Face Type Seals
The face seal design chart below explains the hardware dimensions to use for an O-ring seal when the groove is cut into a flat surface.
For designing systems which contain internal pressure, like the example below, the groove's outside diameter (OD) is primary, and the groove's width then determines the inside diameter.
Conversely, when designing systems which seal equipment from external pressure, the groove's inside diameter (ID) is primary. This design technique minimizes movement of the O-ring in the groove due to pressure, thereby virtually eliminating wear and pumping leakage. If this principle is used, groove diameters larger or smaller than indicated may be used.
Two possible groove widths are shown in this chart, one for liquids, and the other for vacuum and gases, the extra width for liquids allows for some minimal volume swell. In vacuum applications, the narrower width allows for faster pump down and reduces dead volume in which gas can be trapped. In sealing a liquid that is known to cause no swelling of the O-ring elastomer, the narrower groove would be suitable.
Face type seals are sometimes rectangular in shape. These types of seals are often referred to as "racetrack grooves" due to their shape. When designing such a seal to receive a standard O-ring, the inside corner radii of the groove should be at least three times (ideally six times) the cross-section diameter of the O-ring to avoid over-stressing the ring or causing corner creases that would potentially leak.