Question 7 3AE02 - Third Assistant Engineer (Alt)

The Correct Answer is C. ### Why Option C ("rising and falling with each stroke") is correct: The testing procedure involves isolating the compressor from the main high-pressure discharge line by turning the discharge service valve fully "in" (closed to the system, but opening the gauge port to the compressor discharge head). The high-side pressure gauge is now only monitoring the pressure within the compressor's discharge plenum (the space directly above the discharge valves). When the compressor is turned over by hand: 1. **Compression Stroke:** As the piston moves up, it attempts to push refrigerant out of the cylinder and past the discharge valves into the discharge plenum. If the discharge valves are **not** leaking, the pressure in the plenum will slightly increase as the gas is momentarily compressed and released. 2. **Return (Suction) Stroke:** As the piston moves down, the cylinder pressure drops, and the discharge valves should seat (close) to prevent high-pressure gas from flowing back into the cylinder. 3. **If the Discharge Valves are Leaking:** During the suction stroke, the poorly seated (leaking) discharge valves allow high-pressure gas from the discharge plenum (where the gauge is attached) to leak back into the cylinder. This sudden backflow momentarily **drops** the pressure shown on the gauge. During the subsequent compression stroke, the pressure is restored and might even rise slightly higher as new gas is compressed, causing the pressure to **rise**. This continuous cycle of leakage during the suction stroke and re-compression during the compression stroke causes the gauge needle to visibly **rise and fall (oscillate)** with each turn of the compressor's flywheel, confirming the leakage. ### Why the other options are incorrect: **A) decreasing with each stroke:** If the pressure were simply decreasing with each stroke, it would indicate a massive leak out of the system (perhaps at the service valve) or a complete failure of the suction side, rather than a valve leak *cycling* pressure between the cylinder and the discharge plenum. **B) decreasing to a vacuum:** The pressures being monitored are on the high side of the system (discharge side), which always operates above ambient pressure and usually far above zero psig. There is no mechanism in this test setup that would cause the gauge pressure to pull into a vacuum. **D) increasing with each stroke:** If the pressure were only increasing with each stroke, it would imply that the compressor is effectively building pressure against a closed system (which is what the service valve does) and that the valves are seating perfectly (no leakage back), simply compressing the gas more tightly into the discharge head space. This would indicate the valves are **good**, not leaking.