Many fluids are pumped close to the temperature at which they will vaporize, solidify, coke, crystallize etc. It is critical that you determine the desired operating range for the fluid before you make any attempt to alter it. I Number 1 of this series explained this problem in great detail. These seals add heat at the worst possible location. Grease or lip seals will also cause shaft wear at the point the seal material touches the rotating shaft.
Corrosion is a chemical reaction and therefore corrosion increases with temperature.
This is the best reason for converting any acid pump from packing to a mechanical seal. In the metric system it grows 0,001 mm. Open impellers must be set to a specified clearance from the pump case or back plate. Unfortunately all materials do not grow at the same rate and in the same direction. This is important to remember when you make critical settings and interferences and one of the main reasons we should do everything we can to keep down excessive temperature rises within the system.