What occurs at 30% tank level in a mercury system?

At a 30% tank level in a mercury system, the correct action is for the duty pump to turn on. This is typically a part of the system's automated controls designed to maintain an adequate level of mercury in the tank. When the tank reaches this threshold, it indicates that the level is low enough that the system needs to actively pump mercury to avoid potential operational issues or to ensure the continuous functionality of processes that depend on the mercury supply.

The activation of the duty pump ensures that the levels are maintained without interruption. It is a safety and operational measure that helps prevent the tank from becoming too empty, which could lead to more significant problems, such as loss of pressure in the system or equipment damage.

In the context of this question, other options like the standby pump starting or low-level alarms sounding may also be part of the system’s overall management but would generally not be the primary action taken at exactly the 30% mark. For instance, the standby pump might be activated under different conditions or levels, while the low-level alarm serves as a warning rather than a direct response to initiate pumping. Thus, the choice of the duty pump turning on aligns correctly with operational protocols in a fluid management system.