5.2 Detection, Ventilation, and Emergency Shutdown

The Detector Ladder Is Already Designed — Your Job Is the Human Response

Quick Answer: Use the ANSI/IIAR 2-2021 numbers you already learned, and turn them into operator actions. 25 ppm: investigate, restrict unnecessary entry, find the source. ≥150 ppm (½ IDLH): emergency ventilation is latched; do not enter without PPE and a written plan. Upper detection limit or 40,000 ppm (25% LFL), whichever is lower: refrigerant compressors and pumps shut down; emergency ventilation stays on. Outside the principal machinery-room door you have a refrigeration e-stop and an emergency-ventilation ON/AUTO switch. Do not kill the fans with the refrigeration e-stop.

Chapter 2 taught the design ladder: at least two detectors with identical sensing ranges, audible and visual alarms inside the room and outside each entrance, 25 ppm to a monitored location (may auto-reset if concentration falls), ≥150 ppm visual/audible plus latched emergency ventilation, and the high-range trip that also stops refrigerant rotating equipment. This section does not invent new setpoints. It answers, what does the person in charge do when those setpoints hit?

25 ppm — investigate and restrict

Twenty-five ppm is not a shrug, and it is not a bunker alarm. It is the concentration where IIAR 2 requires notification so someone can act, and it matches the NIOSH REL TWA and ACGIH TLV-TWA you just labeled. Operator response:

• Treat the alarm as real. One detector in alarm is a leak until a competent check says otherwise. Do not wait for the second detector, the smell, or a second shift to confirm.
• Restrict the room. Stop casual walk-throughs, contractors grabbing tools, and truck drivers cutting through the engine room. Investigation is not a tour.
• Account for people. Who was inside? Who might be in a pit, penthouse, or attached locker? The 25 ppm event is how you find out someone is missing before the reading is 200 ppm.
• Investigate with a plan. Use the plant's leak procedure: portable detector, buddy or radio check-in, correct PPE for the expected concentration, and an isolation path you can run without standing in the cloud. Look high and low.
• Do not reset and walk away because the 25 ppm alarm may automatically clear when the reading dips. Auto-reset is a design feature of the Level 1 warning, not proof the packing stopped leaking.

If the portable instrument is already climbing through tens of ppm toward 100 ppm while you are still in street clothes, you are no longer in an investigation. You are in a withdraw decision. 25 ppm is the start of control, not a license to linger.

150 ppm — emergency atmosphere, ventilation latched, no casual entry

150 ppm is one-half of the 300 ppm IDLH. IIAR 2-2021 requires visual indicators, an audible alarm, and emergency ventilation, then latches emergency ventilation and visual indicators until manual reset from the machinery-room reset switch. Operator response:

• Believe the latch. Emergency fans do not get to stop because someone thinks it smells better. They run until a person resets them under the procedure — not because the trend briefly dipped to 149 ppm.
• Do not enter without PPE and a plan. 150 ppm is not IDLH, but it is on the doorstep, the atmosphere is uncontrolled, and a liquid spray can jump the reading into the IDLH range in seconds. SCBA (or an equivalent supplied-air plan with escape) is the IDLH/unknown tool. A cartridge respirator is not a ticket to wander a 200 ppm room hunting a hiss.
• Keep people out. Entrance horns and strobes mean do not open the door to see. Rescue is a trained, equipped function under the emergency plan, not a hero entry.
• Confirm emergency ventilation actually started. Look at the ON/AUTO switch position, fan status lights, and discharge. A latched command that never spun a fan is a failed safeguard, not a successful alarm.
• Non-emergency fans are designed to de-energize at 150 ppm unless they are part of the emergency system. Do not restart a comfort fan to clear the room by hand.

The CIRO trap is treating 150 ppm like a loud 25 ppm. It is not. 25 ppm is investigate/restrict. 150 ppm is an emergency ventilation event and a do-not-enter-without-PPE event.

