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Engineering Seals – Static Joint Design with Reinforced Rubbers |
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Topics Covered |
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Static Joint Design with ‘0’ Rings and Other
Automatic Seals |
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Rules of Thumb |
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There are a number of
‘rules of thumb’ and formulae, developed by manufacturers, which are useful
in the design of a seal using reinforced rubbers: ·
The thinnest
practicable gasket is best, greater thicknesses increase both stress
relaxation and the amount of material exposed to the environment ·
Inside dimensions
of the gasket and the flange should be the same ·
Clean cut edges
are essential as ragged edges always favour leakage ·
The lateral
compliance of a gasket (proportional to its thickness and, inversely
proportional to the area and shear modulus) can be helpful in preventing
fretting caused by vibration. Alternatively, dowels may be used to prevent
relative lateral movement of the flanges. ·
Presoaking of
gaskets has been shown experimentally to be undesirable. Gasket manufacturers
have empirical formulae for joint design. These were developed for asbestos
reinforced rubber and the use of aramid or other reinforcing fibres requires
different values for some factors. The following
expression is suggested as a starting point, S=Y+mP+PA/A'where S is the minimum
assembly stress, Y is the minimum stress needed to force the gasket into the
flange surface roughness and consolidate its bulk (typical values are 14MPa
for liquid tight joints or 28MPa for gas tight joints), P is the internal
fluid pressure, A is the enclosed area and A' is the gasket cross sectional
area. Thus mP is the
residual stress in the gasket having to withstand the radial force generated
by the internal pressure P (a typical value for m would be 1.1). and PA/A' is
the force expressed as a stress on A' needed to contain P acting over A. After trials
adjustments may be needed, particularly if the application involves any temperature
cycling. Flange Fixation
The choice of the
number of bolts around the flange is usually dictated by the risk of flange
distortion. Generally doubling their number will decrease bowing to one
eighth. The diameter of the bolts, and the torque used in assembly, is
dictated by the need to contain the pressure plus the excess stress for
sealing estimated by the equation above. Gasket Width
Gaskets resist leakage
or blowout purely by frictional forces so the gasket width should be at least
twice its thickness and the assembly stress as calculated in the equation
above should be at least two times the internal pressure. Other Design Aspects
Anti-friction
treatments (such as graphite) applied to a gasket may cause problems, but by
reducing adhesion they can make disassembly easier. Room temperature
vulcanising (RTV) silicones can be useful as anti-adhesion supplements to
gaskets, especially for damaged flanges, as well as serving as useful formed
in place gaskets in their own right. Non-asbestos gaskets
crush more readily than the asbestos-based variety and should not be
subjected to a stress greater than 100MPa or so. Formed in Place Gaskets
Many possibilities are
now available including: ·
PTFE cord wound
round bolts which exhibits high ‘cold flow’ under a load, even at room
temperature. This forces it into the threads, sealing them, but the joint has
low blowout pressure ·
Sealant beads,
usually of silicone rubber. which are useful for sealing lubricating oils but
not fuels ·
Foamed epoxy
resins are generally good for sealing against air, water, oils and
conventional fuels to about 120°C but useless against alcohols ·
PVC plastisols
gelled by heat have a similar range but above 70°C there is a risk of
corrosion, especially for aluminium surfaces Anaerobic Resins
Anaerobic resins which
cure in the absence of oxygen provide the most useful ‘liquid gaskets’.
Perhaps more familiar as 'superglue' these materials are increasingly used
for automotive and other applications (e.g. gearboxes), giving very stiff
joints with many bolts, but are not suitable for pressed steel covers or
surfaces that tend to move laterally. They are stiff in shear but are not
intended to be ‘adhesives’. Specialised types exist which can give service up
to 180°C. Static Joint Design with ‘0’ Rings and Other Automatic Seals
As an alternative to
gaskets, static flange joints may be sealed using an elastomeric ‘O’ ring in
a machined recess (figure 1). In these configurations the sealing action is
said to be ‘automatic’ since the action of the pressure differential itself
‘energises’ the seal to generate the sealing pressure.
The flange faces
should be fully in contact after assembly, with a compression of about 20%
applied to the ‘O’ ring. This determines the relationship between the depth
of the recess and the ring cross-section. The width of the recess should be
sufficient to accommodate the thermal expansion of the ‘O’ ring since the
rubber can be regarded as incompressible and the coefficient of thermal expansion
of most rubbers is twenty times that of steel. Automatic face seals
are also manufactured with various non-circular cross sections, including
U-shaped sections with the open end of the U facing the high pressure side. If the faces tend to
separate, for example under the action of a pulsating pressure, it may be
necessary to fit anti-extrusion rings on the low pressure side, Figure 2.
These rings must have a much higher modulus than the elastomeric ring, but
elasticity is less important. Suitable materials are phosphor bronze or
plastics such as nylon or filled PTFE.
Pipe Seals
Softer rubbers, such as lightly filled natural rubber, come into their own against very rough surfaces such as clay pipes. Natural rubber also has the key property of low rate of stress relaxation, and case histories of a service life of 100 years exist for sewer pipe seals. |
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