The union joint will not seal if this happens. If the O-ring has become stiff or shrunken due to age or cold it may sag out of position and become pinched in the joint. If this happens use a dab of grease or adhesive to keep the O-ring in place. If the rubber has become compressed and stiff it may tend to fall out of the valve body during installation. Make sure that O-ring does not disengage from the counterbore area during installation. The joint needs to be tight enough to close the extrusion gap for O-ring to maintain its seal. In this case the O-ring will finish the job. This causes resistance to further turning of the nut and may leave insufficient tightness in the metal to metal seat of the union for sealing.

However, if there is significant misalignment in the piping arrangement then some of the “umpf” experienced during make-up has been diverted into deflecting the piping. Union will seal pressure without need for O-ring if alignment is good enough and if the installation torque has been efficiently transmitted into tightening the joint Union is designed to crush solid metal along a high ridge, creating a “line” seal between the taper and the flat surfaces.

Shattered union manual