SUBJECT : The classification of
mechanical seals. 2-11
Talk to a seal manufacturer's sales representative, send for all
of the brochures, and you'll learn that the subject is very
confusing. What do we really know about mechanical seals? A few
things for sure:
- All legitimate seal companies have the essentially the same
materials available to them. There are no secret or "mystery
materials". There are, however, companies that purchase standard
materials and then change the name to give the impression that
they're supplying special materials and proprietary compounds.
Shame on them!
- We're not sure of what is happening between the lapped seal
faces. Testing has shown that :
- Sometimes there is a film of lubricant
- Sometimes the faces are running on vapor.
- Sometimes the faces run dry.
- Sometimes the faces run on a combination of all three
For all practical purposes seals should not leak visibly. Look at
the seal in the water pump of your automobile, or the seal in the air
conditioning unit in your car and ask yourself if you think either of
them is leaking. Fugitive emission regulations have changed the
definition of leakage to less than a few parts per million, depending
upon the chemical involved.
- More than 85% of all mechanical seals fail long before they
wear out. In other words there's plenty of wearable face left on
seals when they're removed because of leakage.
- Seals are affected by pressure
- Extra heat is created at the faces. Heat is almost always
- The lapped faces will be distorted at some pressure. If
they're distorted beyond five light bands ( 58 millionths of an
inch or 1,5 microns) of flatness they'll leak.
- The elastomer will be extruded and become damaged at some
- Seals are affected by heat.
- All of the seal materials have an upper temperature limit.
The elastomer (the rubber part) has the lowest. Its limit is
determined by the type and grade of material being used. Some
grades of carbon and most coated, or plated hard faces have
restricted temperature limits.
- Thermal expansion can misalign components, put lapped
surfaces "out of flat", and alter the seal face load.
- The pumping fluid can be altered if subjected to high heat.
It can crystallize, solidify, vaporize, coke etc. This will
restrict the free movement of the seal components.
- Corrosion always increases with heat.
- Stuffing box, or seal environmental controls are necessary a
great deal of the time.
- With the exception of split seals, most applications
require a large seal chamber with a stuffing box recirculation
line connected to the suction side of the pump.
- The temperature of the fluid in the sealing chamber should
be kept within certain limits. These limits are determined by
the specific gravity. viscosity, concentration, vaporization
point, etc. of the fluid.
- The pressure in the sealing chamber, often installed in
dual seal application, can be controlled to prevent
vaporization of the product or vaporization of the solvent
carrying the product.
- If the product presents too big a sealing problem it can
often be flushed away with a cool, clean, lubricating
- Non-clogging features are desirable and necessary in
mechanical seal design.
- The spring or springs should be located out of the sealing
fluid to prevent clogging and corrosion. Stressed metal
corroded at a rate faster than unstressed metal and the springs
are under a constant stress.
- The elastomer must move to a clean surface as the seal face
- Centrifugal force should be used to throw solids away from
the lapped seal faces.
- The moveable portion of the seal must move to a clean area
as the seal faces wear. "Back to back" double seals and outside
mounted seal designs should be avoided in dirty service.
- Vibration damping is necessary in all mechanical seals.
- To prevent the faces from vibrating open.
- To prevent damage and wear to the driving mechanism (drive
lugs, spring, or bellows) and seal faces.
- To prevent damage (chipping at the outside diameter) to the
- Back up seals or dual seals make sense in several
applications. These include:
- Costly products.
- Dangerous products. The danger could be an explosive, fire
hazard, carcinogen, toxic, bacteria, radiation, etc..
- When there is no spare pump and you can't afford an
unexpected shut down caused by a seal failure.
- To keep oxygen or air away from a product.
- To prevent a product pressure drop across the seal faces,
you can instll a back-up seal and pressurize beween the dual
- Sealing pollutants
- To prevent freezing of the shaft on the outboard side of a
- Proper seal face loading is more critical than you suspected.
It can change with :
- Improper installation.
- Thermal expansion.
- Impeller adjustment. This includes both the initial setting
and the adjustments that have to be made to compensate for
- Face wear
- Axial play in the shaft bearings. Especially the sleeve or
If you like the brand of seal you're using ,have the manufacturer
repair it at his facility, or purchase the spare parts from him to
insure you'll be getting the correct materials and tolerances. It
doesn't make sense to do anything else.
