The family of shaft seals known as mechanical seals is the most advanced type of seal used in the mixing industry. They can handle the highest pressures, maintain nearly leak free operation, and require minimum maintenance if installed and operated properly. The down side is the higher initial cost (both for the seal and for the more complicated equipment required surrounding the seal) and the higher level or expertise needed to service the seals. Mechanical seals are increasing in popularity due to the growing environmental restrictions regarding any leakage from process tanks.

BASIC PRINCIPLES

There are hundreds of mechanical seal designs, but they all are variations of a basic layout consisting of a collar mounted on the shaft which uses springs to push a ring (which also rotates with the shaft) against another ring which is held stationary. The rings rotate against each other riding on a thin layer of lubricant, and the springs hold them so tightly together that leakage through the seal is reduced to an immeasurable amount. The mating surfaces of the rings must be perfectly flat to seal properly, and are manufactured to tolerances measured "light- bands". The rings must also be extremely hard to endure the pressure and wear, so they are usually made up of ceramic, carbon, silicon carbide, tungsten carbide or similar materials. The stationary "seat" is held in place and maintains a static seal with the mounting housing using gaskets or 0-rings. The rotating elements of the seal must attain a static seal with the shaft using 0-rings, wedges or packing.