scotch yoke

Look at the picture. What do you see? I see a spring return actuator with

  • a diaphragm chamber
  • a spring chamber
  • the scotch-yoke mechanism

What are the high level potential failures for a ball valve?

  • Scotch-yoke mechanism breaks, can no longer operate valve (dangerous)
  • Spring breaks/spring cap breaks, spring will no longer drive valve stem (dangerous)
  • Leak in diaphragm/chamber housing, pressure drops, spring will move connected valve (TBD)

What is the rough definition of a safe failure? A failure that causes a spurious trip, i.e. a failure that initiates the safety function without a demand from the process. Does anybody disagree that if the pressure drops in the diaphragm chamber that the spring will move the connected valve to the safe position (assuming de-energize to trip, why would you otherwise use a spring return actuator)? Doesn’t this mean that the spring return scotch yoke actuator has safe failures?

If so, then why do end-users and engineering companies blindly accept a third party certificate where the spring return scotch-yoke actuator is supposed to have NO safe failures? The information on the certificate does not make any sense. If the analysis that resulted in the certificate does not make sense, why would you trust any data on the certificate? Is it because the claimed dangerous failure rate is an order of magnitude lower than exida concluded for a similar device through its predictive analysis methodology?

In this particular case, reviewing the spring return scotch-yoke actuator does not yield any indication why this specific device would be any better than other devices we have analyzed, definitely not an order of magnitude. There is definitely no indication why this device should not have any safe failures.

Before you use any information from a certificate, ask yourself if it makes sense. If it does not, don’t use the information, you are fooling yourself. You are calculating results based on bogus input. Garbage in is garbage out. Check out exida’s Safety Automation Element List for an overview of safety certified equipment with adequate failure information from exida as well as other 3rd party assessors.


Tagged as:     Safety Automation Element List     Safe Failures     SAEL     Iwan van Beurden     exida  

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