During an accumulation test, the boiler is fired at which of the following?

The correct choice is based on the requirement to assess the boiler's ability to safely handle and discharge steam during an accumulation test. During this test, the boiler is subjected to a maximum firing rate with the main steam stop valve closed. This process allows for the measurement of pressure buildup, ensuring that the safety valves activate at the correct pressure settings.

Using the maximum firing rate is crucial because it demonstrates the boiler's capacity under extreme conditions. By keeping the main steam stop valve closed, the steam produced cannot exit the boiler, which leads to a rapid increase in pressure. Observing this behavior ensures that both the boiler and its safety mechanisms function properly, giving confidence in the equipment's safety and reliability under operational stress.

Other mentioned firing rates, such as minimum or low, wouldn't adequately test the safety mechanisms or the performance of the boiler under maximal stress conditions. Therefore, firing at maximum capacity with the steam stop valve secured provides the necessary data for safety compliance and operational validation.