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Packing & Mechanical seal

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MECHANICAL SEALS

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COMPARISON BETWEEN PACKING & MECHANICAL SEALS

The major functional differences are that: 1. Packing, sealed in an axial mode.2. Mechanical seals, sealed in a radial mode. Packing throttles pressure down over a relatively long axial area of the shaft.

Mechanical seals throttle pressure which shorter radial distance. Because of the different sealing modes, there is a difference in the amount of friction caused by the sealing device.

Anything from two to ten times more horsepower is consumed by packing compared to mechanical seals.

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The basic parts of mechanical seal

1. Primary seal

2. Secondary Seals

3. Hardware

4. Seal Plate

5. Shaft Sleeve

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Seals Parts

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Mechanical Seal Parts

Primary seal components:

These consist of an axially adjustable sealing ring and an axially fixed mating ring against which it slides, forming a seal.

Depending on the seal configuration, either the sealing or the mating ring may be the rotating or stationary element.

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Mechanical Seal Parts

Secondary seals components:

These consist of O-rings or other suitable gasketmaterials.

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Common Shapes For Secondary Seals

a) O Ring b) Wedge c) V- Ring d) U Cup

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Mechanical Seal Parts

Hardware

This consists mainly of the spring (s) and a retainer normally made of a series 300 stainless steel.

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Working Principle

* Mechanical Seal, seal in a radial mode

* Two flat sealing faces at right angle

* Rotating face is flexibly mounted

* Two faces are loaded by the spring(s)

and drive collar

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Working Principle

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Working Principle

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Working Principle

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Working Principle

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Working Principle

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Common leak path

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1. Shaft sleeve to shaft.2. Rotating seal face to shaft sleeve.3. Stationary element to gland plate.4. Gland plate to pump case.

Potential Secondary Seal Leakage Points

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Types of Mechanical Seals

Classified into two groups

1. By Arrangement

2. By design

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Types of Mechanical Seals

2. Classified by designa) Single Springb) Multiple Springc) Pusher typed) Non-pusher typee) Balance Sealf) Unbalance Sealg) Conventional typeh) Cartridge type

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Types of Mechanical Seals

1. Classification by arrangement

1.1 Single seal installation

a) Internal (inside) seal

b) External (outside) seal

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Types of Mechanical Seals

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Types of Mechanical Seals

1. Classified by arrangement

1.2 Multiple seal installation

a) Tandem sealsb) Double sealsc) Dual seals

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Types of Mechanical Seals

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Types of Mechanical Seals

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Types of Mechanical Seals

2. Classified by designa) Single Springb) Multiple Springc) Pusher typed) Non-pusher typee) Balance Sealf) Unbalance Sealg) Conventional typeh) Cartridge type

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Spring

Function

To supplied pressure between the two seal faces

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Spring

SINGLE SPRINGS - ADVANTAGES1. Variations in face loading can be attained with a single coil spring, but

the rate of deflection is such that it requires a large axial displacement

to effect a relatively small force change.2. A corrosion allowance can be built into larger cross section wire.

DISADVANTAGES1. Shaft rotation is a big factor in single-spring seal applications. The

spring should be placed in a retainer to resist unwinding. The retainer

is large and heavy, introducing a balance and vibration problem.

2. The unwinding of the spring can be coped with by providing either a

right-hand or left-hand coil so that rotation tends to tighten the coil.

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Single spring

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SpringMULTIPLE SPRINGS - ADVANTAGES1. The multiple-spring design has a shorter axial stuffing box space

requirement than a single coil spring.2. Multiple combinations of the same spring can be used with seals of

many sizes.3. Face loading (about 30 lb/f) can be more readily varied simply adding

or subtracting springs.4. Multiple-springs also resist unwinding from centrifugal forces to a much

higher degree than single coil springs, since the forces act differentlyand the springs are caged in retainers.

DISADVANTAGES1. The small cross section of the wire can corrode easily. However,

stainless steels and Hastelloy can compensate for much of this disadvantage.

2. The small springs are more likely to clog from debris.

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Multiple spring

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Spring Metal Bellows

Creates a more uniform and constant face loading between the rotating and stationary seals

More readily conforms to shaft misalignment than spring type seals

Can handle a range of pumping temperatures from a - 320 to + 8000 F, and pressures greater than 300lb/in2. The main disadvantage of metal bellows seals is the initial cost.

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Bellows

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Pusher Type

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Pusher Type

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Elestomeric bellow non pusher seal

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Metal bellows non pusher seal

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Unbalance Seals

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Liquid film wedge between seal faces

Thin film of fluid must exist across the faces, providing seal face lubrication.

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Pressure acting on sealing ring

The unit loading between seal faces of the unbalanced seal increases as the hydraulic pressure being sealed increases.

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The balance seal

Casing pressure also acts on the opposite raised surface of the sealing ring. Two end areas of the sealing ring above the shaft's balancing diameter are equal

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Pressure acting on sealing ring

Rotating face is in perfect hydraulic balance

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Single Balance Seal

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Multiple Seals

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Cartridge type

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Cartridge type

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Bellows

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Tandem Seals ArrangementBarrier fluid pressure < Inner seal flush pressure

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Dual SealsBarrier fluid pressure > Inner seal flush pressure

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Double SealsBarrier fluid pressure > Seal camber pressure

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Multistage centrifugal pump

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Tandem Seal

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Flushing line