What are the methods for adjusting centrifugal pump flow?

For a single pump, it is usually the regulating valve on the outlet pipeline, then the pump outlet valve, the regulating valve or stop valve on the return line, and the inlet is generally not regulated, right? What's the discussion about this? If the adaptability of the pump is concerned, there are methods such as pump selection, series connection, and parallel connection to adjust the flow rate.

1. Add a control valve 2. Add a flow limiting plate 3. Change the motor to use variable frequency to control the speed. The above three are the centrifugal pump flow adjustment that can be implemented concretely. I have used most of them.

1. Use the pump outlet valve to manually adjust. 2. Install a pneumatic or electric regulating valve on the pump outlet pipe. Automatic adjustment or remote adjustment can be set.

The opening of the outlet valve can also increase the flow rate by connecting multiple pumps in parallel.

Change motor frequency to achieve regulation； Adjust via regulating valve.

1. Pump outlet control valve 2. Regulating valve 3. Orifice plate. Pay attention to the valve opening of the backup pump.

(1) Install a regulating valve on the outlet pipeline of the centrifugal pump to pressurize the valve； Or close the outlet valve. 　　The advantage of this adjustment method is that it is easy to operate and flexible. The disadvantage is that when the valve is closed, the resistance in the pipeline increases and the energy loss increases, so that the pump cannot work in the highest efficiency area, which is uneconomical. The method of changing the valve opening to adjust the flow rate is often used in situations where the flow adjustment range is not large and adjustment is often required. 　　(2) Change the speed of the pump, that is, change the characteristic curve of the pump. 　　(3) Turning the outer diameter of the impeller also changes the characteristic curve of the pump. The curve of the pump can be changed using the above two methods. Using these methods to adjust the flow rate within a certain range can ensure that the pump works in a high-efficiency zone, and energy utilization is more economical, but it is inconvenient and the flow adjustment range is not large, so it is not widely used.

The centrifugal pump has a fixed head at a fixed speed. Adjusting the outlet valve adjusts the diversion area. This method can be used to adjust the flow rate. The advantage is that it is simple and easy to implement, but the disadvantage is that the throttle valve consumes energy. Using a frequency converter to adjust the motor speed can also adjust the flow rate, which has the advantage of saving electric energy.

