Ammonia Leak Alarm in the Machine Room at Shift Change
At 05:55, during shift change, the fixed ammonia detection system in the machine room alarms at 25 ppm on the low-level sensor near the compressor skid. Two operators are in the room. One smells ammonia and sees a faint white plume near a flanged joint on the high-pressure liquid line. The room has a mechanical exhaust fan interlocked to the detector, but the fan is on a manual-off switch that the night operator turned off an hour earlier because it was noisy. The day supervisor arrives and wants to send the junior operator to tighten the flange with a wrench while the plant keeps running.
1. List, in order, the immediate actions you would take, and identify the unsafe instruction in the supervisor's plan.
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Immediate actions, in order:
- Treat the alarm as real. Do not investigate alone; both operators move to a safe position and one accounts for personnel.
- Start or restore the mechanical exhaust (the fan is off on a manual switch, which defeats the interlock) and confirm it is running.
- Notify the supervisor and follow the site emergency plan; evacuate or shelter as the plan directs, and keep people out of the plume path.
- Don respiratory protection only if trained and fit-tested; otherwise do not enter to tighten anything.
- Isolate the leaking section if it can be done from outside the hazard (close valves, pump down, reduce pressure) before any repair.
The unsafe instruction is sending a junior operator to tighten a live high-pressure flange while the plant runs. That exposes an untrained person to ammonia at pressure, risks a sudden release and a face/eye injury, and tightening a leaking flange under pressure can worsen the leak. Repair only after isolation, depressurisation, and with proper PPE and a second person.
2. The exhaust fan was switched off manually. What does this tell you about the safety system, and what should change?
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The detector interlock is only as good as the fan it controls. A manual-off switch that lets an operator disable the exhaust removes the ventilation the alarm depends on, so the room can accumulate ammonia while the detector is the only warning. That is a design and management-of-change problem, not just an operator error.
The fix is to remove or control the manual-off (key switch with permit, or alarm on fan-off), verify the interlock actually starts the fan on alarm, add a fan-running status to the alarm panel, and retrain on why ventilation is not switched off for noise. Noise should be fixed at the fan, not by defeating it. Also check whether the 25 ppm alarm should have escalated given a visible plume.