How to Select Pipe Diameter for Micro Gear Pump Systems

Sometimes, customers report that the pump's flow rate does not meet requirements. In such cases, Suofu ask them one question: "What is the inner diameter and length of your inlet pipe?"

Generally, when designing large-scale systems, users pay close attention to pipe diameter selection. However, this aspect is often overlooked in micro pump systems. Furthermore, for large industrial flow systems, there are clear specifications in various manuals for selecting the appropriate pipe diameter based on known parameters like flow rate, temperature, and viscosity. However, these manuals typically do not provide detailed specifications for pipe diameters smaller than DN25. So, how should the pipe diameter be selected for applications with flow rates ranging from several tens of milliliters per minute to several tens of liters per minute? Here is a brief discussion:

Considering purely from the perspective of liquid flow velocity:

I. Outlet Pipe Diameter (Inner Diameter):

For the pump outlet pipe diameter, two main points should be considered:

1. The pipe diameter should not be too small, and the flow velocity should not be too high, to avoid unnecessary flow resistance losses.

2. The pipe diameter need not be excessively large, to prevent unnecessary pipeline costs.

We recommend that for liquids with a viscosity around 1 cP, a reasonable flow velocity is between 1-3 m/s. For example, for a flow rate of 5 L/min, the suitable pipe inner diameter range is approximately 6mm (corresponding to 3 m/s) to 10mm (corresponding to 1 m/s). If the viscosity increases, the allowable flow velocity can be reduced accordingly.

The selection of the outlet pipe diameter should primarily ensure appropriate flow resistance within the pipe. The pipe diameter can be appropriately increased for longer pipelines or those with numerous bends, and vice versa. Of course, reducing the outlet pipe diameter will increase the outlet pressure. Money saved on piping costs may then be offset by increased costs for purchasing a higher-pressure pump and higher operational motor power consumption, as the pump's power consumption (W) is proportional to the product of flow rate (Q) and pressure (P).

II. Inlet Pipe Diameter (Inner Diameter):

In most cases (when the inlet is not pressurized), the pump can only draw liquid in by relying on atmospheric pressure. The maximum pressure difference from the inlet pipe end to the pump inlet end = 101 kPa - NPSHr - the height difference between the liquid level and the pump (assuming the liquid tank is at atmospheric pressure). In other words, the inlet conditions for the pump are far more constrained than the outlet conditions, as the maximum theoretical pressure difference is only about one atmosphere. Therefore, we recommend that for liquids with a viscosity around 1 cP, a reasonable flow velocity within the inlet pipeline is between 0.5 - 1.5 m/s. For example, for a flow rate of 5 L/min, the suitable pipe inner diameter range is approximately 8mm (corresponding to 1.5 m/s) to 14mm (corresponding to 0.5 m/s). If the viscosity increases, the allowable flow velocity can be reduced accordingly.

If Handling Volatile Liquids:

If the liquid being transferred is prone to volatilization, such as Freon, an overly small inlet pipe diameter can easily create a certain degree of vacuum within the pump inlet pipeline. This can exacerbate liquid vaporization and may ultimately prevent the system from functioning properly. In such cases, the pipe inner diameter should be appropriately increased. The pump should be placed as close as possible to the liquid tank, and the number of bends, diameter changes, and valves in the pipeline should be minimized. It is also advisable to install a vacuum gauge on the pipeline to monitor the vacuum level in real-time and reduce liquid vaporization.

Here is a chart for pipeline selection for your reference, primarily considering the outlet.