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

• A. Welded
• B. Axially swaged
• C. Memory metal
• D. Non-removable push-fit

Answer: (D)

Vacuum distribution pipelines must have joints that are permanently sealed and able to withstand continuous negative pressure. Welded joints fuse the tubing into one solid piece, providing a true, leak‑free connection with no disassembly risk. Axially swaged joints form a tight, lasting bond by cold-forming the ends of the tubing to fit together, which also yields a permanent, leak‑tight seal. Memory metal joints use shape‑memory alloys to create a durable seal that remains secure under the temperature and pressure cycles typical of vacuum service.

Non-removable push‑fit fittings, even when described as non‑removable, rely on a mechanical grip and seals that can creep, loosen, or leak under vacuum and over time, and they complicate sensing and repairing leaks. For vacuum systems, these kinds of joints do not meet the requirement for a permanent, leak‑tight connection, so they are not permitted.