For how long may an operator be away from the boiler without risking safety?

The correct answer indicates that the safe time an operator may be away from the boiler is defined by the time it takes for the sight glass to go from half full to empty. This concept is crucial in maintaining safety and operational integrity in boiler management.

The sight glass provides a visual indication of the water level within a boiler. If an operator leaves the station while the water level is at half and does not return before it completely depletes, this could lead to a potentially dangerous situation. If the water level drops too low, it can result in overheating, boiler damage, or even explosion due to the lack of water to absorb the heat generated by the boiler. Therefore, the right choice emphasizes vigilance and the necessity of keeping a close eye on the water level to prevent any mishaps.

Other options may suggest arbitrary time frames based on guesswork or average circumstances, which do not sufficiently factor in the real-time operational dynamics of the boiler. The correct answer aligns with a practical and immediate assessment tool, ensuring that operators maintain a close watch over critical parameters affecting boiler operation.