The ConFlat (CF) flange was developed in the early 1960s in response to the needs of the semiconductor industry for a gas impermeable vacuum connector. This metal-to-metal seal does not suffer from the permeation issues encountered with elastomeric O-rings such as nitrile, silicone, and fluorocarbon based materials. The flange works by physically deforming a soft metal gasket, typically ¼ hard copper, by cutting into the top and bottom of the seal with a stainless steel knife edge.
A lot of people are concerned about the torque specification for the bolts in a ConFlat flange. This flange is not the cylinder head on a ’68 Ford F100 so bolt torque is not the critical issue. It is more important that the flanges are tightened metal-to-metal (flange faces touching; there is about a 1 mm gap before tightening) than to tighten them to a particular torque. After the first round of tightening them to a particular torque, the next pass indicates they are no longer tightened to that torque because the flange knife edges have bit into the copper gasket.
Following is the process used at the Lesker Company to tighten all size ConFlat flanges. Remember to WEAR GLOVES, wipe everything down with IPA in advance of assembly, and:
1) Ensure the CF flange knife edges are visibly not damaged and that no radial scratches can be felt with a fingernail in the copper gasket.
2) Apply an anti-seize thread lubricant (for example VZTL available at www.Lesker.com) to each bolt to prevent galling between the bolts and nuts, tapped hole, or plate nut.
3) Make sure the top and bottom of the leak check port are aligned in the flange faces. Assemble the bolts nuts and washers or plate nuts into each hole. If nuts are used, I use two washers, one on each side, but this is not critical.
4) Tighten each bolt finger tight (with your gloved hands!).
5) Tighten each bolt 1/12 of a turn (that can easily be determined if one is using a hex head bolt) and then moving in either a clockwise or counterclockwise fashion around the flange, tighten each bolt. One can tighten the bolts in a star pattern if desired, but keeping track on a large flange may be difficult.
6) Continue to tighten each bolt 1/12 of a turn until the CF flange faces touch all around. The assembly is now complete.
7) To ensure that the flange components are metal-to-metal a feeler gauge can be used to determine if there is a gap between the metals.
Regarding plate nuts and washers—do not use washers with plate nuts. It is optional to use washers on the bolt head side. Washers reduce the friction of the bolt head on the flange, making it marginally easier to tighten. Washers also prevent the bolt head from digging into the flange, which makes the flange less attractive for those who may care about such a thing.
The gaskets in ConFlat flanges are not reusable. There are users who have developed techniques which may provide one reuse but there is risk associated with bending this rule.
Category: Flanges and Components
Sub-Category: CF Flanges & Components
Related Topics: Assembly, Maintenance
