Mechanical Seal

1. Mechanical Seal For Pumps
Prepared By: Engr. Muhammad Shahbaz

2. Mechanical Seal
• Everywhere where pumps with rotating shafts are used, a shaft seal is
involved. The shaft seal forms a barrier between what is inside the pump
and the atmosphere.
• A pump with a through-shaft is not completely sealed. It is a challenge to
the entire pump industry to minimize leakage.

3. Stuffing box
• A braided stuffing box packing is
the simplest type of shaft seal. The
packing is placed between the shaft
and the pump housing as shown
• In the stuffing box housing a soft
packing ring is axially compressed
until it makes contact with the
shaft. After the soft packing has
been exposed to wear the stuffing
box must be further compressed to
prevent excessive leakage.
• Vibrations and misalignment will
cause this seal type to leak.
Stuffing box
housing
Soft packing
Shaft

4. Lip seal
A universal lip seal type is a
rubber ring sliding against the
shaft. This type of seal is
primarily used in connection
with a low differential pressure
and low operating speed.
Lip seal

5. Mechanical shaft seal
A mechanical shaft seal consists of
two main components
1. Rotating part
2. Stationary part
The rotating part is axially pressed
against the stationary part
The clearance between the
stationary and rotating part of the
seal must be small in order to
reduce leakage
Stationary part Rotating Part

6. Mechanical shaft seal with two axial seal faces
• The best possible way of making
a seal with a minimum of
clearance and thus a minimum
amount of leakage is by pressing
two axial surfaces against each
other
• These axial surfaces can be
obtained with a stepped shaft,
running against a flat surface on
the pump housing
• The shaft and pump housing
must be highly wear resistant
and well aligned
Pump housing
Seal faces
Stepped shaft
Atmosphere

7. Mechanical shaft seal with rotating seal ring and stationary
seat
• A more practical solution is
obtained by fitting a rotating seal
ring on the shaft and a stationary
seal ring (seat) in the pump
housing.
• The tiny space between the seal
faces is called the seal gap.
• This design allows the use of a
wide selection of materials for the
rotating seal ring and stationary
seat.
Stationary seat
Seal gap
Rotating seal ring

8. Secondary seals
• Secondary seals consist of rubber parts such
as O-rings or bellows, used to avoid leakage
between the shaft and the rotating seal ring
as well as between the stationary seat and the
pump housing.
• To minimize leakage, the rotating seal ring
must be pressed against the seat. Therefore
the rotating seal ring must be able to move
axially on the shaft.
• To obtain axial flexibility, the secondary seal
must either be a bellows or an O-ring sliding
on the shaft.
• The secondary seal that seals between the
rotating seal ring and the shaft rotates
together with the shaft.
• The secondary seal that seals between seat
and pump housing is static.
O-ring, stationary
O-ring, rotating

9. Spring
• The rotating spring presses the
rotating seal ring against the seat and
the rotating O-ring along the shaft
Spring

10. Torque transmission element
• A torque transmission element
ensures that the rotating seal ring
rotates together with the shaft
Reasons for using Mechanical
seal:
• To minimize leakage
• To prevent toxic fluids escaping
to atmosphere
• To reduce power loss
Torque
transmission
element

11. Operating principle
• The rotating part of the seal is fixed on the pump shaft
and rotates in the liquid during pump operation.
• The compression of the rubber bellows between the shaft
and one of the two torque transmission rings fixes the
rotating part to the shaft.
• The spring transfers the torque between the torque
transmission rings . The rotating seal ring is mounted
together with the rubber bellows . The torque
transmission ring compresses the rubber bellows to the
rotating seal ring . The rubber bellows prevents leakage
between the shaft and rotating seal ring and ensures axial
flexibility despite contamination and deposits.
• In a rubber bellows seal, axial flexibility is obtained by
elastic deformation of the bellows. However in an O-ring
seal, the O-ring slides along the shaft.
• The compression force from the spring keeps the two
seal faces together during pump standstill and operation
• This flexibility also keeps the seal faces together, despite
axial movements of the shaft, surface wear, and shaft
run-out. Stationary part Rotating Part
Pump housing
Stationary secondary
rubber seal
Stationary seat
Rotating seal ring
Torque transmission Ring
Spring
Torque transmission ring
Rubber bellows (rotating
secondary seal)
Shaft
Lubricating film in
sealing gap

