I was at a customer site recently and they were encountering intermittent contamination throughout the plant but could not isolate the source…
Upon review, the company has instituted very effective procedures for cleaning all of the material delivery, storage, and drying systems. I observed the entire cleaning process and realized they forgot to clean the feedthroat shut-off. Believe it or not, most companies do not give proper attention to the slide just below the hopper. In this case, they actually remove the hopper from the slide and vacuum the feedthroat, but they neglect the small fines that become trapped between the surfaces of the feedthroat shut-off.
The important lesson is to ensure that you look at any problem as objectively as possible and to see if there is a new cause to your problem that you may not have previously thought about.