Digital Dosing Enables Full Potential of Metering Pumps

By Klaus Müller, Grundfos Pumps Corp.

Metering pumps, also called dosing pumps, have come a long way from the earliest generation of mechanical machines, to the products of today that employ digital dosing technology to meter liquids more accurately, more reliably, and more economically.

“Key innovations incorporated into Digital Dosing™ include variable speed drive which provides water treatment facilities with significant energy savings and considerable life cycle cost reductions over the life of a project”
-Klaus Müller

By definition, a metering pump is a positive displacement pump used to accurately pump a controlled volume of liquid, with an internal means of adjusting the capacity of the pump. 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.

Tracking the evolution of the various technologies used in metering pumps demonstrates how changes have driven the industry to better control functions. Inventors of metering pumps actually thought of them as instruments, rather than pumps, because they envisioned metering pumps to be capable of much more than just pumping. However, it is likely that the pioneers in the metering pump field never imagined what robotic technology would someday do to achieve their vision. The newest generation of digital dosing pumps offers a whole host of benefits, permitting more accurate control over the process, and lowering capital costs as well as operation and maintenance expenses.

The evolution of metering pump technology
Metering pumps began as rudimentary machines that relied on comparatively crude mechanical adjustments to control pump capacity. Every convenience wanted by early users of metering pumps, such as flow rate confirmation and pulsation dampening, had to be supplied as an external accessory. As technology improved, many innovations were introduced to control capacity and leakage, improve the handling of difficult liquids, and lower the cost of operation.

Though metering pumps were first introduced in the U.S., many technology advancements were paralleled by concurrent engineering efforts in Europe and Asia. Today, users of metering pumps around the world are witnessing entirely new digital design platforms that the first developers and users of this technology likely never imagined.

1940s and 50s – The First Packed Plunger Metering Pumps
The first commercial metering pump was developed in the late 1930s by Milton Sheen and his son Robert. At the time, both were employees of what is now Betz Laboratories, a Philadelphia based manufacturer of specialty chemicals. The first metering pumps used a simple packed plunger, driven through a gear box by a foot mounted motor and a slider crank mechanism. The Sheens added a method of changing the stroke length of the plunger by adjusting the crank arm, which in turn varied the pump’s capacity. With the addition of some easily serviceable check valves that Sheen designed to fit onto the pump head (he called them step valves), the first dosing pump was born. These early pumps, at that time called chemical feed pumps, were first produced commercially in the 1940s. The biggest improvements of the 1940s and 1950s were modifications in the way the stroke length was mechanically adjusted.

1960s – Diaphragms Introduced to Metering Pumps
The packed plungers used on all metering pumps to that point typically leaked around the packing, which ultimately reduced the accuracy of the metering pump, as well as leaking expensive and in some cases harmful chemicals. 

To actuate the diaphragm, metering pump designers introduced a closed volume of clean hydraulic oil located between the plunger and the diaphragm. The plunger and the many moving parts of its drive system were lubricated by this oil, and the oil pressurized by the plunger in turn actuated the diaphragm. PTFE diaphragms made the pumps leakproof and chemical resistant.

A variation on the hydraulically actuated diaphragm metering pump was introduced to permit control of capacity by adjusting the amount of hydraulic fluid activating the diaphragm. This bypass, known as “lost motion” alters the effective stroke length without requiring expensive mechanical linkages. Lost motion designs eliminated mechanical linkages associated with stroke length adjustment, thus reducing metering pump cost.

As a less costly alternative to the hydraulic system, mechanically actuated diaphragm pumps were developed which mechanically attached the reciprocating plunger to the diaphragm.  There were direct drive versions of the mechanically actuated pump which changed stroke length by adjusting mechanical linkages in the pump. There were also lost motion versions of mechanically actuated diaphragm metering pumps, with the mechanism driven forward by an offset cam and the flow regulated by limiting the return of the mechanism.

These designs further reduced the complexity and the cost of diaphragm metering pumps, though were in some ways more vulnerable than their hydraulically actuated cousins. Aside from pressure limitations, directly attaching the plunger and diaphragm created significant stress points and reduced diaphragm life. Despite these vulnerabilities, mechanically actuated designs met an important need of lowering the cost of metering pumps, especially for relatively low pressure applications.

1970s – Solenoid Driven Metering Pumps Significantly Lower Metering Costs
The 1970s brought an important innovation to the world of metering pumps: the solenoid drive, which made metering pumps less complicated and considerably more affordable. This design still used a diaphragm, but the diaphragm was actuated by energizing and de-energizing a solenoid. The result was a dosing pump that was relatively simple, had few moving parts, accepted an electronic signal, and achieved accurate dosing of chemicals at a much lower cost than earlier mechanically and hydraulically actuated versions.

