Drain cock, n.
A drain cock is a device for venting liquid from a pressurized vessel. When the drain cock is in an open position, liquid may exit from the vessel though an opening in a valve stem. The drain cock includes primary and back-up seals that form by threading a stem through a housing aperture to a closed position. The primary seal is formed by a resilient sealing member tightly contacting the housing valve seat. The back up seal is formed by the rigid, conical surface of the valve stem mating with a rigid valve seat in the housing aperture. The double seal arrangement provides a dependable occlusive seal because the back up seal safeguards leakage in the event of primary seal failure. The drain cock reduces the likelihood of such failure because the surface to surface back up seal alleviates the transverse forces to which the primary resilient seal is subjected. Additionally, because the valve and valve seat are rigid members, the backup valve is not susceptible to deformation or costly repair.
Source: Introduction to United States Patent 6062540 (Public Domain)