SUBJECT: The most asked questions about
mechanical seals 10-2
What is considered good life for a
- 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
- 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're 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.
Why do most seals fail
- One of the seal components becomes damaged.
- The seal faces open.
What are the most common causes of component
- Corrosion of one of the seal components.
- Physical damage that includes the affects of high heat or
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 you are sealing are clogging the
- The product changed state and interfered with the free
movement of the seal. It:
- Became viscous.
- Built a film on the sliding components and the lapped
- The product vaporized across the lapped faces blowing them
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
- 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
Do seal faces have to be kept
- 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
- 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
When should you use two hard
- 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
- 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.
Why not standardize on two hard
- They generate higher heat than the carbon/ hard face
- They are not very forgiving. If the faces are not dead flat at
installation, they seldom lap them selves flat in operation.
Do seals have to leak.?
- Any good quality mechanical seal should run without visible
- 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.
Why do most original equipment seal designs
frett and damage the shaft under the dynamic elastomer or spring
- 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.
Do you have to flush most slurry
- It depends upon the percentage of solids. Most fluid with
entrained solids can run without flush if you have met the
- 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
- 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
- You are using suction recirculation to get flow in the
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.
If I put a higher fluid pressure barrier
fluid between dual seals, shouldn't that keep the faces
- 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
- 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 seals should be hydraulically balanced in both directions
so that they will stay shut regardless of the direction of the
How does seal hydraulic balance
- 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
- Hydrodynamic forces generated between the seal faces
because for all practical purposes liquids are not
- We balance these forces by reducing the closing area of the
seal faces and thereby reduce the closing force. This is usually
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.
Is it O.K. to have a third party rebuild my
- Not really. If you're 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 and
the third party doesn't own the molds for your carbon/graphite
face. Machined carbons don't have the density required for good
- There are many grades of elastomers. How do you insure you
have the right grade. You can't tell by looking at the
- Lapping is a real art. The temperature has to be closely
controlled to get the right flatness.
Should I be using split mechanical
- 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
- 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
- If you have to remove a lot of pump insulation to get to
- If the pump is in an awkward location, split seals make
- 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.
If I touch the lapped faces, are they
- Not at all. Touching seal faces seldom causes 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.
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'll never have this problem.
Why not standardize on Teflon as the
preferred rubber part in a mechanical seal?
- Teflon® is not an elastomer, it doesn't have a memory and
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.
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 where centrifugal force will throw solids into the
- Solids will pile up in front of the seal preventing the faces
from moving forward when the sacrifical carbon wears.
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 installation process and allow you to
make impeller adjustments without upsetting the seal face
Do I need the new 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.
Why does my outside mounted seal make a
- The seal faces are running dry. The product you are trying to
seal is not a lubricant.
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.
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
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