Internal‑pressure bellows: medium inside bellows, inner liner inside, tie‑rods needed for large displacement.
External‑pressure bellows: medium outside bellows, nested sleeve structure, good anti‑instability, tie‑rods normally not required.

NBR rubber expansion joints fail in high-temperature thermal oil systems because thermal oil runs above 300°C, while NBR can only handle ~90–140°C. This extreme heat causes rapid oil-swelling (softening and seal failure) and thermo-oxidative aging (hardening and cracking). The joint quickly fails, leading to thermal oil leaks and fire hazards. Use stainless steel or specially insulated expansion joints instead.