mechanical seals operating principles.docx

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7/27/2019 Mechanical Seals Operating Principles.docx http://slidepdf.com/reader/full/mechanical-seals-operating-principlesdocx 1/19 Mechanical Seals Operating Principles Essential elements of a mechanical seal  These are the three essential elements of a mechanical seal:  Seal faces: one rotating with the shaft and one stationary in the pump casing, cover or flange. Secondary seals: one to seal the rotating face to the shaft and one to seal the stationary face to the pump cover or flange. Metal parts: to transmit torque and to provide an axial mechanical force to load the faces.  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.  

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Mechanical Seals OperatingPrinciples Essential elements of a mechanical seal These are the three essential elements of a mechanical seal: Seal faces: one rotating with the shaft and one stationary in the pump casing, cover orflange. Secondary seals: one to seal the rotating face to the shaft and one to seal the stationaryface to the pump cover or flange. Metal parts: to transmit torque and to provide an axial mechanical force to load the faces. 

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. 

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 The fluid in the pump and seal area Key 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 amixture. 

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.   The large majority of mechanical seals never wear out and are removed from service

for some other reason.   Seal failures occur for a wide range of reasons. Some failures occur as an

interaction with the tribology of the interface. Effective forces in a Mechanical Seal These are the forces operating in mechanical seals: 

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

Leakage of a liquid lubricated mechanical seal  

Key point: leakage rate Q strongly depends on the gap height h   The gap height is determinate by several factors: materials, manufacturing quality,

lubrication regime, face distortions.   The leak rate of a contacting seal is also influenced by other pump related factors

such as run outs and vibration levels. 

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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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Lubrication regimes of liquid lubricated mechanical seals 

Seal Balance To reduce the axial face contact force which allows to seal high pressures, i.e. up to 3000psig with one set of faces. It is the ratio (k) of 2 geometric areas: the closing (Ah) and opening area (Ac) For unbalanced seals k = 1 For balanced seals k = 1 

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 Mechanical seals are classified by arrangement and configuration 

Mechanical seals are classified by arrangement and configuration. The wide variety of seal types is due to the diversity of applications each utilizing differentmachinery, fluids and processes. Selection of the best type is not always easy and straight forward as there is usually acompromise between economical and technical factors. Mechanical seal classification by arrangement:

Single seals 

Inside mounted = pressure on outsidediameter of parts  Outside mounted = pressure on inside

diameter of parts 

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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 hazardousgases, dangerous chemicals or flammable liquids, will generally use single seals. Mechanical seal classification by arrangement:

Dual seals 

Pressure between seals is higher than sealchamber 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". 

Pressure between seals is lower than sealchamber pressure (typically atmospheric). External fluid only lubricates the most

outside set of faces. The most inside faces

are lubricated with the pumped fluid. Themost outside seal serves as a safety seal orcontainment device. Leakage to the atmosphere is external fluid,

possibly mixed with small amounts of pumped fluid. Is also called a "Tandem seal".

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

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Mechanical seal classification by arrangement, i.e.design 

Classification by pusher vs. non-pusher and balanced vs. non-balanced Pusher vs. Non-pusher Pusher 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 orsleeve. 

Defined by the secondary seal type: o-ring or polymer wedge versus bellow, rubber ormetal. Application fields of each type overlap. Most apparent distinction is the pressure limit. Acquisition cost can vary widely. 

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Balanced vs. Unbalanced 

Reduced closing forces Reduced power consumption For pressure up to 3000 psig Always recommended for volatile liquids 

High closing forces Low leakage For pressure up to 200 psig Not recommended for volatile liquids 

Classification by 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. Stationery rotating seals and rotating spring seals 

Stationary spring seals are recommended by high speeds > 5000 ft/min. Stationary spring seals are more suitable for machinery with inherently larger tolerancessuch a heavy duty slurry pumps and older pumps which have looser tolerances. 

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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. 

