Mechanical seals are widely used in pumps and rotating equipment to prevent fluid leakage along the shaft. They replace traditional packed glands and lip seals, offering improved efficiency, reliability and longer service life.
A mechanical seal operates by maintaining a controlled leakage path between two flat sealing faces—one rotating with the shaft and one fixed to the housing. These faces are typically separated by a very thin lubricating film, allowing low-friction operation while maintaining sealing performance.
Mechanical seals prevent leakage by maintaining contact between two precision flat faces. One face rotates with the shaft while the other remains stationary. A thin fluid film between the faces provides lubrication and reduces wear.
The balance between sealing force, fluid pressure and lubrication is critical. If the film thickness becomes too small, excessive wear occurs. If it becomes too large, leakage increases.
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.
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 three static seals.
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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 sealing faces are generally pressed together using some form of spring loading. Several
different spring loading systems are available.
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For conventional mechanical seals the single spring arrangement is used.
The other spring arrangements are used if the space is restricted.
It is vitally important that the sealing surfaces are perfectly flat and are parallel.
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.
The 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 bronze, cast iron, stainless steel etc.
The seal 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 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 desirable to have locally hardened surfaces or to use sleeves.
There are a number of mechanical seal options
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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.
