What may happen if a pipe or vessel full of liquid refrigerant is isolated?

Isolating a pipe or vessel full of liquid refrigerant can lead to a situation where pressure builds up due to thermal expansion of the liquid or heat transfer from the surroundings. In such cases, if the pressure reaches a certain threshold, the relief valve is designed to open to prevent excessive pressure buildup. This prevents potential hazards, such as the rupture of the pipe or vessel, which could occur if the pressure continues to rise unchecked. The relief valve thus serves a critical safety function in maintaining the integrity of the system and protecting against overpressure conditions.

The other options present scenarios that are less likely under the circumstances described. For example, while an explosion is a serious risk in some contexts, it is generally a more extreme outcome that would not be the immediate consequence of isolating a refrigerant system equipped with safety measures like relief valves. Additionally, liquid refrigerant may not evaporate if it is isolated in a sealed system under certain pressure and temperature conditions, and stating there would be no effect is misleading, as any system can experience changes in pressure over time.