When must safety relief valves (SRVs) have their spindles oriented vertically?

Safety relief valves (SRVs) need to have their spindles oriented vertically when they are installed on power boilers. This orientation is critical because it ensures that the valve operates correctly under high pressure conditions typically present in power boilers. The vertical alignment allows for optimal flow and prevents any accumulation of water, which could impair the valve's functioning.

Power boilers operate under specific safety regulations that dictate the installation guidelines for safety equipment to ensure reliable performance and maintenance of safe operation standards. In this context, the vertical orientation of SRVs is emphasized to maintain their reliability and efficiency in preventing overpressure situations.

The other scenarios might allow for different orientations based on design and operational characteristics, but for power boilers, adhering to the vertical orientation of SRVs is essential to meet compliance with safety standards and ensure proper operation.