|
| Seal
Failure Analysis |
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
|
| |
| 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.
|
| |
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
|
|
| Common
Causes of Seal Failures |
|
|
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. |
|
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. |
|
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. |
|
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. |
|
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. |
|
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
|
| 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. |
|
| |
|
.gif){kind=spacer}
