Often confused · Type III section
The rupture disc on a low-pressure chiller
The rupture disc is the one-time over-pressure protection on a low-pressure chiller: a membrane that bursts to relieve pressure the shell wasn't built to hold. The pressure figures the exam attaches to it come from industry practice and exam answer keys, and the EPA rule text doesn't contain them.
Part of Type III section in the Study guide
What the disc is, and what it isn't
A low-pressure appliance uses a refrigerant with a saturation pressure below 45 psia at 104 °F, such as R-11, R-113, R-123 or R-245fa (40 CFR 82.152, eCFR as of October 5, 2026). It's built to work near and below atmospheric pressure, which is why its relief point sits at a pressure a high-pressure system would pass through without anyone noticing on a gauge.
Where 15 and 10 psig come from
Prep sheets often present both as EPA rules. They're an exam key / industry practice, not in the rule text: EPA's test topics name the maximum leak-test pressure for low-pressure appliances as a Type III subject without printing a value, and 40 CFR Part 82 gives none (eCFR as of October 5, 2026). Learn 15 psig for the disc and 10 psig for leak tests because the exam keys them. On a real machine, the manufacturer's data and instructions set the limits.
Where the disc meets the rest of Type III
The same low design pressure explains the chiller's other habits. Running in a vacuum lets air in through leaks, which is the job of purge units. Recovery ends at 25 mm Hg absolute, a level compared with drying vacuums on deep vacuum and dehydration. The full section is Type III low-pressure chillers, and equipment-room safety is on refrigerant safety.
Three pressures to keep apart
Low-pressure numbers and where each comes from
Pressures a Type III candidate needs, with their source
| Number | What it marks | Source |
|---|---|---|
| Below 45 psia at 104 °F | What makes an appliance low-pressure | 40 CFR 82.152 |
| 25 mm Hg absolute | Required recovery level for low-pressure appliances | 40 CFR 82.156(a), Table 1 |
| 0 psig | The level a low-pressure appliance is pressurized to for a non-major repair | 40 CFR 82.156(a) |
| 10 psig | Maximum leak-test pressure | Exam key / industry practice, not in the rule text |
| 15 psig | Rupture disc burst pressure | Exam key / industry practice, not in the rule text |
Rule citations: 40 CFR Part 82, Subpart F, eCFR as of October 5, 2026.
Questions on low-pressure relief
Note whether the chiller in each stem sits in a vacuum or above atmosphere before you choose.
0 right · 0 of 7 answered
A low-pressure chiller's rupture disk has blown. What should the technician do?
Why each option is right or wrong
Answer: A. Ventilate the area, replace the disk, then evacuate and recharge
- ACorrect: a blown disk can release refrigerant into the room, so ventilate first, then replace the disk, evacuate the air that got in, and recharge.
- BWrong: after the disk opens, air and moisture enter the chiller, so it has to be evacuated before it runs again.
- CWrong: charging a chiller with an open relief port sends the refrigerant straight back out; the disk goes in first.
- DWrong: adding refrigerant to hunt for the failure wastes it and releases more; the failed part is already known.
A blown rupture disk can release refrigerant into the machinery room, where the vapor displaces oxygen. Ventilate the space first. Then replace the disk, evacuate the air and moisture that entered the open shell, and recharge.
A technician replaces a low-pressure chiller's rupture disc. What hazard comes from treating a rupture disc as if it were a check valve?
Why each option is right or wrong
Answer: A. Losing required overpressure protection
- ACorrect: a rupture disc is a one-time overpressure relief device, not a check valve; treating it as one can leave the system without overpressure protection.
- BWrong: a rupture disc doesn't sit in the recovery vapor path.
- CWrong: subcooling has nothing to do with the rupture disc.
- DWrong: the disc doesn't create a pressure differential; it bursts only when pressure exceeds its setting.
A rupture disc gives one-time overpressure protection; it isn't a one-way flow device like a check valve. Treating it as one risks losing that protection.
During vapor introduction into a low-pressure chiller, the technician closely monitors system pressure. Why must the manufacturer-specified psig limits never be exceeded during this procedure?
