what is unbalanced seal ?
Almost all mechanical seals can be used in unbalanced (Reference) or balanced versions. The term “unbalance” used to close the sealing surface is greater than the average pressure (pressure gradient) between the sealing surfaces multiplied by the contact area between the surfaces when the packing tank pressure is multiplied by the area (closing force) exposed to the pumping fluid. In other words, an unbalanced mechanical seal exhibits a net hydraulic closing force generated by the actual pressure to seal.
For example, if the stuffing box pressure is 50 psig (3.4BARG), the spring load must be increased. Therefore, the “surface load” or closing force on the surface will even be 50 psig times higher than the surface area. Of course, this restricts the pressure seal capacity of the unbalanced seal.
When subjected to vibration, dislocation and cavitation, the unbalanced seal is more stable than the balance seal. The drawback is their relative low pressure limit. If the closing force applied on the sealing surface exceeds the pressure limit, the lubricating film between the two sides is extruded, and the high load dry running seal fails.
what is balanced seal ?
When it comes to the “balance” of mechanical seals, we are not talking about mechanical or rotational balancing. On the contrary, we refer to hydraulic balance. For hydraulic seals, a step is usually placed in the seals, allowing a wider sealing surface. Some seals are “online” balanced, which means that the step is designed to be sealed. Due to the space limitation in the packing box, the on-line balancing seal can not completely achieve the effect that the real balancing seal can achieve, but the on-line sealing improves the sealing pressure capacity.
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