Shutdown valves in critical SIS applications.

Fisher™ Digital Isolation™ solutions are SIL-certified as full assemblies by exida and can improve critical performance specifications in safety instrumented systems by up to 50%. 

The first valve assemblies that meet the design process requirements of Safety Integrity Level (SIL) 3 per the International Electrotechnical Commission’s IEC 61508 standard have been introduced by Emerson. These Fisher™ Digital Isolation™ final element solutions serve the needs of customers for shutdown valves in critical safety instrumented system (SIS) applications.

Without this solution, users must specify all the individual valve components, procure each one, and assemble them into a working whole. Even if these steps are done correctly, this type of custom assembly will still not provide all the benefits of the Digital Isolation assembly.

“We were excited to work with Emerson to accomplish the first certified valve assembly,” said Loren Stewart, exida evaluating assessor for this project. “By providing a complete SIL solution for valve assemblies, Emerson is reducing unknown variables while helping ensure safety is always paramount.”

Engineering a safety shutdown valve is a complicated task. The normal and upset process conditions must be carefully evaluated and understood when choosing valve and actuator components. In addition, the proper combination of solenoids, brackets, couplings and other critical hardware must be specified and carefully matched to the selected valve. Each of these components must function individually and in concert to operate.

Emerson addresses these and other issues by providing an engineered Digital Isolation shutdown valve assembly, designed for each particular process. The various components are specifically selected to satisfy the application requirements. The entire assembly is sold as a fully tested and certified unit, with a single serial number and associated documentation delineating the details of every part of the assembly.

Because the assembly is built as a complete solution in Emerson facilities, it boasts a significantly improved probability of failure on demand (PFD) rate. In some cases, the failure rate of the assembly will be up to 50% less than the combination of the same valve components purchased individually and assembled by an end user.

This improved PFD results from the assembly being certified as a whole system by exida, a global leader in product certification and knowledge, to meet their rigorous Remote Actuated Valve Assembly requirements. By using this certified assembly, SIS designers may realise a substantial reduction in dangerous undetected failure rates (λDU).

In one example, an Emerson customer was hoping to achieve a six-year test interval for a SIL 2 safety interlock. When they performed a component SIL calculation on a standard shutdown valve, they could only achieve a SIL 1 level reliability at a six-year test interval. However, the Digital Isolation assembly exceeded SIL 2 requirements using the same components. This allowed the plant to safely extend their run time to six years, generating millions of dollars in additional revenue.

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