mechanical seals operating principles.docx
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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