Hydraulically and mechanically actuated diaphragm metering pumps remained popular, though, because of several significant shortcomings of solenoid pumps. The movement of the diaphragm by means of the solenoid produced much higher diaphragm velocity compared to other designs. This significantly added to diaphragm stress, which reduced diaphragm life and increased the intensity of pressure pulsations. Upper limits of both pressure and capacity for solenoid metering pumps were considerably lower than that of other diaphragm dosing pump types. Solenoid pumps were not as accurate as the other metering pump types. Despite their shortcomings, the simplicity and lower cost of solenoid metering pumps resulted in exponential growth of the number of applications of diaphragm metering pumps. 

1980s and 90s – New Designs Accept Electronic Signals and Utilize Speed Control
During the next two decades, dosing pump manufacturers introduced changes that built on the improvements of the previous decade. These changes focused on metering pump controls, cost, and improved accuracy. 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 concept. It incorporated electronic controls that could have power to the pump simply turned on or off, could accept a pulse signal such as a water meter would put out, or could accept a 4-20 mA signal to cause the pump to turn on or off. All of these could be done in addition to traditional stroke length adjustment. 

Late in the 1980s and throughout the 1990s, manufacturers began to incorporate variable frequency drives, stepper motors, and servo motors. These variable speed technologies permitted the capacity of the pump to be controlled two different ways, by varying the stroke length and the pump speed. This combination provided increased capacity range for a metering pump, and broadened the application capabilities of a single pump, reducing the number of pump sizes that a manufacturer needed to offer.

Early versions of the combination of stroke length and speed control could only be adjusted manually. Even after automatic stroke length adjustment mechanisms were introduced, they were still hampered by the absence of software controls to seamlessly switch back and forth between stroke length adjustment and speed adjustment. If the pump user or the process called for an increase in the capacity of the pump, this increase of flow didn’t follow a smooth curve, but rather ramped up in a series of uneven steps. Thus, the advertised high turndown ratios were, practically speaking, much narrower than the theoretical limits of speed and stroke adjustments would suggest. During these years, the ability to internally confirm that the capacity setting of the pump was indeed being achieved, was still elusive. Users of metering pumps that claimed high accuracy still relied on external flow meters to confirm that the capacity they desired was being achieved.

Push-button control to set required dosage

A New Design Platform for Metering Pumps – Digital Dosing™
Recent years have seen the introduction of an entirely new generation of diaphragm metering pumps, incorporating digital technology to improve accuracy and process control, to save users money by lowering the amount of chemicals used, and to reduce the number of pump sizes required by large users of dosing pumps. All of this is achieved while simplifying the operation of metering pumps, and, at the same time, opening up an almost limitless number of potential new features and benefits. What started 70 years ago as a convenient form of capacity adjustment for a positive displacement pump has finally evolved into the instrument that Sheen and other pioneers in the metering pump industry foresaw: a new level of intelligence within the dosing pump, where flow inducement is less important than measurement of what is being dispensed.

Using state-of-the-art stepper or brushless D.C. motors, in combination with software to vastly improve the intelligence of the electronic control, the new generation of digital dosing pumps is able to consistently operate at full stroke length. Elimination of the need for stroke adjustment, and the ability to vary capacity solely by means of a software-controlled speed adjustment, permit huge turndown ratios (e.g. 1000:1). This is achieved without the complexity and inaccuracy of earlier versions of high turndown metering pumps. This feature alone offers significant savings, since the number of pump sizes needed to cover the entire range of flow is greatly reduced without any sacrifice in accuracy.

The precise speed control also permits easier priming, more accurate handling of difficult liquids like sodium hypochlorite that tend to off-gas, and the metering of more viscous fluids.  Eliminating the need for stroke control also means the pump has fewer parts and doesn’t require knobs or other forms of mechanical adjustment. These changes all result in lower costs.

Key innovations that are being incorporated into the new generation of digital dosing pumps include the ability to precisely monitor pressure, flow, and amp draw on the motor on a continuous basis. Monitoring pressure allows the user to determine when the accuracy of the pump is being compromised because of air or gas bubbles, cavitation, or leaking check valves. It also permits the pump to self-adjust to compensate for these effects. Monitoring flow continuously within the pump gives the feedback needed to adjust pump speed as system conditions change or as air or gas is introduced into the pump. Combining dosing, measuring, and regulating into one machine means that the user doesn’t have to make any calculations to determine the pump’s flow setting, but merely inputs the desired flow rate. The flow rate that is programmed in is what the pump delivers, without the need of costly independent flow measurement. This new generation of digital dosing pumps has lower pressure pulsations and higher accuracy than its predecessors, even at extremely low flow rates.

Digital Dosing™ with variable speed drive

Digital Dosing™ – Today’s Metering Pumps at Full Potential
The new platform of digital dosing opens up a whole new world of possibilities to designers of metering pumps and to the companies that use them, limited only by the imagination of the designers and users. 

Use of metering pumps as precise control instruments allows more concentrated chemicals to be used, and allows many accessories such as flow meters, back pressure valves, and pulsation dampeners to be eliminated.  Metering pumps are becoming more user friendly and costs have been further reduced as advances in control software continue and as economies of scale kick in for the new generation of digital dosing pumps.

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.