Abstract: A balanced mechanical seal between two zones having different fluids, at least one of which may be corrosive, abrasive, of different lubricity, of elevated temperature and with other undesirable properties or characteristics. A buffer fluid is introduced into one zone at a constant rate so as to flow to a location radially inwardly of contacting seal faces and then radially outwardly of said seal faces. The flow serves to cool and lubricate the seal faces and substantially prevent the flow of fluid from the other of the zones radially inwardly across the seal faces. The ratio of areas which determines the seal balance is such that in the direction of buffer fluid flow it is of relatively low value. However, in the opposite direction of fluid flow, the balance is a much higher value. Thus in the event of buffer fluid failure, flow of the undesirable fluid across the seal faces is substantially prevented, thus preserving the faces from substantial damage from that undesirable fluid.