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Single
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Single
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Metal
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Other mechanical seals we supply "equals to" and repair
 Anchor  Angst
Pfister Borg
Warner
Busack
Burgmann
Chesterton n
Depac
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Star
Flexibox Flygt
Merkel |
G Hecker
Werke Latty
N.O.K.
Pacific
Robco
Roten
Rotoflex
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Sepco
Weston
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A mechanical seal must contain four
functional components: 1) Primary sealing surfaces, 2) Secondary sealing
surfaces, 3) a means of actuation and 4) a means of drive.
1) The primary sealing surfaces are the heart of the device. A common
combination consists of a hard material,
such as [silicon carbide]] or tungsten carbide,imbedded in the pump casing and a
softer material, such as carbon
in the rotating seal assembly. Many other materials can be used depending on the
liquid's chemical properties,
pressure, and temperature. These two rings are in intimate contact, one ring
rotates with the shaft, the other ring is
stationary. These two rings are machined using a machining process called
lapping in order to obtain the
necessary degree of flatness.
2) The secondary sealing surfaces (there may be a number of them) are those
other points in the seal that require a
fluid barrier but are not rotating relative to one another.
3) In order to keep the two primary sealing surfaces in intimate contace, a
means of actuation must be provided.
This is commonly provided by a spring. In conjunction with the spring, it may
also be provided by the pressure of
the sealed fluid.
4) The primary sealing surfaces must be the only parts of the seal that are
permitted to rotate realtive to one
another, they must not rotate relative to the parts of the seal that hold them
in place. To maintain this non-rotation
a method of drive must be provided.
SUBJECT: The most asked questions about mechanical seals
1. What is considered good life for a mechanical seal? The only part of a
mechanical seal that is supposed to be
sacrificial is the carbon face. The seal should run leak free until the carbon
face is worn away. If the seal leaks for
any other reason we consider it a premature failure and always correctable. Two
hard faces are selected when
carbon is not acceptable in the application and you have run out of options. You
are then trying to get the longest
life you can. The only variable in seal life should be the lubricating quality
of the product you are sealing. Hot
water, many gases and most solvents are typical non-lubricants. With all of that
said, the fact is that in excess of
eighty five percent of mechanical seals fail prematurely. When seals are removed
from the running pump most of
the carbon face is still intact. Little face wear is the rule not the exception.
2. Why do most seals fail prematurely? One of the seal components becomes
damaged or the seal faces open.
3. What are the most common causes of component damage Corrosion of one of the
seal components and physical
damage that includes the affects of high heat.
4. What are the most common causes for the lapped seal faces to open?The seal
was set screwed to a hardened
shaft. Solids in the product are clogging the moveable components. The product
changed state and interfered with
the free movement of the seal. It: Crystallized, became viscous or solidified. A
film formed on the sliding
components or the lapped faces. The product vaporized across the lapped faces
blowing them open.
5.Do seal faces have to be lubricated? Can they run dry? The graphite in the
carbon/graphite face is a natural
lubricant. In operation the graphite separates from the mixture and transfers to
the hard face. This means that the
seal face combination you are normally running is carbon on graphite. The hard
face is just some place to put the
graphite. Moisture must be present for the graphite to separate from the
carbon/graphite mixture. Running dry
means higher heat at the faces. If you are using a good unfilled carbon/graphite
(and you should be) the faces are
not going to be your problem. The elastomer and the product you are sealing can
be very sensitive to a
temperature change in the stuffing box, or an increase of temperature at the
seal faces.
6. Do seal faces have to be kept cool? Most carbons and hard faces can tolerate
a lot of heat. The elastomers
(rubber parts) are the parts you have to watch. They are the most sensitive to a
change in stuffing box temperature,
especially if they are positioned in the seal face. Hydraulically balanced seals
generate very little heat between
the faces. Unbalanced seals usually require cooling because of the excessive
heat they can generate. Some face
combinations generate more heat than others. Two hard faces as an example. Some
seal materials conduct heat
better than others. Ceramic is a poor heat conductor and carbon is not much
better. Tungsten carbide and silicone
carbide are excellent conductors of heat.
