All mechanical seals are hydraulically balanced to control the opening and closing forces on the seals rings. The measure of hydraulic balance is called balance ratio. A mechanical seal with a balance ratio of one or more is an unbalanced seal, whereas a seal with a balance ratio of less than one is a balanced seal.

The degree of hydraulic balance is determined by controlling the dimensions of the radially disposed areas of the seal rings. The balance ratio is the area of the seal face compared to the net hydraulic closing area.

An unbalanced seal typically has a balance ratio of around 1.2. In this case, the pressure on the seal face is 1.2 x the sealed pressure, plus the pressure created by the spring force. Therefore, the face pressure on an unbalanced seal is always greater than the pressure being sealed. For this reason, unbalanced seals are limited to a pressure of 10 barg and are used on relatively simple, low pressure applications.

A balanced seal typically has a balance ratio from 0.65 to 0.85, so that the seal face pressure is always less than the sealed pressure. This enables the seal to generate sufficient fluid film between the seal faces and therefore allows operation at high pressure.