Introduction of Mechanical Seals

Mechanical seals are being used increasingly on fluid pumps to replace packed glands and lip seals. Pumps with mechanical seals perform more efficiently and generally perform more reliably for extended periods of time

Mechanical seals are provided to prevent pumped fluids from leaking out along the drive shafts. The controlled leakage path is between two flat surfaces associated with the rotating shaft and the housing respectively. The leakage path gap varies as the faces are subject to varying external loads which tend to move the faces relative to each other.

The mechanical seal requires a different shaft housing design arrangement compared to that for the other type of seals because the seal is a more complicated arrangement and the mechanical seal does not provide any support to the shaft.

In order for the mechanical seal to perform over an extended time period with low friction the faces are generally hydrodynamically lubricated. The fluid film will need to carry substantial load. If the load becomes too high for the film surface contact will take place with consequent bearing failure. This lubricating film is generally of the order of 3 micrometres thick , or less. This thickness is critical to the required sealing function. Mechanical seals often have one face of a suitable solid lubricant such that the seal can still operate for a period without the fluid film.

The mechanical seal generally includes three static seals.

1.The sleeve seal - this is usually an O-Ring
2.The seals between the moving seal member and the shaft or sleeve.- This is often an o-ring but can be a wedge or vee seal. This seal may not be used for bellows type mechanical seals
3.The housing seal is generally an o-ring of a gasket.

All of these seal must be compatible with the fluid being contained and the associated environment. These seals may limit the design for high temperature applications. In this case the bellows type alternative may be the best option.

The use of mechanical seals generally involve the use of additional equipment primarily for the flushing /coolant systems. This includes pumps, coolers, strainers, filters etc.