If an extremely corrosive liquid has satisfactory lubricating properties, an outside mechanical seal offers an economical alternative to the expensive metallurgies required for an inside seal to resist corrosion.
The illustration below shows a typical outside seal arrangement in which only the insert, seal ring and secondary seals are exposed to the product. All these components can be non metallic. The metallic rotary unit parts are exposed to the atmosphere.
Outside mechanical seals can also be used in equipment with stuffing boxes which will not accommodate inside seals, and they are easier to access for adjustment and trouble-shooting.
The outside mechanical seal does have disadvantages. Because it is exposed, the seal is vulnerable to damage from impact. Of greater importance, however, is an outside mechanical seal's pressure limitation.
The hydraulic pressure on an unbalanced outside mechanical seal works to open, rather than to close, the seal faces. The seal, relies upon the springs to maintain face contact. Even though a degree of balance can be designed into a hydraulically balanced outside seal, all outside seals are limited to applications having moderate pressures (150 PSIG maximum, depending on shaft size and speed).
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