Metering pump

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A metering pump is a special type of positive displacement pump that can meet the requirements of various stringent process workflows; its flow rate can be adjusted continuously within the range of 0-100%, and it is used for transporting liquids, especially corrosive ones. As is well known, a metering pump mainly consists of three parts: power drive, fluid delivery, and regulation control. The power drive unit drives the fluid transfer diaphragm (piston) to move back and forth through a mechanical linkage system; during the first half of each stroke, the diaphragm (piston) draws in the fluid to be transferred, and during the second half of the stroke, it expels the fluid from the pump head. Image 1. Working principle of the metering pump: The metering pump consists of three parts – the drive base, the pump head, and the motor. The drive base component is driven by the motor to rotate the worm, and through the worm gear and lower sleeve, the power is transmitted to the N-shaped shaft. The N-shaped shaft and the eccentric block drive the crank-slider mechanism and the crosshead, causing the plunger to move back and forth; the stroke of the plunger is adjusted by modifying the rotational eccentricity through the N-shaped shaft and the stroke adjustment mechanism, thereby controlling the flow rate ； As the plunger moves back and forth within the hydraulic cylinder, the suction and discharge valve assemblies open and close alternately, allowing liquid to be continuously drawn in and discharged from the hydraulic end ； During the suction stroke of the pump, a negative pressure is created in the liquid chamber of the pump head, which causes the suction valve to open and allows liquid to flow into the cylinder chamber ； During discharge, the movement of the plunger increases pressure on the liquid, causing the discharge valve to open and release the liquid. Based on the overcurrent protection mechanism, they can be divided into four types: plunger type, piston type, mechanical diaphragm type, and hydraulic diaphragm type. Based on their driving method, they can be divided into three types: motor-driven, electromagnetic-driven, and pneumatic-driven. Based on their working principle, they can be divided into three types: reciprocating, rotary, and gear-type. Other categories include electronically controlled, pneumatically controlled, insulated, heating-type, and high-viscosity types, among others. II. How to select a metering pump: Determine the pressure: The rated pressure selected should be slightly higher than the actual maximum pressure required, typically by 10–20% ； Do not choose a value that is too high; excessive pressure will result in energy waste, as well as increased costs for equipment investment and operation. 2. Determine the flow rate: The flow rate of the selected metering pump should be equal to or slightly greater than the flow rate required by the process. It is preferable for the flow rate of a metering pump to be within the range of 30% to 100% of its rated flow rate, as this ensures high repeatability and precision for the metering pump. For economic and practical reasons, it is recommended to select the actual flow rate required by the metering pump to be 70–90% of its rated flow rate. Image 2: Determining the material of the pump head (hydraulic end): Once the specific model and specifications of the metering pump are known, it is necessary to select the material for the parts that come into contact with the fluid, based on the properties of that fluid. This step is very important; an incorrect choice can lead to corrosion of the components due to the action of the fluid, or to fluid leakage that contaminates the system. In severe cases, it can also lead to major accidents. 3. Precision level: When selecting a metering pump, it is also necessary to consider the required precision level of the pump; the higher the precision level, the greater the investment required. The normal operating temperature for metering pumps is between –30 and 100°C; special metering pumps have a wider operating temperature range (for example, high-temperature liquid metering pumps with insulation jackets can handle temperatures up to 500°C). Regarding the particle size of the medium, it is required to be less than 0.1 mm; for media with a particle size greater than 0.1 mm, the flow-through structure of the pump can be modified accordingly to meet the requirements. For the viscosity of the medium, it should generally be between 0 and 1000 mm/s; special metering pumps can achieve values up to 6000 mm2/s. III. Troubleshooting of common issues
1. When the outlet of the metering pump is at atmospheric pressure, how can the pump’s repeatability in metering be improved? Installing a valve spring on the discharge valve of the metering pump can enhance its repeatability. The most effective solution, however, is to install a backpressure valve at the end of the pipeline. 2. When measuring hydrogen peroxide using the self-priming method, cavitation occurs within the system. The metering pump is equipped with a self-venting pump head, enabling self-priming liquid suction. Keep the suction line as short as possible. 3. The metering pump does not measure the liquid medication after installation and operation. (1) Is an exhaust pipe installed and is the exhaust valve closed? During the priming stage of the metering pump, the discharge valve needs to be opened. Note: Not all metering pumps have a drain valve. (2) There may be a gas leak at the suction side of the metering pump. The suction side connector on the hydraulic end may be missing an O-ring or the suction valve connection may be loose ； (3) The bottom valve of the metering pump may be blocked, preventing the measured medication from passing through ； (4) The stroke length of the metering pump is not set properly ； (5) The measured chemical substances may crystallize at the hydraulic end, preventing the check valve ball and seat from functioning properly. Image 4: How to prevent the stroke positioning motor from burning out? When adjusting the stroke position, make sure that the motor of the metering pump is always running. If the metering pump motor is not running, the stroke length adjustment rod must resist the tension of the compression spring, which can lead to premature damage of the stroke positioning motor. 5. How to replace the diaphragm of a metering pump? (1) Remove the 4 screws that hold the pump head in place. The screws are located on the back of the metering pump. (2) After the pump head becomes loose, but before removing it, adjust the stroke length to 0% position. It ensures that the electromagnetic shaft has sufficient pressure to maintain a secure connection, thereby allowing the diaphragm to be removed. (3) Pull the hydraulic end outward to disengage the screw from the socket. Grab the liquid end and rotate counterclockwise. With a little resistance, the diaphragm can be unscrewed. (4) Once the diaphragm has been removed, inspect the safety diaphragm of the metering pump to ensure it is intact and free from any damage. Install a new diaphragm; rotate the backplate and diaphragm clockwise until they fit tightly. Adjust the backplate so that the liquid leakage outlet is located at the very bottom of the pump. (5) After the diaphragm has been installed and the leak drainage holes on the backplate are in a vertical position, install the pump head. Ensure that the suction valve is aligned with the liquid leakage discharge hole, and that the screws on the hydraulic side are aligned with the corresponding 4 holes. (6) Rotate to the 100% stroke length position. This allows the entire assembly to rotate so that the leak outlet on the backplate aligns with the lowest point of the pump. Adjust the hydraulic end and diaphragm to the appropriate positions during pump operation. (7) Once the hydraulic end along with the backplate are in the correct position, the 4 bolts are tightened diagonally until they are properly secured. Apply even pressure when completing this task.