A SIF description includes three valves. To protect against the hazard, valve one must be closed to

Question:

A SIF description includes three valves. To protect against the hazard, valve one must be closed to cut off the fuel supply from natural gas. Valve two must be closed to preserve heat in a process unit. To protect against the hazard, valve three must be opened to vent natural gas. Which valve(s) are to be included in the SIF verification?

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