12. • The stationary part of the seal is fixed in the
pump housing . It consists of a stationary seat
and a stationary secondary rubber seal .
• The secondary seal prevents leakage between
the stationary seat and the pump housing .
• It also prevents the seat from rotating in the
pump housing .
• The pumped medium to be sealed is generally
in contact with the outer edge of the rotating
seal ring . When the shaft starts to rotate, the
pressure difference between the pumped
medium in the pump housing and the
atmosphere forces the medium to penetrate the
sealing gap between the two flat rotating
surfaces. The lubricating film is generated.
Pumped
medium
Rotating seal ring, pumped
medium side
Rotating seal ring,
atmospheric side
Atmosphere

13. Types of seals
There are two types of seam
1. Static Seal
2. Dynamic Seal
1. STATIC SEALS: Sealing takes place between two parts that don’t move in relation to each
other.
• Application: Pipe flanges ,vessel /Tower nozzles, pump casing joint. - Fan /Blower casing
joint , Compressor casing joint. - Turbine casing joint, Heat exchanger joints
• Types: Gaskets , O-rings etc.
• GASKET: Packing designed to go between two rigid parts in stationary conditions May be in
form of sheet , strip , bulk. Properties: Impermeability, Ability to flow into joints, Corrosion
resistance
2. DYNAMIC SEALS: Used for sealing fluid between parts that move in relation with each
other.
• Application: Centrifugal pump gland, valve gland , bearing housing Turbine/compressor inter
stage and end sealing , Reciprocating compressors cylinder sealing
• TYPES: Gland packings, Mechanical contact seals, Labyrinth seal, Oil seal, Oil film seals

14. Essential elements of a mechanical seal
The basic design of the Mechanical seal consists of the following elements:
1. Flexibly mounted seal face
2. Rigidly mounted seal face
3. Compression device
4. Secondary seal
Mechanical seal Components
• Rotary seal face
• Stationary seal face
• Springs
• Retainer
• Sealing /flushing media
• O-rings

15. Essential Requirements for Proper Operation of a Mechanical Seal
These are the essential requirements:
• 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

16. Effective forces in a Mechanical Seal
These are the forces operating in mechanical seals
1. Axial and radial forces
2. Closing and opening forces
3. Hydrostatic and hydrodynamic forces
Material of Seal Faces
• Few materials are suitable for seal faces. To keep leakage as low as possible, the
seal gap must be very small. As a result, the lubricating film is very thin.
• Consequently, the seal face materials must be able to withstand rubbing against
each other at high load and speed.
• The best seal face materials have low friction, high hardness, good corrosion
resistance and high heat conductivity.
• Examples: Carbon graphite, Aluminum Oxide (Alumina), Tungsten carbide,
Silicon carbide, Diamond coatings

17. Classification of Mechanical Seal
Mechanical seals are classified by arrangement and configuration.
Selection of the best type is not always easy and straight forward as there is usually a
compromise between economical and technical factors.

18. Mechanical seal classification by Configuration i.e. Design

19. By Design: Pusher vs. Non-pusher and Balanced vs. Non-balanced
Pusher vs. Non-pusher
Pusher seals utilize a dynamic secondary seal which moves axially with the major
seal face. Non-pusher seals have a static secondary seal which stays stationary
against the shaft or sleeve.
Defined by the secondary seal type: O-ring or polymer wedge versus bellow, rubber or metal.
Balanced Seal:
• Reduced closing forces
• Reduced power consumption
• For pressure up to 3000 psig
• Always recommended for volatile liquids
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Non- balanced:
• High closing forces
• Low leakage
• For pressure up to 200 psig
• Not recommended for volatile liquids
Balance Seal vs. Non Balance Seal

20. Cartridge seals and split seals
Cartridge seals
• Seal are pre-assembled with sleeve and flange in one unit.
• Easy to install.
• No measurements during installation.
• Spring load is preset.
• May be factory tested with air, water or oil.
• More costly as compared to component seal.
Split seals
• Seat is axially split.
• Does not require disassembly of the pump to install , reduce down time.
• Leaks more than a conventional seal.
• More costly as compared to conventional seal.

21. Factors need to be considered in the application of a
mechanical seal
• Pressure & speed (PV limit = Pressure x Velocity).
• Temperature.
• Fluid properties or characteristics.
• Run out of the shaft.
• Seal chamber type, available space radial and axial.
• Flushing/cooling arrangements, utilities in the plant.
• Mode of operation of the pump in the plant: continuous, cyclic, multi-
• purpose.
• Static versus dynamic pressure.
• Test requirements.
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