The also require a substantial difference in pressure between the discharge and the clean liquid connections. In a pump application these pressures are too close together. No, the clean fluid always takes the path of least resistance. That is the same reason that higher pressure air does not keep dry solids from penetrating the lapped faces. Centrifugal force will pack solids in front of the inboard seal face and restrict its movement.
Higher pressure is called barrier fluid, lower pressure is called buffer fluid.
They also put the pumping fluid at the inner seal outside diameter where it belongs. Dual seal should be hydraulically balanced in both directions so that they will stay shut regardless of the direction of the fluid pressure. There are two forces closing the seal faces. A spring force caused by the spring, springs, or bellows pushing on the seal face. A hydraulic force caused by the pressure of the fluid acting on the closing area of the seal faces.