Which of the following materials is prohibited for use in CNG service?

In the context of Compressed Natural Gas (CNG) service, the use of plastic tubing is prohibited due to its inability to withstand the high pressures associated with CNG systems. Plastic materials can degrade over time when exposed to natural gas, potentially leading to leaks, failures, or hazardous situations. Additionally, plastic may be affected by environmental factors like UV light and temperature, which can further compromise its integrity under pressure.

In contrast, materials such as steel piping, brass fittings, and stainless steel tubing are commonly used in CNG applications because they possess the strength and durability required to handle high-pressure environments safely. These materials are designed to withstand the stress and are less susceptible to deterioration when in contact with natural gas, making them suitable for both the infrastructure and vehicle components involved in CNG service.