What is Mechanical Seal?

Parts of a mechanical seal

1.Rotary seal ring
2.Stationary seal ring
3.Rotary seal ring secondary seal
4.stationary seal ring secondary seal
5.spring
6.Gland Plate
7.Clamp Ring

Mechanical Seal 4 main points sealing system.

Primary Seal:

The sealing Between Rotating and stationary Faces is know as primary seal

Secondary Seal

  • The seal between the rotary part and shaft with o ring
  • The seal between the statioary part and gland seal plate
  • The seal between gland plate and stuffing box is usually o ring or gasket

Stationary seal ring

The stationary seal ring is usually attached to the gland plate and is made of softer material like carbon graphite

Rotary seal Ring

The Rotay seal ring is fixed to the rotating shaft and is made of harder material usally tungsten carbide and silicon carbide.

The fluid film:

In mechanical seal faces are lubricated by thin film between rotating and stationary

The distance between the stationary and Rotary Faces are 0.0001mm

Essential Requirement of proper operation of Mechanical Seal

1.Seal faces must be flat and polished.
2.seal faces must be perpendicular to the shaft.
3.sufficient spring force for maintaining the contact
4.sufficient lubrication between seal faces

Flatness of seal faces 0.0008mm obtained by lapping

Mechanical seal faces material :

Usually dissimilar materials are used for seal faces. one face will be soft material like carbon and another material will be harder like silicon carbide and tungsten carbine. However for abrasive fluids both faces are harder materials are used.

Requirement of spring in mechanical seal

A spring member is used to maintain the contact between the faces.It pushes one face to contact with the other.

1.single spring
2.multiple springs
3.metal bellows

Types of Mechanical seal:

Pusher
Non pusher or bellow seals

Balanced
Unbalance seal
inside mounted
outside mounted
catridge design

Seal Orientation

1.face to back used in tandem un pressurized dual seals
2.Back to back: both flexible seal elements between seal faces used in pressurized dual seal
3.face to Face : Both seal faces are between flexible elements used in pressurized dual seal

Codification Based on API 682

Based on API 682 all mechanical seal shall be cartridge type
C2 A1 C 11 62

C2 category
A1 seal arrangment
C Type
11 Flush Plan
62 Quench Plan 62

Category 1 seal chamber temperature from -40 to 260c and pr upto 22kg2

category 2 seal chamber temperature from -40 to 400c and pr upto 42kg2

A1 Single seal
A2 Dual un pressurized seal
A3 Dual pressurized seal

Dual pressurised seal

On this kind of seal the lubricating fluid is generated by the barrier fluid, which is 1.5 - 2kg more than the product pressure. If there is any leakage the barrier fluid penetrates the product.

Dual Mechanical seal Application:

In high pressure and temperature aggresive fluids

acid fluids
vaccum lines
chemical
food processing industry.

Mechanical Seal selection Basis

Metal parts: must be corrosion resistant steel, stainless steel , haste alloy, bronze.

Mating Faces: Must also be corrosion and wear resistant ceramic, carbon ,tungustun carbide, silicon carbide

pressure: Depending on pressure seal may be balanced or unbalanced.

Seal flushing

Seal Flushing is based on the operating condition of the pump. commonly used seal flushing plan are plan 11 and 23

Plan 11 - seal flush from pump discharge through orifice

plan 23 - seal flush plan from internal pumping device in seal chamber through cooler. its is used in hot water service

plan 32 - seal flush plan of injecting cool and clean water from external source.

seal flushing:

It prevents abrasive particles from entering seal faces.
It prevents sediments build up in seal faces.

seal quenching:

It prevent leaked water from contacting the atmosphere
It lubricates and cools the seal faces.

Identification of mechanical Seal Failure:

There is excessive loss of sealed product
Excessive loss of barrier fluid
There is excess process contamination

Common causes of Mechanical Seal Failure

Incorrect Fitting and Alignment
Loss of barrier fluid for lurication
incorrect seal selection
Most of mechanical seal fail due to process change or fault.
Shaft runout
Bearing failure

Single Mechanical Seal:

A single mechanical seal has one mating surfaces which is pressurized and lubricated by the sealing product. Its is the preferred solution for many applications.

Tandem Mechanical seal

It consist of two mechanical seal arranged in succession. seal near product is called primary seal or inboard seal.primary seal is lubricated and pressurized by product fluid.

The seal near atmosphere is called outboard or secondary seal.The outboard seal is lubricated by unpressurized buffer fluid.If primary seal fails , secondary seal will protect the leakage.The secondary seal is used only for additional protection

Tandem mechanical seal Application:

Volatile liquids, chemicals, hazardous liquids, food processing and pharmaceutical field.

Double Mechanical Seal:

It consist of two Mechanical seal with presurized barrier fluid circulating between them.The barrier fluid is always higher than the product fluid.The inboard and outboard seal are pressuried and lubricated by barrier fluid.

IF inner seal fails barrier fluid leaks into product . If outer seal fails barrier fluid leaks to the atmosphere, In both cases the product never leaks. This type of seal is used in hazourdous products.

Double Mechanical Seal application

High toxic and Explosive Hazardous fluids.

Most commonly used seal faces:

Carbon & silicon carbide

Where carbon faces cannot be used;

In most areas carbon faces are used. but in some areas two mating faces are metal like silicon and tungsten carbide.seal faces are selected according to the product characteristics.

In abrasive and acidic area, volatile fluids areas carbon cannot be used.

Reasons for selecting two hard faces:

carbon faces are succeptable to acid attacks
carbon faces cannot be used in abrasive applications like slurry, sea water,crude oil
food processing and pharmaceutical were carbon wear are not accepted
Duty temperature exceeding the carbon temperature range
Higher viscosity will tend to bond the faces together.

Causes of component Damage:

Mechanical Damage to seal face due wrong alignment and installations.
Chemical and corrosion attack due to incompability
cracks may devolpe in seal ring and o ring may get harder and brittle due to high temperature.
wear due to excessive misalignment and abrasives

Mechanical Seal life:

Normally a mechanical seal life will be five years when operated in clean stable media , service life may exceed five years, when operated in viscous and abrasive media it may fail within few months

Springs used in mechanical seal:

Single spring
Multi spring
Wave spring
bellows

mechanical seal types by arrangement

single seal:
inside mounted
outside mounted
Dual seal
pressurised (double seals)
un presurized seal( tandem seals)