Dosing Technology Digital Diaphragm Dosing Pumps — Stepper Motor Makes the Difference
Water treatment and purification is a technologically demanding process. Dosing is a core task here: Additives and auxiliary agents or highly effective chemicals need to be added both cost-effectively and safely for the environment, which means that the requirements for dosing pumps are just as high. Digitalization is the order of the day here as well.
Positive displacement pump or centrifugal pump? While centrifugal pumps are preferred for water resources management — just like in industry in general — there is no question that the design of positive displacement pumps makes them much better suited to dosing processes, particularly in water treatment — whether for drinking water, wastewater, cooling water or boiler feed water.
Whereas conventional compact dosing pumps use stroke length adjustment and/or dosing gaps to deliver the correct dose, Grundfos has taken a different approach with its diaphragm dosing pumps and uses stepper motors “as a shift in technology,” as Rainer Schmitz calls it. The Business Development Manager at Grundfos is convinced: “The days of adjusting the stroke length and stroke frequency are over!” Grundfos has given its new drive concept a special name: Digital Dosing. “Our dosing pumps always operate at full stroke length. Any change in the dosing amount is achieved solely via the pressure stroke speed,” says Schmitz, underlining the key difference.
Thanks to the speed-controlled stepper motor, the digital dosing pumps achieve an adjustment range up to 1:3,000. According to the dosing expert, this is around 30 times higher than on conventional dosing pumps. As a result, a single type of pump can cover a wide throughput range — e.g. 7.5 to 0.0025 ml/h — and thus reduces product/spare part diversity.
Another advantage is the low-pulsation and uniform dosing. He explains that this delivers significantly better process quality, as a result of which there is no need for downstream mixing sections. In the process, the repeat accuracy is up to ± 1 % across the entire dosing range, which permits accurate and uniform dosing of chemicals even if the amounts added are tiny, e.g. in the case of concentrated chemicals.
Intelligent Control Electronics
The integrated flow control dosing monitoring system — a unique feature in the dosing pumps market, as Schmitz adds — allows the most common causes of dosing errors to be accurately diagnosed. These are defective valves, air bubbles, cavitation and overpressure. Faults are displayed in plain text in the alarm menu or can be forwarded via Fieldbus. Troubleshooting options are quickly available, which reduces downtimes.
In addition, the auto flow adapt function ensures that the dosing process continues to run at the defined setpoint dosing volume flow even if the process is impaired by external factors (air bubbles, cavitation, fluctuating back pressures). If the actual value deviates from the setpoint value, the controller regulates the speed of the drive and thus the dosing amount. This means that, in most cases, there is no need for long-winded calibration processes or back pressure valves. In the case of air bubbles or outgassing, for example, the motor is automatically adjusted so that they can escape at the dosing head in a controlled manner. If the dosing pump is out of use over the weekend, the diaphragm will vibrate every 20 minutes so that air bubbles can be carefully expelled during the next dosing stroke. The time-consuming process of bleeding the dosing head on a Monday morning is thus rendered a thing of the past. In addition, the slow mode function can also be used to perform the suction stroke more slowly overall (25% / 50%), which is ideal for outgassing or viscous media.
Thanks to the integrated flow measurement, there are no additional costs for corresponding measuring equipment. The current measurement of the volume flow is shown on the display. The values can also be transmitted via the analog output or via a bus protocol if required.
For simple and fast integration in Fieldbus systems (for example Profibus or Profinet), pre-programmed function blocks are available to download that will significantly shorten the integration time.
Last but not least, Grundfos also offers a streamlined and optimized range of accessories for everything you may require between the dosing container and the injection point. Thanks to its simple operation, dosing technology has now also been discovered by the trade, so that Grundfos now offers dosing sets for this area that are very simple to select and contain everything needed for dosing.
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