Why must the pressure used to leak test a low-pressure appliance be kept to a very low ceiling?
Correct Answer
B) The machine's relief device is set only slightly above atmospheric pressure and will rupture
A low-pressure machine is a thin walled vessel designed to operate near atmospheric pressure, and it is protected by a rupture disc set just above that. Pressurize the shell past the disc rating and the disc bursts by design, venting whatever is inside and leaving the machine open until a new disc is installed. That is why nitrogen leak testing on these machines is held to a very low ceiling, far below what would be routine on a high-pressure system.
Why This Is the Correct Answer
The rupture disc protects a vessel built for near atmospheric service, so its rating sits only a little above atmospheric pressure. Any test pressure approaching that rating risks bursting the disc, which vents the machine, creates a release if refrigerant is still inside, and turns a leak test into a parts order and a shutdown.
Why the Other Options Are Wrong
Option A: The chiller tubes will collapse if shell pressure exceeds the water side pressure during the test
The tubes are not at risk from modest shell pressure in that direction, and a leak test does not load them. The relief device is the limiting component, not the tube bundle.
Option C: Higher pressure would drive refrigerant permanently into the compressor oil in the sump
Refrigerant does dissolve into oil, but that is a normal and reversible condition that boils back out. It is not why a low-pressure machine has a strict test pressure ceiling.
Option D: The purge unit cannot operate at pressures above the test ceiling once it is set
The purge unit's operating range is beside the point during a leak test, since the machine is shut down and the purge is not the component the pressure limit protects.
Memory Technique
Thin shell, thin margin. The disc pops long before a high-pressure machine would even notice.
Reference Hint
Low-pressure appliances carry rupture discs rated just above atmospheric pressure, which is why leak test pressure is strictly limited.
More epa-608-type-3 Questions
Some jobs on a chiller count as a major repair under the refrigerant rules, which changes what a technician is allowed to do with a badly leaking machine. Which job falls into that category?
An owner asks how many pounds of refrigerant the rules allow to be left inside a chiller once the recovery has been signed off. What is the correct answer to give him?
The motor of a hermetic centrifugal chiller sits inside the pressure boundary rather than outside it on a shaft. What follows from that construction?
An oxygen monitor is used to decide whether a chiller machine room is safe to enter after a refrigerant release. Below what oxygen reading is the atmosphere treated as oxygen deficient?
A low-pressure chiller's rupture disc has a discharge line running from it. Where does that line have to terminate?
A refrigerant vapor detector in a chiller machine room reaches its alarm level. Besides sounding, what is that alarm expected to do?
Which piece of equipment requires a technician to hold Type III certification before it may be serviced?
What property of the refrigerant is what actually places an appliance into the low-pressure category?
During normal operation, what condition exists inside the evaporator of a running low-pressure centrifugal chiller?
In what unit is the required final recovery vacuum for a low-pressure appliance normally expressed?
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