Which type of pipe is prohibited for use underground?

Threaded pipe and fittings are prohibited for use underground primarily due to the risk of corrosion and leakage over time. In underground environments, where moisture can accumulate, threaded fittings may not provide the same level of sealing and durability as other types of connections.

Corrosion can gradually wear down the threads, weakening the integrity of the connection and increasing the likelihood of failure. Furthermore, any leaks that occur at these threaded joints can lead to significant safety hazards, especially when dealing with compressed natural gas, where the potential for explosions or harmful emissions exists.

In contrast, other options listed—such as plastic pipes, flexible hoses, and steel pipes—tend to have better resistance to environmental factors, or are designed specifically for underground installations. Understanding the properties and limitations of various pipe materials is crucial for ensuring safe operations in the CNG delivery industry.