Which type of fittings shall not be used on supply or return lines unless certain conditions are met?

In the context of plumbing and fuel gas systems, compression fittings are designed to create a seal through the compression of a ring against the pipe. While they are commonly used in various applications, there are specific conditions where their use on supply or return lines may be limited or prohibited.

The reason compression fittings should be approached with caution relates to their potential for becoming less secure over time, especially in high-pressure systems or in situations where vibration is present. In these environments, the integrity of the connection can be compromised, leading to leaks. For this reason, the codes often outline that compression fittings should be used only if specific conditions—like being installed in locations where movement and vibration are minimal—are met.

This highlights the importance of understanding the implications of various fittings in system design and maintenance. The use of compression fittings might be restricted in certain situations due to concerns over reliability and safety, emphasizing the need for alternatives that are better suited for high-pressure or fluctuating conditions.