To protect a large compressor against excessive compressor discharge pressure we could use:

• A. A fusible plug
• B. Either a fusible plug or a rupture disc
• C. A spring-loaded pressure relief valve
• D. A magnetic check valve

Answer: (C)

The idea being tested is overpressure protection that can respond automatically and repeatedly. A spring-loaded pressure relief valve is designed to vent gas when the discharge pressure exceeds a preset limit and then reseal once pressure drops back down. This makes it ideal for a large compressor, where you want ongoing protection without needing to replace parts after each relief event. The spring holds the valve closed until the set pressure is reached; when released, the valve opens to relieve, and as pressure falls, the spring re-closes it, ready for the next surge.

Fusible plugs rely on temperature and aren’t precise for controlling pressure, and they’re not resettable in normal operation. Rupture discs provide a one-time release and must be replaced after each event, so they don’t offer ongoing protection. A magnetic check valve prevents backflow rather than relieving excess pressure, so it doesn’t serve the same protective purpose.