [safety instrumented system] application? I am looking for other useful documents and technical requirements when it is possible to use only control valves as the final element instead of on/off valves and not in combination with it in the SIF [safety instrumented function] application.
Riyaz shared back with me that this question is a common struggle for many process engineers. Generally, shutdown valves are not well designed, unlike control valves, where valve bodies have simpler geometrical designs. Rotary valves have splined shaft connections with appropriate trim/seating/sealing, etc.
As the trend toward digitalization continues to grow, more manufacturers and producers are moving towards a predictive maintenance strategy for the final element in a SIF. At the same time, these users feel the need to install and operate control valves to improve operational reliability and increase plant availability. However, a few constraints prohibit the use of control valves.
The major ones are:
- Cost Economy
- Size considerations (Compare to butterfly vs ball or ball vs globe valves, etc.)
- Maintenance philosophy
- Project budget, etc.
As Riyaz concluded in the blog post highlighted above:
If control valve is designated to carry out a safety function, then it should meet the SIL level of the Safety Instrumented System Function loop. In this case, failure rate numbers will be required to compute the total PFDavg of the loop. The end user may possibly ask for third party certification to comply with IEC 61508 requirements to meet certain SIL suitability.
However, if a control valve is designated for normal process control than as per IEC61511-3 part 1, section 3.2.3, Basic Process Control System, definition does not designate control valves to have SIL suitability.
The probability of failure upon demand (PFDavg) must be performed, including digital valve controllers to perform periodic Partial Stroke Tests to increase the diagnostic coverage factors used in the PFDavg calculations.
You can see some different architectures in a Chemical Engineering article back in 2011, Safety-Instrumented Systems: Control Valves as Final Elements. Another resource to assist you is to sign-up to download the Safety Lifecycle Management Workbook with the safety system planning, implementation, and management of the IEC 61511 / ISA 84 safety lifecycle.