Virtually all mechanical seals are available in either unbalanced ( Ref. Figure) or balanced versions. The term “unbalanced” is used when the stuffing box pressure times the area exposed to the pumped fluid (closing force), acting to close the seal faces, is greater than the average pressure between the seal faces (pressure gradient)times the area of contact between the faces. In other words, unbalanced mechanical seals exhibit net hydraulic closing forces which are generated by the actual pressures to be sealed.
For example, if there were a stuffing box pressure of 50 psig (3.4barg), the spring load would have to be added. Hence, the “face load” or closing force on the faces would be even higher than 50 psig times the face area. This, of course, limits the pressure sealing capacity of an unbalanced seal.
Unbalanced seals are often more stable than balanced seals when subjected to vibration, misalignment, and cavitation. The disadvantage is their relatively low-pressure limit. If the closing force exerted on the seal faces exceeds the pressure limit, the lubricating film between the faces is squeezed out and the highly loaded dry running seal fails.
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