Why each option is right or wrong
Answer: C. To prevent rupture disc failure and mass venting
- AWrong: desiccant in a drier doesn't depend on system pressure.
- BWrong: lubrication limits don't set the charging pressure ceiling.
- CCorrect: exceeding the manufacturer's limit can burst the rupture disc and release the charge.
- DWrong: subcooling isn't the concern; overpressure and the rupture disc are.
Technicians must never exceed specified psig limits during pressure testing or charging. Excessive pressure in low-pressure chillers easily bursts the rupture disc, causing total refrigerant loss.
What is the purpose of a rupture disk in a low-pressure chiller?
Why each option is right or wrong
Answer: B. To protect the system from excessive pressure
- AWrong: the chiller's normal operation keeps it in a vacuum; the disc does nothing to hold that vacuum.
- BCorrect: the rupture disk bursts at its set pressure to protect the low-pressure chiller from excessive pressure.
- CWrong: flow is controlled by the metering device and valves, not by a rupture disc.
- DWrong: filter-driers and strainers remove contaminants; a rupture disc is a pressure-relief device.
A rupture disk is a safety device that protects low-pressure chillers from excessive pressure by bursting at a predetermined pressure to prevent catastrophic failure of the system.
What can happen if a rupture disk activates on a low-pressure chiller?
Why each option is right or wrong
Answer: B. Refrigerant is released and air enters the system
- AWrong: nothing recharges the system automatically; a burst disc means a refrigerant loss that must be fixed by a technician.
- BCorrect: a burst disc vents refrigerant, and once system pressure falls back below atmospheric, air and moisture can enter through the opening.
- CWrong: compressor damage is not the direct result; the disc opens the refrigerant circuit to the atmosphere.
- DWrong: the disc releases refrigerant vapor from the system, not just oil.
If a rupture disk activates on a low-pressure chiller, the refrigerant will be released into the atmosphere and air will enter the system, requiring extensive service to remove contaminants and recharge the system.
A low-pressure chiller is operating below atmospheric pressure. What is the danger of replacing or testing rupture discs without following the manufacturer's safety sequence?
Why each option is right or wrong
Answer: D. Atmospheric air is rapidly drawn into the system
- AWrong: the recovery compressor isn't involved when replacing a disc.
- BWrong: in a vacuum there's no high-pressure vapor waiting to condense and fracture the tubing.
- CWrong: the purge unit doesn't lock out because of a disc change.
- DCorrect: a chiller in vacuum will suck in air and moisture through an open disc fitting if the sequence isn't followed.
Replacing or testing a rupture disc while the chiller is below atmospheric pressure, without the manufacturer's sequence, lets air and moisture rush into the system.
During a routine inspection, a technician discovers a chiller's rupture disc has an illegible set-pressure rating. What is the safest immediate response?
Why each option is right or wrong
Answer: A. Isolate the system and apply tags
- ACorrect: you can't confirm the disc protects the system, so isolate it and tag it out until the disc is replaced or verified.
- BWrong: running with an unknown relief rating risks an unprotected or premature burst.
- CWrong: overriding a relief device removes the safety protection.
- DWrong: pressurizing with an unknown relief setting may burst the disc or exceed safe limits.
If the disc's set pressure is unknown, isolate the system, tag it out, tell the owner, and get the correct replacement disc before it runs again.
More practice across all four sections: Practice test · Timed mock
Before you pressurize a chiller
Why heat a low-pressure chiller before using nitrogen for a leak test?
EPA's Type III test topics list hot water or a built-in heating device as the first way to raise pressure, with nitrogen after (EPA test topics, checked October 7, 2026). Heat raises the refrigerant's own pressure without adding anything to the circuit.
Can compressed air or oxygen be used to pressure-test a chiller?
No. EPA's Core safety topics call for nitrogen delivered through a pressure regulator with a relief valve, and they rule out oxygen and compressed air (EPA test topics, checked October 7, 2026).
Which refrigerants put a chiller in Type III?
Those with a saturation pressure below 45 psia at 104 °F, such as R-11, R-113, R-123 and R-245fa (40 CFR 82.152, eCFR as of October 5, 2026).