Interlocks
Interlock circuits and their components should be designed to actuate the final devices (e.g. control valves) in the direction required to cause the process to fail-safe upon loss of power.
A good principle to follow is this: “every system is to fail to its lowest energy state, or to a state away from its critical operating limit”. In other words, each process should be analyzed to determine the major source of energy for operation, e.g. steam to reboiler, exothermic (i.e. heat releasing) reaction.
Decreasing the amount of energy reduces the risk of an equipment exceeding the design limits, or at least minimizes the potential damage if the limits are exceeded. For the reboiler example, a fail-safe design would trip the steam supply to the reboiler. In the exothermic reaction example, a fail-safe design would trip the feed to the reactor and/or the fuel to the reactor heater. The pressure control loop seen earlier is also another example of fail-safe design.
Protection systems should indicate that a demand to perform a safety function has been made and that the necessary actions have been performed.