JLSeagull,
Semantics can be important. "Fail Safe" as an engineering concept is not important, and is often worse than worthless. Designing a system that fails in the safest possible manner (taking into account both personnel safety and protection of equipment) is very important.
The analysis you alluded to above is exactly what needs to be done. The designer needs to look at every credible failure and determine how he wants actuated valves to perform during that failure. Every ESD that I've ever installed has been pressure to open, pressure to hold open, loss of pressure to close (I've always done it with the huge-spring kind of actuator). That way you don't have an operator standing in a fire trying to pump a manual actuator. But that is just one facet of the analysis. How do I want the solenoid that supplies gas to the actuator to fail on loss of signal? Loss of power? Does it need to just close or does it need to vent downstream? I say that the answer to every one of these questions is valve-specific and the PHA (or one of the less formal alternatives) needs to address all of these questions instead of being a PSV-Minette that is so scripted that no one would consider asking a question that was not on the script.
Although a Lot of discussion and commentary has been in this thread by many of the forum giants.
I add my little bit to it;
The concept is basically meant to define certain condition(s) for some solitary valves as regards
1)their position(s) 'Open' or 'shut' when the failure of some particular nature occur;the scenario is considered to yield safer process conditions.
2)sometimes Multiple signal actuation scheme(s)are supplemeted to take care and ensuure that valve's operation(almost)never fails;this also has its own limitations.but may be nick named as 'Fail safe'strategy.
3)there is an overall Mega level Hazop,Hazan,What-If and Failure mode analysis supported and Fully PLC,DCS based Highly beefed-up overall risks minmization strategy.
The jargon of 'Fail-safe' might be somwhere in here but could not be pin-pointed as such for any particular &specific valve under all conditions and circumstances
