When overall loop performance, from system to final element, is quantified,
it is easy to see that the weak link of most safety loops is the potential nonoperability
of safety valves, since they alone account for roughly one half of
the probability of a dangerous failure. Emergency shutdown (ESD) valves
are the final line of defense and are critical to minimizing the chance of fire
or explosion during process upsets. As shutdown valves are rarely cycled,
however, there is always concern over whether they will operate if actually
needed. In fact, if these valves are not periodically stroked, it can almost be
guaranteed that they will not work when called upon