A minimum flow valve, also known as an automatic recirculation valve, is a pump protection valve installed at the outlet of a water pump. It prevents damage caused by overheating, excessive noise, instability, and cavitation when the pump is operating under load. As long as the pump flow rate falls below a certain threshold, the valve's bypass return port automatically opens, ensuring the minimum flow rate required by the pump.
I. Working Principle
A minimum flow valve is connected to the pump outlet. Like a check valve, it relies on the thrust of the medium to open the valve disc. While the main channel pressure remains constant, the valve disc opens to a certain position depending on the flow rate in the main channel. The main valve disc is then positioned at a certain position. A lever transmits this movement to the bypass, switching the bypass on and off.
II. Working Process
When the main valve disc opens, it drives the lever, which closes the bypass. When the flow rate in the main channel decreases and the main valve disc cannot be opened, the main valve disc returns to its sealing position, closing the main channel. The disc then actuates the lever, opening the bypass. Water flows from the bypass to the deaerator. Under pressure, the water flows to the pump inlet, where it is recirculated, thus protecting the pump.
III. Advantages
The minimum flow valve (also known as an automatic control valve, automatic recirculation valve, or automatic backflow valve) is a valve that integrates multiple functions.
It has the following significant advantages:
1. The minimum flow valve is a self-operated control valve. The lever automatically adjusts the bypass opening according to the flow rate (regulating the system's flow). It is a fully mechanical structure, relying on the flow control valve, and does not require additional energy.
2. The bypass flow rate is adjustable, making the overall valve operation highly economical.
3. Both the main channel and the bypass function as check valves.
4. The three-way T-shaped structure is suitable for recirculation pipelines.
5. The bypass does not require continuous flow, reducing energy consumption.
6. Integrated multi-functions reduce design workload.
7. Significant cost advantages in initial product procurement, installation and commissioning, and ongoing maintenance. Installation and maintenance costs are reduced, resulting in overall costs lower than traditional control valve systems.
8. Reduced failure potential, minimizing the possibility of failure caused by high-speed fluid flow, eliminating cavitation issues and electrical wiring costs.
9. Stable pump operation is ensured even under low-flow conditions.
10. Pump protection requires only a single valve, eliminating the need for additional components. Because the pump is unaffected by failures, the main and bypass channels are integrated, making it virtually maintenance-free.
IV. Installation
The minimum flow valve should be installed at the pump outlet, as close as possible to the centrifugal pump being protected. The distance between the pump outlet and the valve inlet should not exceed 1.5 meters to prevent low-frequency water hammer caused by fluid pulsation. Flow direction is bottom-to-top. Vertical installation is preferred, but horizontal installation is also acceptable.
V. Maintenance, Care, and Usage Precautions
1. Valves should be stored in a dry, ventilated room, with both ends of the valve channel blocked.
2. Valves stored for long periods should be inspected regularly to remove dirt. Particular attention should be paid to cleaning the sealing surfaces to prevent damage.
3. Before installation, carefully check that the valve markings match the intended use.
4. Before installation, inspect the valve interior and sealing surfaces. If any dirt is present, wipe them clean with a clean cloth.
5. After use, valves should be inspected regularly, including the sealing surfaces and O-rings. Any damage or failure should be repaired or replaced promptly.