Minimum pump flow rate

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What is the minimum flow line of a pump? The metal-colored dividing line represents the minimum discharge flow required for the pump to start safely and operate properly; this is what is known as the minimum flow rate for a centrifugal pump. When the pump’s operating flow rate is 30% below the rated flow rate, it is necessary to establish a minimum flow line that allows the pump to operate properly at its lowest flow rate (that is, a return line is added to the outlet pipe of the centrifugal pump), which is known as the minimum flow line. Speaking in detail is unnecessary; the diagram makes it clear: Curve 3 represents a similar parabola for the minimum continuous flow rate Qmin ； Curve 4 represents a similar parabola for the low flow rate Qmin in the optimal region. Every type of centrifugal pump has a minimum flow rate; below this rate, cavitation occurs in the pump. There is also a maximum allowable operating flow rate, and the range between these two flow rates constitutes the pump’s allowable operating area. 1 When the flow rate is below the pump’s minimum flow rate, the vibration speed increases. To prevent cavitation in the pump, the minimum return line is opened to maintain stable operation of the pump (this function is similar to that of the anti-vibration line in compressors). 2 When starting a high-pressure centrifugal pump, the minimum return line is opened before starting the pump in order to avoid damage caused by pressure buildup. 3 Some centrifugal pumps cannot be stopped, such as boiler feed pumps. Boilers are usually supplied with water by multiple pumps. Depending on the demand levels – for example, more water is needed in the early hours of winter compared to during the day, and less water is required at night – some of these pumps are used to supply water. With this minimum flow pipeline, the pumps that need to be turned off can operate at the minimum flow rate, thus consuming the least amount of power; there is no need to repeat the process of turning the pump off, filling it with water, and then turning it back on. Minimum flow protection for pumps: Metal-style dividing line. There are generally three ways to provide minimum flow protection for pumps. They are: designing a continuous circulation system, designing a control circulation system, or designing a pump protection valve system. 1 Advantages of designing a continuous circulation system: Lower investment required ； When a low-flow condition occurs during operation, the flow-limiting orifice bypass automatically eliminates it. Disadvantage: Regardless of the flow rate required by the process, there is always flow passing through the bypass route. When selecting a pump, the bypass flow rate should be added to the operating flow rate; otherwise, it is likely to result in insufficient pump output ； The operating costs are high ； When the pump head is very high and the liquid is at its saturation temperature, vaporization may occur behind the orifices of the flow control plate. 2 Advantages of designing a control loop system: When selecting a pump, there is no need to consider the additional value associated with bypass flow ； The bypass is activated automatically only under low-flow conditions; it remains closed at other times to save energy. Disadvantage: Adding a set of automatic control circuits is too costly. 3 Design of a self-circulating control valve protection system: Advantages: ① The self-circulating control valve automatically detects changes in flow rate, and automatically activates the main circuit and the bypass circuit, enabling fully automatic control ； ②No additional external devices or components are required for control ； ③Save costs.