I. Overview As an ideal device for precision fluid metering and dosing, metering pumps are now widely used in various fields including the pharmaceutical, food and beverage and petrochemical industries in the process of being highly corrosive, poisonous, High viscosity and high pressure media metered addition task. After more than half a century of practical application and technical improvements, dosing pumps have now entered their rapid growth period. The proven power-driven and liquid-delivery (pump head) material technology now enables the new metering pump to deliver almost any conventional and specialty media with the same operating pressure and capacity to meet most industrial requirements.
With the general increase in the requirements and automation requirements of the production process, the safety and controllability of the metering pump, as the ultimate implementing agency for the metering and adding of chemicals, have become increasingly important. Known for its high degree of automation in manufacturing processes, the petrochemical industry is also one of the most widely used areas for distributed, distributed and intelligent computerized control systems, requiring matched actuators - metering pumps With flexible and diverse control modes, it is easy to form various control loops with the computer system to realize more complicated and accurate process control. To comply with this new trend, internationally renowned metering pump manufacturers such as ProLogis Germany to ensure that the traditional performance of their products continue to lead the world at the same time, with the help of embedded microprocessor system, a variety of regulatory control functions and Data communication protocol integrated into the metering pump, truly from the stroke frequency to stroke length of the two-dimensional adjustment, making it the world's first product of intelligent precision metering pump.
Two. Metering pumps basic principle is well known, the metering pump is mainly driven by the power, and the adjustment control the fluid delivery of three parts. Power drives drive the fluid delivery diaphragm (piston) via a mechanical linkage system for reciprocation:
The diaphragm (piston) will be suctioned by the delivery fluid during the first half of the stroke and will be expelled from the pump head during the second half of the cycle; therefore, the frequency of reciprocation of the stroke or the length of stroke per reciprocation may be adjusted for fluid delivery . Precision machining accuracy to ensure that each pump to achieve the precise measurement of the medium being delivered.
Because of its power-driven and fluid delivery methods are different, metering pump can be roughly divided into two major types of plunger and diaphragm.
2.1 Plunger metering pump There are two common valve pump and valveless pump. Plunger metering pump because of its simple structure and high temperature and pressure and other advantages have been widely used in the field of petrochemicals. Aiming at the shortage of high-pressure medium under the condition of high pressure, the valve-less rotary plunger metering pump has been paid more and more attention. It is widely used in high viscosity medium such as syrup, chocolate and oil additives Metering added. Due to the structural disadvantage of being unable to achieve complete isolation between the metering medium and the pump lubricant, the plunger metering pump suffers from a number of limitations in high contamination prevention fluid metering applications.
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, reciprocating in the drive mechanism to achieve the suction - discharge process. Due to the isolation of the diaphragm, the structural isolation between the fluid being metered and the drive lubrication mechanism is truly achieved. High-tech structural design and selection of new materials have greatly increased the service life of the diaphragm. Coupled with the excellent corrosion resistance of composite materials, diaphragm metering pumps have now become the main pump in fluid metering applications. In the diaphragm metering pump family members, hydraulic driven diaphragm pump due to the use of hydraulic oil evenly driven diaphragm to overcome the mechanical direct drive pump diaphragm over-concentration shortcomings, enhance the diaphragm life and maximum working pressure. In order to overcome the possible malfunction caused by diaphragm breakage in single diaphragm metering pump, some metering pumps are equipped with diaphragm breakage sensor to realize automatic interlock protection when diaphragm ruptures. The measurement of pump head with double diaphragm structure further enhances its safety For applications that are particularly sensitive to safety.
As a diaphragm metering pump, electromagnetically driven metering pump generates pulsating driving force with electromagnet, which saves the motor and transmission mechanism, makes the system small and compact and is an important branch of low pressure metering pump with small range.
Now, precision metering pump technology has been very mature, the fluid metering capacity of up to 0-100,000 l / h, working pressure up to 4000bar, the scope of work covers all areas of industrial production requirements.
Third, the metering pump control Metering pump fluid pump out of each decision its measurement capacity. At a certain effective diaphragm area, the volume flow of the pump's output fluid is proportional to the stroke length L and the stroke frequency F: VÎ±A * F * L
In the measurement of medium and work pressure to determine the circumstances, by adjusting the stroke length L and stroke frequency F can be achieved on the metering pump output two-dimensional adjustment.
