1、 Overview

As an ideal equipment for precise measurement and dosing of fluid, metering pump has been widely used in various fields including pharmaceutical, food and beverage and petrochemical industries. It is responsible for the measurement and addition of strong corrosive, toxic, high viscous and high-pressure media in the process. After more than half a century of practical application and technical improvement, now the metering pump has entered its high-speed growth period. At present, the mature power drive mode and the material technology of liquid delivery end (pump head) make the new metering pump can transport almost any conventional and special medium, and its working pressure and capacity can also meet most requirements of industrial production. With the general improvement of production process index and automation level, the safety and controllability of the metering pump become increasingly important as the Zui final actuator in the process of chemical measurement and addition. Petrochemical industry has always been famous for the high automation of production process, and it is also one of the widely used fields of distributed, distributed and intelligent computer control system. Therefore, it is required that the matching actuator metering pump should also have flexible and diverse control modes, which can easily form a variety of control loops with the computer system to realize more complex and more complex processes Control. In order to comply with this new trend, international metering pump manufacturers such as German promint company, while ensuring the traditional performance of their products, integrate a variety of adjustment and control functions and data communication protocols into the metering pump with the help of embedded microprocessor system, which truly realizes the two-dimensional adjustment from stroke frequency to stroke length, making its products become the world* Intelligent precision metering pump.

As we all know, the metering pump is mainly composed of power drive, fluid transmission and regulation control. The power driving device drives the fluid conveying diaphragm (piston) to realize reciprocating motion through mechanical linkage system

Diaphragm (piston) in the first half of the stroke will be transported fluid inhalation and in the second half of the cycle will be discharged from the pump head; therefore, changing the frequency of reciprocating motion of the stroke or the stroke length of each reciprocating motion can achieve the purpose of adjusting the fluid delivery capacity. The precise machining accuracy ensures the output of each pump, thus realizing the precise measurement of the transported medium.

Due to the difference of power drive and fluid transmission mode, metering pump can be roughly divided into plunger type and diaphragm type.

2.1 plunger metering pump

There are two kinds of common valve pump and valveless pump. Plunger metering pump is widely used in petrochemical industry because of its simple structure and high temperature and pressure resistance. In view of the shortcomings of ordinary plunger pump for high viscosity medium under high pressure condition, a valveless rotary piston metering pump has been paid more and more attention, and has been widely used in the metering of high viscosity media such as syrup, chocolate and petroleum additives. Due to the structural defect that the metering medium and the lubricant in the pump can not be completely isolated, the plunger metering pump is limited in the application of high anti pollution requirements.

2.2 diaphragm metering pump

As the name implies, the diaphragm metering pump uses a specially designed and processed flexible diaphragm to replace the piston, realizing reciprocating motion under the action of the driving mechanism, and completing the suction discharge process. Because of the isolation function of diaphragm, the separation between the metered fluid and the driving lubrication mechanism is realized in structure. The high-tech structural design and the selection of new materials have greatly improved the service life of the diaphragm. Combined with the excellent corrosion resistance of composite materials, the diaphragm metering pump has become the main pump type in the application of fluid metering. In the family of diaphragm metering pumps, the hydraulic driven diaphragm pump uses hydraulic oil to drive the diaphragm evenly, which overcomes the shortcomings of excessive stress concentration of pump diaphragm under mechanical direct drive mode, and improves the diaphragm life and working pressure upper limit. In order to overcome the working failure of single diaphragm metering pump caused by diaphragm breakage, some metering pumps are equipped with diaphragm rupture sensor to realize automatic interlock protection when diaphragm breaks; the metering of pump head with double diaphragm structure further improves its safety, and is suitable for the application occasions with special sensitivity to safety protection.

As a kind of diaphragm metering pump, electromagnetic driven metering pump uses electromagnet to generate pulsating driving force, which saves the motor and speed changing mechanism, making the system compact and compact, which is an important branch of small range low-pressure metering pump.

Now, the technology of precision metering pump has been very mature. Its fluid metering and conveying capacity can reach 0-100000 L / h, and the working pressure Zui can reach 4000 bar. The working range covers the requirements of all fields of industrial production.

3、 Control of metering pump

The metering capacity of metering pump is determined by the volume of fluid pumped each time. Under a certain effective diaphragm area, the volume flow rate of the pump output fluid is proportional to the stroke length L and stroke frequency f: V ∝ a * f * L. under the condition of determining the metering medium and working pressure, the two-dimensional adjustment of the output of the metering pump can be realized by adjusting the stroke length L and the stroke frequency f.

Although both stroke length and frequency can be used as adjustment variables, in engineering application, stroke length is generally regarded as coarse adjustment variable, and stroke frequency is fine adjustment variable: adjust stroke length to a certain value, and then realize fine adjustment by changing its frequency, so as to increase the flexibility of adjustment. In relatively simple applications, the stroke length can also be set manually, and only the stroke frequency can be used as the adjustment variable to simplify the system configuration