Steve,
Topic of LENS gasket.
My field of working in the past (nowadays I work for
hydrocarbons) was detailed engineering for fertilizer
plants. I knew the use of lens gaskets from the early
seventies.
The application was in high pressure (PN 160 and PN
320 in the DIN specification) for the highly corrosive
urea and ammonium carbamate (by default, liquid
ammonia also) solution lines, where almost all known
materials had failed; and the leakage from flanges was
a perennial problem. I do not wish to mention the
process licensors for these projects. I cannot say
that the lens gasket was the ultimate solution. The
fact remains that it used to be very effective against
the leakages. For the ratings below and for PN100, the
conventional type of gaskets was used.
Now, coming to the point; what really makes the lens
gasket sought after? The answer is that in the good
old days, the urea synthesis pressure used to be in
the range of 250 bar and in the environment of severe
corrosion. Nowadays, by the improvement in technology,
this level has been brought down to a very reasonably
manageable level, wherein, conventional gaskets are
effective. That could be the reason that this topic is
looking to be new to many pipers.
Let me explain the structure of a typical lens. I am
avoiding the use of a sketch. The mating pipe ends
(nozzles) have a wall thickness in excess of the one
required for withstanding the internal pressure. This
extra is kept for the threads to be cut on the outer
surface. The threads engage on a pair of collar
flanges with the studs on either side. The outer and
the inner diameters of the lens are the same as those
of the pipe, with a vertex formed at the outer tip.
The surface of the gasket is such that both the
external surfaces have a spherical contour with their
centres lying on either side (opposite) along the axis
of the pipeline; with radii large enough to lave a
delta type cross section at the rim. The surface
finish is that of a triple delta or lapped one.
Extreme care is observed while handling the gasket so
as not to cause any scratch or score on the surface,
due to any reason. The pipe end, a pair of special
nozzles made for the purpose has a reverse bevel
surface that too with the same quality of surface
finish. When both of them are brought together,
literally, the contact is over a line, a sphere
meeting a hollow cone, that too over a super finish
texture, at both the contact faces. When the studs
apply pressure, during tightening, contact stress is
theoretically, infinite. That seals the leak. This is
the secret of a lens gasket. Obviously, the material
used to be special urea grade stainless steel.
Looking back on the sealing philosophy, to have an
effective sealing pressure, we should have 1.5 times
the system pressure at the contact face. The purpose
of having less and less contact area from a low rating
towards the high pressure, is through, a flat gasket,
then a raised face, still further, are the tongue and
groove, male and female, double male and female, ring
type jointâ¦. The lens is the highest end. A similar
situation exists in equipment (particularly, heat
exchangers) body seals as delta gaskets. The bolt
circle has a limit to manageable levels (which
contains the number of studs) and the stud size is
another factor, which limits the external FORCE
applied to achieve a closure. That means we are left
with only one option that is to reduce the contact
width of the gasket.
All the components (except the flange and studs) in
this case are made of special urea grade stainless
steel.
I hope the matter has been well explained. If anyone
has got further doubt, please do mail back to me.
C. V. Gangadharan, located at Athens.
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