Protective systems comprise process sensors, equipment condition or position sensors, logic performing systems, shutdown valves with actuators, interconnected wiring, tubing, air supply, hydraulic supply, power supplies, bypass switches, indicator lights, and horns. See Figure 1300-2 for a detailed drawing of an equipment shutdown system. There is a wide variety of equipment available that can perform each of these functions. The overall reliability of the system depends upon what type of equipment is selected, how it is installed, how well it is maintained and tested, and whether any redundancy has been provided.
Protective systems can be powered pneumatically, electrically, hydraulically, or by a combination of all of these. Pneumatic systems have the advantages of being simple, reliable, and easy to maintain, and of requiring no electrical power. For these reasons they have been used extensively in the past on offshore platforms as well as in refinery process plants. As plants get bigger and shutdown systems become more complex, pneumatic components can become cumbersome. Electronic components, including programmable controllers, have proven their durability under severe operating conditions. Redundant and triple-redundant systems continue to improve. Most of the recently installed large shutdown systems have been electronic, regardless of location, and use programmable controllers to perform the logic.