In this blog series, I will cover the emerging trends in the valve assembly market, how these trends relate to the IEC 61511 requirements, and what's in a final element. When you look at the different devices that go into some of these safety systems, the final element becomes quite complex.

I will also cover requirements traceability, the verification and validation of that final element, and practical paths to show compliance in a real environment.

Emerging Trends

Over the past 30 months, exida and OEM equipment suppliers have seen a marked increase in the number of requests for Certified Valve Assemblies. Five years ago, I would occasionally hear an OEM say “my customer just asked me if we had a certificate for this whole assembly”. Today, the request is a lot more frequent.

I think it reflects that, as the market is maturing, more process owners are looking at what they need to do to be fully compliant with IEC 61511.

These requests are coming from EPC and Process Owners for specific valve assemblies related to specific projects. This indicates that Process Owners are looking to satisfy the requirements of IEC 61511 that relate to the integration and validation of final elements.

It might seem like overkill. Is it really necessary to buy a certified valve and a certified actuator? When we look at what goes into these final elements, you'll see quite a bit of complexity, which leads to the requirement within 61511 to demonstrate that the equipment used in these safety systems will function correctly.

What is Required by the Standards?

Take a look at this graphic. Is the assembly on the left-hand side equivalent to what might come fully integrated on the right hand side?

The answer is -- it absolutely can be equivalent.

In fact, sometimes, it can be better. But, there is a need for visibility, transparency, and traceability for what gets installed at the plant and run. Does it meet all the functional requirements of the safety requirements spec? Can it be audited? Does it demonstrate what it needs to do?

There's no prescriptive path in the standard saying that you must buy an integrated valve assembly or you can't do certain things, but there are requirements to show that traceability, and to show a reasonable amount of evidence for verification and validation that the system will function correctly.

IEC 61511 Requirements

When we look at 61511, we can pull out some areas that, looking at a complex final element, we’ll touch upon.

  1. SIL Verification -  Anything that's part of the safety critical path has to be included in that SIL verification calculation. We'll dig into each of these bullets a little more.
  2. Design -  Anything used in a safety instrumented function has to be assessed to 61508 or justified based on prior use. Depending on the complexity of some of these things, that may not be covered.
  3. Assembly/Integration - Third is the assembly and integration of that final element as requirements, validating that it's working correctly functionally.
  4. Functional Validation - Finally,  being able to show the evidence at functional assessment number three. That is also a specific requirement.

Anatomy of Final Elements

This blog is mostly directed at the process industry, so we're primarily going to talk about valves and actuators, but it could be extended to other applications as well.

When we think about the final element, what immediately comes to mind is the valve, the actuator, and probably some type of control device like a solenoid. That's the minimum you need in your final element, but most times, there's additional things.

You're certainly going to need some type of integration in the sense of a mounting bracket, which may or may not be part of the valve assembly, and in coupling devices.

Sometimes, some OEMs have these couplings and these connector devices included in their valve or actuator. They would then be part of the valve actuator assessment. However, many don't. There's a stem that could be connected to an actuator stem, but then you need an independent way of coupling it. Or you need the mounting bracket or you need the hardware to do it.

Understanding if all those parts are included in the OEM scope is important. Beyond that, there can be things such as feedback devices for automated testing or proof testing. There can be functional devices like quick-exhaust valves or volume boosters to make sure that the assembly is moving quickly enough. There can be double-acting actuators, or hydraulic / pneumatic-assist return actuators.  In that case, the hydraulic supply or the pneumatic supply needs to be considered as well.

A final element can have quite a few things in addition to the valve and actuator.

In addition to making sure we have the right parts included, we need to be able to show requirements traceability down through the design of the safety instrumented function and then to the specific devices in the final elements.  I will cover that in part 2.

Tagged as:     valves     SIL verification     IEC 61511     IEC 61508     Chris O'Brien  

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