Why do pumps burn up when it runs dry?

Friction of the mechanical seal faces running together without lubrication or the shaft packing running against the shaft without cooling by water seepage are the result of pumps running dry. Impeller clearances are usually wide enough that the impeller and volute aren't damaged when dry running occurs, but if the pump uses the fluid media to maintain a balance or if tolerances are extremely tight and there is contact with the impeller and volute then galling, damaging, and possibly seizing the impeller could happen.