T
here are four different types
of gasketing material for the
main head seal in Parr reactors
and pressure vessels, each
with its own advantages and
limitations. Some of these are
recent additions which have sig-
nificantly expanded the choices a
user can consider when selecting
a closure and gasket material
for the intended operating
conditions.
Confined and Contained
Flat PTFE Gaskets for
Temperatures to 350 °C
The traditional and most
popular main head gasket for
Parr vessels is a flat gasket made
of a PTFE fluoropolymer. In Parr
flat gasket closures, the gasket
is held in a recess in the vessel
cover. The mating lip on the cyl-
inder closes the recess, leaving
the gasket completely confined
with only a small inside edge
exposed to the reactants within
the vessel. This combination of
complete gasket containment
and the exceptional properties
of PTFE materials produces a
reliable closure for working
temperatures up to 350 °C.
Flat contained gaskets require
an initial loading pressure in
order to develop and to maintain
a tight seal. In Parr designs this
is produced by tightening a ring
of cap screws in a split-ring
cover clamp. Fortunately PTFE
is slightly “plastic” and will flow
under pressure, producing a seal
that improves with each use as
the gasket is forced into the faces
on the head and cylinder. It also
is a very forgiving seal which
does not require the special care
needed to achieve a uniform
loading, which is essential when
working with a metal or other
non-plastic gasket material.
An equally important advan-
tage of the PTFE gaskets is their
essentially universal chemical
resistance.
Self-Sealing O-rings
Parr has greatly expanded its
offerings of reactors and vessels
which feature self-sealing O-ring
closures. In these designs the
sealing force on the gasket is
developed from pressure within
the vessel itself, eliminating the
need for cap screws in the split
ring to pre-load the seal. In these
self-sealing closures the split ring
sections simply lock the head
and cylinder together.
Users who select the self seal-
ing O-ring design must consider
two important characteristics
of elastometric materials. First,
they will not withstand operating
temperatures as high as the PTFE
gaskets. Secondly, none of these
materials offers the universal
chemical resistance of PTFE poly-
mers. The chemical resistance
is especially important since the
O-ring is directly exposed to the
contents of the vessel.
Although there are a number
of available O-ring materials, the
real choice comes down to two.
Fluoroelastomer (FKM) O-rings,
such as Viton, are a first choice
for Parr self-sealing closures.
They have good chemical resis-
tance and a working temperature
up to 225 °C. Perfluoroelastomer
(FFKM) O-rings, such as Kalrez,
have extremely broad chemical
resistance and can be used at
working temperatures up to 275
°C. Unfortunately, this material
Flat PTFE Gasket
O-ring FKM Seal
High Pressure Metal Gasket
Flat Flexible Graphite Gasket
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P a r r I n s t r u m e n t C o m p a n y
Gasket and Seals
