Cast iron fittings shall not be used in ______ piping.

• A. Oil
• B. Water
• C. Gas
• D. Steam

Answer: (A)

Understanding fluid compatibility and toughness is essential for piping design. Cast iron fittings are strong in compression but are brittle and have limited toughness under thermal cycling and shocks. Petroleum products in oil piping can attack cast iron joints and promote leaks, and the internal pressures common in oil systems require a material with greater ductility and resistance to fracture. For these reasons, oil piping typically uses steel or ductile iron fittings that can safely handle the hydrocarbons, pressures, and temperatures involved. In contrast, cast iron fittings are more common in some water or steam systems and are generally avoided for oil lines due to reliability and compatibility concerns. Gas piping follows its own material requirements focused on safe containment and corrosion resistance, not cast iron, reinforcing that cast iron is not the right fit for oil piping.