Parr Instrument 4700 - Manual (Page 7)

General Purpose Pressure Vessels
w w w . p a r r i n s t . c o m 7
PARR PRESSURE VESSEL SEALS
Several different head sealing arrangements are
used in Parr Pressure Vessels, each selected for easy
access to the interior of the vessel as well as for
safe operation within the pressure and temperature
limits for which the vessel is designed. Both flat
compression type gaskets and O-rings are used in
these designs.
The various gasket materials used in Parr pressure
vessels are listed in Table I. Since several of these
materials are produced by different suppliers under
different trade names, the ASTM generic designa-
tion is used in these instructions to identify the type
of sealing material (or materials) recommended for
each Parr vessel.
Flat Gasket or Self Sealing O-ring Closure
The flat gasket is held in a recess in the vessel head
and a machine pilot on the cylinder closes the recess
to completely contain the gasket. The split ring clo-
sure used with this gasket has cap screws which must
be tightened to develop the loading on the gasket.
The self sealing design features an O-ring retained
in a groove on the vessel head. This design is self
sealing and the split ring used with this sealing
system does not require nor have the cap screws
used with the flat gasket.
The flat PTFE gasket can be used to operating tem-
peratures as high as 350 °C. The flat flexible graphite
(FG) gasket can be used to operating temperatures
as high as 600 °C.
The maximum temperature of the vessels equipped
with O-ring seals depends upon the material used
for the O-ring. The most common material is a fluo-
roelastomer (FKM) which has a 225 °C maximum
operating temperature limit.
Seals for Operating Temperatures up to 350 °C
Parr uses both flat contained gaskets and O-rings
made of different materials, each with a different
maximum working temperature. Flat gaskets made
of PTFE fluoropolymer resins are the recommended
choice for many applications since PTFE materi-
als are inert to most chemicals. PTFE gaskets will
provide good seals under repeated opening and
closing of the vessel if the gasket temperature does
not exceed 350 °C.
O-rings are available in several different materials
for use within the temperature limit listed in the
following table.
Pressure and Temperature Limits
O-Ring Material Maximum Temperature
NBR 150 °C
FKM 225 °C
FFKM 300 °C
PTFE 350 °C
Gaskets for Operating Temperature above 350 °C
Parr uses a flexible form of graphite which has
proven to be an excellent high temperature sealing
material. It has almost unlimited temperature range,
retaining its structure at temperatures well above the
maximum at which a metal gasket can operate, and
offering broad corrosion resistance as well.
Metal Gaskets can be furnished if required for special
applications. These are usually made of stainless
steel machined to a unique diamond shape with
edges which fit into supporting grooves in the head
and cylinder of the vessel. This type of gasket re-
quires careful maintenance and a uniform loading ap-
plied by tightening a ring of compression bolts with a
torque wrench. For easier handling, Parr has replaced
its diamond shaped metal gaskets with flat, flexible
graphite gaskets (FG) described above. But it will
continue to furnish metal gaskets in custom vessels
for applications in which a metal gasket appears to
offer the best solution to a difficult sealing problem.
Sealing the Vessel
Vessels with split ring closures are sealed by tight-
ening the compression bolts in the split ring sec-
tions with a wrench furnished with the apparatus. To
ensure uniform loading, turn down each bolt finger
tight, then tighten to the limit described below for
the type of gasket being used. Do not over-tighten
the compression bolts as this can generate exces-
sive strain on the closure.
Vessels with a Flat PTFE Gasket
Tighten the compression bolts using a criss-cross
pattern, applying a firm but hard pull to each
screw. Use a torque wrench to apply 25 ft-lbs to
each compression bolt. Let the vessel stand for
about five minutes after the initial tightening, then
tighten again to 25-ft lbs. This will compensate for
any tendency of the PTFE gasket to flow under the
loading pressure.