The following content is excerpted from Huang Chunfang, Sinopec Press 2. Adjustment of centrifugal pumps The working point of a centrifugal pump is the intersection point of the pump characteristic curve and the pipeline characteristic curve. If the number of revolutions of the pump is constant, there is only one characteristic curve of the pump (except for axial flow pumps with adjustable blades)； If the pipeline device remains unchanged and the valve in the pipeline does not move, the characteristic curve of the device will not change. The two unchanged curves have only one intersection point. Therefore, when the pump is running in the pipeline network, the working point is fixed. However, when the pump is running in the system, sometimes due to the influence of multiple factors such as the collaborative work of two or more pumps and the pipeline system, the actual operating operating point of the pump does not match the optimal working point of the pump. In this case, the characteristics of the pump need to be adjusted to make it operate economically. Sometimes, the pump also needs to be adjusted to meet certain flow and head requirements. This method of artificially changing the operating operating point of the pump is called pump adjustment. There are only two ways to change the working point of a centrifugal pump: One is to change the pump characteristic curve, and the other is to change the pipeline characteristic curve. 1. Adjustment by changing the pipeline characteristic curve 1) Outlet valve throttling adjustment Figure 10-9-1 Regulating valve throttling adjustment The outlet valve throttling adjustment is to adjust the working point by changing the opening of the regulating valve installed on the discharge pipeline of the pump, that is, by changing the characteristic curve of the pipeline device. As shown in Figure 10-9-1, if the characteristic curve of the pump is HQ, and you want to adjust the flow rate to decrease from Q1 to Q2, you can adjust the opening of the valve, that is, close the valve, so that the pipeline characteristic curve is adjusted from Q-Σh1 to Q-Σh2. At this time, h1 in the pipeline changes to h2, the flow rate decreases from Q1 to Q2, and the head changes from H1 to H2, achieving the purpose of changing the flow rate. The disadvantage of throttling adjustment by turning down the outlet control valve is large energy loss, which increases the throttling loss of the valve and reduces the overall efficiency of the device. This adjustment is uneconomical in the long term. Especially for centrifugal pumps with steep drop-shaped head performance curves, this method is even more uneconomical. However, because this method is simple and easy to operate, as long as a regulating valve is installed on the pressure line and the valve opening is changed, the adjustment requirements can be met and a wider adjustment range can be obtained. At present, this method is still widely used. 2) Bypass valve adjustment: Use the pump inlet and outlet bypass valve to adjust the flow. When the bypass valve is opened, part of the liquid flow flows back to the pump inlet through the bypass pipe. At this time, the lift of the pump decreases and the flow rate on the discharge pipe decreases. The purpose of flow regulation is achieved. Generally, this adjustment method is very uneconomical because the flow in the bypass consumes energy in vain. However, for some centrifugal pumps and vortex pumps with high specific speeds, as the power of these pumps does not increase or decreases as the flow rate increases, it is more appropriate to apply this adjustment method. 2． Changing the performance curve of the pump for adjustment In addition to the use of series and parallel operation of pumps to change the characteristic curve of the pump to achieve the purpose of adjusting the working conditions mentioned in the previous section, there are also commonly used methods for changing the performance curve of centrifugal pumps.: 1) Change the speed to adjust the performance of the pump. One way to change the speed of the pump is to use the prime mover of the pump that can adjust the speed, such as a steam turbine. Diesel engine or DC motor, two-speed (low speed and high speed) motor. The most common and widely used centrifugal pump is the use of variable frequency motors for speed regulation. Speed-adjustable steam turbines, diesel engines or speed-adjustable motors have many auxiliary equipment, which are inconvenient to use, operate and maintain, and the structure of the equipment itself is also complex. Especially in large-capacity units, these prime movers are bulky, heavy, and inconvenient to maintain. Figure 10-9-3 Changing the speed adjustment Changing the speed of the pump means changing the characteristic curve of the pump. As shown in Figure 10-9-3, when the pump speed drops from n1 to n2, the flow rate drops from Q1 to Q2, and the head drops from H1 to H2. When the valve opening remains unchanged, that is, the pipeline characteristics remain unchanged, the performance change rules before and after changing the rotation speed are as follows:: (10-9-1) In the formula, Q1, H1, P1——are the flow rate, head and power when the revolution is n1. Q2, H2, P2——are the flow rate, head and power when the revolution is n2. 2) Turning the outer diameter of the impeller to expand the range of use Turning the outer diameter of the impeller is actually an adjustment method to change the characteristic curve of the pump, but it is only a one-time change and cannot be adjusted repeatedly. Figure 10-9-5 Cutting the outer diameter of the impeller Usually in order to expand the use range of a centrifugal pump, the outer diameter of the impeller can be cut into several sizes. In this way, according to the changes in the high efficiency area of ​​the pump and the throttling adjustment, there is an economical use range of small squares as shown in Figure 10-9-5, which can be found on the pump sample. 3) Disassemble the impeller series. Multi-stage pumps can disassemble the impeller series to achieve the purpose of adjusting the head. For example, the 250YS100×2 oil pump has a head of 600m. After removing the two-stage impeller, the head is reduced to 400m. The purpose of reducing energy consumption is achieved by reducing the head of the pump. When disassembling the impeller of a multi-stage pump, two points should be noted:: Because the suction device of the centrifugal pump is installed in front of the first-stage impeller, when disassembling the impeller of a multi-stage pump, the first-stage impeller must not be disassembled. ； When disassembling more than one stage, try not to disassemble adjacent impellers. Instead, disassemble them in sections to prevent uneven flow paths between stages and reduce pump efficiency.