What should an operator have while blowing off a boiler?

The correct choice emphasizes the importance of monitoring the gauge glass level during the blow-off operation of a boiler. The gauge glass provides a visual indication of the water level inside the boiler. Maintaining an appropriate water level is critical for both safety and the efficient operation of the boiler. If the water level is too low, it can lead to overheating and potential damage to the boiler; conversely, if it's too high, it can cause water carryover into the steam lines, leading to operational issues.

Monitoring the gauge glass while performing a blow-off ensures that the operator can react promptly to any changes in water level, which could indicate a problem that needs immediate attention. This practice aligns with safe operating procedures for boiler management.

In contrast, options like opening or closing valves without attention to the gauge glass could potentially result in unsafe conditions or equipment damage. Therefore, the focus on the gauge glass reflects a critical safety measure during boiler operations.