The Mokveld axial flow shut-off valve complete with the Mokveld spring return piston actuator is the first valve to receive TÜV Certification for use as a Final Element in a Safety Instrumented System in compliance with IEC61508, up to Safety Integrity Level SIL3.

Although of course for any actual safety application the validity of the underlying data has to be checked, this TÜV report and certificate does validate the reliability data of the Mokveld Final Element.
The quantitative requirements for use in SIL3 (a Probability of Failure on Demand –PFD- better than 1*10-3), are fulfilled by a single Final Element, but Mokveld fully supports the qualitative requirement for using two Final Elements in a SIL3 loop.

When two Final Elements are used in a SIL3 loop, the very low failure rates of the Mokveld valves offer the possibility of selecting longer test intervals, increasing the availability of the process system. With the same two Final Elements also the SIL4 requirements are fully met.

Since the Final Element is only a part of a complete Safety Instrumented System, the Final Element can not consume the complete PFD of the Safety Instrumented System, because also other elements in the system can fail.

For a SIL3 system the total PFD should not exceed 1*10-3. Within this system-PFD also the initiators and voting logic need to be allocated a certain PFD. Therefore, TÜV has allocated 50% of the SIL3 budget to the Final Element. The PFD of the Mokveld Final Element as certified is 0,5*10-3 to allow use in a SIL3 system.

The TÜV Certificate and underlying report cover the important aspects such as:

• Field of Application
• Limitations of use
• Failure rates
• Distinction between clean and untreated medium
• Test intervals
• Default stroking time (2 seconds including instrumentation response time)