Mechanical Seal

Presented by

Saumya Lohani

FOPE EG Plant

Principle of Sealing

Thin liquid film between a rotary and stationary face.

One soft face and one hard face rotating against each other

No interuption in liquid film No gas phase to prevent face contact No solid particles to damage seal face

Mechanical seal

The function of mechanical seal is to permit a rotating shaft to penetrate an enclosure while maintaining separation of the environments on the inside and outside of the enclosure.

primary Ring

Snap Ring

O Ring

Disc

Spring

Retainer

Set Screw

Mating Ring

O Ring (Static Seal)

Dynamic Seal Faces

Design and selection of mechanical seal

Chemical and physical properties of sealing fluid. Corrosiveness Density and viscosity Vapor pressure and B.P. Seal cavity pressure Seal cavity temperature. Equipment design and Installation. Material of concentration.

Mechanical Seal Classification

By arrangement

Single multiple

Inside outsideDouble back to

back

Double face to

facetandem staged

By design

unbalanced Single Spring Pusher Type

BalancedMultiple Spring

Non Pusher Type

Flexible Rotor Face on Rotor

Flexible Stator

Face on stator

Inside and Outside Mounted Seals

fluid

housing housing

Pusher seal Dynamic O ring at the rotating seal face O ring moves axially along the shaft to compensate for miss alignment and face

wear Can be used for very high pressure application ( 3000 psi) In corrosive service, constant relative motion wears away oxide layer of the shaft

or sleeve material, causing fretting corrosion Hang up- where sealed fluid can oxidize, the normal seal weepage can build

under the seal faces and prevent forward movement

Non Pusher Seals Has a static O ring in the drive collar of the rotating seal face

unit No fretting corrosion and seal face hang up. Welded metal bellows limits pressure to 250 psig.

Unbalanced seals

ao

ac

•Pressure limitation 10 bar•Plain shaft is used.•Comparatively cheaper

•b=ac/ao >100% (seal balance).

Balanced seals•Pressure 85 bar•Stepped shaft is used.•Expensive.

•b=ac/ao <=100% (seal balance).

ao

ac

Stationary Seal Head Limitation of being applied in services with less than 5% solids. Higher cost and increased space requirements. Can handle severe face misalignment. Used when shaft speed exceeds 5000 fpm (52.4 m/s) When mating ring rotates and the seal head is stationary.

Seal Seat Rotating

Housing

Primary Seal Ring ( stationary)

Rotating Seal Head Low cost, less axial space, self cleaning due to centrifugal

force. Speed limitation up to 4500 fpm. Unable to handle severe face misalignment. Seal rotates and mating ring is stationary

Primary seal rotating

Seal seat stationary

Single Spring and Multiple Spring Determined by space limits and liquid sealed. Single - openness of design

-non clogging component of the assembly -large dia coils withstand greater degree of corrosion

Multiple- shorter axial space -face loading is accomplished by combination of springs.

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