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What Are Mechanical Seals?Sap Parts
Manufacturer-Designer-OEM Supplier
http://www.sapparts.com/
Meaning of Mechanical Seals
• A mechanical seal is a device that helps join systems or mechanisms together by preventing leakage (e.g. in a plumbing system), containing pressure, or excluding contamination.
• The effectiveness of a seal is dependent on adhesion in the case of sealants and compression in the case of gaskets.
• A stationary seal may also be referred to as 'packing'.
Types of Seals
• Induction sealing or cap sealing
• Adhesive, sealant
• Bodok seal, a specialized gas sealing washer for medical applications
• Bridgman seal, a piston sealing mechanism that creates a high pressure reservoir from a lower pressure source
• Bung
• Compression seal fitting
Types of Seals
• Diaphragm seal
• Ferrofluidic seal
• Gasket or Mechanical packing
• Flange gasket
• O-ring
• O-ring boss seal
• Piston ring
• Glass-to-metal seal
• Glass-ceramic-to-metal seals
Why Use Mechanical Seals?
• Mechanical seal is a sealing device for any rotating equipment which prevents the inner fluid from leaking through the clearance between the rotating shaft and the housing case.
• There are various types of mechanical seals for each application and operating condition of the equipment but in principle, mechanical seal is a face type seal, which performs sealing by two sealing faces vertical to the shaft.
• Mechanical Seal incorporates various components such as the spring giving contact force and the packing which eliminates intrusion of the fluid from the outside.
Why Use Mechanical Seals?
• The seal face is an accurately flat face, and so if it is worn, it does not cause the leakage of the sealant as far as wear is within the maximum allowable amount.
• Therefore it can be used for a long time free of maintenance, and is very effective and efficient in terms of reliability and economy.
• Mechanical seal has been said that it requires high initial cost and is hard to handle, but it is now highly valued as an important functional part of the equipment, which enables long continuous operation of the production process of equipments and thus finally gives considerable profit to the operation.
Why Use Mechanical Seals?
Comparison Between Mechanical Seal and Gland Packing
Mechanical Seal Gland Packing
Leakage Extremely small or noneCertain leakage is necessary to avoid
seisure
LifeLong with appropriate design and
material
Comparatively short as it wears and
periodical tightening is required
Wear of shaft and sleeveSpring load keeps faces in contact if faces
wear
Periodical tightening required and must
be replaced if wear of shaft or sleeve
becomes excessive
Power LossSmall as frictions area are coefficient and
small (Energy Saving Type)
Big as friction area and coefficient area
are large
Operating Range
(Pressure Temp.)
Applicable widely with appropriate
materials and design
Limited application due to keeping
leakage small
CostRunning cost is low as maintenance is not
necessary
Running cost is high due to necessary
maintenance
Mechanical Seals Operating Principles
• Essential elements of a mechanical seal:
• These are the three essential elements of a mechanical seal:
• Seal faces: one rotating with the shaft and one stationary in the pump casing, cover or flange.
• Secondary seals: one to seal the rotating face to the shaft and one to seal the stationary face to the pump cover or flange.
• Metal parts: to transmit torque and to provide an axial mechanical force to load the faces.
• Essential requirements for proper operation of a mechanical seal:• Seal faces must be flat and polished.
• Seal faces must be installed perpendicular to the shaft.
• Spring force must be sufficient to maintain contact of the faces.
A few facts about the leakage (and wear) behaviour of contacting mechanical seals:
• It is essential for proper lubrication and wear of the faces.
• Normal leak rates range between immeasurably small to steady drips or temporary to even small steams. Some seals leak some of the time, some seals never leak (measurably), and some leak all the time. Leakage patterns can be constant, progressive or erratic in nature.
• It can be in liquid, gaseous and/or solid state.
• Successful contacting seals tend to have very low wear rates and low leakage rates.
A few facts about the leakage (and wear) behaviour of contacting mechanical seals:
A few facts about the leakage (and wear) behaviour of contacting mechanical seals:
• Some forms of contact is necessary for low leakage rates. Non-contacting or “full lift off” seals (hydrostatic or hydrodynamic tend to have visible, sizeably larger leakage rates.
• The large majority of mechanical seals never wear out and are removed from service for some other reason.
• Seal failures occur for a wide range of reasons.
• Some failures occur as an interaction with the tribology of the interface.
MECHANICAL SEAL COMPONENTS :
• The Basic components in a mechanical seals include the following
1. A stationary sealing face.
2. A Rotating sealing face
3. A Static secondary seal for stationary face.
4. A Static secondary seal for rotating face.
5. A spring or a bellow to press the sealing faces together.
6. A system to flush seal area.
7. A method to prevent slippage of sealing faces (Keyways, pins or secondary seal friction).
ADVANTAGES OF MECHANICAL SEALS :
1. Handles all types of fluids (Acids, salts and abrasive particles)
2. Handle slightly misaligned / non concentric
3. Handle Bi-directional shaft rotation, large pressure, temperature and speed excursions.
4. Shaft condition is not critical (Finish roughness, roundness, hardness and material)
5. Operation does not cause shaft wear.
6. Long operating life.
7. Positive sealing for food processing, hazardous chemicals and radioactive fluids.
DISADVANTAGES OF MECHANICAL SEALS :
1. Requires more space than radial lip seals.
2. Cannot handle axial end play.
3. Sealing faces must be finished smooth (0.08 to 0.4 micrometer) and can get easily damaged.
4. High initial cost.
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