2008

Improved Relief Valve Would Be Less Susceptible to Failure

Opening force and, hence, opening piston speed would be reduced.

The balanced-piston relief valve with side vented reaction cavity has been proposed as an improved alternative to a conventional high- pressure, high-flow relief valve. As explained below, the proposed valve would be less susceptible to failure.

The left side of the figure shows a typical conventional high- pressure, high-flow relief valve, which contains a piston that is exposed to the upstream pressure across the full valve-seat diameter and is held against the valve seat and the upstream pressure by a large spring. In the event of an increase in upstream pressure to a level above the valve set point (the pressure above which the valve opens), the opening force on the piston can be so large that the piston becomes accelerated to a speed high enough that the ensuing hard impact of the piston within the valve housing results in failure of the valve.

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In the Proposed Relief Valve, the net fluid-pressure opening force on the piston would be much less than in the conventional relief valve because the upward force of the fluid pressure on the bottom piston surface would be mostly counteracted by the downward force of the fluid pressure on the top piston surface.
For a given flow cross section, the proposal would significantly reduce the force, thereby reducing susceptibility to failure. A basic version of the proposed balanced-piston relief valve with side vented reaction cavity is depicted on the right side of the figure. The piston would contain a central hollow that would allow the pressurized fluid to flow into the spring cavity above the piston, so that the pressure in the fluid would act against both the upper and lower piston faces.

The outer diameter of the piston at the upper end would be somewhat less than the outer diameter of the piston at the lower end, the two diameters meeting at a shoulder on the side of the piston. A sleeve filling the annular space between the two diameters would surround the upper end of the piston. Therefore, the upper piston face would be slightly smaller than the lower piston face, the difference between the areas of these faces being equal to the annular cross-sectional area of the sleeve or, equivalently, of the shoulder.

The reaction cavity (the annular side volume between the shoulder and the sleeve) would be vented to either the atmosphere or other source of reference pressure below the valve set point. As a result, the upward (opening) fluid pressure force on the piston would exceed the downward (closing) fluid pressure force on the piston, the net upward fluid pressure force being equal to the annular area of the shoulder and the gauge pressure (absolute fluid pressure less atmospheric or other reference pressure). Because the annular shoulder area could be made less than the area of the lower piston face, the opening force could be tailored to a suitably low value through design choice of the upper and lower piston diameters. (Of course, for a given valve set point, it would be necessary to choose a spring of correspondingly reduced stiffness.) The fluid in the spring cavity would present inertial impedance that would further reduce the opening acceleration of the piston. As an additional benefit, it may be possible to reseat the valve at a greater fraction (perhaps as much as 100 percent) of the valve set point than that of a conventional relief valve.

This work was done by Bruce R. Farner of Stennis Space Center.

Inquiries concerning rights for the commercial use of this invention should be addressed to the Intellectual Property Manager, Stennis Space Center, (228) 688-1929. Refer to SSC-00232-1.

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