Take as reference a centrifugal pump, where a shaft connects the motor to the impeller. Rotating parts are shown in grey, stationary parts in green. Shaft 3 rotates in chamber A (motor side) and chamber P (impeller side). A rotating ring (grey) 1 is fitted to the shaft and rotates with the shaft; any leakage is prevented by the static seal 4 (usually an O-ring). The stationary part is fitted with a stationary ring (green) 2. Here too, a static seal 4 prevents leakage. The two rings 1 and 2 are in contact by means of faces machined with extreme precision and smoothness; usually the contact force is guaranteed by springs, which compensate for any differential expansion of the various parts.

The pressure in chamber P, together with the centrifugal effect obtained thanks to the rotation of shaft 3, causes a film of liquid (a few microns thick) to be created between the contact faces, which prevents direct contact between the faces of the rings, lubricating them. The chambers A and P are therefore isolated from each other, despite their relative motion.

In addition to internal variants on process side and external variants on atmosphere side, there can be a differentiation according to the management of the working environment of the seal, for example the barrage with an inert liquid in the chamber, called flushing, immediately upstream of the seal ring. The purpose of this flushing can be the cool seal, to promote circulation in the seal chamber, to avoid solid deposits, chemical reactions or other undesirable phenomena. The simple mechanical seal is universally used in cooling water circulation pumps in endothermic engines, in circulating appliances and in many other applications where work is carried out at relatively low pressures and with reasonably clean, not corrosive and not hazardous fluids.

Multiple mechanical seals are used when it is necessary to have an absolute guarantee that the process fluid is sealed; infact, the external seal, on the atmosphere side, can operate in the event of failure of the internal seal. The vast majority of multiple seals are actually double seals. These are actually two simple seals, one downstream of the other, between which there is an intermediate chamber bounded by the two seals. The intermediate chamber usually contains an inert liquid, called a barrier fluid, often water. If this liquid is kept at atmospheric pressure, it is called a tandem seal; if it is kept at a higher pressure than that existing in the process, it is called a double seal.