For boilers with bare tube heating surface over 500 ft², which of the following is required?

For boilers with a bare tube heating surface exceeding 500 square feet, the requirement for two safety relief valves is rooted in safety regulations and engineering practices designed to ensure the safe operation of high-capacity systems. Safety relief valves are critical safety devices that prevent excessive pressure buildup, which can lead to catastrophic failures.

When a boiler has a large heating surface area, the potential energy stored within the system is significant, and having only one relief valve could leave the system vulnerable in the case of malfunction or blockage. Two safety relief valves provide redundancy; if one valve fails to operate correctly, the other can take over to ensure that excess pressure is safely released. This is particularly important in maintaining the overall safety and integrity of the boiler system.

Other choices regarding sources of feedwater may pertain to operational efficiency and redundancy, but they do not specifically address the critical safety aspect that multiple safety relief valves do. Therefore, the requirement for two safety relief valves for larger boilers is a vital precautionary measure to protect against high-pressure hazards.