BS ISO 3669:2020 pdf download – Vacuum technology — Dimensions of knife-edge flanges
This document specifies the dimensions of fixed or rotatable bolted knife-edge flanges used in vacuum systems for pressures ranging from atmospheric to as low as 10 −11 Pa.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
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knife-edge flange
metal sealed flange used for high and ultra-high vacuum service
Note 1 to entry: Sandwiching one metal gasket between two knife-edge flanges and securely bolting these together makes a vacuum tight joint. The seal is made when the identical circular (triangular profile) knife-edges are bolted together. A deformable metal gasket captured between the knife-edge flanges establishes the sealing surface.
Note 2 to entry: Originally developed as ConFlat® flanges. The widespread and continued use of knife-edge flanges has made these a de facto international standard, codified by this document.
nominal bore
value intended to both identify the flange and specify the largest practical size of tubing that can be accommodated by the flange
Note 1 to entry: See Table 1, in which the convention of identifying original flanges by the outside diameter of the  flange (historically in inches) has been maintained.
leak check groove
groove machined into the seal side of the flange to facilitate the free passage of trace gas from the outer perimeter of the flange to the seal zone near the metal gasket
5 Requirements
5.1 Materials
5.1.1 Flange
The selection of the material shall be compatible with the requirements for the flanges. Considerations may include service temperature, sealing capacity, corrosion resistance, magnetic permeability, type of seal gasket used and dimensions.
NOTE Austenitic stainless steel is commonly used, but it is not the intent of this document to specify or limit the choice of flange material to austenitic stainless steel.
5.1.2 Bolt holes
The flange may have either clearance or tapped bolt holes.
NOTE As a number of flanges in use currently originated in the United States, the tapped flanges often have English tapped holes. Of increasing use are flanges with metric threads. Both are presented in this document (see Table 1).
5.1.3 Grooves
Leak check grooves should be used. The grooves shall be arranged equidistantly between the bolt holes.
5.1.4 Gasket
In general, the gasket should be softer than the flange to avoid dulling of flange knife-edge.
NOTE Oxygen-free high conductivity (OFHC) copper is commonly used, but it is not the intent of this document to specify or limit the choice of gasket material to OFHC copper.
5.2 Dimensions
Flange dimensions are shown and specified in Figures 1 to 3 and in Table 1 and Table 2. See Figure 4 for the recommended dimensions of leak check grooves.BS ISO 3669 pdf download.