Stable, Soft-Opening/Soft-Closing Pressure-Relief Valves

Stennis Space Center, Mississippi

Monday, March 01 1999

The risk of ignition in systems containing oxygen is reduced.

Improved pressure-relief valves have been developed for systems that contain gases and liquids in a variety of pneumatic, hydraulic, and cryogenic applications. These valves could prove especially beneficial in both cryogenic and noncryogenic systems that contain oxygen. The improved valves are designed to suppress instabilities that shorten operational lifetimes and create hazards in the operation of older pressure-relief valves.

A typical older pressure-relief valve exhibits instability that can result in oscillation ("chatter"), which degrades the valve beyond the normal anticipated wear of parts. Oscillation can result in hard impact; in the presence of oxygen, hard impact can lead to ignition, with resultant catastrophic failure of the valve and possibly of the entire system. A valve of the present improved type is stable over its entire operational range from fully closed to fully open. It does not oscillate or generate hard impacts; instead, it opens and closes softly.

The key to stable, soft-opening/soft-closing operation is a concept of upstream control. A conventional "pop"-type pressure-relief valve is characterized as operating under downstream control: Once the valve has opened, the flow is controlled mainly by an effective cross-sectional area downstream of the valve seat. In a valve of the improved type, the flow-limiting cross section remains upstream of the valve seat at all times, and so the valve is said to operate under upstream control.

The figure illustrates the basic design and principle of operation of a valve of the improved type. As in a conventional relief valve, excessive upstream pressure opens the valve by lifting of a poppet from a seat in a valve body; however, the similarity with a conventional pressure-relief valve ends here. The poppet in the improved valve includes a conical portion and a paddle (essentially a disk) upstream of the conical portion. When the valve is closed and the upstream pressure is below the set point, the conical portion of the poppet engages about half the thickness of a main valve seat, forming a tight seal. In this condition, the paddle engages the wall of a cylindrical passage upstream of the main valve seat.