Mechanical seals is 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 mechanical seals faces are subject to varying external loads which tend the move the mechanical seals faces relative to each other.

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

In order for the mechanical seals 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 mechanical seals can still operate for a period without the fluid film.

Pressure Balance mechanical seals

It is possible to reduce the mechanical seals contact pressure by using a pressure balanced mechanical seals design of off-set a proportion of the force generated by the pumped fluid pressure.   This principle is illustrated in the sketch below.

Design Features of mechanical seals

The mechanical seals 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.

All of these mechanical seals 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 metal bellows type mechanical seals may be the best option.

The sealing faces are generally pressed together using some form of spring loading. Several different spring loading systems are available.

• Single spring mechanical seals

• Multiple springs distributed around mechanical seals body

• Disc Springs mechanical seals

• Metal Bellows mechanical seals

• Magnetic

For conventional mechanical seals the single spring arrangements is used.   The other spring arrangements are used in the space is restricted.

The seal faces are usually dissimilar materials with the softer face being the narrower surface. For abrasive applications similar hard materials are used e.g tungsten carbide. Mechanical seal surfaces must have sufficient strength to withstand the hydrostatic fluid forces and must be able to remove the heat generated by sliding action. Carbon is often used against ceramic, tungsten carbide, silicon carbide, bronze, cast iron, stainless steel etc.

Mechanical seals surface must be flat, smooth and square to the shaft.  Both surfaces are normally lapped to a high quality finish. The harder surface is most important because the softer surface is designed to run-in over the initial operating period.

The shaft design is critical. It must be rigid enough to support mechanical seals 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.

Mechanical seals Assembly Options

There are a number of mechanical seals options

• External Mechanical Seals. This design is installed on the outside of the stuffing box with the sealed pressure inside. This provides good access allowing mechanical seal components to be cleaned.
• Internal Mechanical Seals. Generally mechanical seals are mounted inside the stuffing box with the sealed pressure outside the mechanical seals.
• Double mechanical seals. mechanical seals mounted in pairs are used for sealing hazardous, toxic or abrasive fluids and are often provided with clean flushing fluid between the mechanical seals.   Double mechanical 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 mechanical seals assembly options as listed below
  • In Series Double Mechanical Seals- Used primarily to overcome the risk of failure of a single mechanical seals.
    
    
    
    
    
    
  • Face to Face Double Mechanical Seals- Used when a cooling fluid interface is required . One mechanical seals is used for the process fluid the other seal is used for the coolant.
    
    
    
    
    
    
  • Back to Back Double Mechanical Seals- Used when an abrasive fluid is being contained and both mechanical 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 mechanical seals.   Improved systems are constantly being introduced onto the market

Additional Equipment for Mechanical Seals

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