7. When should you use two hard faces? With any of the oxidizing agents. When
sealing any of the halogens. If the
product tends to stick the faces together. If you are sealing hot oil and you
have to pass a fugitive emission test.
Some de-ionized water will attack carbon in any form. When you are not allowed
anything black in the system
because of the possibility of color contamination. Any time carbon/graphite will
not work for some reason. If the
specifications call for two hard faces.
8. Why not standardize on two hard faces? They generate higher heat than the
carbon/ hard face combination.
They are not very forgiving. If the faces are not dead flat at installation,
they seldom lap them selves flat in
operation.
9. Do seals have to leak.? Any good quality mechanical seal should run without
visible leakage. Single, stationary,
(the springs do not rotate) hydraulically balanced mechanical seals can pass a
fugitive emission test as long as the
rotating portion of the seal is designed to be located square to the shaft.
Rotating seals (the springs rotate with the
shaft) seldom can pass a fugitive emission test. They are too sensitive to
various forms of misalignment. Cartridge
mounted stationary seals usually fail fugitive emission testing because the set
screwing of the cartridge to the shaft
prevents the rotating face from positioning its self square to the shaft. Some
seal companies offer some type of a
self aligning design to solve this problem.
10. Why do most original equipment seal designs frett and damage the shaft under
the dynamic elastomer or
spring loaded PTFE.? Corrosion resistant shafts and sleeves protect themselves
from corrosion by forming a
protective oxide (ceramic) layer on the metal surface. The dynamic elastomer in
the seal polishes this layer away
as the shaft slides through the elastomer because of shaft vibration, pipe
strain, misalignment etc. The ceramic
protective oxide that is removed by the polishing action imbeds its self into
the elastomer causing it to act as a
grinding wheel that increases the sleeve or shaft damage.
11. Do you have to flush most slurry applications? It depends upon the
percentage of solids. Most fluid with
entrained solids can run without flush if you have met the following conditions:
The packing stuffing box has been
replaced with a larger inside diameter version. Centrifugal force will throw the
solids away from the lapped seal
faces. You are using a hydraulically balanced seal that generates low heat. The
seal springs are not located in the
fluid. The fluid is at the seal outside diameter. The dynamic elastomer moves to
a clean surface as the carbon
wears. You are using suction recirculation to get flow in the stuffing box.
12. I am looking for a simple solution to a difficult problem. Do discharge
recirculation filters or cyclone separators
installed between the pump discharge and the stuffing box make sense in slurry
applications? I wish they did!
Filters clog and then there is no circulation in the stuffing box. Cyclone
separators were never intended to be a
single pass devise. The also require a substantial difference in pressure
between the discharge and the clean
liquid connections. In a pump application these pressures are too close
together.
13.If I put a higher fluid pressure barrier fluid between dual seals, shouldn't
that keep the faces clean? No, the
clean fluid always takes the path of least resistance. That is the same reason
that higher pressure air does not keep
dry solids from penetrating the lapped faces. Centrifugal force will pack solids
in front of the inboard seal face and
restrict its movement. Do you need a higher pressure barrier fluid between dual
seals? Higher pressure is called
barrier fluid, lower pressure is called buffer fluid. The only dual seals that
require a barrier fluid are the "back to
back" rotating, unbalanced versions, and you shouldn't use them any way.
Balanced tandem seals (one behind the
other) use a buffer fluid that will not dilute your product if the inner seal
fails. They also put the pumping fluid at
the inner seal outside diameter where it belongs. Dual seal should be
hydraulically balanced in both directions so
that they will stay shut regardless of the direction of the fluid pressure.
