Which requirement typically applies to the relief valves on a boiler?

In the context of boiler safety and design, relief valves are critical components that ensure the safe operation of the boiler by preventing overpressure conditions. The requirement for relief valves to be installed with spindles vertical when on a power boiler is essential for their reliable and effective function.

When relief valves are positioned vertically, they are able to operate as designed, allowing pressure to escape in a manner that protects the boiler system. This vertical orientation reduces the likelihood of improper sealing and helps to ensure that the valve can open fully when needed. Additionally, this installation practice aligns with standard codes and safety regulations, which emphasize that relief valves must be installed in a way that facilitates their correct operation without obstruction.

Other choices do not align with the industry standards for relief valve installation. For instance, if relief valves were manually operated, it would pose a significant safety risk as they would require human intervention to function during emergency situations. Monthly calibration may be critical for certain types of equipment, but it is not specifically a standard requirement for every relief valve. The proximity of relief valves to the boiler, while important for system design, does not have a strict limit of 10 feet as a general rule. Thus, option D stands out as the most accurate and widely accepted requirement.