Traditional solenoid dosing pumps use an electromagnetic solenoid to shift the shaft connected to the diaphragm. A spring returns the shaft to a "relaxed" state. These pumps regulate the dosed quantity by adjusting stroke length and / or stroke frequency. But reducing the stroke length does create disadvantages regarding dosing accuracy. In cases where the dosing pump cannot operate at 100% stroke length, both the suction and discharge valves will suffer from reduced performance leading to inefficient dosing. Variations in stroke frequency also lead to non continuous dosing. As there are many possibilities of adjusting those two settings (stroke frequency vs stroke length) to get the same output, many times an optimal setting will be found through continued trial and error.
By using a stepper motor, the volume dosed is altered by the discharge stroke speed while continuously utilizing 100% of the stroke length - which leads to optimum dosing accuracy and better handling of degassing liquids. How? Well as you can see in the graphic below, assuming we have the pump set to a 100% stroke length, both pumps have very similar outputs when pumping @ 100% frequency. But as you slow the pump down, the difference becomes more apparent. The traditional pump at the top shows how there is more time where the pump is doing nothing between strokes, where as a pump with the stepper motor, indicated by the graphic on the bottom, is discharging the entire time in between suction strokes. The stepper motor also improves system performance: through continuous dosing of chemical, lower pulsation in the system, and by completely filling and evacuating the pump cavity.
It's the complete filling and evacuating of the pump cavity that also makes it have fewer issues when pumping a liquid that off gasses. Let's face it, if we ran at 100% stroke & frequency all the time, we would have very few problems even with the traditional dosing pump because most of the gas can be pulled in and evacuated in a single full stroke. But most of the time these pumps are selected because it's known that the demand will be no more than X gph, so we size the pump knowing it can meet that max and be turned down to meet the real required volume. It's when people begin to mess with lower volumes that most issues arise. Most people understand the need for continuous dosing, so they don't mess with the frequency, instead opting to change the delivery volume by adjusting stroke length. Then problems begin to arise. In fact if you have a traditional dosing pump, you can probably walk over to it right now and see where the stroke length adjustment knob may have some different pattern, like a solid bar, going from 0 to 20 or even 30%, like the pic below. The purpose of that is to indicate that the pump shouldn't be run that low due to the inconsistencies or inaccuracies of flow at those low stroke lengths. And accuracy is why metering pumps are used.
By utilizing the 100% stroke length all the time and varying flow by slowing down the discharge stroke, stepper motor pumps are proving to have many advantages over traditional dosing pumps, including being able to more effectively dose chemicals that off-gas. If you'd like to read a case study, click here. If you'd like to see and learn more about this technology, then contact Steve to setup a demo for you.
