Performance of O-Ring Seals in Demanding Operating Conditions
Achieving a Longer Service
Life with the Right Material
GÜNTER DÖRNER
SULZER INNOTEC
4198
Plastic components display lower mechanical and
thermal load-bearing capacities than metals and
thus have a limited range of applications in the
field of mechanical engineering. However, they are
used both for secondary components, such as
housings and covers, and for main components.
Components made of polymer materials are also
used in the manufacture of diesel engines. Sulzer
Innotec assisted an engine manufacturer in the
search for a suitable material for O-ring seals for
the cooling water passage in its engines. The use
of different materials, based on the findings of
Sulzer Innotec’s research, significantly extended
the useful life of the O-rings.
SULZER TECHNICAL REVIEW 1/2007
15
additives and fillers gives the basic material extra properties or improves its existing qualities. This
type of elastomer composition frequently contains more than 5 different aggregates, while fillers often account for over 25% of the
weight of the mixture.
A Diverse Range of Materials
1 Plastic seals in marine diesel engines are exposed
to intense loads due to temperatures and environmental media. Sulzer Innotec assisted a manufacturer of diesel engines in its search for materials
that can withstand these loads.
In diesel engines, seals are
frequently made of synthetic
elastomer materials due to the special demands placed on them.
Only a few basic elastomers are
generally utilized; they differ
mainly in terms of the range of
temperatures at which they can be
used and their resistance to chemicals. Each of these basic materials
has specific characteristics that determine its variety of potential
uses. The addition of processing
2 Dismantled
cylinder liner of a
marine diesel
engine.
Metal alloys have a defined composition. However, the composition of plastics and elastomers is
known to the public in only very
few cases. In fact, manufacturers
use their expertise to formulate the
composition of these compounds
and do not therefore wish to reveal
this information for reasons of
competition. Because countless
suppliers offer seals with many
different formulations, it is extremely difficult to select the right
sealing materials for a specific application. Due to a lack of alternatives, the limited comments on the
material data sheets are usually
used as selection criteria. Moreover, users rely on the supplier’s
statements because the examination of material quality for a specific application can prove very expensive. As the unit price of sealing rings is usually very low, this
level of expenditure would be unjustifiable from an end-user perspective.
Systematic Search
for Causes of Failure
A manufacturer of diesel engines
had already enlisted the services
of Sulzer Innotec on several occasions in order to examine O-rings
that are used to seal the cooling
water passage and had suffered
operational damage (Fig. 1). Because the majority of the systems
functioned smoothly until that
16
SULZER TECHNICAL REVIEW 1/2007
3 Silicone O-ring damaged during
operation in a marine engine.
point, there did not appear to be
any need for further action. When
the rate of failure of the O-rings increased substantially, however, the
client decided to trace the cause of
the O-ring failure—which occurred after relatively few hours of
operation—using a systematic investigation.
Thermochemical Processes
During an onsite inspection on
board a diesel-powered container
ship, Sulzer Innotec was able to
document the operational factors
that affect O-rings (Fig. 2). On this
occasion, an O-ring composed of
silicone was extracted in order to
serve as the basis for further investigation (Fig. 3). Additionally, the
O-ring groove was inspected and
both its surface and geometry
were found to be of good quality.
The client supplied detailed information about operating temperatures and the surrounding media
(water quality) as well as data on
the mechanical loads involved.
This information is required in order to determine the cause of damage. Based on the damage to the O-
ring, it was immediately possible
to rule out the collective mechanical loads acting on the component
during operations as the primary
cause of damage. Instead, the deformation of the O-ring and the
numerous tears along its entire circumference indicated that the
damage was being caused by a
thermochemical process.
Simulation of
Operating Conditions
Sulzer Innotec was asked to simulate the damage in the laboratory
on the basis of these observations.
Exposure tests under intensified
test conditions such as increased
temperatures or elevated concentrations of chemicals, which make
it possible to shorten the testing
periods in the laboratory, are especially suitable for this purpose.
The efficiency of the investigations
was increased by simultaneously
conducting exposure tests for 4 Oring materials used by the client
(HNBR, EPDM, silicone, Viton).
