An finish-face mechanical seal is a tool used on a rotating shaft to keep fluids in and contaminants out. It prevents the fluid moved through an asset, most often a centrifugal pump, from leaking. These seals are positioned in the asset’s stuffing box or seal chamber. This is the area of the pump where the pump shaft connects to the drive (an electric motor, for example).
Aside from air seals, which will be discussed later, most types of mechanical seals include two flat faces which might be installed perpendicular to the shaft. One of many faces is mounted stationary to the seal chamber housing. The opposite face rotates with the shaft to provide the primary seal. Axial mechanical force and fluid pressure keep the contact between the seal face materials. This contact prevents leakage and retains the fluid within the pump.
The Three Types of Mechanical Seals
Three types of mechanical seals are used in process equipment:
A cartridge-mounted, finish-face mechanical seal is a totally self-contained unit that houses the sealing components—a gland, sleeve, and hardware. A cartridge seal permits the unit to be preassembled and preset by the manufacturer. With the manufacturer dealing with these tasks, installation and upkeep are simplified. Cartridges may be outfitted with one or two seals, depending the application’s requirements.
Part, finish-face mechanical seals encompass a separate rotating member and a stationary seat that mount in a gland or housing. Since they are not preset, set up and maintenance are more complicated than cartridge seals. Installing these requires experienced technicians who can properly install and adjust them.
Air seals are noncontacting, pneumatic devices engineered to seal rotating shafts. These seals are primarily put in in dry powder or slurry applications. They protect in opposition to product loss, emissions, and contamination by using small amounts of air or inert gas. This air is throttled to create positive pressure and an effective seal.
Most mechanical seals have 5 parts:
Rotating main face – Fixed to and rotates with the shaft and seals towards the stationary main sealing element
Stationary major face – Fixed to the stationary housing of the pump, mixer or other equipment by means of which the rotating shaft passes and seals in opposition to the rotating primary sealing element
Mechanical loading gadgets – Biases the primary sealing elements in contact to initiate sealing. These can be a single spring, a number of springs, wave springs, or metal bellows.
Static and/or dynamic secondary seals – Seal between the mechanical seal parts and the equipment shaft and housing that compensates for any shaft movement that may damage the seal faces.
Drive mechanisms – Set screws and drive pins are examples of drive mechanisms used to provide rotation to the rotating main seal face
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