High-range — shutdown of refrigerant machinery, fans stay up

At the detector's upper detection limit or 40,000 ppm (25% LFL), whichever is lower, IIAR 2-2021 requires emergency ventilation and shutdown of refrigerant compressors and pumps (and the specified normally closed automatic refrigerant valves). 40,000 ppm is 4% by volume — 25% of LFL as IIAR 2 states it — and it is not 25 ppm, not 150 ppm, and not the OSHA-cited 15–28% flammable window itself. Operator response:

• Expect rotating refrigerant equipment to be down. Screws, recips, and refrigerant pumps should be off. If they are still running, the shutdown circuit failed — that is now part of the emergency, not a debate about production.
• Expect emergency ventilation to be running. High-range is a richer cloud, closer to a flammable mixture. Dilution and exhaust are still the design intent. If someone already hit a miswired master kill and the fans are dead, you have a flammable-and-toxic box with no exhaust.
• Do not enter. This is well above IDLH. Entry is emergency-response PPE and the plant emergency plan, not operations curiosity.
• Do not use the high-range trip as your first shutdown. If you can safely stop a leak from outside at 40 ppm, you do that at 25 ppm investigation, not after the room is at percent-level ammonia.

Two switches outside the principal door — do not wire them as one

IIAR 2 places clearly identified emergency controls outside and adjacent to the designated principal machinery-room door:

1. Refrigeration emergency shutdown (e-stop). Off-only control of refrigerant compressors, refrigerant pumps, and normally closed automatic refrigerant valves in the machinery room. Clearly marked, readily operable, guarded against accidental bumping, manual reset. It is not a room-wide electrical kill. Lighting, controls that must stay alive, and — critically — emergency ventilation are not what this handle is for.
2. Emergency ventilation switch. ON / AUTO. ON forces emergency ventilation regardless of detector state. AUTO returns control to the detection system. Signage must say what the switch does. This is the handle a responder uses without entering a 200 ppm room when they need fans now.

Do not kill ventilation with the refrigeration e-stop. That is a design rule and an operator rule. A supervisor who slams the red mushroom and watches the exhaust fans coast down has made the room worse: ammonia stays, ignition sources may still be present, and the next person who opens the door walks into a richer cloud. If a contractor landed both functions on one breaker so it is simpler, that is a finding, not a convenience.

How a CIRO uses the pair in a real leak:

• Need the screws and liquid pumps stopped from outside? Refrigeration e-stop.
• Need the emergency fans running from outside, even if detectors have not yet latched, or you do not trust AUTO? Ventilation ON.
• Event over, atmosphere verified, procedure complete? Reset per the written procedure. AUTO is not a silent I hope it is fine. The 150 ppm latch still needs the in-room manual reset for ventilation and visual indicators; do not sneak in to slap the reset while the portable still reads 180 ppm.

Ventilation that operators can fail even when the design is correct

Emergency rate in IIAR 2-2021 machinery rooms is 30 air changes per hour on gross volume unless a documented analysis supports less. You do not re-calculate that during a leak. You keep the path open:

• Do not park a trailer against the exhaust stack or chain the makeup louver shut for winter comfort.
• Do not block the principal door so the outside switches are unreachable.
• Do not pull fuses on the emergency fan because it was noisy during a detector test and nobody put them back.
• Makeup air must replace exhaust. A room that sucks doors shut may be starved; a room that dumps exhaust into a dock canopy is ventilating the dock, not the neighborhood.

Normal/occupied ventilation is a different mode. Comfort fans and the emergency system are not interchangeable. At 150 ppm, non-emergency fans drop out. Your remaining airflow should be the emergency system.

Supervisor script when the horn sounds

1. Read the concentration and which zone. 25 ppm on a remote penthouse is not the same event as 160 ppm in the engine room, but both are real.
2. Restrict access. Entrance strobes mean stop inbound traffic.
3. At 25 ppm, start the leak procedure: PPE appropriate to the reading, portable instrument, isolate from the safe side, notify the monitored location if they do not already have the alarm.
4. At ≥150 ppm, assume emergency ventilation must be running and latched. Do not enter without PPE and the emergency plan. Verify fans from outside. Use ventilation ON if they are not running.
5. If rotating refrigerant equipment should be down and is not, use the refrigeration e-stop — and confirm the fans did not die with it.
6. High-range or unknown IDLH: SCBA-level response, plant emergency plan, no freelance packing changes.
7. After the event, do not treat auto-reset of a 25 ppm warning as a root-cause analysis. Find the leak. The 150 ppm latch is reset only when the procedure says the room is ready.

That is the human side of a ladder the standard already published. Memorize the numbers from IIAR 2; grade yourself on whether the fans were still turning after the e-stop.