Years ago, most pump shafts were sealed using rings of soft packing, compressed by a packing gland, but this type of shaft seal required a fair amount of leakage just to lubricate the packing and keep it cool. Then came the development of the “mechanical seal,” which accomplishes the job of restraining product leakage around the pump shaft with two very flat surfaces (one stationary and one rotating). Even though these mechanical seal faces also require some (very small) leakage across the faces, to form a hydrodynamic film, this leakage normally evaporates and is not noticeable. Most pump shafts today are sealed by means of mechanical seals. However, because of the delicate components used for this new sealing method, mechanical seal failures are the greatest cause of pump down time. This requires a better understanding of this seal type and its application.