Which pipe types shall not be used unless allowed?

• A. Cast-iron pipe
• B. Copper pipe
• C. Stainless steel pipe
• D. Glass pipe

Answer: (A)

Materials for clean agent piping must be compatible with the agent and approved for use in the system by the authority having jurisdiction. Cast-iron is not suitable for fixed-discharge piping in most cases because its durability and compatibility with modern clean agents are limited. It tends to be brittle, can corrode or scale inside, and makes reliable, leak-tight joints harder to achieve with the types of fittings used in clean agent systems. Because of these drawbacks, cast-iron pipe should not be used unless a specific exception is granted by the AHJ or the system design explicitly allows it.

Copper and stainless steel are commonly used options when approved, offering good corrosion resistance and reliable joints. Glass piping is not standard for typical installations and would generally require special approval, but the key here is that cast-iron is the material that is not used unless explicitly permitted.