We'll now look at the various methods of classifying mechanical
seals and in the process learn which to specify for our applications.
I'll give a brief description of each type and list the most obvious
advantages and disadvantages of each. Needless to say the advantage
of one is almost always the disadvantage of the other
seal. The springs/ bellows rotates with the shaft
- Advantage. Lowest cost and reasonably self cleaning,
especially in those designs where the springs are located outside
of the sealing fluid.
- Disadvantage. Sensitive to
misalignment between the shaft and the stuffing box face. This
causes the seal to move back and forth twice per shaft revolution.
Gaskets and thermal expansion increase the misalignment problem.
Most of these designs cannot pass a fugitive emissions test.
The stationary seal The
springs/bellows do not rotate with the shaft.
- Advantage. Misalignment and sealing fugitive emissions is not
a problem unless the seal is cartridge mounted. Cartridge mounted
stationary seals need some type of self aligning feature.
- Disadvantage. Alignment requires
that the rotary unit be absolutely square to the shaft and, in a
cartridge seal, this is very difficult to accomplish because the
cartridge tends to cock the face when the set screws are tightened
to the shaft.
- Not your first choice in slurry applications because
centrifugal force will not throw the solids away from the
moveable components. Slurry is defined as solids in liquid.
Their size and quantity are not important.
- You must be careful when introducing cooling to this type
of seal because the unit does not rotate, causing an uneven
cooling of the lapped face.
- In recent years this seal has only been available in
cartridge designs adding to the misalignment problem and
increasing the initial cost considerably.
inside mounted seal. All components are in the pumping
- Advantage. The elastomer can move to a clean surface as the
seal face wears. Centrifugal force throws solids away from the
seal components allowing the lapped seal faces to stay in co
- Disadvantage. All the metal
components must be corrosion resistant to the pumping fluid.
- If the product solidifies or crystallizes when the pump is
stopped, the seal can become inoperable.
outside mounted seal. None of the metal components are in
contact with the pumping fluid. Most designs clamp to the shaft
rather than using set screws that damage ceramic or glass coated
- Advantage. This is the most common solution to non-metallic
- Disadvantage. Centrifugal force
throws solids into the lapped seal faces and prevents the sliding
components from moving freely.
- Higher stuffing box pressure can cause the retaining clamp
to slide on the shaft
seal. It has only one set of sealing faces.
- Advantage. Lowest cost and usually a simple installation.
- Disadvantage. The equipment will
be shut down with a seal failure. In most cases the resultant
excessive leakage cannot be tolerated.
More than one set of faces are installed in a variety of
- Back to back. The worst possible choice if used in the
rotating seal version. Stationary versions are acceptable because
the sealing fluid is located at the outside diameter of the seal
faces where we can take advantage of centrifugal force
- Tandem. One seal behind the other requiring a low pressure
buffer fluid between the seals. This arrangement cannot be used if
a higher pressure barrier fluid is required or desirable.
- Face to face. All of the advantage of tandem sealing without
the assembly problems. Usually the two seals share a common
stationary face. This can be dangerous if the stationary face
fractures because you'll lose both seals
- Concentric. One seal mounted inside the other, sharing a
common mating face. These seals require a lot of radial space and
are therefore usually limited to mixer applications. If the common
face fractures, you'll lose both seals.
In all of these configurations two-way balance should be specified
for safe operation.
- Advantage. Back up protection that will almost guarantee no
unexpected seal failure.
- Disadvantage. Higher cost, and in
some instances, space restrictions.
unbalanced seal. The seal faces are subjected to full system
hydraulic and surge pressures in addition to the spring
- Advantage. None, other than lower cost
- Disadvantage. Limited
application. Usually requires a larger seal inventory because both
balanced and unbalanced versions would have to be stocked for the
common shaft sizes.
balanced mechanical seal. The design allows the seal faces to
see only a small portion of the system hydraulic pressure.
- Advantage. WCan handle a wde range of operating conditions
from vacuum to high suction pressure, as well as unexpected
pressure surges in the system.
- Disadvantage. The inside version
(the most popular one) requires more radial room because of the
need for a balance sleeve. The sleeve also adds to the initial
cost. In cartridge seals it would be silly to build an unbalanced
version, because the sleeve is always present, but some companies
do it any way.
elastomer type of seal , utilizing an o-ring, wedge, chevrons
or a U-cup, with the o-ring type having the most advantages.
