In Part 1, we covered the emerging trends in the valve assembly market, how these trends relate to IEC 61511 requirements, and the anatomy of final elements. In Part 2, we discussed the need to show requirements traceability down through the design of the safety instrumented function and then the specific devices in the final elements. In this last installment, we will go over what typical mechanical design documents look like and what the exida team looks for in those documents.

Typical Mechanical Design Documents 

Typically, mechanical design documents will be ISO certified. There will be a written design procedure, a requirement specification, and a design file that has drawings. 

Questions that are raised include: 

• How do they manage their suppliers? 
• How do they integrate the valve and actuator?
• How do they test it?

This can vary, which could be why more process owners are coming back and asking for a certified assembly; there are areas where a documented design process needs to be used.

While a semi-formal or reputable valve integrator may be OK for general process requirements, it doesn't meet the rigor and traceability that IEC 61508 requires. Once you get those valves in the plant, it’s very hard to go back and collect that evidence.