Although both stroke length and frequency can be used as manipulated variables, the stroke length is generally considered as a coarse variable in engineering applications and the stroke frequency is a fine-tuned variable: the stroke length is adjusted to a certain value and fine-tuned by changing its frequency to increase Adjust flexibility. In relatively simple applications, the stroke length can also be set manually, using only the stroke frequency as a control variable, simplifying the system configuration.
3.1 conventional analog / switch signal conditioning methods Process control applications widely used 0 / 4-20mA analog current signal as a sensor, controller and actuator exchange of standards between the external control function of the metering pump is also mainly used this way, External adjustment of stroke frequency and stroke frequency.
Position servos are the most common way to achieve stroke length adjustment. The integrated servos are designed to accept 0 / 4-20mA control signals directly from the regulator or computer, thus automatically adjusting the stroke length to vary from 0-100%.
Relatively speaking, there are quite a few ways to realize the stroke frequency adjustment, mainly including inverter motor control and direct relay contact control. Driven by a 0 / 4-20mA current signal, the variable frequency drive operates the metering pump motor at the desired speed to adjust the stroke frequency. For solenoid driven and partially motor driven metering pumps, the external contact signal can also be used to adjust the stroke frequency.
3.2 base control mode In some special occasions, such as ph adjustment, metering pump as the actuator, add acid or alkali under the control of the regulator. To simplify system configuration and improve reliability, a microprocessor-based embedded control system is integrated directly into the dosing pump so that only a pH sensor is required to provide a complete conditioning system. The concept of a base smart metering pump is also suitable for controlling other process parameters such as ORP and residual chlorine concentration.
3.3 Set Programmed Control As a result of the internal integration of the microprocessor, some metering pump products have been fully improved in control performance and operability. In addition to real-time metering and flow control with external control commands, they also have a quantitative addition, time-series trigger Program addition, event sequence triggering program addition, time-event hybrid triggering program addition and automatic calibration and other modes of operation, and can provide to pump the total amount of fluid, the number of remaining strokes and to be transported fluid capacity, set the stroke Length and other relevant metering pump working parameters and other useful information.
Time series triggers programmable working mode to make metering pump fully based on real-time clock, according to the pre-planned task list, according to the set number of strokes or time to add the working medium quantitatively, time can be hourly, daily, working day, Day, week and two weeks for the cycle of flexible set; and the sequence of events triggered program-based work so that the pump is based on real-time triggering event, according to pre-planned list of tasks in the event of a specific incident (contact input) by setting The number of strokes or time to add the working medium. The above two methods can be mixed organically to complete the more complex time-event hybrid triggering mode of operation. Up to 81 events can be set.
A simple example is a cooling tower circulating water disinfection and algae removal system. Disinfectant to a specific concentration according to the proportion of added water flow rate added; and process requirements, in the proportion of the process of adding a weekly pulse should also be a large dose of disinfectant (such as set Friday 12-13), and asked At this point to stop adding the role of proportion. The whole programming is very simple, so complex and fluid addition tasks can be easily and accurately achieved.
3.4 Fieldbus ProFibus control mode In the petrochemical industry and other large-scale high degree of automation applications, the use of digital communications protocols for high-speed transmission of data between automation devices and then form a networked control system, large capacity, high reliability, has become the development The mainstream. After being incorporated into the German DIN standard in the early 1990s, the fieldbus ProFibus has become the European standard (EN50170) in the late nineties and has been widely adopted worldwide. It has become one of the representative protocols in fieldbus technology .
Fieldbus ProFibus consists of three situations. ProFibus-DP, ProFibus-PA and ProFibus-FMS. ProFibus-DP (DecentralizedPeriphery) is defined as a decentralized peripheral fieldbus designed specifically for the exchange of high-speed data between process control systems and decentralized peripherals. The transmission medium is twisted pair or fiber optic connection RS485 transmission standard, baud rate 9.6-12Mb / s, low system cost, high speed and reliability.
Fieldbus ProFibus instruction set streamlined, programming is very convenient. Metering pumps with fieldbus ProFibus communication function, with all manual adjustment functions, can easily be incorporated into advanced, distributed computerized control systems to fully utilize their various functions and become modern precision metrology Pump leader. ProMax metering pump family that is, there are three series of dozens of models equipped with fieldbus capabilities.
IV. Conclusion Today, the research and development of metering pump has gone beyond its traditional pursuit of pure capacity and material category, and turned to the direction of high-precision, multi-function and intelligent development. This trend has become the world's major metering pump manufacturers Consensus.
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