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Load and Spring back
The selection of the most
suitable seal for extreme service conditions is
often a delicate trade off between load and
spring back of the seal. The compression load or
seating load has a direct relation with the
obtainable tightness where as the spring back of
the seal determines how well this tightness is
maintained with varying temperatures and
pressures.
For a given seal cross
section and seal type it is generally true that
with maximum load the spring back is lowest. And
of course that seal with minimum load will
generate the highest spring back.
Looking at a Metal O Ring
made from stainless steel, one with a thin wall
and the other with a thick wall, the one with
the thin wall will require lower load to be
compressed to groove depth than the one with the
thick wall, where the spring back of the latter
will be lower than the one with the thin wall.
Again looking at the same
cross section Metal O Ring, a seal made from
Inconel X750
®
or Inconel 718
®
will have a higher
load and higher spring back than a similar wall
thickness Metal O Ring made from stainless steel.
Going over to Metal C Rings,
there is a certain similarity with spring back
and seating load. The big difference is that
Metal C Rings compared with the same cross
section Metal O Rings, typically generate a
lower load, and thus a better spring back than
Metal O Rings.
To overcome the low load of
Metal C Rings, the spring energised Metal C Ring
was developed. This seal, generates a relative
high and uniform seating load with a relative
high spring back.
The Oysterseal ®
or Metal V Ring, by its shape and material
selection generates a high spring back, but
gives only a moderate seating load.
Compression or Seating Load
The compression or seating
load is expressed in N/mm seal circumference.
The higher the load, the better the sealing
performance will be. Soft plating requires lower
load then harder plating. Rough surface finish
of the sealing faces require a higher load to
obtain similar seal performance.
Non plated seals typically
require extreme high loads to obtain technical
tightness. It will be difficult, regardless the
applied load to obtain a high tightness with
unplated seals.
Tightness, Plating
Depending on the required tightness, medium to be
sealed and temperature, a different plating or
coating may be selected.
For tightness better than
10–9 mbar.l/s a soft plating is always required.
Indium and Tin are soft platings. PTFE is a soft
coating too, but the obtainable tightness is
limited to 10–6 mbar.l/s., because of the own
porosity of PTFE.
Silver, Gold and Copper are
medium soft and require a higher load seal to
obtain tightness ranges of 10–9 mbar.l/s. Silver
is the most common used plating. Nickel plating,
being the hardest plating requires a high load
seal selection.
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Flange Rotation or Lift off
Big diameters and high
temperatures play a decisive role in the
selection of a metal seal. Flange rotation or
lift off occurs under high pressure.
The higher the pressure and
the bigger the flange diameters, subjected to
this pressure, the bolds will be subjected to a
quite high stress and strain, resulting in
flange rotation or lift off.
For such service conditions
the biggest possible cross section for the given
diameter should be selected, in order to have
the best possible springback characteristics.
Summarizing
The groove surface finish for
any selected seal should be equal or better then
Ra = 1,8. For tightness equal or better than
10–9 mbar.l/s, a surface finish better than Ra =
0,8 should be applied.
Always select the biggest
possible cross section for a given diameter.
Select the softest plating
allowed for the application. If bold load
permits, choose the highest seal load available
when medium soft plating such as silver, gold or
copper is selected.
For temperatures above 350°C
it is advised to use Inconel as base materials.
For pressures above 35 MPa it is advised to use
C Ring type seals or vented O Rings, both with
or without spring energising, depending on the
required tightness.
For high temperature
applications and when an O Ring type seal is
required a gas filled seal may be the better
option.
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