Volume 10, Number 5 • September/October 2002 • Small Business/SBIR

NASA Helps Industry Relieve Pressure Safely

A high-performance pressure-relief valve developed under an SBIR contract with Stennis Space Center provides stability over the entire operational range, from fully closed to fully open. Artwork courtesy of Stennis Space Center.

In many industrial applications, pressure relief valves (PRVs) perform the critical function of safely releasing pressure before potentially damaging buildups occur. Conventional relief valves, however, have proven unstable, leading to premature wear and devastating consequences. A high-performance pressure relief valve, the PRV95, now being manufactured by Marotta Scientific Controls, Inc. of Montville, NJ, provides the answer to premature wear and instability. Using an improved valve design developed under a NASA SBIR contract from John C. Stennis Space Center (SSC), Marotta’s PRV95 pressure relief valve provides stability over the entire operational range, from fully closed to fully open. The valve employs upstream control for valve positioning, which makes the valve more stable and affords excellent repeatability with minimal lag time.

“It opens and closes softly, and does not oscillate or generate hard impacts; oscillation can result in a hard impact pressure release, which can lead to an explosion in the presence of oxygen,” says Bill St. Cyr, chief of the Test Technology Branch at Stennis Space Center.

Marotta’s PRV95 design is unique in its ability to maintain a seal near the set point of the relief limit. Typically, relief valves seal tightly up to 90 percent of set point and then reseat when pressure is reduced to 85 percent of set point. The PRV95 technology maintains seal integrity until 98 percent of set point and will reseat a 95 to 97 percent of set point. This allows the operator to protect the system while not exceeding its limits.

The key to stable, soft-opening/soft-closing operation is 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 the PRV95, the flow-limiting cross-section remains upstream of the valve seat at all times, thereby creating upstream control. The figure below illustrates the basic design and principle of operation of a controlled pressure relief valve.

As in a conventional relief valve, excessive upstream pressure opens the valve by lifting a poppet from a seat in the valve body. However, the poppet in the PRV95 includes a conical portion and a paddle 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. When the upstream pressure rises to approximately the set point, the poppet moves downstream a little, but the valve is not yet open; the conical portion of the poppet remains partly engaged with the main valve seat, while the paddle remains in the cylindrical passage in the retainer. As pressure rises above the set point, the conical portion of the poppet moves out of the main valve seat, and the paddle moves out of the cylindrical passage in the retainer.

Primarily designed to operate in systems that contain gases and liquids in a variety of pneumatic, hydraulic and cryogenic applications, the PRV95 valve offers several advantages over valves with different operating characteristics. These advantages include a smooth transition from fully closed to fully open; noise and wear reduction through elimination of chatter; reduction in the risk of product fire and explosion through elimination of hard impact; and corresponding reductions in the uncontrolled venting of hazardous fluids and products. In summary, the increased stability of the valve operation results in better performance, with wider operating ranges and control. All of these attributes translate into the additional advantage of lower lifecycle costs. Q

For more information, contact the Office of Technology Transfer at Stennis Space Center, phone: 228/688-1929. Please mention you read about it in Innovation.

NASA Official: Jonathan Root • Web Design: Printing & Design Office, NASA Headquarters • Credits