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What are High Vacuum (HV) and Ultra-High Vacuum (UHV) Flanges?

Vacuum Flanges such as CF or Conflat, ISO, ASA and KF, are detachable connectors similar to connectors in Electrical Technologies.  The design and fabrication of these flanges take into account assembly and disassembly, relatively light weight for ease of handling and the right sealing mechanism for the job.  There are many types of flanges used in the Vacuum Industry.  They are generally categorized as High Vacuum (HV) or Ultra-High Vacuum (UHV).

The HV type include ISO-K, ISO-F, ASA Rotatable, ASA Non-Rotatable, and KF or NW flanges.  These flanges use a soft gasket squeezed between harder flange surfaces to form a leak free joint.  

The UHV flanges are Knife Edge type called CF or Conflat or Wire Seal Designs .  They are truly metal to metal seal types using a soft copper or aluminum gasket seal for baking at high temperatures.

Sealing Mechanism of High Vacuum (HV) Flanges?

The Sealing Mechanisms for HV type is designed for optimal performance based on the applications.  HV flanges use rubber, elastomers and soft polymers on a metal ring.  

KF and ISO sealing systems are identical.  The flange face is counter-bored or grooved to hold a metal centering ring which has an elastomeric O-ring on the outer diameter.  The centering ring aligns the two flanges and keeps the O-ring in position.  For larger diameter flanges, to avoid roll off of the O-ring, a spring-loaded retainer is used.  The seal performance is limited by the elastomeric properties of the O-ring. Temperature range recommended is 0 to120-180 deg F and pressure range atmosphere to 10 to -8 Torr.

ASA flanges adapted from the 150 lb steam pipe flange are gender based design with one flange having a groove for the O-ring and one smooth face for compressing the O-ring.  The vacuum performance of these flanges is 10 to -7 Torr, slightly less than the KF and ISO type flanges. Due to the size of these flanges being larger, they carry larger gas load permeation.

 

Sealing Mechanism of Ultra High Vacuum (UHV) Flanges?

The UHV flanges such as CF or Conflat, are genderless, meaning both flanges are identical.  The seal is made with a knife edge that is machined below the flat surface on the flange.  As the bolts of a flange-pair are tightened, the knife edge makes annular groove on each side of a soft metal gasket.  The metal extruded in the process makes a tight seal by filling in all surface marks and defects.  The performance range of UHV flanges is fairly wide compared to the HV flanges.  The seal operates from 750 Torr to 1×10 to -13 Torr within temperature range of -196 deg C to 450 deg C.

Types of Sealing Materials

For UV flanges the sealing materials are:  Buna-N which is Nitrile rubber, Silicone, Fluorocarbon.  The specific properties depends on the performance desired involving unique combination of inert materials, pure elastomers for mechanical strength and the cross linking agent for curing.

For UHV applications  of CF or Conflat flanges, the most popular material is ¼ hard, high purity, oxygen-free (OFHC) copper stock.  When the seal is compressed between the knife edges a lateral cold flow of the gasket material occurs.  The vertical wall of the knife edge prevents further flow while increasing the interface pressure.  These pressures cause the gasket material to fill in surface imperfections producing a high vacuum seal.  This trapped energy within the seal material provides high contact seal pressures during thermal cycling when the gasket material softens.

Flange Assembly Hardware

Each type of flange require an assembly system designed for optimal seal performance.  Following are the major types:

  1. CF or Conflat  Flanges:

High Tensile Strength nut and bolt made from low magnetic permeability Stainless Steel is strongly recommended.  12 point Cap Screws are better choice for high temperature bakeout.  Plate Nuts with two matching holes allow ease of assembly.

  1. ISO Non-Rotatable and Optional Fastener:

A variety of Bolt/Clamps options are available for ISO flange joints.  These are Single Clamp/Bolt, Double Clamp Bolt, Bolt Ring and Bolt Nipple types.

  1. ASA Rotatable and Non-Rotatable:

The ASA assembly is much simpler compared to other types of flanges.  Bolt sets include Hex Head bolts, Flat Washers and Hex Nuts.

  1. KF or NW Flanges:  

These flanges are smaller diameter and the assembly is much simpler by design.  Preferred methods are with hinged clamp or bulkhead head clamps with bolts.