Fundamental of Mechanical Seal

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PEM 3 KNOWLEDGE SHARING PRESENTATION

FUNDAMENTALS OF MECHANICAL SEALPresented by

Emmanuel Raphael Essen (MNIMechE, MNIM, ISMN)

October, 2014PEM 3 Knowledge sharing Presentation, October 2014

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Tables of content. Historical development of a Mechanical seal What is Mechanical Seal Reason for using Mechanical seal Advantages of Using cartridge seal. Types of seals: Essential elements of a mechanical seal Mechanical seal Components Essential requirements for proper operation of a mechanical seal The fluid in the pump and seal area, Leakage, Seal failures and Leakage Rate. Effective forces in a mechanical sealPower consumption of a liquid lubricated

mechanical seal Seal Face Material Power Consumption Classification of Mechanical Seal Many factors need to be considered in the application of a mechanical seal Mechanical seal application limit Seal faces loading devices Flush arrangement Selecting a Mechanical seal Mechanical seal piping plan

PEM 3 Knowledge sharing Presentation, October 2014

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Historical Development of a Mechanical seal

At the beginning of the nineteenth century, many endeavours were made to develop a replacement for the conventional, braided packing used for piston pumps and rotating shafts.A more reliable system for different kinds of liquid-conveying rotating machinery was desired.

By the 1930’s, the James Walker Group came up with a mechanical shaft seal for refrigeration compressors. At the same time, the John Crane company invented the first automotive mechanical shaft seal. In the early 1940’s, the company developed and introduced the patented elastomer bellows axial shaft seal, today known as “Type 1”.

After this breakthrough in sealing technology, other types of mechanical shaft seals were developed. With several types of mechanical shaft seals, the John Crane company adopted the tagline, “The right seal for the right application”.

Today, John Crane is still a leading seal manufacturer alongwith Grundfos, Burgmann, Flowserve, etc.


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What is Mechanical Seal Mechanical seal is a sealing device for any rotating equipment which prevents the inner fluid from leaking through the clearance between the rotating shaft and the housing case. There are various types of mechanical seals for each application and operating condition of the equipment but in principle, mechanical seal is a face type seal, which performs sealing by two sealing faces vertical to the shaft.

Mechanical Seal incorporates various components such as the spring giving contact force and the packing which eliminates intrusion of the fluid from the outside. The seal face is an accurately flat face, and so if it is worn, it does not cause the leakage of the sealant as far as wear is within the maximum allowable amount.

Therefore it can be used for a long time free of maintenance, and is very effective and efficient in terms of reliability and economy

Mechanical seals are used not only in pumps but also in other various equipment such as mixers, agitators, compressors, rotary unions, submersible motors, etc.


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Reasons for using Mechanical seal: To minimize leakage To prevent toxic fluids escaping to atmosphere To reduce power loss.

Advantages of Using cartridge seal. By using cartridge seals the following can be achieved: Reduced skill base Increased Reliability Fits multiple types of pumps Reduced downtime Increased operating profits.

Types of seals: Static Seals & Dynamic Seals

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


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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

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


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Essential elements of a mechanical sealThe basic design of the Mechanical seal consists of the following elements: Flexibly mounted seal face, Rigidly mounted seal face, Compression device and Secondary seal

Mechanical seal Components Rotary seal face Stationary seal face Springs Retainer Sealing /flushing media O-rings


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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

The Fluid in the Pump and Seal AreaKey Point: The fluid contacts the seal faces and other parts in wide open areas, in very small gaps and at the exit of the seal faces. Pressure and temperature of the fluid will depend on its location and determine its respective state, i.e. liquid, gaseous, solid or a mixture.


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Leakage:

A few facts about the leakage (and wear) behavior of contacting mechanical seals:It is essential for proper lubrication and wear of the faces. Normal leak rates range between immeasurably small to steady drips or temporary to even small steams. Some seals leak some of the time, some seals never leak (measurably), and some leak all the time.

Leakage patterns can be constant, progressive or erratic in nature. It can be in liquid, gaseous and/or solid state. Successful contacting seals tend to have very low wear rates and low leakage rates. Some forms of contact is necessary for low leakage rates. Non-contacting or “full lift off” seals (hydrostatic or hydrodynamic tend to have visible, sizeably larger leakage rates.


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The large majority of mechanical seals never wear out and are removed from service for some other reason. Seal failuresSeal failure occur for a wide range of reasons. Type of seal failures

Lubrication failures: Dry running and poor lubrication

Contamination failures: Clogging, Hang-up, Particles deposits, Sticking and seizure.

