Double jeopardy

[ernesto1962]

Dear Sir* API 521 states:
4.2.3 Double Jeopardy
The causes of overpressure are considered to be unrelated (i.e. independent) if no process or mechanical or
electrical linkages exist among them or if the length of time that elapses between possible successive occurrences of
these causes is sufficient to make their classification unrelated. The simultaneous occurrence of two or more
unrelated causes of overpressure (also known as double or multiple jeopardy) is not a basis for design. Examples of
double jeopardy scenarios are fire exposure simultaneous with exchanger internal tube failure* fire exposure
simultaneous with failure of administrative controls to drain and depressure isolated equipment* or operator error that
leads to a blocked outlet coincident with a power failure. On the other hand* instrument air failure during fire exposure
may be considered single jeopardy if the fire exposure causes local air line failures. See 4.2.6 for additional guidance.
This standard describes single jeopardy scenarios that should be considered as a basis for design. The user may
choose to go beyond these practices and assess multiple jeopardy scenarios* particularly for severe consequence
events. Because such assessments are outside the basis for design* the user is not required to meet accumulations
allowed by the pressure design code for these scenarios. Acceptance criteria are the sole responsibility of the user.