14. How does seal hydraulic balance work? There are two forces closing the seal
faces. A spring force caused by
the spring, springs, or bellows pushing on the seal face. A hydraulic force
caused by the pressure of the fluid acting
on the closing area of the seal faces. There are three forces opening the
mechanical seal: A hydraulic force
caused by fluid or vapor trapped between the lapped faces. Centrifugal force
that is causing the rotating portion of
the seal to try and become perpendicular to the rotating shaft. Hydrodynamic
forces generated between the seal
faces because for all practical purposes liquids are not compressible. We
balance these forces by reducing the
closing area of the seal faces and thereby reduce the closing force. This is
normally done by a small sleeve
inserted into the seal or as step machined into the shaft. Metal bellows seals
have an effective diameter measured
through the bellows to accomplish the same thing.
15. Is it O.K. to have a third party rebuild my mechanical seals? Not really. If
you are happy with your seal have the
manufacturer, or the company that sold it to you do the rebuilding. Here are a
couple of reasons why:
Carbon/graphite has to be molded in a sintering process. The third party does
not own the molds for your
carbon/graphite face. Machined carbons do not have the density required for good
seal faces. There are many
grades of elastomers. How do you insure you have the right grade. You cannot
tell by looking at the part. Lapping
is a real art. The temperature has to be closely controlled to get the right
flatness.
16. Should I be using split mechanical seals? There are places where they are
the only logical solution: Double
ended pumps. If one seal is leaking why take the pump apart and change both?
Change only the one that is
leaking. Large vertical pumps. Sometimes you have to take the roof off the
building to remove the solid
mechanical seal. Large size shafts are a natural for split seals. Changing a
seal means doing a re-alignment. Why
go through that again? If you have to remove a lot of pump insulation to get to
the seal. If the pump is in an
awkward location, split seals make sense. Many split seal designs can run with
no visible leakage, but they seldom
can pass a fugitive emission test that calls for leak rates in the order of
parts per million.
17. If I touch the lapped faces, are they ruined? Not at all. Touching seal
faces seldom causes any problems. We
are trying to keep solids from penetrating between the lapped faces so the less
you handle them the less likely
solids will be deposited on the faces.
18. Why should you not use stainless steel springs or stainless steel bellows in
mechanical seals? Chloride stress
corrosion is the problem and chlorides are every where. Use hastelloy "C"
springs and metal bellows and you will
never have this problem.
19. Why not standardize on PTFE as the preferred rubber part in a mechanical
seal? PTFE� is not an elastomer, it
does not have a memory. It has to be spring loaded to the sleeve or shaft. This
spring loading interferes with the
flexibility of the seal and prevents the elastomer part from flexing and rolling
to compensate for minor shaft
movements.
20. Why not mount the seal outside the stuffing box and then dirt and solids
will not get into the springs and sliding
parts of the mechanical seal? The sealing fluid will be at the inside diameter
of the lapped faces and centrifugal
force will throw solids into the faces. Solids will then pile up in front of the
seal preventing the faces from moving
forward when the carbon wears.
21. What is a cartridge seal? The rotating portion of the seal is mounted on a
cartridge sleeve and this assembly is
connected to the stationary portion of the seal along with the seal gland to
form a cartridge assembly. Cartridge
seals simplify the assembly process and allow you to make impeller adjustments
without upsetting the seal face
loading.
22.Do I need gas seals if I want to seal fugitive emissions? Not really.
Rotating seals do not pass fugitive emission
tests because of their sensitivity to misalignment. Stationary seals usually do
not have this limitation. The difficulty
arises when you try to install a stationary seal on a cartridge sleeve. When you
tighten the sleeve set screws to the
pump shaft you introduce misalignment between the rotating seal face and the
rotating shaft. Hysteresis (delay or
lag) problems take over and the result is the stationary seal design fails to
pass the fugitive emission test. Any good
cartridge mounted self aligning seal can resolve this problem. Although a single
seal can pass the test, a dual seal
is recommended with a low pressure buffer fluid between the seals to act as a
back up when the first seal wears
out or fails. The buffer fluid will prevent unwanted product dilution and
simplify the installation because there is no
need for a compatible high pressure barrier fluid that is often hard to find.
