Types of Mechanical Sealsby Pauline Gill
Mechanical seals are used in industrial pumps, compressors, and other applications where a spinning shaft protruding into a wetted process is likely to have continuous, rigorous duty. Mechanical seals move the point of the seal away from the shaft to specially designed sealing faces that gradually wear down. This provides a longer service life than moderate duty seals such as lip seals, which seal directly against the circumference of the shaft itself. Lip seals may wear a circumferential groove into the spinning shaft, requiring its eventual costly replacement, while mechanical seals gradually sacrifice their sealing faces.
Mechanical Seal Operation
Mechanical seals provide the sealing reconciliation between a spinning shaft and a stationary pump or compressor housing. They typically have a key part such as a ring or bushing which is mounted on and has a static seal provided by an o-ring to the spinning shaft. Another part is attached to and has a similar o-ring static seal to the stationary housing. Either or both may have sliding bushings and springs that press the seal faces together with a pre-determined effacement force. It is at this effacement, that the actual sealing action takes place. Mechanical seals allow a minuscule almost drop by drop leakage past the seal face to lubricate it to slow seal wear.
Mechanical Seal Types
There are a number of mechanical seal types, each providing advantages for specific applications: the pusher type, the non-pusher, unbalanced, balanced, conventional, and cartridge type.
Pusher and Non-Pusher
Pusher type seals use an axially mounted spring on the shaft sealing assembly to impart a fixed sealing force to the seal effacement. Non-pusher springs use a sealed bellows instead of a spring for pushing the effacement halves together. Both are of modest complexity and cost, and tolerate slight amounts of misalignment and vibration.
Balanced and Unbalanced
Balanced mechanical seals handle higher pressures and are better suited for liquids that have lower lubrication qualities. Unbalanced seals also work better in situations with higher vibration or misalignment.
Conventional mechanical seals are generalized for use on applications that may not have originally been designed with mechanical seals, instead being designed with lip seals. They are mainly intended for retrofits and upgrades of pumps and compressors and may require some engineering and maintenance effort to effect the change.
Cartridge mechanical seals provide the entire mechanical seal package in an easy to replace package that neatly fits inside the cartridge type housing. Cartridge type seals provide the required static sealing to both the spinning shaft and stationary housing. Many modern pump builders incorporate these common cartridge type mechanical seals into their newer products to provide easier maintenance.
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