When is more than one safety valve commonly used on a boiler?

More than one safety valve is commonly used on a boiler when it has over 500 sq ft of heating surface. This is primarily due to safety regulations and engineering practices aimed at ensuring adequate pressure relief capability. A boiler of this size produces more steam and operates at higher thermal conditions, which increase the risk of pressure build-up. Having multiple safety valves allows for a more reliable and effective way to relieve excess pressure, thereby enhancing the safety of the operation. This redundancy is crucial because it ensures that if one valve fails to operate due to mechanical issues or blockage, others can still function to prevent dangerous over-pressure situations.

In contrast, a boiler with under 500 sq ft of heating surface typically does not require multiple safety valves, as the pressure and steam production levels are usually manageable with a single valve. Similarly, while continuous operation and variable system pressures are important factors in boiler design and safety, they do not in themselves dictate the necessity for multiple safety valves as directly as the size and heating surface area of the boiler do.