Which of the following fittings is NOT permitted for joining vacuum distribution pipeline systems?

In vacuum distribution pipeline systems, the use of fittings must ensure both the integrity of the system and its ability to maintain a proper vacuum pressure. Non-removable push-fit fittings are not permissible in this context primarily because they may not provide the necessary reliability for maintaining a vacuum. These types of fittings can be less secure than threaded, welded, or flanged connections, leading to potential failure points where air could enter the system, compromising its functionality.

On the other hand, removable threaded fittings, welded connections, and flanged connections are accepted because they offer secure, durable methods for joining pipes that can be easily assembled or disassembled when maintenance is required. These methods provide the necessary strength and sealing capabilities to handle the vacuum conditions effectively. Thus, while the importance of easy assembly and disassembly is recognized, it must be balanced with the requirement for the system to remain leak-free and operational under negative pressure conditions.