CARO Piping Systems and Valves 2 — Questions and Answers
Question 1: What is the purpose of a wet suction riser in an ammonia refrigeration system?
- To carry high-pressure liquid refrigerant to the evaporator
- To carry a mixture of vapor and liquid upward without trapping liquid at the compressor (Correct answer)
- To separate oil from liquid ammonia in vertical piping
- To equalize pressure between vessels at different elevations
Correct answer: To carry a mixture of vapor and liquid upward without trapping liquid at the compressor
A wet suction riser is designed to carry two-phase refrigerant (liquid and vapor) upward in the piping while preventing liquid slugs from reaching the compressor.
Question 2: Which valve type is best suited for throttling or regulating flow in a refrigeration system?
- Gate valve
- Globe valve (Correct answer)
- Check valve
- Ball valve
Correct answer: Globe valve
Globe valves are designed for throttling service because their disc and seat design allows fine adjustment of flow, unlike gate valves which are intended for full open or closed positions only.
Question 3: What does 'pipe schedule' refer to in refrigeration piping specifications?
- The maintenance interval for pipe inspections
- The wall thickness of the pipe relative to its nominal diameter (Correct answer)
- The flow velocity specification for the system
- The installation sequence for pipe sections
Correct answer: The wall thickness of the pipe relative to its nominal diameter
Pipe schedule designates the wall thickness of a pipe; higher schedule numbers indicate thicker walls and higher pressure ratings, which is critical for high-pressure refrigeration applications.
Question 4: Why is it important to pitch refrigerant suction lines toward the compressor?
- To increase refrigerant velocity and capacity
- To ensure oil and liquid refrigerant drain back to the compressor rather than trap in low spots (Correct answer)
- To reduce pressure drop in the suction line
- To prevent condensation on the outside of insulated piping
Correct answer: To ensure oil and liquid refrigerant drain back to the compressor rather than trap in low spots
Suction lines are pitched toward the compressor so that oil and any liquid refrigerant flow back naturally, preventing oil logging in the evaporator and liquid slugging in the compressor.
Question 5: What is a liquid seal in ammonia refrigeration piping used for?
- To prevent vapor from migrating from the high-pressure side to the low-pressure side through liquid piping (Correct answer)
- To lubricate valve stems in cold service
- To trap oil before it enters the evaporator
- To equalize pressure across the expansion valve
Correct answer: To prevent vapor from migrating from the high-pressure side to the low-pressure side through liquid piping
A liquid seal (liquid leg) prevents high-pressure hot gas or vapor from blowing back through liquid lines, ensuring that only liquid refrigerant enters the expansion device.
Question 6: When installing ammonia refrigeration piping, copper fittings must NOT be used because:
- Copper is too expensive for industrial applications
- Ammonia reacts with copper to form copper amide, which causes corrosion and system contamination (Correct answer)
- Copper cannot handle the low temperatures of refrigeration systems
- Copper fittings do not meet ASME pressure ratings for refrigerants
Correct answer: Ammonia reacts with copper to form copper amide, which causes corrosion and system contamination
Ammonia reacts with copper and its alloys to form copper amide, a corrosive compound that degrades fittings and contaminates the system, which is why only steel or iron fittings are used.
Question 7: What is the function of an oil drain valve on an ammonia refrigeration vessel?
- To release refrigerant pressure before maintenance
- To drain accumulated oil from the vessel back to the oil pot or drain point (Correct answer)
- To sample refrigerant quality for purity testing
- To equalize pressure between the vessel and atmosphere
Correct answer: To drain accumulated oil from the vessel back to the oil pot or drain point
Oil drain valves allow accumulated compressor oil, which is denser than ammonia and settles at the bottom, to be drained from vessels and returned to the oil management system.
CARO Piping Systems and Valves 2
What is the purpose of a wet suction riser in an ammonia refrigeration system?