Klaus Müller explains the history behind metering pumps and details how digital dosing enables their full potential.
“Flow monitoring allows pump speed adjustment as system conditions change”
From rudimentary machines relying on crude mechanical adjustments for controlling pump capacity in the 1940s to the vast improvements of handling difficult liquids and reduced operational costs of today, the evolution of technologies used in metering pumps demonstrates how changes have driven the industry to better control functions.
Applications where precise metering of liquids is important include the dosing of chemicals into water for pH adjustment or disinfection; addition of chemicals in process applications; and the metering of colors, flavors, or vitamins into foods, drugs, or personal care products.
The first commercial metering pump, developed in the late 1930s by Milton Sheen and son Robert used a packed plunger, driven through a gearbox by a foot mounted motor and a slider crank mechanism. The Sheens added a method of changing the plunger’s stroke length by adjusting the crank arm to vary the pump’s capacity. Later, Sheen added check valves to fit onto the pump head, and the first dosing pump was born.
Packed plungers used on all metering pumps typically leaked around the packing, reducing the pump’s accuracy so in the 1960’s diaphragms were introduced. To actuate the diaphragm, designers applied a closed volume of hydraulic oil between the plunger and the diaphragm. PTFE diaphragms made the pumps leak proof and chemical resistant.
A variation of this metering pump permitted control of capacity by adjusting the amount of hydraulic fluid activating the diaphragm, and eliminated mechanical linkages associated with stroke length adjustment, thus reducing metering pump cost.
A less costly alternative to the hydraulic system, mechanically actuated diaphragm pumps were developed which mechanically attached the reciprocating plunger to the diaphragm. This reduced the complexity and cost, especially for relatively low-pressure applications.
The 1970s introduced solenoid drive technology. The diaphragm was actuated by energizing and de-energizing a solenoid, and with only a few moving parts and an electronic signal, accurate dosing of chemicals was achieved at a much lower cost. Despite shortcomings of the solenoid pumps ― reduced life caused by diaphragm stress and increased intensity of pressure pulsations ― the simplicity and lower cost resulted in exponential increases of the number of applications of diaphragm metering pumps.
In the 1980s, another low cost alternative to the solenoid metering pump employed a constant speed synchronous A.C. motor and a mechanically actuated design. Throughout the 1990s, variable frequency drives, stepper motors, and servo motors were incorporated, permitting the pump’s capacity to be controlled by varying the stroke length and pump speed. This provided increased capacity range for a metering pump which broadened a pump’s application capabilities.
Introduced next were automatic stroke length adjustment mechanisms, but due to the absence of software controls, switching back and forth between stroke length and speed adjustment was cumbersome. Also, confirming the capacity setting of the pump being achieved was still elusive.
In recent years, a new generation of diaphragm metering pumps, incorporating digital technology to improve accuracy and process control, saves users money by lowering the amount of chemicals used, and reduces the number of pump sizes required.
Using stepper or brushless D.C. motors and software for improved electronic control, achieves consistent full stroke length, allowing for huge turndown ratios (eg. 1000:1), easier priming and more accurate handling of difficult liquids.
Key innovations incorporated into digital dosing pumps include precise and continuous monitoring of pressure, flow and amp draw on the motor. If the pump’s accuracy is being compromised due to air or gas bubbles, cavitations or leaking check valves, the pump can be set to self-adjust to compensate for these effects. Flow monitoring allows pump speed adjustment as system conditions change. By combining dosing, measuring, and regulating into the pump means that the user doesn’t have to calculate to determine the flow setting, but merely inputs the desired flow rate.
New generation metering pumps allow for more concentrated chemicals to be used, eliminate the need for other accessories, and significantly reduce costs.
Klaus Müller is a mechanical engineer with Grundfos Alldos Pumps and has worked with the company’s research and development of metering pumps from 2001 to present. Prior to this, he had worked with Alldos Pumps since in 1983, until Grundfos acquired the company in 2005.