What condition determines that two or more safety valves are required on a boiler?

Two or more safety valves are required on a boiler when the heating surface exceeds a specified area, primarily due to safety and regulatory standards that aim to prevent excessive pressure buildup. The specified area of 500 square feet is significant because it corresponds to the increased potential for pressure and steam generation in larger boilers. As the heating surface area increases, so does the risk of pressure exceeding safe operating limits, necessitating additional safety measures.

By requiring two or more safety valves for boilers with over 500 square feet of heating surface, safety regulations ensure that there is sufficient capacity to relieve pressure effectively. This is crucial because a single valve may not be able to release the built-up pressure quickly enough in high-pressure systems, which could lead to catastrophic failures. Therefore, this threshold helps maintain safe operational conditions in larger boilers by ensuring they are equipped to handle potential pressure scenarios adequately.