Pumps and Systems, June 2009
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Rising and falling pressure, as well as the need to manually recalibrate metering pumps to ensure dosing rates are maintained at constant levels and specific set points, is time consuming for operators handling fluctuating system pressures. With conventionally controlled volume metering pumps, any change to the system pressure can result in dosed volume changes, incorrect addition of metered chemicals to the process and reduced accuracy, quality and cost effectiveness. In conventional dosing pumps, this problem can only be avoided through manual monitoring and frequent recalibrations or the use of costly, independent flow measurement and control technologies.
Relative deviation from the volume flow set point depending on the system pressure.
Metering pumps that self calibrate and compensate for fluctuating application conditions are new. Such advanced technology usually eliminates troublesome stroke length control as a capacity variation means. Using advanced motor technology with a multi-sensor control circuit, the latest digital dosing pump technology controls suction and discharge while using a constant 100 percent stroke length.
The advantage of digital dosing is that suction is always performed in conjunction with a full stroke volume, changing only motor speed. The direct effect is a reduction on pressure spikes along with smooth and almost continuous injection, which are key design factors to improve metering pump applications. These pumps usually operate based on a stepper motor that allows them to control the stroke speed instead of the stroke frequency or stroke length, typically at a discharge stroke speed slower than the suction stroke speed.
To further compensate for the growing demand for tighter controls in chemical dosing applications, this new method combines on-board intelligent pressure monitoring in the dosing head with constant information on the diaphragm's position to provide the latest design platform available. System pressure compensation is designed to boost the convenience and accuracy of chemical injection quality.
For nearly eight decades, metering pump manufacturers have made changes to improve the ease-of-use and dependability of capacity adjustment internal to the pump based on the marketplace's expectations. By using flow monitoring in integrated self diagnostics to sense the stepper motor position, users now have an instant and accurate reaction to system pressure fluctuations. Such new technologies eliminate the need for external monitoring and repeated recalibration under fluctuating conditions.
Pumps equipped with this system intelligence are calibrated at the factory for water-like chemicals. For applications where the media has a different viscosity or density, a single on-site calibration is sufficient. Users can then count on the pumps to compensate for pressure fluctuations by automatically and continuously adjusting to changing conditions.
The pump receives information on changing pressure conditions via a pressure sensor integrated into the pump head, where chemical metering takes place. The pump's integrated microprocessor tracks, in real time, the exact position of the pump's stepper motor and the diaphragm's position inside the head. The collected information enables the pump to immediately and accurately change speed to follow the change in system conditions. Against rising system pressure, which would normally lower the discharge volume of the stroke, advanced metering pumps respond by increasing the stroke speed.
When the system pressure falls, the pump accurately decreases speed, which ensures dosing accuracy in accordance to the set point that the operator programmed into the pump. Because this system is part of the metering pump, and not dependent upon a manual response or external capacity monitoring, the response is instantaneous and more accurate.
Volume flow depending on a fluctuating system pressure-with and without integrated diagnostics.
In addition to the time saved on recalibration, this solution delivers a precise drop of fluid, even with difficult media and fluctuating pressure conditions. In addition to compensating for fluctuating system pressure, these pumps also can monitor and compensate for leakage in the suction and pressure valves, air and gas bubbles in the dosing head, cavitation and inadmissibly high operating pressure.
In the past, monitoring and preventing these inconveniences required external accessories and investment in equipment like flowmeters to do what this next generation of digital dosing pumps can do themselves.