For pressures over 100 pounds per square inch, what material must be used for fittings between the boiler and valves?

For pressures exceeding 100 pounds per square inch, the correct material for fittings between the boiler and valves is steel. This is due to steel's high tensile strength and durability, which make it suitable for withstanding the high pressures and temperatures often found in boiler systems. As the operating pressure increases, the materials used in the plumbing and fittings must also be capable of handling greater stress without failing.

Steel fittings ensure that there is minimal risk of leaks or breaks, which could lead to severe safety hazards in a boiler system. Unlike other materials, such as brass, aluminum, or cast iron, steel provides the necessary strength and resilience required for high-pressure environments. While brass can be used in some lower-pressure applications, and aluminum and cast iron might be suitable for specific conditions, they do not meet the safety and performance standards required for fittings in high-pressure boiler systems.