What is the minimum steam drum pressure on an operating boiler?

The minimum steam drum pressure for an operating boiler is typically set to 85% of the boiler's operating pressure. This standard is established to ensure safe and efficient operation of the boiler system. Maintaining the steam drum pressure at this level helps to prevent issues such as water carryover into the steam system, which can lead to equipment damage or operational inefficiencies. Additionally, it allows sufficient pressure to maintain proper steam quality and to support the requirements of downstream processes or equipment.

In the context of the other choices, lower percentages would not provide the necessary pressure to ensure safe and efficient operation, and higher percentages exceed the minimum requirement and could lead to unnecessary stress on the boiler components. Overall, setting the minimum steam drum pressure at 85% strikes a balance between safety, efficiency, and reliability in boiler operation.