The opening force is always the area of the sealing faces. The balance ratio is then Ac/Ao. A seal with a balance ratio less than 100 percent is called a balanced seal. A seal with a balance ratio greater than 100 percent is called an unbalanced seal. Most balanced seals have a balance ratio between 60 and 90 percent. Most unbalanced seals have a balance ratio between 110 and 160 percent.
Pusher seals normally require a step in the shaft/sleeve or internal hardware to achieve a balanced design. Metal bellows seals do not require this step. The balance diameter, or mean effective diameter (MED), of metal bellows seals is located near the middle of the convolution. When pressure is applied to the outer diameter of the seal, the MED shifts downward, lowering seal balance. The opposite is true when the seal is subject to internal pressure. The rate of change in the balance depends on the face width and the bellows leaflet design.
Pusher seals can be designed to withstand pressure from either direction. This is accomplished by trapping the O-ring between two diameters as shown in Figure 4.1. The cavity must be long enough to allow the O-ring to move, allowing pressure to act on the primary ring. These designs allow the seal to withstand system upsets.
Figure 4.1. Seal balance for pusher and non-pusher seals
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