Author: Lin Qiangxing, Shanghai Suofu Industrial Co., Ltd

Sometimes customers say that the flow of the pump cannot meet the requirements. At this time, I will ask him a question: what is the inner diameter and length of your inlet pipe.

When designing a large-scale system, the general user will pay attention to the selection of pipe diameter, but when it comes to the micro pump system, it is easy to be careless. At the same time, for the selection of pipe diameter of large industrial flow system, there are some clear specifications in some manuals. As long as the flow, temperature and viscosity are known, it is easier to select the appropriate pipe diameter. However, the minimum pipe diameter in these manuals is generally DN25, so there is no further subdivision. So how to select the appropriate pipe diameter for applications with a flow range of tens of milliliters to tens of liters per minute? Here we will briefly discuss:

If the flow rate of the liquid is considered purely:

1、 Outlet pipe diameter (inner diameter):

For the outlet pipe diameter of the pump, two points should be considered: 1. The pipe diameter should not be too small and the flow rate should not be too fast to avoid unnecessary flow resistance loss. 2. The pipe diameter need not be too large to avoid unnecessary pipeline cost. We suggest that the reasonable flow rate is 1-3m / s for a liquid of about 1cp. For example, for a flow rate of 5 L / min, the inner diameter of the pipe can be selected from 6 mm (3 M / s) to 10 mm (1 m / s). If the viscosity increases, the flow rate can decrease accordingly. The selection of outlet pipe diameter shall be subject to the proper flow resistance of the liquid in the pipe. If the pipe is long and has many bends, the pipe diameter shall be appropriately increased, otherwise it can be reduced. Of course, if the outlet pipe diameter decreases, the outlet pressure will increase. The money saved in the pipeline will be spent on the increase in the cost of purchasing high-pressure pumps and the increase in motor power during operation. Because the power consumption w of the pump is proportional to the multiplication of the flow rate Q and the pressure P.

2、 Inlet pipe diameter (inner diameter):

In most cases (when there is no pressure at the inlet), the inlet of the pump can only suck liquid by the action of atmospheric pressure. The maximum pressure difference from the inlet pipe end to the inlet end of the pump = 101kpa NPSHr - the height difference between the liquid level and the pump (assuming that the liquid tank is under normal pressure). That is to say, the inlet condition of the pump is far less than the outlet condition, because the maximum theoretical pressure difference is 1 atmospheric pressure. Therefore, we suggest that the reasonable flow rate in the inlet pipeline is 0.5-1.5m/s for the liquid of about 1cp. For example, for a flow rate of 5 L / min, the inner diameter of the pipe can be selected from 8 mm (1.5 m / s) to 14 mm (0.5 m / s). If the viscosity increases, the flow rate can decrease accordingly.

If volatile liquid is transported:

If the transported liquid is easy to volatilize, such as Freon. If the inlet pipe is too small, it is easy to generate a certain degree of vacuum in the inlet pipe of the pump, which will aggravate the vaporization of the liquid, and eventually the test system may not work normally. At this time, the inner diameter of the pipeline shall be properly increased, the pump shall be as close to the liquid tank as possible, and the elbows, reducers and valves on the pipeline shall be less. It is better to install a vacuum gauge on the pipeline to monitor the vacuum degree in the pipeline in real time and reduce the vaporization of liquid.

Here is an icon of pipeline selection for your reference, mainly considering the outlet:

        Then, some friends may have the following question: is the inlet pipe as large as possible? When the pump is self-priming, is the pump speed high or low? I will explain these problems in future articles.