What is the minimum flow valve?
What is the minimum flow valve?
The minimum flow valve, also known as the automatic recirculation valve, is a pump protection valve installed at the outlet of the feed pump, which is used to prevent the damage of the feed pump caused by overheating, serious noise, instability and cavitation when the feed pump is running under the load. As long as the flow of the feed pump is lower than a certain value, the bypass return port of the valve will automatically open to ensure the minimum flow required by the liquid feed pump.
The minimum flow valve is used in the hot rolling descaling system of iron and steel enterprises, the medium is water, and the working temperature is ≤ 100 ℃; it mainly protects the normal operation of the water pump. The valve can only be fully opened or fully closed when in use.
- Working principle of minimum flow valve
- Working process of minimum flow valve
- Advantages of minimum flow valve
- Installation of minimum flow valve
- Precautions for maintenance and use of minimum flow valve
- Application of minimum flow valve in power plant
The minimum flow valve is generally installed on the branch pipe at the outlet of the water pump. When the water pump operates normally and the external pipe supplies water, the minimum flow valve is closed. When the normal operation of the water pump does not supply water to the outside, it does not stop the pump. The minimum flow valve is controlled by one of the three ways of temperature, pressure and interlock with the outlet valve to open and close, discharge a part of high temperature water with small flow, prevent cavitation of the water pump, and ensure the normal and continuous operation of the water pump.
When the main valve disc is opened, the valve disc drives the lever action, and the lever force makes the bypass close. When the flow of the main channel decreases and the main valve disc cannot be opened, the main valve disc will return to the sealing position to close the main channel. The valve disc once again drives the lever action, the bypass is opened, and the water flows from the bypass to the deaerator. Under the pressure, the water flows to the inlet of the pump for recycling action, so as to protect the pump, as shown in the figure below.
Figure 1: main valve closed, bypass open status
Figure 2: main valve open, bypass closed status
The minimum flow valve (also known as automatic control valve, automatic recirculation valve and automatic return valve) is a valve with multiple functions.
It has the following significant advantages:
- 1. The minimum flow valve is a self-contained control valve. With the function of lever, the bypass opening (flow regulation of the system) will be automatically adjusted according to the flow. It has a complete mechanical structure and relies on the flow control valve. No additional energy is required.
- 2. The bypass flow can be adjusted and controlled, and the overall operation of the valve has high economy.
- 3. The main channel and bypass have the function of check valve.
- 4. Tee T-shaped structure, suitable for recycling pipeline.
- 5. Bypass does not need continuous flow to reduce energy consumption.
- 6. Multifunctional integration to reduce the design workload.
- 7. It has significant cost advantages in the aspects of early product procurement, installation adjustment and later maintenance, reducing installation and maintenance costs, and the overall cost is lower than the traditional control valve system.
- 8. Reduce the possibility of failure, minimize the possibility of failure caused by high-speed fluid, without cavitation problem and electrical wiring cost.
- 9. The pump can still work stably under low flow condition.
- 10. The protection of the pump only needs one valve and no other additional components. Because it is not affected by the fault, the main channel and bypass become a whole, almost maintenance free.
The minimum flow valve shall be installed at the outlet of the pump as near as possible to the protected centrifugal pump. The distance between the outlet of the pump and the inlet of the valve shall not exceed 1.5m to prevent the low-frequency water hammer caused by the pulsation of the liquid. The flow direction is from bottom to top. Vertical installation or horizontal installation is preferred.
- 1. The valve shall be stored in a dry and ventilated room, and both ends of the valve channel shall be blocked.
- 2. Valves stored for a long time shall be inspected regularly to remove dirt. Special attention shall be paid to the cleaning of sealing surface to prevent the damage of sealing surface.
- 3. Before installation, carefully check whether the valve mark is consistent with the use requirements.
- 4. Before installation, check the inner chamber and sealing surface of the valve. If there is dirt, wipe it with a clean cloth.
- 5. Check the valve regularly after use, and check the sealing surface and O-ring. If the damage fails, it shall be repaired or replaced in time.
All centrifugal pumps have a minimum flow limit. When they operate in this flow direction, the temperature of the pumped liquid will gradually rise and vaporize until the equipment is damaged and unable to operate. Therefore, in order to ensure that the total flow at the pump outlet is not less than the minimum flow, a bypass should be set, and the minimum flow valve plays a regulating role on this bypass. Generally, the bypass should go through large circulation as much as possible, directly connect the root pipe from the outlet to the inlet of the pump, the effect may not be very good, at least return to the front tank, and make it more ideal to allow to reach the previous heat exchanger. The minimum flow valve is installed on the road beside the pump to form a circulation system with the pump, which is mainly to prevent the pump from cavitating and cavitating during start-up and causing damage to the pump. The main selection requirements are that the valve flow is 30% of the pump, the valve has the requirements of high-pressure differential resistance (differential pressure up to 15MPa), anti scour (at present, the advanced technology is multi-stage pressure reduction method), when the valve is closed, it must reach zero leakage, and the flow curve is usually straight. General electric actuator is used for control, and DCS is gradually used for intelligent control.
Source: China Minimum Flow Valve Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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