Types, characteristics of metering pumps and flow regulation methods

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A metering pump is also known as a dosing pump or proportional pump. A metering pump is a reciprocating positive-displacement pump used for precise measurement, and it is generally required that its stability accuracy be no more than ±1%. Types and characteristics of metering pumps Based on the structural type of the hydraulic section, metering pumps are commonly divided into four types: piston-type, hydraulic diaphragm-type, mechanical diaphragm-type, and bellows-type. 　　①Plunger-type metering pump. Its structure is basically the same as that of a conventional reciprocating pump; its hydraulic end consists of a cylinder, plunger, suction and discharge valves, sealing packing, etc. In addition to meeting the design requirements for the hydraulic end of conventional reciprocating pumps, components such as the suction valves, discharge valves, and seals, which affect the pumping accuracy, must also be carefully designed and selected. 　　②Hydraulic diaphragm metering pump. It is commonly referred to as a diaphragm metering pump. A single-diaphragm metering pump is equipped with a diaphragm at the front end of the plunger (the plunger does not come into contact with the diaphragm), which separates the hydraulic section into a fluid delivery chamber and a hydraulic pressure chamber. The infusion chamber is connected to the pump’s suction and discharge valves; the hydraulic chamber is filled with hydraulic oil (a light-type oil) and is in communication with the hydraulic oil tank (refilling tank) located at the upper end of the pump. As the plunger moves back and forth, pressure is transmitted to the diaphragm through hydraulic oil, causing it to bend back and forth and thus resulting in changes in volume; this mechanism enables the transfer of liquid while meeting the requirements for precise measurement. 　　③Mechanical diaphragm metering pump. Its diaphragm is connected to the plunger mechanism; there is no hydraulic oil system, and the back-and-forth movement of the plunger directly causes the diaphragm to bend back and forth. 　　④Bellows-type metering pump. Its structure is similar to that of a mechanical diaphragm metering pump, with the diaphragm replaced by a bellows, and the end of the plunger being fixed to the bellows. As the plunger moves back and forth, it stretches and compresses the bellows, thereby changing the volume of the cylinder and achieving the purpose of fluid delivery and measurement. Flow rate regulation methods for metering pumps The common methods for regulating the flow rate of metering pumps include adjusting the stroke of the plunger (or piston), adjusting the number of reciprocations of the plunger, or combining both of these methods; among them, adjusting the stroke is the most widely used approach. This method is simple and reliable; it can still maintain high metering accuracy at low flow rates. There are three ways to adjust the travel distance. 　　①Manual adjustment for parking. Manually adjust the stroke of the metering pump when parking. 　　②Manual adjustment during operation. During pump operation, the axial displacement is changed to indirectly adjust the crank radius, thereby achieving the purpose of regulating the stroke length. Common methods include N-shaped crankshaft adjustment, L-shaped crankshaft adjustment, and eccentric cam adjustment, etc. 　　③Automatically adjusts during operation. The common types are pneumatic control and electric control. Pneumatic control achieves automatic adjustment of the stroke by changing the air supply pressure signal. Electric control achieves automatic adjustment of the stroke by changing electrical signals. The three valves of the metering pump: In diaphragm metering pumps, in order for the diaphragm to function properly, it is necessary to maintain a constant volume of hydraulic oil within the closed space of the pump’s hydraulic chamber ； This ensures that the volume created by the reciprocating motion of the diaphragm always equals the displacement volume of the plunger, thereby maintaining a constant flow rate in the diaphragm metering pump. However, in the actual use of metering pumps, leaks are unlikely to occur at the areas where the plunger is sealed, and if a leak does happen, gas may enter the hydraulic chamber. If too much oil is added or if the pressure in the discharge line rises unexpectedly, it will cause a change in the volume of the liquid within the hydraulic chamber, which in turn leads to unstable flow rates from the metering pump and reduces its accuracy in measuring volume. Therefore, to address the above issues, each metering pump is equipped with a so-called three-valve device, which actually refers to a device or a combination of devices that serve three functions: a compensation valve, a vent valve, and a safety valve, all of which are associated with the hydraulic chamber. Based on its different working principles, it is also known as an automatic compensation three-valve device. 　　The automatic compensation three-valve device is widely used in diaphragm metering pumps, especially in situations where a suction lift requirement exists; only such a compensation device can be employed in such cases. As shown in the figure, it consists of three separate valves (safety valve, vent valve, and oil fill valve). 1. Function of the metering pump safety valve The function of the metering pump safety valve is to prevent abnormal pressure levels in the hydraulic chamber, thereby protecting the diaphragm. When the hydraulic oil in the hydraulic chamber reaches a level beyond the acceptable limit due to excessive filling, and the diaphragm has reached the front limit plate while the plunger has not yet reached the end of its discharge stroke, the cylindrical plunger will continue to move forward, causing the pressure in the hydraulic chamber to rise sharply. At this point, the safety valve of the metering pump should open immediately. At the same time, when the pressure in the discharge line rises abnormally, the safety valve of the metering pump can also serve to protect the metering pump from damage. The opening pressure of the metering pump safety valve should be able to be adjusted with sufficient precision and accuracy easily. Figure 4-39 shows a common structure for safety valves. 2. Function of the discharge valve of the metering pump: Since it is impossible to completely prevent gas from entering due to the plunger seal, gas can also enter the hydraulic chamber in other situations, such as during refueling; this formation of air bubbles can significantly affect the accuracy of measurement. Therefore, a vent valve needs to be installed. The vent valve should be located at the highest point of the hydraulic chamber to facilitate the release of gas. 3. Function of the compensating valve in a metering pump During long-term operation of a metering pump, as the amount of working fluid leaking from the plunger seal increases, the oil volume in the hydraulic chamber gradually decreases. During the suction stroke, when the diaphragm has reached the rear limit plate but the plunger has not yet reached the rear dead center, an instantaneous vacuum is created in the hydraulic chamber, resulting in a decrease in flow rate. Therefore, a compensation valve is necessary to automatically make up for the loss of oil in the hydraulic chamber when such an instantaneous vacuum occurs. 　　　In the three-valve device for forced compensation, regardless of whether oil top-up is required, whenever the plunger reaches the dead center, the device connects the oil top-up chamber with the hydraulic chamber; at this point, whether oil top-up takes place is determined by the pressures in these two chambers. The forced compensation three-valve is also located at the highest point of the hydraulic chamber, serving both to refill oil and to release air. 　　A metering pump is a special type of reciprocating positive-displacement pump whose flow rate can be adjusted continuously within the range of 0-100%. It is used for transporting liquid materials, especially corrosive and hazardous liquids. Metering pumps are also known as dose pumps or proportional pumps; in some cases, they are referred to as dosing pumps as well.