What a Fire and Gas System Does
The three jobs of a fire and gas system — detect, alert and act — and where it sits among the other layers of protection.
11 min
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Three jobs, one system
A fire and gas system has three jobs. It must detect a hazard: flammable gas, toxic gas, smoke, heat or flame. It must alert people: the control room operator, people in the area, and people in buildings nearby. And it must act: shut down and isolate equipment, depressurise, start fire water, close ventilation dampers or release a fire suppression agent.
If any one of these jobs fails, the system fails. A detector that sees gas but whose alarm is lost among two hundred other alarms has not protected anyone. A system that alarms correctly but never closes the air intake dampers on the control building lets gas reach the people it was meant to protect.
Where it sits among the layers of protection
Process plants use several independent layers to prevent and limit harm. The basic process control system keeps the plant in its normal range. Alarms ask the operator to act. The emergency shutdown (ESD) system and other safety instrumented systems stop the process when limits are passed. Relief valves protect against overpressure.
The fire and gas system is different from all of these. It does not prevent the loss of containment. It acts after gas or fire is already present. Its purpose is to limit the consequence: to stop a small leak growing into a large fire, to stop a fire escalating to other equipment, and to give people time to escape. This is why engineers call it a mitigation layer, not a prevention layer.
Prevention and mitigation compared
Act before anything is released. Examples: process control, high-pressure trips in the emergency shutdown system, relief valves. They watch process variables such as pressure, level and temperature.
Why is a fire and gas system described as a mitigation layer?