Metal Jacketed and Microwave Styles
Parr PTFE lined acid digestion vessels are made in two different styles: There is a line of metal-jacketed vessels that can be heated in an oven, sand bath, oil or water bath and in other conventional ways, and a line of digestion vessels developed specifically for rapid heating in a microwave oven. Both styles are made in several sizes, offering a broad selection of vessels ranging from economical models for routine tests to massive styles for procedures involving extreme conditions or potentially hazardous samples. Each of these vessels has a thick walled PTFE liner that completely isolates the charge from the supporting body. These liners are removable and can be charged or emptied while outside of the vessel to eliminate any possible contamination from acid contact with metal parts. They can be replaced if they become damaged or contaminated.
The Nature of PTFE
Unique Closures
Several designs are used to keep Parr acid digestion vessels tightly sealed during consecutive heating and cooling cycles when conventional seals might leak. One of these designs uses a tapered closure between the PTFE cup and cover with an unusually long sealing face that tends to become self-sealing. This long path also minimizes the tendency for acid migration through the joint. vessels using this design can be sealed by simply turning down a knurled cap by hand without using a wrench or spanner. For applications requiring higher temperatures and pressures, the PTFE liners are made with a broad, flanged closure and sealed by tightening the vessel cap with a spanner or, in a larger design, by using cap screws to develop the sealing force. Tight closures for the PTFE cup in Parr microwave vessels are developed and maintained with self-sealing PTFE O-rings.
Spring-Loaded Seals
Since PTFE has a much larger coefficient of thermal expansion than the materials within which a liner is enclosed, a PTFE liner will expand and contract more than its enclosure when these vessels are heated and cooled. To compensate for this difference and to maintain a constant loading on the PTFE seal, all Parr acid digestion vessels have spring-loaded closures which will continue to maintain pressure on the liner seal throughout the operating cycle, and particularly during cooling when PTFE parts would otherwise relax and leak.
Safety Blow-Off Discs
Safety blow-off assemblies are provided on most Parr Acid Digestion Vessels to protect the vessel and the operator from the hazards of unexpected or dangerously high internal pressures. The blow-off feature consists of an area with an intentionally weakened structure allowing the vessel to fail in a predictable and reasonably safe manner in the event it becomes over pressurized. The burst pressure in each installation is established by a frangible metal disc installed as a sandwich with a matching inner disc which serves as a corrosion barrier to protect the main disc from corrosive vapor.