Which type of pipe fittings should not be used for metallic pipelines and tubing?

Cast iron pipe fittings should not be used for metallic pipelines and tubing due to their brittleness and susceptibility to corrosion, especially in the presence of moisture and certain gases found in propane systems. When used in metallic pipelines, cast iron fittings can suffer from thermal expansion and contraction, leading to stress fractures or breaks.

In applications involving propane gas or other fuels, durability and the ability to withstand pressure are critical for safety. Brass, plastic, and steel fittings are typically more flexible and resistant to the corrosion that can occur in the environment surrounding gas systems, making them more suitable for this purpose. Hence, using cast iron fittings in these contexts can pose significant safety risks, as they may fail, causing leaks and potential hazards.