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Check valve slow closing structural design
**Abstract:**
To address the issue of water hammer and pressure surges caused by the rapid closure of conventional check valves, this paper proposes a design improvement involving the addition of an auxiliary slow-closing valve. The enhanced check valve is capable of smooth opening and closing, significantly reducing the impact of water hammer. A standard check valve functions by opening when the flow is in the downstream direction and closing when reverse flow occurs, primarily to prevent backflow of the medium, protect pumps and motors from reversal, and ensure safe discharge of fluid from containers. Common types include swing check valves, where the flap swings around a pivot, and lift check valves, where the flap moves vertically along the centerline. However, both structures close abruptly when backflow stops suddenly, leading to a rapid pressure increase on the inlet side, which can cause damage due to water hammer. Hence, traditional check valves are typically manufactured in sizes ranging from 50 to 500 mm.
To mitigate the water hammer effect and expand the application range of check valves, this study introduces a slow-closing auxiliary valve based on the original design. This modification effectively reduces the pressure surge during pipeline operation. For instance, in a swing check valve, a secondary relief valve is installed on the side or both sides of the main flap using threaded connections. Under normal flow conditions, the main valve opens, and the media pressure pushes open the auxiliary valve, allowing fluid to pass through its orifice and relieve pressure at the inlet. As the main flap opens, the pressure before and after the valve becomes balanced. When backflow occurs, the main flap closes quickly, while the auxiliary valve gradually closes via a buffer cylinder. The small hole in the auxiliary valve cover allows controlled flow into the upper chamber, and part of the fluid returns through the gap between the flap and the piston rod, enabling a slow closure of the auxiliary valve. This dual-phase closure—fast for the main valve and slow for the auxiliary—effectively prevents water hammer.
Similarly, in a lift check valve, an additional slow-closing mechanism is introduced between the valve body and the flap. A pad ring increases the flap’s travel, and a rubber seal is used for the buffer piston and cylinder. The valve shaft is rigidly connected to the piston, with minimal clearance between the cylinder and the flap. During forward flow, the flap opens, and the buffer piston rises. In reverse flow, the buffer cylinder, already filled with fluid, gradually releases it through the small gap, allowing the main flap to close slowly and dampen the pressure surge.
Testing was conducted using a calibrated pressure gauge and a pressure transmitter connected to an oscilloscope. Results showed that a standard check valve without an auxiliary valve produced a maximum water hammer pressure increase of 0.53 MPa, while the improved version with a slow-closing auxiliary valve only increased pressure by 0.04 MPa under the same conditions (100 mm diameter, 0.5 MPa system pressure, 5 L/s flow).
In conclusion, the proposed design offers smooth operation, reliable performance, and effective water hammer prevention. It is simple to manufacture, easy to replace, and allows for maintenance of the auxiliary valve without disrupting the main function of the check valve.
References:
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4. Shenyang Valve Institute, Hefei Institute of General Machinery. Valve. Beijing: Mechanical Industry Press, 1994. 75–80