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
In order for the mechanical seal to perform over an extended time period with low frictional the faces are generally hydrodynamically lubricated. The fluid film will need to carry substantial load. If the load becomes to 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.
Pressure Balance Seals
It is possible to reduce the seal contact pressure by using a pressure balanced seal design of off-set a proportion of the force generated by the pumped fluid pressure. This principle is illustrated in the sketch below.
The mechanical seal generally includes a three static seals.
-The sleeve seal - this is usually an O-Ring
-The seal 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
-The housing seal is generally an o-ring of a gasket.
-Multiple springs distributed around seal body
The shaft design is critical. It must be rigid enough to support the seal in the correct position and the shaft surface finish must be suitable to ensure good sealing on the static seals (0.4 micrometers CLA or better). The shaft Total Indicated Runout (TIR) should not exceed 0.125mm. There should be minimum shaft vibration. The shaft may be subject to fretting corrosion as a result of micro-movements of the seal and is is often desireable to have locally hardened surfaces or to use sleeves.
There are a number of mechanical seal options
*External Seal.. This design is installed on the outside of the stuffing box with the sealed pressure inside. This provides good access allowing the seal components to be be cleaned.
*Internal Seal.. Generally mechanical seals are mounted inside the stuffing box with the sealed pressure outside the seal.
*Double Seals.. Mechanical seals mounted in pairs are used for sealing hazardous, toxic or abrasiv fluids and are often provided with clean flushing fluid between the seals. Double seals also provide an additional degree of safety were the pressure differentials are likely to reverse and/or there is a high risk of the sealing failing. There are a number of double seal assembly options as listed below
In Series - Used primarily to overcome the risk of failure of a single seal.
*Face to Face - Used when a cooling fluid interface is required . One seal is used for the process fluid the other seal is used for the coolant.
*Back to Back - Used when an abrasive fluid is being contained and both seals are flushed with a clean buffer fluid. The flushing fluid is introduced at a higher pressure the process fluid.
The are a large number of variant mechanical seals e.g split seals. Improved systems are constantly being introduced onto the market
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.