Case in Point. I was called to a plant that uses oil-heated embossing rolls as an integral part of the process. The plant's reject rate was up and production speed was down due to an increase in the temperature difference (T) across the rolls. This phenomenon was noticed on all the rolls in the plant. In the course of the normal barrage of troubleshooting questions, I asked, “When was the fluid last sampled and tested, and may I review those results?” Silence. The fluid, which was a high performance fluid from a well-known manufacturer, had never been tested in the seven-year life of the system. The owner assumed that fluid testing was necessary because he used a “good” fluid and because the system had been specified and designed by a reputable engineering firm. The system had a closed expansion tank with a nitrogen blanket system, and the owner assumed (there's that word again!) that the fluid was adequately protected.
The owner had failed to take into account that, when the embossing rolls were changed (a regular occurrence), the replacement rolls were installed with an appreciable amount of air inside. This air introduced a small amount of oxygen into the system each time the rolls were changed, and over time, the fluid became highly oxidized. One property of oxidized fluid is an increase in viscosity, and in this case, the increased viscosity was sufficient to affect pump performance and reduce turbulence in the rolls, both of which contributed to the adverse changes in system performance.