23.Why does my outside mounted seal make a whistling sound? The seal faces are
running dry. The product is not
a lubricant.
24. Every time I remove a rubber bellows seal from my pump it is stuck to the
shaft. Why? It is supposed to
vulcanize its self to the shaft so that it can drive the rotating face. If you
can remove it easily something is wrong.
You probably used the wrong lubricant on the rubber during installation. This is
a case where the lubricant we use
is supposed to attack the rubber and make it swell.
25. When my metal bellows seal fails because of breakage at the plates, the
break is always near the end fittings
and never in the middle of the bellows. How is that explained? This is the
common mode of failure for excessive
vibration. Metal bellows seals need some type of vibration damping to stop
harmonic and "slip stick" vibration
problems.
� DuPont Dow Elastomer
Seal Categories
All Mechanical seals must contain the four elements described above but the way
those functional elements are
arranged may be quite varied. The standards of modern mechanical seals are
widely defined by API Standard 682
- Shaft Sealing Systems for Centrifugal and Rotary Pumps.
Mechanical seals are generally classified into two main categories. "Pusher" or
spring-type seals operate by a
spring providing a force on the primary seal. "Bellows as "non-pusher" type
seals use a metal or rubber bellows
arrangment to create the pressure on the primary seal.
A "cartridge seals" is a prepackaged seal that is common in more complex
applications.
Dry gas seals are relatively new innovations that do not require lubrication by
the fluid. These are commonly used
in compressors.
Seal Flush Plans
Since the rotating seal will create heat, this heat will need to be carried away
from the seal chamber or else the
seal will overheat and fail. Typically, a small tube connected to either the
suction or the discharge will help
circulate the liquid. Other features such as filters or coolers will be added to
this tubing arrangement depending on
the properties of the fluid, and its pressure and temperature. Each arrangement
has a number associated with it, as
defined by API.
Tandem and Double Seals
Since almost all seals are designed to leak slightly, dangerous chemicals or
flammible petroleums must not be
allowed to leak into the atmosphere or onto the ground. In these applications a
second seal is place after the
primary seal along the pump shaft. The space in between these two seals are
filled with a neutral liquid or gas.
In a tandem seal, the seal will leak to the unpressurized cavity. If the cavity
registers a dramatic increase in
pressure, operator will know that the primary seal has failed. If the cavity is
drained of liquid, then the secondary
seal failed. In both instances, maintanence will need to be performed. This
arrangement is commonly used when
sealing fluids that would create a hazard or change state when contacting open
air.
In a double seal, the liquid in the cavity between the two seals is pressurized.
Thus if the primary seal fails, the
neutral liquid will leak into the pump stream instead of the dangerous pumped
fluid escaping into the atmosphere.
This application is usually used in gas, unstable, highly toxic, abrasive,
corrosive, and viscous fluids.
Origins
The Mechanical Seal was invented by a man named George Cook and the seal he
invented was called a "Cook
Seal." He also founded the Cook Seal Company. Cook's seal (which actually did
not have a means of drive) was
first used in refrigeration compressors.
The Cook Seal company was a sideline product for Cook and he sold the company to
Muskegon Piston Ring
Company where it was renamed as The Rotary Seal Division of Muskegon Piston Ring
Co. Muskegon Piston Ring
sold the Rotary Seal Division to EG&G SEALOL who in turn was largely
acquired by
John Crane Industries of Morton
Grove, IL. John Crane had been in the Mechanical seal business for many years
prior to this acquisition and in fact
had long since exceeded SEALOL in overall market size. Today, Crane's main
competitor is Flowserve.
In 1990 the world market for Mechanical Seals was estimated at $1 billion.
Source: Wikipedia.
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