1.How to select a metering pump
- Determine the pressure: The rated pressure of the selected metering pump should be slightly higher than the actual maximum pressure required, generally 10-20% higher. But don’t choose too high. Too high pressure will waste energy and increase equipment investment and operating costs.
- Determine the flow rate: The selected metering pump flow rate should be equal to or slightly greater than the flow rate required by the process. The use range of the metering pump flow rate is better at 30-100% of the rated flow range of the metering pump. At this time, the metering pump has high reproducibility accuracy. Considering economic and practical considerations, it is recommended that the actual required flow rate of the metering pump be selected as 70-90% of the rated flow rate of the metering pump.
- Determine the material of the pump head (hydraulic end): After the specific model and specifications of the metering pump are determined, the material of the flow part is selected according to the properties of the flow medium. This step is very important. If the selection is improper, it will cause the medium to corrode and damage the flow parts or the medium to leak and pollute the system, etc. In severe cases, it may also cause major accidents.
- Other aspects: When selecting a metering pump, it is also necessary to consider the accuracy level of the required metering pump. The higher the accuracy level, the greater the investment. The general working temperature of metering pumps is -30 to 100°C, and the working temperature range of special metering pumps is wider (such as high-temperature liquid metering pumps with insulation jackets, whose conveying temperature can reach 500°C). For the particle size of the medium, it is required to be less than 0.1mm. For media larger than 0.1mm, the flow structure of the pump can be changed in a targeted manner to meet the needs. For the viscosity of the medium, it should generally be 0 to 1000mm/s, and special metering pumps can reach 6000mm2/s.
- The flow rate of a mechanical diaphragm metering pump is the maximum flow rate measured under the calibrated rated pressure (when conveying clean water at room temperature). If the pressure drops, the output flow rate will be higher than the calibration.
- The output flow rate of the metering pump is a hard characteristic (except for mechanical diaphragm metering pumps). Once the flow rate is adjusted in place, it is a fixed value and is not affected by the discharge pressure.
2.Precautions for the installation and pipeline laying of metering pumps
◆ Prevent pipeline vibration: Due to inertial resistance, the fluid at the discharge end will generate pulsating impact force on the pipeline at the moment the pump starts running. This is a unique phenomenon of reciprocating pumps, which is caused by the sudden acceleration of the liquid in the pipeline.
In order to prevent this phenomenon from occurring, the inertial resistance (Pid) at the discharge end is required to be less than 1.0kgf/cm2. When Pid reaches 1.0kgf/cm2, the pipeline will vibrate, which will have an adverse effect on the pump and its peripherals.
Solution:
- Install a pulsation damper (a pressure vessel used to eliminate the pressure pulsation or flow pulsation of the liquid in the pipeline);
- Increase the diameter of the outlet pipeline and shorten the length of the outlet pipeline.
◆ Prevent oversupply: Oversupply is due to the flow inertia of the liquid in the pipeline, which affects the normal operation of the check valve and makes the check valve ineffective, resulting in excessive and unstable flow of the pump. Whenever the negative pressure at the suction end is insufficient or the pipeline is too long, this phenomenon may occur. Therefore, it is required that: the pressure difference of the pump is greater than the inertial resistance Pi (the larger of the inlet or outlet end).
Solution:
- Install a pulsation damper;
- Install a back pressure valve;
- Try to shorten the length of the inlet and outlet pipelines.
◆ Prevent cavitation: If the cavitation margin (NPSHa) of the device is insufficient, the pump may be damaged due to flow interruption or cavitation. Therefore, the cavitation margin of the device is required, NPSHa> required cavitation margin (NPSHr).