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ASA Flanges Technical Notes

ASA flanges, adapted decades ago from 150# steam-pipe flanges, are simple, sexed vacuum seals—one flange is grooved to accept an o-ring (Figure 1) while the mating flange is smooth. They are made from 304L stainless or a weldable aluminum alloy.

ASA flanges are limited (by the o-ring's properties) to applications with temperatures between ~0° C and 120—180° C and pressure from atmosphere to ~10-7 Torr or mbar. The poorer vacuum level than other o-ring systems is suggested because ASA flanges are frequently larger in diameter and therefore have a larger gas load from permeation.

These flanges are sized by the connecting pipe's nominal I.D. in inches. The normal range is: 1", 1.5", 2", 3", 4", 6", 8", and 10", although some manufacturers make sizes up to 36". They have large diameter bolt holes through the rim—a legacy from positive pressure uses. Consequently, for any given tube diameter, ASA flanges have much larger O.D.s than other flange types. The adaptation and adjustment to suit vacuum conditions led to a deterioration of standards.

For example:

  • (a) "Large bore" flanges, introduced to increase gas conductance, may have a larger I.D. than the o-ring groove in normal bore flanges.
  • (b) Bolt holes are sometimes reduced in diameter, causing confusion when bolting flanges together.

As with any sexed flange type, additional issues arise when mating two flanges of the same sex. Here, if the mating pair both have o-ring grooves of identical I.D.s, a thin metal ring is placed between them. If both flanges are smooth, o-rings molded into carriers are installed.

When buying ASA flanged fittings and components, always compare dimensions carefully. When purchasing flanges to mate to an existing item, have that item's flange O.D., I.D., number of bolt holes, bolt circle diameter, and the handy o-ring groove I.D. for quick reference.

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