Split seals Seat is axially split. Does not require disassembly of thepump 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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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 - Characteristics 

A - Clean Lubricating

B - Clean Non-lubricatingC - ViscousD - Clogging / Scaling / Polymerizing / Fibrous

E - Crystallizing

F - Molten LiquidG - Corrosive - Acids

H - High Vapor Pressure

I - Cryogenic

J - 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). Typical dynamic pressure and temperature limits of common seal types 

Seal Type  Pusher  Non-pusher  Balanced  Unbalanced  Max. Pressure

(kPag/psig)  Temperature Range

(ºC / ºF) Elastomeric

bellows  x  x  2070 / 300  -40 to 205 / -40 to

400 Elastomeric

bellows  x  x  6900 / 1000  -40 to 205 / -40 to

400 Metal bellows  x  x  2070 / 300  -75 to 425 / -100

to 800 

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O-ring

secondary

seal  x  x  1380 / 200  -40 to 260 / -40 to

500 

O-ring

secondaryseal  x  x 

6900 / 1000  -40 to 260 / -40 to

500 Polymer

secondary

seal  x  x  1380 / 200  -75 to 260 / -100

to 500 

Polymer

secondary

seal  x  x  5070 / 500  -75 to 260 / -100

to 500 

Stationaryslurry  x  x  2670 / 400  -40 to 205 / -40 to

400 Split seal  x  x  1380 / 200  -40 to 205 / -40 to

400 Dual gas seal  x  x  2070 / 300  -40 to 260 / -40 to

500 Dual gas seal  x  1725 / 250  -40 to 260 / -40 to

500 

Typical PV – Limits of face material combinations in non-lubricating fluids,

i.e. watery substances PV = face pressure x velocity Is an indicator for the severity of an application Is limited in usefulness For lubricating fluids multiply number by 1.5 

Primary Ring  Mating Ring  PV Limit

(MPa x m/s)  PV Limit

(psi x ft/min) 

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Glass-Filled PTFE  Ceramic / Silicon

Carbide  6.13  25,000 

Carbon  Cast Iron  24.52  100,000 Carbon  Ceramic  24.52  100,000 Carbon  Tungsten Carbide  122.59  500,000 Carbon  Silicon Carbide  147.11  600,000 Tungsten Carbide  Tungsten Carbide  24.92  120,000 Silicon Carbide  Silicon Carbide  85.81  350,000 

Typical angular misalignment limits Shaft Speed

(rpm)  Pusher & Metal

Bellows

(mm) Pusher & Metal

Bellows

(in) Elastomer

Bellows

(mm) Elastomer

Bellows

(in) 500  0.152  0.006  0.279  0.011 1000  0.127  0.005  0.254  0.010 2000  0.089  0.0035  0.191  0.0075 3000  0.064  0.0025  0.152  0.006 4000  0.051  0.002  0.127  0.005 5000  0.038  0.0015  0.089  0.0035 6000  0.025  0.001  0.151  0.002 

Overview of mechanical seal drive mechanisms Wide variety of methods which will depend on the component: drive collar, seal face andsleeve. Drive mechanisms transmits torque from the shaft to the rotating face, keep the stationaryseal face from spinning in the pump flange and fix the drive collar to the shaft. 

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In cartridge seals the sleeve will have an axial force from the hydraulic piston effect. Itsdrive mechanisms is used to keep the sleeve from moving axially. Space in the pump may be an important factor. Shaft material hardness may be critical. Drive mechanisms can wear out prematurely if excessive run out occurs in the seal area of the pump. 

Drive Mechanisms for drive collars and seal faces  

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Drive mechanisms for seals sleeves of cartridge type seals 

Description of seal faces loading devices Wide variety of types but they can be categorized as either a spring or a bellows of somekind. Seal face loading devices impart an axial load to maintain contact when there is nohydraulic 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 thecentrifugal effects. 

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 Mechanical seal mating ring types Wide variety of shapes. Its function is to provide a flat surface for the other face to run against. Be aware of clamped designs since bolt forces can create waviness. Support surfaces for mating rings may require a high degree of flatness to avoid waviness. 

Mechanical Seal Mating Ring Types 

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 Functions of mechanical seal glands Support stationary components. Contain throttle bushing. Allow for seal setting. Provide centering of seal components. Provide port location for flushes. 

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Bushing types

The purpose of bushings are the following: They direct leakage from seal. Bushings minimize leakage under seal failure. They provide isolation for quench. They protect seal from radial sleeve motion. 

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Cartridge Seal With Fixed Bushing 

Cartridge Seal With Floating Bushing