Save file captures a situation where a loop of what was originally approximately 40°C crude oil eventually freezes and breaks. I suggest an unstable model because the temperature of the crude oil in the pipe is (as seen in the save file): 41.4 > 42.6 > -26.9 > 55 > 36.4. I suggest liquid as the crude oil temperature abruptly changes from a normal temperature to a very cold temperature (for example, 42.6°C to -30.1°C as seen in the two screenshots).
If you wait long enough, the pipe will break. I have seen this happen a few times, but was never able to catch it in the act - until now!
Unknown; however, save file captures the issue as it is occurring