Chemical, physical degrading and wear: Ageing and swollen rubber parts, Corrosion, wear, Explosive decompression

Installation failures: Shaft misalignment, seats not mounted perpendicular to the shaft, axially moving shaft and wrong assembly length.

System failures: The pressure in the seal chamber, the temperature around the shaft seal in the seal chamber, the pumped medium, the speed, the shaft seal dimensions.


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Leakage rate:

The leakage rate of a mechanical shaft seal depends of a number of factors such as:

Surface roughness of seal faces Flatness of seal faces Vibration and stability of pump Speed of rotation Temperature, Viscosity and Type of pumped medium Pump pressure Seal and pump assembly.

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Effective forces in a Mechanical Seal

These are the forces operating in mechanical seals:

Axial and radial forces Closing and opening forces Hydrostatic and hydrodynamic forces

Seal Face Materials: 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 of seal face materials: Carbon graphite, Aluminium Oxide (Alumina), Tungsten carbide, Silicon carbide, Diamond coatings


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Power Consumption of a liquid lubricated mechanical seal

Important Points:Face friction, churning and soak in heat.Flush to dissipate the heat in order to control the gap temperature.Coefficient of friction can swing considerably during operational transients.The key is to maintain the gap profile as parallel as possible, i.e.minimize distortions.


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Classification of Mechanical Seal

Mechanical seals are classified by arrangement and configuration.

The wide variety of seal types is due to the diversity of applications each utilizing different machinery, fluids and processes.

Selection of the best type is not always easy and straight forward as there is usually a compromise between economical and technical factors.


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CLASSIFICATION BY ARRANGEMENT: 1. SINGLE SEALS: (a) Single internally mounted seal, (b) Single externally mounted sealInside mounted = pressure on outside diameter of partsOutside mounted = pressure on inside diameter of parts The inside mounted mechanical seal is most popular type of single mechanical seal.Most seals are designed to leak so that the liquid or gas will lubricate the seal faces. Applications that do not utilize substances that must be contained, such as hazardous gases, dangerous chemicals or flammable liquids, will generally use single seals. E.g P-1560, P-1570

Inside Mounted-Pressure on outside diameter of the parts

Outside Mounted-Pressure on inside diameter of the parts


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2. Dual sealsPressure between seals is higher than seal chamber pressure (typically min. 30 psig).External fluid lubricates both sets of faces.Leakage to the atmosphere is external fluid.Is also called a "Double seal".


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Pressure between seals is lower than seal chamber pressure (typically atmospheric).External fluid only lubricates the most outside set of faces. The most inside faces are lubricated with the pumped fluid. The most outside seal serves as a safety seal or containment device.

Leakage to the atmosphere is external fluid, possibly mixed with small amounts of pumped fluid.Is also called a "Tandem seal".

DOUBLE SEALS: (a) Back to back seals (b) Face to face seals © Tandem seals


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Faces can be configured in several ways: face to back, face to face and back to back.

Mechanical seal classification by Consfiguration i.e.design


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1. Design: Pusher vs. Non-pusher and Balanced vs. Non-balancedPusher vs. Non-pusherPusher 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 Vs Non- balanced

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

2. Face Pattern

Examples are hydro-grooves, wavy faces, tapered faces.Intended to increase opening forces in order to improve lubrication.Friction is reduced at the expense of a higher leak rate.

3. Stationary Spring Seals and Rotating Spring seals


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Stationary spring seals are recommended by high speeds > 5000 ft/min.

Stationary spring seals are more suitable for machinery with inherently larger tolerances such a heavy duty slurry pumps and older pumps which have looser tolerances.

4. 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.


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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.

Classification by containment devices


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Factors need to be considered in the application of a mechanical seal Here are some of the factors that need to be considered: 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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Mechanical Seal Application Limit

General application guide per seal type

Seal Type Applications Non-pusher elastomeric bellows seal A - B - D - E - L Non-pusher metal bellows seal A - D - E - F - I - J - L Pusher O-ring secondary seal A - B - G - H - K Pusher polymer seal A - B - G - K Pusher stationary slurry seal A - B - C - D - E - F - M Pusher split seal A - B - K Pusher dual gas seal A - B - E - F - G - H - L

Fluid - CharacteristicsA - Clean LubricatingB - Clean Non-lubricatingC - ViscousD - Clogging / Scaling / Polymerizing / FibrousE - CrystallizingF - Molten LiquidG - Corrosive - Acids


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H - High Vapor PressureI - CryogenicJ - High Temperature (> 260 ºC / 500 ºF)K - Solids (< 0.1% by volume and less than 10 micrometers (394 micro inches) in size.L - Solids (< 2% by volume and less than 10 micrometers (394 micro inches) in sizeM - Solids (> 2% by volume).


