skv seals presentation.ppt

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Mechanical Seals

Need for seal in a pump:

When a shaft extends through a wall of a container and the

fluid in the container must not escape through the gap

between the container and the shaft emerging out, a seal

is required.



Mechanical Seals



Earliest Method of sealing:

Packing:



Packing :

Mechanical packing will function properly only if it

operate with a certain amount of leakage.This leakage is

mandatory, but for which the packing gets damaged dueto the excessive heat produced due to friction. So packing

is just a restricting device but not a complete seal.

Disadvantages:

Leakage

Sleeve/Shaft wear

Excessive friction and power loss

Can’t be used in case of hazardous substances as

certain amount of leakage is a must.

Increased down time due to frequent packing

replacements



A Mechanical seal is a machine element that prevents

fluid from escaping a pump at a rotating shaft while maintaining

the contacts of mating faces relative to each other.

Mechanical seal

All these difficulties faced with the packing have led

to the development of the Mechanical seal.



Advantages of Mechanical Seals:

• The leakage ratio of packing to seals is 1080:1

• Reduced friction and hence power losses

Elimination of wear on Shaft or Shaft sleeve

• Zero or controlled leakage over a long period

• Relatively insensitive to shaft deflection or end

play

• Can be used for hazardous & toxic fluids.

• Freedom from periodic Maintenance.

Disadvantages :

• High initial cost

• Requires services of expert personnel.

• Calls for employment of good flushing methods



Parts of a simple mechanical seal



Leakage paths in a Mechanical seal

1

4

2

3



Types of Mechanical seals

Inside

Outside

Single

Back to Back

Face to Face

Tandem

Double

By Arrangement

Single spring

Multi Spring

Unbalanced

Balanced

By Design

Classification



Classification of Mechanical Seals

Unbalanced :

These seals are generally used only in low pressure

applications.Their usage is limited because of the

high friction developed due to the high closing force

developed by fluid pressure.They are used for pressures below 10 bar.



Balanced seals:

In this type of seal, the effect of fluid pressure on the closing

force of the seal ring is eliminated.The closing force is

obtained only by the spring force.Thus the life of the mating

faces is enhanced and can be used for any stuffing box

pressure.



Single coil spring Seals: These are simple in

design with a commendable performance in

environments that do not exceed the mechanical

limitations.The main limitation of single coilspring is that it distorts at high surface speeds

which can lead to excessive wear and

leakage.Hence the usage is limited to low speeds.

Multi- coil: This is a more sophisticated design

utilizing a number of small coil springs neatlyspaced around the rotary unit.These small springs

don't distort at high speeds and exert an even

closing pressure on the seal ring all times.



Inside Seal: An inside seal is fitted inside the stuffing

box.Installation can’t be adjusted without dismantling the

pump.The material of construction are selected to withstandthe passing corrosive liquids in the stuffing box.

Single seals



Outside Seals : An outside seal is fitted outside the

stuffing box.It is easily accessible for adjustments and

trouble shooting.It can be used for moderate pressures.It is found suitable in case of an extremely

corrosive liquids having satisfactory face lubricating

properties.



Double Seals

When selecting a seal for a pump, liquids are

sometimes encountered that are not compatible with a

single stage mechanical seal.Often these liquids are

toxic and may carry abrasive materials in suspension

that would rapidly wear the faces or the liquid may be

so corrosive that extreamly expensive materials would

have to be used for the seal components.In such cases

it is a privilege to use these seals in pairs of two singleseals with a pressurized compatible buffer fluid

circulation system.



Tandem seal : The purpose of this seal is to provide a back up seal

in the event the inner seal fails.The cavity between the inner and

outer seal is flooded from a closed reservoir. If the inner seal fails,

the resulting pressure rise in the area between seals is sensed atthe reservoir where it can either be registered in a gauge or

activate an alarm.



Double seal(face to face):

Due to the availability of a limited space inside

the stuffing box by the way of depth, face to face seals

are used.The limitation of this type is exposure of inner seal to the process media. Viscous, abrasive,

thermosetting or corrosive products can damage the

inner seal.



Seal Selection Criteria

The following are some parameters based on which

the type and materials of construction of amechanical seal are selected -

Media

Pressure

Temperature

Speed

P V factor

F il i h i l l



Failures in mechanical seals

• Overall Chemical attack

• Chemical attack on O- rings

• Fretting

• Leaching

• Face Distortion

• O- ring extrusion

• Erosion of seal face

• Blistering

• Clogged Spring

• Thermal failure

SOME CAUSES FOR SEAL FAILURE



SOME CAUSES FOR SEAL FAILURE :

1.Operating pump at near shutoff conditions.

2. Cavitation

3. Inadequate amount of liquid at sealing faces.

4.Liquid film flushing and evaporating between the seal

faces and leaving residue which can grind away the carbon.

5.Pressure inside the stuffing box too high for type of seal

and product.

6. Improper installation of seal.

7. Misalignment of equipment driver.

8. Carbon insert cracked or face of insert or seal ring

chipped during installation.

9.Scoring of seal faces due to foreign particle entry.

E i d



Equipment start up procedure

• Observe safety precautions.

• After installation, check for the shaft for free movement.

• Start all auxiliary systems( i.e. flush,quench, abrasive

separators etc.).During actual start up,first activate auxiliary

systems, then start shaft drive motor.

• Make sure all valves to the seal are open so that the sea is

ensured of a positive flow of liquid at all times

• For environments that solidify when exposed to

atmosphere, turn heating arrangements prior to start up.

• Observe and correct vibration and any other equipment

abnormality.

• Ensure that the product temperature and operatingressures a free with in the recommended limits.



Running Checks

• Excessive heat should not be generated.This may

indicate stationary parts rubbing against rotating parts.Stop and correct immediately.

• There should be no noise coming from the stuffing

box.

• Check for leakage around the gland ports.

• If any thing appears abnormal, stop equipment and

take corrective actions.

• Gland icing, carbon dust accumulation on the outside

of the gland,squealing- all these conditions are

symptomatic of an inadequate fluid between the two

seal faces.

S l Fl h Pl



Seal Flush Plans









DRY GAS SEALS

• The working principle of dry gas seals is the hydrostatic andhydrodynamic force balance.

• Hydro Static = spring load + gas pressure

these are present even when the faces are stationary or inrotation

• Hydro Dynamic = opening force

this is generated only upon rotation.

• The spiral grooves play a vital role in the generation of opening force.

The gas enters the groove and is induced towards the center

where it is compressed meeting the resistance of the sealing dam.The spirals are recessed into the seat to a depth of 0.0025mm

• Pressure is increased causing the flexibly mounted face to lift off,thus setting the sealing gap.

During normal running the lift- off will be three microns.

ADVANTAGE OF DRY GA EAL OVER OIL EAL



ADVANTAGES OF DRY GAS SEALS OVER OIL SEALS

1. Conventional seal oil systems comprise complicated spaceconsuming equipment like over head tank, pumps, de gasifyingtanks etc. These are not required in a dry gas seal.

2. Contamination of gas with oil and oil with gas can becompletely eliminated.

3. These are NON- CONTACT seals. So less power consumptionand zero wear.

4. Conventional oil seals need regular maintenance where as drygas seals need nominal maintenance such as o-ring replacementafter four or five years.

ADVANTAGES OF DRY GAS SEALS OVER LABY SEALS

In high pressure applications the number of stages oflaby seals should be more to dissipate the pressure energy inthe form of eddies.During this process, the shaft lengthincreases which causes some other problems like its supportingetc.

This is overcome by employing dry gas seals

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