SUBJECT : A quick reference
guide for mechanical seal failure 4-11
Of all the seal related activities, analyzing
mechanical seal failure continues to be the single greatest problem
for both the consumer and the seal company representative. I've
addressed this problem in several of my other technical papers. If
you will take a little bit of time to familiarize yourself with the
following outline you should feel a lot more comfortable the next
time you're called upon to do some seal troubleshooting.
As you look over the failed seal components, keep
in mind that a rebuilt seal may have some marks that occurred during
a previous failure, making them especially difficult to analyze, but
regardless of the design, mechanical seals fail for only two
- Damage to one of the components
- The seal faces open prematurely.
We'll start with damage. This damage is almost
always visible. Look for :
- The elastomer swells or the
other seal parts become "sponge like" or pitted.
product you're sealing is attacking one of the seal
- The attack is coming from the cleaner or
solvent used to clean the lines between batches, or at the end
of a "run".
- The attack is coming from lubricants put on
the elastomers or seal faces. Petroleum grease on Ethylene
Propylene O-rings will cause them to "swell up".
- Galvanic corrosion - Happens with dissimilar
materials in physical contact and connected by an electrolyte. As
an example: stainless steel can attack the nickel binder in a
tungsten carbide face.
and Halogens attack all forms of carbon
including black o-rings.
- Corrosion always increases with an increase in
- Wear or rubbing of a flexible
- Thermal shock of some seal face materials.
Especially those that are hard coated or plated.
- Thermal expansion of the shaft or sleeve can
break a stationary seal face or interfere with the free movement
of a dynamic elastomer.
- The rotating seal hits something because of
- Temperature extremes (both high and cryogenic)
will destroy elastomers and some seal face materials.
- Erosion from solids in the product you are
caused by the dynamic elastomer removing the passivated layer from
the corrosion resistant shaft or sleeve.
- Fluid abrasion that can weaken materials and
destroy critical tolerances.
- A discharge recirculation line circulates high
velocity liquid with entrained solids that can break a metal
bellows and injure lapped seal faces, as well as interfere with
the free movement of the seal.
- The elastomer or rubber part can swell and
breaks the face.
- Problems at installation. They includes
mishandling, setting at the wrong compression, putting the wrong
lubricant on the elastomer etc.
- Fatigue of the springs caused by
The seal faces opening prematurely is the second
Scoring or wear of the hard face is the most
common symptom of this failure. The scoring occurs because the solids
imbed into the softer carbon face after they open. The seal faces
must stay in contact, but there are all kinds of conditions that are
trying to force or pull them open.
- Axial shaft movement (end play or thrust).
This is normal at start up.
- Radial shaft movement (run out or
- Operating off of the pump's best efficiency
- Hysteresis caused by a viscous (thick)
- Centrifugal force tries to separate the faces
in a rotating seal application.
- Hydrodynamic forces generated between the
- Pressure distortion caused during pressure
peaks such as water hammer and cavitation.
- Thermal distortion that can cause the seal
face to separate from its holder or "go out of flat".
- A failure to provide equal and opposite
clamping across the stationary seal face will cause
- A hardened sleeve can cause the seal set
screws to slip.
- A wrong initial setting of the face
- Springs can clog if they are located in the
- Loose set screws. If the sleeve is too soft
they can vibrate out.
- Shaft tolerance and finish is out of
- The rotating shaft or seal hits
- A discharge recirculation line can force open
- Outside springs painted by maintenance
- A cartridge seal installation method can
compress one set of faces and open the other.
- Fretting hang up.
- Cartridge mounted stationary seals flex
excessively unless they have some type of "built in" self-aligning
problems . With a loss of an environmental
control the fluid can:
- Vaporize between the lapped faces forcing them
open and causing a "chipping" of the carbon outside diameter as
well as leaving solids between the lapped faces.
- Become viscous preventing the faces from
following normal "run out".
- Solidify between the lapped faces or around
- Crystallize between the faces or around the
dynamic portions of the seal.
- Build a film on the sliding components or
between the faces causing them to separate.
- Be a slurry and/ or abrasive
- Operate in a vacuum causing the ingestion of
air between the faces of some unbalanced seal designs.
- Swell up the dynamic elastomer, locking up the
- Cause slipstick between the faces if the
sealed fluid is a non, or poor lubricant
common causes of shaft displacement.
- Operating off the pump's best efficiency point
- Misalignment between the pump and its
- The rotating assembly is out of
- A bent shaft.
- A non concentric sleeve or seal.
- Harmonic vibration
- Passing through, or operating at a critical
- Water hammer in the lines.
- The stuffing box is not square to the shaft,
causing misalignment problems.
- Pipe strain.
- An impeller adjustment is made to compensate
for normal impeller wear.
- Thermal growth of the shaft in both a radial
and axial direction.
- Bad bearings or a poor bearing
- Two direction axial thrust at start up is
- The motor is finding its magnetic
- Cavitation - there are multiple types of
damage that can be observed.
- The sleeve moved when the impeller was
- The unit is pulley driven causing excessive
- The impeller is positioned too far from the
bearings. This is a severe problem in mixer or agitator
to prevent product problems that cause
premature seal failure.
