A connection may be leak-proof when the installation is hot and start leaking after it cools down. Thermal cycles cause expansions and contractions that affect flanges, gaskets, and threads.
Thermal expansion
As the temperature changes, the length of the piping and the forces on the connections change.
Gaskets
Certain gaskets may undergo relaxation or permanent deformation after repeated thermal cycles.
When to perform the check
In critical installations, re-checking in both hot and cold states may be necessary, according to the maintenance procedure.
Points of attention
Flanges near burners, regulators, or areas of high temperature fluctuations require increased attention.