What should be done before removing a manhole cover?

Before removing a manhole cover, it is crucial to ensure that the boiler shell is vented and drained. This step is essential for safety, as it helps release any trapped gases, steam, or pressure that may be present within the boiler. Proper venting minimizes the risk of an explosive release of steam or other substances during the manhole cover removal process, thus protecting personnel from injury and preventing damage to equipment.

Additionally, while the other options may involve safety practices in different contexts, they do not directly address the specific hazards associated with accessing the interior of a boiler through a manhole. Ensuring that the pressure or hazardous substance risk is mitigated is the priority when preparing for this task.