Control the environment in the stuffing
Build the seal to compensate for operating
- Control the temperature in the seal
- Use the correct spring or bellows
- Use only hydraullically balanced
- Select a low friction face
- Avoid "dead ending" the stuffing
- Jacket the stuffing box
- Quench behind the seal with the correct
temperture steam or fluid
- Use a gland jacket
- Utilize two seals with a barrier fluid between
- Use heat tape around the stuffing
- Use a heat pipe to remove heat from the
- Vent the stuffing box, especially in a
- Flush in a cool compatible liquid.
- Control the pressure in the seal area
- Be sure to use only hydraulically balanced
- Discharge recirculation will raise the
pressure if you put a restrictive bushing into the bottom of
the stuffing box.
- Suction recirculation will lower the
pressure in the stuffing box.
- Use two seals and let the barrier fluid
control the pressure between the seals.
- Cross connect the stuffing boxes to
equalize the stuffing box pressures in a multi stage
- Stage the stuffing box pressure with tandem
- Impeller pump out vanes will lower stuffing
- Give the seal more radial space
- Bore out the existing stuffing box if it is
- Make or buy a new back plate with the large
stuffing box cast into it.
- Make or buy a large bore stuffing box and
attach it to the back plate after you have machined the old one
- Flush the product away if you're unable to
- Suction recirculation will bring fluid into
the stuffing box from behind the impeller, where it is usually
cleaner. This works on most closed impeller pump applications
and those open impeller pump applications where the impeller
adjusts to the volute rather than the back plate.
- Flush with a clean liquid from an outside
- A pressurized barrier fluid between two
seals can keep solids from penetrating between the faces, if
the faces should open. This application will also work if the
solid particles are less than one micron in diameter (Kaoline
is such a product).
Slurry features that can be part of seal
- Springs out of the fluid
- Teflon coating the metal parts so particles
will not stick to sliding components.
- The elastomer moves to a clean surface as the
- Keep the sealing fluid on the outside diameter
of the seal to take advantage of centrifugal force that will throw
solids away from the lapped faces.
- Rotate the fluid with the seal to prevent
erosion of the seal components. A simple vane arrangement can
- Use two hard faces if you find it impossible
to keep the lapped seal faces together.
- Use a pumping ring to keep solids away from
- Mount the seal closer to the bearings to
diminish the affect of shaft deflection.
Design for higher temperature
- Eliminate elastomers when ever
- If you cannot eliminate elastomers, the O-ring
location becomes important. Try to move the elastomer away from
- Hydraulically balanced seals generate less
- Select low friction faces.
- Fool proof, correct installation dimensions
are necessary. A cartridge design is your best choice.
- Keep a good product circulation around the
- A good lapping technique will keep the faces
flat at high and cryogenic temperatures.
- Pumping rings will keep fluid circulating
between two seals. If you are using balanced seals a simple
convection tank is usually more than adequate. An air operated
diaphragm pump can be used in the line to increase the
circulation. Try to avoid the use of petroleum based fluids as the
barrier or buffer fluid between the seals. Petroleum based fluids
have a very low specific heat that will increase the temperature
between the seals,
- Gland features such as quenching,
recirculation, venting and flushing help.
- Choose well designed faces that will resist
thermal distortion. The closer you get to a "square block" design,
the better off you're going to be.
- Do not insulate the faces with an
Design for pressure resistance
- Limit the number of diameters in any single
- Laminated bellows will allow you to keep a low
spring rate while maintaining pressure capability, if you are
using a welded metal bellows design.
- Finite element analysis of the seal components
will prevent pressure and temperature distortion.
- Use more mass to resist hoop
- Higher modulus materials will resist bending
- Use a tandem seal design for pressure break
down between two seals.
Design for corrosion resistance
- Choose good materials, clearly identified by
type and grade.
- Eliminate elastomers when possible. Elastomers
are the most corrosion sensitive part of the seal.
- Design non stressed parts when ever
- Try not to weld any of the metal components.
If it is necessary, monitor the temperature to prevent inter
- Control the temperature. Corrosion increases
- Use non metallic materials for non metallic
- Watch out for galvanic corrosion when using
- Do not use stainless steel springs. Stick with
Hastelloy "C" if the metal parts of the seal are manufactured from
iron, steel, stainless steel, or bronze. If the seal is
manufactured from a different metal, use springs manufactured from
- Do not depend upon flushing to provide
corrosion resistance. Use the correct materials, keeping in mind
that solvents and steam are sometimes used to flush the lines. Any
materials that you select must be compatible with these flushing
or cleaning fluids also.
you need cryogenic capability
- Go to a welded metal bellows configuration to
eliminate all elastomers.
- You will need a special carbon/ graphite face
that has an organic material impregnated to assist in the release
of the graphite.
- Avoid plated or coated hard faces.
Differential expansion will cause them to crack.
- Always lap the faces at a cryogenic
- Do not coat the faces with grease or oil. It
will freeze at cryogenic temperatures.
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