| The
failure of Seals can be minimized
through proper design, material
selection and maintenance. Attention
to the condition of replaced seals,
as well as the equipment performance
over time, will result in improved
process reliability, reduced operating
costs and a safer work environment.
Seals often fail prematurely in
applications because of improper
design or material selection.
This section would provide you with
some onf the common causes of seal
failure. By correctly identifying
the cause of failure, changes in
the design or seal material can
be made, thus leading to a better
seal output.
From the end-user’s point
of view, a seal can fail in three
ways:
1> Leaking
2> Contamination
3> Change in Appearance
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| Environment
Analysis :-
One major factor in possible seal
failure is the extreme and harsh
environment in which seals are put
to perform. The sealing environment
can consist of virtually anything
from inert gases at room temperatures
to aggressive chemicals at very
high temperatures. The sealing environment
may result in chemical degradation,
cracking or swelling of the sealing
components. Extreme temperatures
may cause seal degradation, swelling
or outgassing. And the pressure
or more often, the vacuum environments
can cause outgassing and weight
loss.
Contributing factors to seal failure
in the sealing environment include:
The types of Chemical the seals
are exposed to.
The temperatures in which the seals
have to perform.
The pressure and vaccum levels to
which the seals are exposed to in
the process. |
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| Seal
Design Analysis :-
Analysis of the seal application
is crucial to the understanding
of possible failure. Most seal design
is performed by component suppliers
and equipment manufacturers. The
designs are improvised occasionally
based on the past experiences. However,
with the process technology changing
so quickly, the experience gained
with seal design may not be relevant.
Vacuum applications have historically
relied on high levels of compression
and gland fill to reduce permeation
and trapped gases. These techniques,
when applied to new materials, or
at higher operating temperatures,
can result in premature seal failure.
This section provides information
about the common cause of failure:
Static Seals - axial and radial,
confined or unconfined
Dynamic Seals - axial (open-close)
or radial (reciprocating or rotary)
Sealing Gland Dimensions
shape (square, trapezoidal, etc.)
compression
gland fill
stretch
Installation Procedures - stretch
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| Abrasion |
Description:
The seal or parts of the seal
exhibit a flat surface parallel
to the direction or motion.
Loose particles and scrapes
may be found on the seal surface.
Contributing Factors: Rough
sealing surfaces. Excessive
temperature. Process environment
containing abrasive particles.
Dynamic motion. Poor elastomer
surface finish.
Suggested Solutions: Use recommended
gland surface finishes. Consider
internally lubed elastomers.
Eliminate abrasive components.
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| Compression
Set |
| Description:
The seal exhibits a flat-sided
cross-section, the flat sides
correspoding to the mating
seal surfaces.
Contributing Factors: Excessive
compression. Excessive temperature.
Incompletely cured elastomer.
Elastomer with high compression
set. Excessive volume swell
in chemical.
Suggested Solutions: Low compression
set elastomer. Proper gland
design for the specific elastomer.
Confirm material compatibility.
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| Chemical
Degradation |
| Description:
The seal may exhibit many
signs of degradation including
blisters, cracks, voids or
discoloration. In some cases,
the degradation is observable
only by measurement of physical
properties.
Contributing Factors: Contributing
Factors: Incompatibility with
the chemical and/or thermal
environment. Suggested Solutions:
Selection of more chemically
resistant elastomer. View
Chemical Compatiblity Section |
| Explosive
Decompression |
| Description:
The seal exhibits blisters,
pits or pocks on its surface.
Absorption of gas at high
pressure and the subsequent
rapid decrease in pressure.
The absorbed gas blisters
and ruptures the elastomer
surface as the pressure is
rapidly removed.
Contributing Factors: Rapid
pressure changes. Low-modulus/hardness
elastomer.
Suggested Solutions: Higher-modulus/hardness
elastomer. Slower decompression
(release of pressure). |
| Extrusion |
| Description:
The seal develops ragged edges
(generally on the low-pressure
side) which appear tattered.
Contributing Factors: Excessive
clearances. Excessive pressure.
Low-modulus/hardness elastomer.
Excessive gland fill. Irregular
clearance gaps. Sharp gland
edges. Improper sizing.
Suggested Solutions: Decrease
clearances. Higher-modulus/hard-ness
elastomer. Proper gland design.
Use of polymer backup rings.
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| Installation
Damage |
| Description:
The seal or parts of the seal
may exhibit small cuts, nicks
or gashes.
Contributing Factors: Sharp
edges on glands or components.
Improper sizing of elastomer.
Low-modulus/hardness elastomer.
Elastomer surface contamination.
Suggested Solutions: Remove
all sharp edges. Proper gland
design. Proper elastomer sizing.
Higher-modulus/hardness elastomer.
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| Outgassing
/ Extraction |
| Description:
This failure is often very
difficult to detect from examination
of the seal. The seal may
exhibit a decrease in cross-sectional
size.
Contributing Factors: Improper
or improperly cured elastomer.
High vacuum levels. Low hardness/plasticized
elastomer.
Suggested Solutions: Avoid
plasticized elastomers. Ensure
all seals are properly post-cured
to minimize outgassing. |
| Over
Compression |
| Description:
The seal exhibits parallel
flat surfaces (corresponding
to the contact areas) and
may develop circumferential
splits within the flattened
surfaces.
Contributing Factors: Improper
design failure to account
for thermal or chemical volume
changes, or excessive compression.
Suggested Solutions: Gland
design should take into account
material responses to chemical
and thermal environments.
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| Plasam
Degradation |
| Description:
The seal often exhibits discoloration,
as well as powdered residue
on the surface and possible
erosion of elastomer in the
exposed areas.
Contributing Factors: Chemical
reactivity of the plasma.
Ion bombardment (sputtering).
Electron bombardment (heating).
Improper gland design. Incompatible
seal material.
Suggested Solutions: Plasma-compatible
elastomer and compound. Minimize
exposed area. Examine gland
design. |
| Spiral
Failure |
| Description:
The seal exhibits cuts or
marks which spiral around
its circumference.
Contributing Factors: Difficult
or tight installation (static).
Slow reciprocating speed.
Low-modulus/hardness elastomer.
Irregular O-ring surface finish
(including excessive parting
line). Excessive gland width.
Irregular or rough gland surface
finish. Inadequate lubrication.
Suggested Solutions: Correct
installation procedures. Higher-modulus
elastomer. Internally-lubed
elastomers. Proper gland design.
Gland surface finish of 8-16
microinch RMS. Possible use
of polymer backup rings. |
| Thermal
Degradation
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| Description:
The seal may exhibit radial
cracks located on the highest
temperature surfaces. In addition,
certain elastomers may exhibit
signs of softening a shiny
surface as a result of excessive
temperatures.
Contributing Factors: Elastomer
thermal properties. Excessive
temperature excursions or
cycling.
Suggested Solutions: Selection
of an elastomer with improved
thermal stability. Evaluation
of the possibility of cooling
sealing surfaces. |
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