What is the primary purpose of multiple safety valves on a large boiler?

The primary purpose of multiple safety valves on a large boiler is to prevent overpressure. Safety valves are critical components designed to ensure that the pressure within a boiler does not exceed safe operational limits. In the event of excessive pressure build-up due to factors such as equipment failure or abnormal operating conditions, these valves automatically open to release steam or water, thereby alleviating the pressure.

Having multiple safety valves provides redundancy and enhances reliability, ensuring that if one valve fails or gets stuck, others are available to perform the necessary function of pressure relief. This multi-valve setup is essential for maintaining the integrity of the boiler and for protecting personnel and equipment from the dangers associated with overpressure, which can lead to catastrophic failures.

The other choices, while relevant to boiler operation or functionality, do not emphasize the primary safety focus that the question highlights. Increasing heating efficiency, controlling water levels, and balancing heat distribution involve different mechanisms and equipment configurations that do not directly address the critical issue of controlling pressure, which is vital for operational safety.