What are Mechanical Seals

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What Are Mechanical Seals?

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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'.

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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

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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

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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.

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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.

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Why Use Mechanical Seals?
Comparison Between Mechanical Seal and Gland Packing Comparison Between Mechanical Seal and Gland Packing Comparison Between Mechanical Seal and Gland Packing
  Mechanical Seal Gland Packing
Leakage Extremely small or none Certain leakage is necessary to avoid seisure
Life Long with appropriate design and material Comparatively short as it wears and periodical tightening is required
Wear of shaft and sleeve Spring load keeps faces in contact if faces wear Periodical tightening required and must be replaced if wear of shaft or sleeve becomes excessive
Power Loss Small 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
Cost Running cost is low as maintenance is not necessary Running cost is high due to necessary maintenance
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Why Use Mechanical Seals?
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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.

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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.

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A few facts about the leakage (and wear)
behaviour of contacting mechanical seals
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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.

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MECHANICAL SEAL COMPONENTS

• The Basic components in a mechanical seals
  include the following
• A stationary sealing face.
• A Rotating sealing face
• A Static secondary seal for stationary face.
• A Static secondary seal for rotating face.
• A spring or a bellow to press the sealing faces
  together.
• A system to flush seal area.
• A method to prevent slippage of sealing faces
  (Keyways, pins or secondary seal friction).

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 ADVANTAGES OF MECHANICAL SEALS

• Handles all types of fluids (Acids, salts and
  abrasive particles)
• Handle slightly misaligned / non concentric
• Handle Bi-directional shaft rotation, large
  pressure, temperature and speed excursions.
• Shaft condition is not critical (Finish
  roughness, roundness, hardness and material)
• Operation does not cause shaft wear.
• Long operating life.
• Positive sealing for food processing, hazardous
  chemicals and radioactive fluids.

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 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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