- Advantage. The elastomer acts as a natural vibration damper to
prevent face chipping and separation. Only the o-ring, version can
be used for either vacuum or pressure. The o-ring, configuration
has the widest selection of materials available and is the most
precision elastomer you can purchase.
- Disadvantage. All elastomers have
temperature limits. Some modern elastomers have an upper limit of
about 700 degrees Fahrenheit (370 C.)
metal bellows seal. All elastomers have been eliminated from
- Advantage. Wider range of temperature sealing. Excellent in
Cryogenic sealing and most hot fluids with the exception of
petroleum products. These petroleum products must be cooled to
- Disadvantage. Higher cost than
comparable elastomer sealing. Problems with vibration unless
dampers have been installed. All of the low expansion metals used
in these designs are not considered corrosion resistant. 316
Stainless steel is not acceptable because of Chloride Stress
problems. In abrasive, slurry applications the thin plates are
sensitive to wear and eventual fracture.
cartridge seal. All components of the seal are mounted on a sleeve that can be
secured to the shaft from out side the seal chamber.
- Advantage. An easy method of seal installation and a necessary
feature for impeller adjustment. These designs allow you to change
the seal with out emptying a side entering mixer application. Seal
centering is provided for in most designs. There is an advantage
to specifying A.P.I. type glands to take advantage the
environmental controls and safety these glands provide.
- Disadvantage. Larger space
requirement and higher initial cost. When used with stationary
seals, you lose the advantage of total misalignment compensation
unless the seal has some type of "built in" self-aligning
non-cartridge seal. The seal attaches directly to the pump
shaft or sleeve, or in some cases, against a shaft shoulder.
- Advantage. None at all, except for lower initial cost and the
fact that it takes less radial room than most cartridge
- Disadvantage. Subject to all the
errors that can be made at an installation. Longer installation
time and the seal is unable to compensate for temperature growth
or impeller adjustment.
seal. In a true split seal all the seal components are split
in half so that the seal can be installed without taking the
equipment apart. Rubber components are not allowed to be glued
together as this would cause a "hard spot", interfering with the free
movement of the dynamic elastomer.
- Advantage. The advantages are obvious. No one wants to
disassemble any piece of rotating equipment unless it's absolutely
- Disadvantage. Limited seal
materials available and some designs cannot be used in
applications that cycle between pressure and vacuum because
pressure assists in holding the components together.
seal. The equipment must be disassembled to install the
- Advantage. Can be used in alternating pressure/vacuum
applications and, for the time being has a wider range of
- Disadvantage. Insulation must be
removed. Several trades could be involved, the pump must be
realigned and the list goes on and on...
seals. They have larger internal clearances along with
different spring arrangements and wider hard faces to compensate for
excessive radial and axial motion. A radial movement capability of
plus or minus 1/8" (3 mm) would be typical.
- Advantage. Ideal for mixers, agitators, sleeve bearing
equipment, or any type of rotating equipment requiring excessive
axial or radial movement.
- Disadvantage. Larger radial space
required. 3/4 inch (20 mm) is typical.
Manufactured for 3/8 inch (10 mm) packing space. Some designs will go
into 5/16" (8 mm).
- Advantage. They fit into existing pump stuffing box space, but
there is little to no room for proper operation unless you install
either a larger stuffing box or back plate with the larger
diameter stuffing box cast in.
- Disadvantage. Can handle only
limited axial and radial movement. In slurry applications they
equipment seal designs. The type you get if you don't specify
a specific brand and model number.
- Advantage. Generally the lowest cost seal.
- Disadvantage. No
interchangeability, requiring you maintain a much larger
inventory. In most case these seals will frett and damage
expensive shafts and sleeves. In the majority of cases you'll not
be able to identify the grade of carbon, silicon carbide,
elastomer etc., and they're always the non-balanced type.
the shelf, universal seal designs. Designed to fit into the
thinnest, shortest space and still meet the necessary operating
conditions. Most are non-fretting designs with universal materials
installed as standard.
- Advantage. Lowest cost inventory and no more shaft/ sleeve
- Disadvantage. Since the majority
of these designs incorporate slotted glands, they require a
centering method to prevent shaft/ sleeve contact.
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