Mechanical Seals Operating Principles Essential elements of a mechanical seal These are the three essential elements of a mechanical seal: Seal faces: faces: one rotating with the shaft and one stationary in the pump casing, cover or flange. Secondary seals: seals: one to seal the rotating face to the shaft and one t o seal the stationary face to the pump cover or flange. fl ange. Metal parts: 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.
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 a mixture.
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 i mmeasurably 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:
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.
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.
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 3000 psig 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
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 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.
Mechanical seal classification by arrangement: Single seals
Inside mounted = pressure on outside diameter of parts
Outside 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.
Mechanical seal classification by arrangement: Dual seals
Pressure between seals is higher than seal chamber pressure (typically min. 30 psig).
Pressure between seals is lower than seal chamber pressure (typically atmospheric).
External fluid lubricates both sets of faces.
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. Is also called a "Double seal".
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.
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 or sleeve.
Defined by the secondary seal type: o-ring or pol ymer wedge versus bellow, rubber or metal. Application fields of each type overlap. Most apparent distinction is the pressure limit. Acquisition cost can vary widely.
Balanced vs. Unbalanced
Reduced closing forces
High closing forces
Reduced power consumption
Low leakage
For pressure up to 3000 psig
For pressure up to 200 psig
Always recommended for volatile liquids
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 i mprove 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 tolerances such a heavy duty slurry pumps and older pumps which have looser tolerances.
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 the pump to install = reduce down time. Leaks more than a conventional seal. More costly as compared to conventional seal.
Classification by containment devices
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-lubricating C - Viscous D - Clogging / Scaling / Polymerizing / Fibrous E - Crystallizing F - Molten Liquid G - 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 size M - Solids (> 2% by volume).
Typical dynamic pressure and temperature limits of common seal types Seal Type
Pusher
Non-pusher
Elastomeric bellows
x
Elastomeric bellows
x
Metal bellows
x
Balanced
Unbalanced
x
x
x
Max. Pressure (kPag/psig)
Temperature Range (ºC / ºF)
2070 / 300
-40 to 205 / -40 to 400
6900 / 1000
-40 to 205 / -40 to 400
2070 / 300
-75 to 425 / -100 to 800
O-ring secondary seal
x
O-ring secondary seal
x
Polymer secondary seal
-40 to 260 / -40 to 500
6900 / 1000
-40 to 260 / -40 to 500
1380 / 200
-75 to 260 / -100 to 500
5070 / 500
-75 to 260 / -100 to 500
2670 / 400
-40 to 205 / -40 to 400
1380 / 200
-40 to 205 / -40 to 400
2070 / 300
-40 to 260 / -40 to 500
1725 / 250
-40 to 260 / -40 to 500
x
x
x
x
Polymer secondary seal
x
x
Stationary slurry
x
x
Split seal
x
x
Dual gas seal
x
x
Dual gas seal
1380 / 200
x
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)
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 and sleeve. Drive mechanisms transmits torque from the shaft to the rotating face, keep the stationary seal face from spinning in the pump flange and fix the drive collar to the shaft.
In cartridge seals the sleeve will have an axial force from the hydraulic piston effect. Its drive 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
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 some kind. 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.
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 fl atness to avoid waviness. Mechanical Seal Mating Ring Types
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.
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.
Cartridge Seal With Fixed Bushing
Cartridge Seal With Floating Bushing
