Emerson Exchange

Valve selection for safety instrumented systems

Emergency shutdown systems demand more than a SIL rating from their valves.

By Jim Montague

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One technical area that can help safety instrumented systems (SIS) fulfill their missions is selecting the right SIS valves, and implementing their diagnostic functions and support software. “Why do we need SISs? To keep plants out of the news,” says Tommy Elkins, product manager, ControlWorx services division, John H. Carter Co.

Janine McCormick, refining industry manager, Emerson Automation Solutions, reported that the two main process sector standards for SISs are IEC 61508, the umbrella safety standard passed in 2010 for manufacturers and devices suppliers, and IEC 61511, the process industry standard for SIS designers, integrators and end users. For valves, it’s most important to define the appropriate safety integrity level (SIL). “It’s also important to reach out to a certified process safety professional,” added McCormick.

McCormick and Elkins presented “How Valve Selection Can Impact SIS in Refining Applications” on the third day of Emerson Global Users Exchange 2016 in Austin, Texas.  

“The first task in SIS valves is selecting the right valve for the application,” said McCormick. “You must consider valve type, materials, fluid challenges such as particulate, corrosion, erosion, cavitation, noise, etc., and determine if the valve is SIL-capable to the level you need. In addition, just because you picked the right valve and all related components are certified SIL-capable, that doesn’t mean they’ll all work well together. Often, there’s a different supplier for each component—valve, actuator or positioner—plus an integrator to put it all together, and this could introduce the possibility for error. The ultimate bottom line is: Is my valve going to move when I need it to?”

Partial stroke testing

Because SIL capability begins to slowly deteriorate as soon as a valve is installed, and users can’t just pull and fully test large valves as often as they’d like, Elkins reported that suppliers such as Emerson have developed proof-testing and partial stroke test (PST) methods to bring valves close to their originally verified capabilities. These methods can detect: seized or tight valve packing, crimped or blocked actuator air line, valve stem sticks, valve seat scars, debris in seat and plugged seat. Other failures include increased friction indicated by a travel deviation alert; broken shaft connection indicated by a visible change in valve signature from decreased friction band; increased breakout force required; and air line clogging indicated by supply droop detection.

To handle unusual problems—such as an outboard shaft moving but having no effect because their plug/shaft connection is broken—McCormick added that Emerson can deploy FIELDVUE SIS DVC diagnostics for shaft integrity. “If a low torque alert comes up during a PST, the user knows to question the shaft integrity,” she added.

To improve valve selection for SIS applications, McCormick recommended, “Be sure to review application needs, such as temperature, pressure, fluid, erosive and corrosive, when selecting the final control element, not just SIL capability requirements. Evaluate whether to consolidate to one vendor for SIS valve solutions, or if that’s not the right path, do you feel that your vendors have good communication to get the solution right the first time? Do you need to add some buffer time in case something doesn’t mount quite right? Finally, review how you’re testing your valves. If it’s a visual exam, is that telling you everything you need to know, or would it be helpful to add a positioner to capture diagnostics? Also, are you leveraging the diagnostic capabilities of your DVC?”

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