There are four different types of gasket materials 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 significantly 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 cylinder 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 advantage 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 sealing 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 polymers. 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-Ring, such as Viton, are a first choice for Parr self-sealing closures. They have good chemical resistance and a working temperature up to 225 °C. Perfluoroelastomer (FFKM) O-Ring, such as Kalrez, have extremely broad chemical resistance and can be used at working temperatures up to 275 °C. Unfortunately, this material should probably be considered an “exotic” because it costs approximately 80 times as much as an FKM O-Ring. And while it will raise the allowable working temperature to 275 °C, as a practical matter, most users intending to work at this temperature level would be well advised to choose a closure with a flat PTFE gasket and a 350 °C temperature limit. Other exotic O-Ring materials are available, and there are economically priced materials such as ethylene-propylene that will resist some materials that cause FKM to fail, with only slight sacrifices in operating temperatures.
Contained Flat Flexible Graphite Gaskets for Temperatures to 600 °C
For operating temperatures above 350 °C, Parr uses a flexible form of graphite, called Grafoil®, which has proven to be an excellent high temperature sealing material. It consists of flexible layers of graphite bonded together to produce a gasket that is almost as easy to seal as a flat, PTFE gasket, but with an almost unlimited temperature range and excellent chemical resistance.
Parr has converted all of its standard designs to accept a flat, Grafoil gasket whenever operating temperatures above 350 °C are required, replacing the metal gaskets formerly used for high temperatures. These flexible graphite gaskets are held in grooves identical to the ones used for PTFE gaskets and sealed with the same split-ring closures. This makes it possible to substitute a PTFE gasket whenever the vessel is to be used at temperatures below 350 °C. Grafoil gaskets are reusable, but their service life is shorter than can be obtained with a PTFE gasket.
Metal gaskets have traditionally been the only gaskets available for use at temperatures above 350 °C. Parr has designs for diamond cross-section metal gaskets which can be furnished for special applications, but we would recommend the flexible graphite gaskets described above for most applications.
Trademarks of Sealing Materials
A number of gasket materials have so dominated their product categories that their Trade Names have become more common than the actual material designation itself. In an attempt to respect the value of these Trade Names and their proper usage and to minimize the disruptions in our descriptions, we have adopted the following generic material descriptions and designations for use in this catalog. Where available we have selected the ASTM material designation.
Viton®, Kalrez®, and Teflon® are registered trademarks of DuPont Dow Elastomers.
Grafoil® is a registered trademark of UCAR Carbon Co.Inc.