Sulzer Innotec defined the test parameters so that the damage mechanism occurring during operations begins after a short testing
period. After 4 weeks, no significant changes were detected for the
samples aged in air in terms of
their visual characteristics and
chemical and physical properties;
this was in line with the information in the data sheets provided by
the individual suppliers. However, the samples kept in water at increased temperatures displayed
clear changes. Even visual inspection revealed cracking due to brittleness in both of the silicone samples tested (Fig. 4). Sulzer Innotec
was thus able to simulate the operational damage in the laboratory
within a relatively short period of
time through the selection of suitable testing parameters (Fig. 5).
Material Brittleness
The change in the material is attributable to the decomposition of
the silicone due to the damp environment (hydrolysis) and increased temperatures. This results
in the chemical splitting of the
macromolecules of the silicone
material, which shortens the
chains of molecules and thus causes the material to become brittle.
Because the basic silicone material—and not the filler components—is responsible for the failure of the O-rings under the prevailing operating conditions, the
use of a different type of silicone
will not lead to an improvement in
their operational performance.
The properties of the 3 other types
of material (EPDM, HNBR and Viton) tested together with the silicone material also changed dramatically—a clear indication that
none of these sealing materials are
suitable for the operating conditions that occur in diesel engines.
Identification of
Suitable Materials
These results prompted the client
to search for alternative sealing
materials with the assistance of
Sulzer Innotec. Aflas and Kalrez,
which are probably the bestknown fluoroelastomers other
than Viton, would very likely be
highly suitable for use in seals under the given operating conditions
but had to be ruled out for reasons
of cost. On the basis of data sheets
and discussions with suppliers, 3
grades of fluoroelastomer-based
materials were finally selected as
the most promising substitute candidates.
15 mm
4 Sealing materials after exposure to water at increased
temperatures (from left to right: EPDM gray; HNBR black;
silicone red and yellow; Viton brown). The upper row
shows the materials which did not undergo any significant
changes after exposure to air.
The materials were tested according to the test procedure defined
by Sulzer Innotec (exposure to air
and water at increased test temperatures) in O-rings with varying
cross-section thicknesses, whereby
the impact of the O-ring geometry
was also taken into account. The
aging tests in air showed that all
3 materials would be suitable in
principle. However, only 2 of the
3 materials survived the resistance
5 Silicone seal after exposure to air (left) and water
at increased temperatures.
10 mm
SULZER TECHNICAL REVIEW 1/2007
17
test in water or steam without
undergoing significant changes
(Fig. 6). On visual inspection, the
third material displayed damage
that exceeded the acceptable limits for operational use following its
exposure to water. After its chemical and physical properties were
tested, it was definitively eliminated as an alternative O-ring
material (Fig. 7).
Successful Field Trial
15 mm
6 O-rings after exposure to water at increased temperatures. The material of the seals from suppliers 1 and 2
(black material) did not display any changes.
7 O-rings after exposure to water at increased
temperatures. Clear damage due to aging is
visible in the material from supplier 3 (below).
10 mm
The client then successfully tested
the 2 other materials in a field trial. A laboratory check of the Orings that had been used in operations confirmed these good results.
Because materials from 2 different
suppliers were able to withstand
the operating conditions in the
diesel engine, it was—at the same
time—possible to avoid any dependency on a single supplier.
It was decided that new materials
should be used due, in particular,
to the results of the tests conducted by Sulzer Innotec. After initial
problems with quality were eliminated through quality assurance
measures, the service life of the Orings increased substantially. The
operator of the diesel engine was
able to extend the replacement intervals for the O-rings significantly, leading to a reduction in maintenance costs.
aging and long-term performance
of sealing materials that are already in use. The location in which
the seals are used is irrelevant in
this context. Instead, it is important to be as familiar as possible
with the operating conditions so
that a test program can be created
that is specifically tailored to the
application in question. Sulzer Innotec experts can subsequently
use suitable tools to assess the
long-term performance of the materials under specific operating
conditions.
Investigation Independent
of the Place of Use
10 mm
18
SULZER TECHNICAL REVIEW 1/2007
Taking the operating parameters
temperature, environmental media, mechanical loads, and requisite service life into account,
Sulzer Innotec can create testing
programs that allow for the evaluation of new sealing materials in
existing equipment and enable
statements to be made about the
Contact
Sulzer Innotec
Günter Dörner
Else-Zueblin-Strasse 48
8404 Winterthur
Switzerland
Phone +41 52 262 69 41
Fax +41 52 262 00 12
guenter.doerner@sulzer.com