In any high pressure seal, the tendency for the fluid pressure to extrude the seal into the clearance gap is a major cause of failure. In seals for reciprocating applications, friction between the seal and the wall against which it moves may either augment this effect or oppose it. These circumstances are usually called pump condition and motor condition respectively, since they are found in piston seals on these machines.

Seal performance is likely to be better in motor condition. Often, it is possible at the design stage to arrange for seals to be working in this condition. If so, this should certainly be done, but remember that if it results in the seal having to operate within a bore, this cannot usually be given such a high finish, or be made to such close tolerance, as an external surface.

The condition of lubrication between the seal and the surface against which it slides vary from boundary to thick film lubrication, depending on the viscosity of the fluid, sliding speed and the pressure. With film lubrication normally arising from a combination of viscous fluid, high speed and, perhaps also, low pressure, the amount of fluid left behind a seal as it moves may be considerable. In the pump condition this may be termed leakage, but on the motoring stroke a similar effect may occur if the shaft remains wetted by the fluid, in spite of the fact that this then means that fluid is being moved from a low to a high pressure, that is, negative leakage. It should be noted that negative leakage may result in air ingress, and thus attempts to obtain zero leakage may result in aeration of the fluid to be sealed.

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