What happens if the internal pressure of a low-pressure chiller is driven past the rating of its rupture disc?
- AThe purge unit automatically routes the excess non-condensables outdoors
The purge unit only handles the small quantity of non-condensable gas that collects in the condenser. It has no capacity to relieve an overpressure condition and no connection to the relief path.
- BA relief valve lifts and then reseats once the pressure drops back down
Reseating is what a spring-loaded relief valve does. A rupture disc has no spring and no seat, so once the diaphragm tears it cannot close again and the machine keeps venting.
- CThe compressor unloads and the machine coasts safely to a stop
Compressor controls protect the motor and the bearings, not the shell. On a machine that is shut down for service the compressor is not running at all, yet the disc can still be blown by careless heating or pressurizing.
- The disc bursts and the refrigerant charge escapes to the atmosphere
Why D is correct
Unlike a spring-loaded relief valve, a rupture disc does not reseat. It is designed to burst open and stay open, protecting the shell by dumping everything inside it. Because a low-pressure machine holds a large charge, blowing the disc during careless heating or pressurizing releases that whole charge at once.
What this question is testing
This item checks whether the candidate knows the difference between a rupture disc and a reseating spring-loaded relief valve, and understands what overpressuring a low-pressure machine during service actually costs. It also tests awareness that the hazard exists on a shut-down machine, since careless heating or pressurizing during a leak test is the usual cause.
On the job
This is why crews never put an open flame or uncontrolled heat on a low-pressure chiller, and why any warming done to raise pressure is gentle and closely watched with a gauge on the machine. A blown disc means the plant loses its cooling, the owner loses a charge that is expensive and hard to source, and the crew has to write up a release. Replacing the disc and recharging the machine turns a half-day leak hunt into a multi-day job.
Memory technique
A popped balloon does not un-pop: a rupture disc bursts once and stays open until it is replaced.
Exam tip
Rupture disc equals one shot. Any answer that has it closing again, resetting, or self-limiting is describing a relief valve instead.
Where to look it up
Low-pressure appliances are protected by a rupture disc, a non-reseating device; overpressure during service causes a full-charge release.