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Seal Faces Loading Devices

Description of seal faces loading devicesWide variety of types but they can be categorized as either a spring or a bellows of some kind.

Working of a seal face loading devices

Seal face loading devices impart an axial load to maintain contact when there is no hydraulic pressure from the pumped medium.At higher pressures the spring force is only a small fraction of the overall face pressure.At face speeds above 5000 ft/min the spring element is installed stationary because of the centrifugal effects.


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Flush arrangements It refers to the various methods used to lubricate, cool and remove deposits and heat in mechanical seal.Purpose of Flush: Introduce fluid into seal chamber to improve the lubrication. Work to support the piping plan. Removal of seal generated heat. Removal of vapor bubbles. Protect against erosion

Selecting a mechanical seal The mechanical seal should be selected according to the operating conditions at the seal location. These important factors must be considered when selecting a mechanical seal: • Shaft seal diameter • Type of pumped medium • Temperature • Sealing pressure • Shaft speed of rotation.


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See next slides for

Seal Piping planP-1560, P-1570, P-

1580, P-1101 and P-4101


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For P- 1560, P-1570 and P-1580Propane Reflux Pump (De-propanizer ), De-butanizer Reflux pump and LPG Re-injection Pumps.

The sealant is supplied from a common sealant system (A-1502) which supplies the P-1560, P-1570 and P-1580 pumps.

Mechanical seal system of a single unit has the following:

AccumulatorHeat ExchangerPressure guageTemperature guage.

See Plan 53c


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P-1560 A/B

Duty Propane Reflux Pump (Depropaniser)Manufacturer BW/IP Inter.Co.Driver Electric MotorMotor Motor Speed 2970 rpmGear box NonePump Type Horizontal Barrel TypeSuctions SingleStages Two Seals Pressurised Seal SystemFluid Pumped PropaneDesign Temperature -46C approx. Sealant medium Aeroshell 31/41


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P-1570 A/B

Duty Debutaniser Reflux PumpsManufacturer BW/IP International B.V.Driver Electric MotorMotor Speed 2950 RPMGear box NonePump Type Horizontal Barrel typeSuctions DoubleStages 5 stageSeals Pressurised SystemFluid Pumped ButaneTemperature -8C approx. Sealant medium Aeroshell 31/41


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P-1580 A/B

Duty LPG Re-injection PumpsManufacturer SundstrandDriver Electric MotorMotor Speed 9466 RPMGear Box Yes. 9419 rpm final gearPump Type CentrifugalSuctions SingleStages SingleSeals Pressurised SystemFluid Pumped LPGTemperature -30C approx. Sealant medium Aeroshell 31/41


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P-1101 A/B Amine Charge Pumps

Manufacturer BW/IPDriver Electrical motorMotor Power 850 kWMotor Speed 2980 rpmGear Box NonePump Type CentrifugalSuctions SingleStages 4Seals Crane 8B1/AR151Fluid Pumped Lean Amine with dissolved gasesTemperature 55ºC

It takes fluid from the pump discharge through a pipe and flushes the seal chamber and empty itself back to the product


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P-4101Duty HTF Circulation PumpsManufacturer BW/IP Internationale BVDriver Electric MotorMotor Speed 1450rpmPump Type CentrifugalSuctions DoubleStages SingleSeals Crane 215 metal bellows, mech seal cartridge type.Fluid Pumped HTF Shell Thermia BTemperature 139.6oC.


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P-4111A/B Duty HTF Circulation PumpsManufacturer BW/IP Internationale BVDriver Electric MotorMotor Power 580kWMotor Speed 1450rpmGear box NonePump Type CentrifugalSuctions DoubleStages SingleSeals Crane 215 metal bellows, mech seal cartridge type.Fluid Pumped HTF Shell Thermia BTemperature 191oC

Fluid is taken to the seal chamber from pump discharge, cool the seal facesand discharges it back through a discharge pipe to the product line


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Thank you

Documents

Fundamental of Mechanical Seal