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Digital dosing

The Plug & Play pump is on its way
Digital dosing

To the specialist user, conventional dosing technology is all about controlling parameters like the number of strokes and the stroke length, volume and frequency. Only experts who understand the physics behind the system can use mathematical calculations to adjust their dosing pumps to achieve the required dosing quantity. However, this approach is time-consuming and is associated with numerous disadvantages. This is where dosing pumps using digital technology come in.

Dipl.-Ing. M. Ayatollahzadeh

With conventional pumps, the dosing quantity is controlled manually or electrically by adjusting the stroke length. It can also be controlled electronically by adjusting the motor speed, altering the pulse pause of the dosing stroke, or by setting the correct combination of stroke length and pulse/pause rate. Unfortunately, altering the stroke volume often causes airlocks or suction problems, particularly at the lower end of the dosing scale. Meanwhile, reducing the dosing quantity by adjusting the stroke frequency or by employing the pulse/pause control method can cause miscibility gaps in applications that rely on mass-proportional control tasks. In addition, users often have difficulty in selecting the best way of setting dosing quantity. As dosing processes become more complex, conventional dosing pumps can quickly meet their limits. Insufficient precision and suction problems when dosing minute amounts, bubbling with outgassing media, high chemical consumption with concentrated media, and miscibility gaps when dosing viscous media are all problems which make life difficult for users and seriously affect process quality. Ever-more complex dosing applications are increasingly calling for Plug & Play dosing pumps which take the pressure off the operators and enable them to maintain reliable, cost-effective, high-precision processes.
Digital dosing pumps
Digital dosing pumps featuring the very latest drive technology give users a whole new dimension in operator-friendliness, energy efficiency and dosing functionality. Using EC drives with multi-sensor control circuits facilitates optimum management of suction and dosing stroke speed without having to alter stroke length. Digital technology constantly calculates and controls the dia-phragm position, thus regulating the volumetric flow. The decisive advantage here is that suction is always performed with the full stroke volume. The smaller the desired volumetric flow vis-à-vis the maximum rate, the smoother the dosing operation and the smaller the amount of pulsation in the dosing system. This is very important in ensuring smooth pumping of outgassing media and in installations requiring longer suction lines.
Making light work of complex control tasks
In manual mode, the dosing rate is set directly in litres or millilitres per hour with a resolution of 1/800 of the maximum rate. This information is shown on the display. With mass-proportional contact signal control, the required dosing quantity per input pulse is entered directly in millilitres, and complex calculations are no longer required. The pulse intervals are constantly measured and the dosing speed is adjusted by PID controllers. This overcomes the problem of dosing gaps between the pulse inputs that often occurs with conventional methods.
With analogue signal control, the input signals can be freely assigned to a defined dosing quantity on both sides, right across the proportionality range – vital functionality for applications involving pH-value correction and cascade control of several dosing pumps.
For batch dosing, the batch quantity in litres and the dosing speed in litres per hour are set. This allows dosing to be precisely tailored to the needs of the application – for example, fast dosing for filling jobs or continuous dosing for the duration of the batch. Batch dosing with timer function rounds off this control method.
Together, digital input of the dosing rate (without stroke adjustment) combined with the calibration function to accommodate different dosing media, the positive drive of the diaphragm, and the optimised dosing head and valve geometry deliver very high precision performance of ±1 % for dia-phragm pumps. These are key factors in achieving environmentally-sound, cost-conscious chemical dosing. The dosing quantity setting range of 1/800 considerably reduces the number of variants, allowing dosing rates of 75 ml/h to 150 l/h to be handled with one dosing pump and just two dosing head sizes. Market launch of the next series, which will be able to handle rates of up to 940 l/h, is planned for early 2007.
Process reliability
Compared to PTFE-coated elastomer diaphragms, solid PTFE dosing dia-phragms boast far higher diffusion resistance to aggressive chemicals and are more resistant to pressure. The use of multi-layer diaphragms with pressure sensor control ensures optimum process reliability. If there is a defect in the working diaphragm, the pump continues to dose with an equally robust second diaphragm without any interruption to the process. The system also sends a diaphragm rup-ture signal so that repairs can be carried out at the earliest opportunity.
Precision dosing of viscous media
These days, a growing number of viscous chemicals are being used in industrial production operations, for example for component cleaning and in waste water recycling processes. Coupled with specialist valves and nominal widths and the positive drive of the dosing dia-phragm, targeted control of suction speed facilitates proper management and precise dosing of viscous media without the need for numerous gearing reductions. This function is achieved using two slow-mode steps, which involve reducing the dosing quantity by up to 60 % of the maximum dosing rate for media of 1,000 mPas and by up to 40 % of the maximum dosing rate for media up to 2,600 mPas.
Up until now, dosing control of oscillating dosing pumps was achieved by simple inductive or optical measurement of missing strokes, with transmission of an alarm signal. Now, digital drive technology with precisely-defined diaphragm position uses a pressure sensor in the dosing chamber to provide an exact display and diagnosis of the dosing profile (indicator diagram). Dosing errors – for example as a result of cavitation, air bubbles in the dosing head, leaks in the suction and pressure valve, system overpressure or loss of pressure following a line break – are reliably detected and an error message is triggered. Moreover, when an error message is received, it is now possible to read out representative points on the indicator diagram from the memory and use them for purposes of diagnosing errors.
A solution for continuous monitoring and comprehensive automatic evaluation of the current indicator diagram is currently being developed. A characteristic curve-based Plug & Play diaphragm dosing pump is just around the corner.
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