What is a mechanical seal?
A mechanical seal is a precision sealing element designed to stop fluid leaking between a rotating shaft and a stationary housing such as a pump casing. Unlike the older "soft seals" (packing), it prevents wear of the shaft and delivers maximum efficiency with minimum leakage.
It consists essentially of two main faces: one turns with the shaft (the rotating element), the other is fixed to the housing (the stationary element). The very close contact between these two faces seals the fluid through the thin liquid film between them.
Operating principles and installation basics
Uninterrupted liquid transfer is vital in industrial facilities, chemical processes and water treatment systems. The weakest link in these systems is usually the shaft seal. The mechanical seal steps in at exactly that point and is modern engineering's most effective answer to the sealing problem.
How does a mechanical seal work?
The working principle of a mechanical seal rests on managing hydraulic balance and friction.
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Seal faces: two faces, one hard (usually silicon carbide or tungsten carbide) and one softer (such as carbon graphite), are pushed together by spring force.
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Liquid film: during operation a fluid film at micron level, far thinner than a strand of hair, forms between the faces. This film lubricates and at the same time reduces the heat caused by friction.
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Secondary sealing: O-rings or bellows block the leaks that could otherwise pass by the shaft and the cover.
What types of mechanical seal are there?
Many types exist, chosen according to the application, the chemistry of the fluid and the pressure values:
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Single mechanical seals: the most common type for standard applications.
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Double mechanical seals: used with toxic, abrasive or hazardous fluids, with a "barrier fluid" fed between the two seals.
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Cartridge seals: ready-assembled units that are the easiest to fit and need no setting.
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Bellows seals (metal or elastomer): they withstand axial movement and high temperature.
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Conical spring seals: simple in construction and economical.
How is a mechanical seal installed?
Installation quality decides 90% of the service life of a mechanical seal. Faulty installation can destroy the seal within minutes.
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Preparation: the shaft surface must be smooth, clean and free of burrs. Shaft run-out must always be checked.
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Cleanliness: never touch the seal faces with bare hands. Oil or dirt spoils sealing performance. Use alcohol-based cleaners.
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Lubrication: to ease the O-rings into place, use only the lubricants the manufacturer recommends, usually silicone-based or glycerin. Caution: do not use petroleum-based oil on EPDM rubber!
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Positioning: seat the stationary element squarely in its housing. When fitting the rotating element to the shaft, make sure the spring load sits at the correct distance, the working length.
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Tightening: tighten the bolts crosswise and to an even torque.
How are mechanical seal dimensions taken?
The most critical step in replacing a mechanical seal is establishing the dimensions that will fit your existing equipment perfectly. A wrong measurement can lead to leaks during installation or leave the part completely unusable. You need these three basic dimensions to identify the correct spare part without error:
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Shaft diameter (d1): the exact outside diameter of the shaft, or shaft sleeve, on which the seal sits, measured with a caliper.
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Outside diameter (d3 / d7): the outside diameter of the rotating element, or of the part of the stationary seat that enters the pump housing.
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Installation length (L): the total length of the spring section in its compressed state, which the seal needs in order to seal properly during operation.
Why does your seal fail?
The greatest enemy of mechanical seals is dry running. Turning the shaft while there is no liquid in the pump heats the faces instantly and cracks them through thermal shock. Always make sure the pump has been vented and the suction line is open.
Author:
Industrial Design Engineer