In the American National Standards Institute standard 7.0, the Hydraulic Institute defines controlled-volume metering pumps as “reciprocating positive displacement pumps typically used for the injection of chemical additives, proportional blending of multiple components, or metered transfer of a single liquid. These types of pumps are used in applications requiring highly accurate, repeatable and adjustable rate of flow.”
Image 1. Metering pumps receive an analog input signal from a PLC to proportionately change output based on system demand requirements. Flow verification sensors on the pumps confirm pump feed to the PLC. (Images & graphics courtesy of IWAKI America)Metering pumps were designed to dispense a repeatable amount of fluid during a period of time. This fluid—typically a chemical—could then be delivered at a known, controllable and adjustable rate. Today, several types of metering pumps are available with many options, although much of their design still traces back to their origin. Historically, the diaphragm metering pump has been the most common type and has helped advance metering pump technology to where it is today.
Figure 1. A cross-sectional view of a hydraulically backed metering pump. An eccentric cam is used in the gear reduction assembly to change the stroke length of the piston’s linear motion. The piston pressurizes hydraulic fluid, pressing the diaphragm forward with even pressure and changing the volume in the head cavity. Check valves turn this volumetric change into a repeatable pumping motion. (Courtesy of IWAKI America)Diaphragm metering pumps have progressed a long way since the days of early positive displacement pumps. Modern electric diaphragm pumps started out using large alternating-current (AC) motors coupled to a gearbox to slow the motor speed and transfer rotational direction into reciprocating linear motion. This design is still in use and can be seen in high-pressure piston/plunger pumps, mechanical and hydraulically driven diaphragm pumps, and larger-capacity low-pressure pumps. While these larger, robust, mechanically coupled diaphragm and gearbox-driven pumps are still important in many applications, several factors have changed over the past 40-plus years that required the evolution of pump design and an increase in capabilities to satisfy new demands.
Over time, chemical companies have reduced water content, shipping less water and less weight, which makes operations more cost-effective. The resulting increase in chemical concentrations has driven the demand for pump outputs to become both lower and more accurate. These new requirements contributed to the evolution of the electronic solenoid metering pump, developed to accurately pump lower flow rates at lower costs and with less routine maintenance.
With the advent of electronic solenoid metering pumps, and as electronics technology concurrently evolved, a level of control was introduced that was not previously affordable to many motor-driven pump applications. The addition of electronic control to the pump to power a solenoid enabled easier pump control.
Image 2. Large electronic solenoid metering pumps in an automotive plating facility. Pumps are used to replenish chemicals to maintain concentration levels.Whether by a stop/start inhibit signal, a digital signal proportional to a measured flow rate or the use of an analog control signal, control of electronic metering pumps became a standard and a more economical offering. Proportional control in the metering pump facilitated direct connection to the driving equipment, such as a water meter. This allows for automatic adjustment without incorporating any extra or custom hardware to control the pump motor or changing the setting of a stroke length adjustment knob.
Figure 2. Cross-section of mechanical metering pump wet end and gear reduction assembly. Rotational energy is converted to linear reciprocating motion in the gear assembly, moving a diaphragm back and forth to change the pump head volume. Check valves then control flow into and out of the pump head.Advancements in microprocessor and display technology enabled the internal pump electronics to move from analog to digital, providing more information in a display, improving resolution in control and integrating many of the features. As the electronics evolved, they began to include outputs, such as alarm or proportional output, for pump monitoring and diagnostic purposes.
At the same time solenoid metering pumps saw increases in control capability, other types of metering pumps evolved in a similar manner and for the same reasons. Smaller output requirements and advancements in tube materials transformed large hose pumps into the digitally driven smaller peristaltic pumps available on the market today. Like big motor-driven diaphragm pumps, large hose pumps still have applications in various markets, but new tube materials significantly increased chemical compatibility and dramatically improved life and reliability.
Higher concentrations and the need to pump fewer chemicals also helped smaller peristaltic pumps grow in popularity. The new materials, in combination with smaller tubes, were capable of higher pressures—historically a weakness with peristaltic pumps. Similar to diaphragm metering pumps, the peristaltic metering pump benefited from the same improvements in electronic technology: converting AC motors to direct current (DC) to increase controllability, adding control input and output features, and integrating digital displays. This shift made these pumps strong competitors in what was traditionally a diaphragm-dominated marketplace.
Image 3. Metering pumps with explosion proof motors in coal mine, part of the coal preparation plant. Several types of chemicals may be used to clean and process raw coal to remove inorganic matter, increase heat value, and meet customer specifications.More recently—because of further advancements in motor technology at lower costs—metering pumps have come full circle, back to the motor-driven variety. Instead of large, bulky and expensive AC-voltage induction motors, brushless DC-voltage motors are used to drive the diaphragm or plunger. These brushless DC and step-driven motors have tremendously increased motor speed and position control, improving pump accuracy and flow rates, while making it easier to fully control the pumps with inputs and outputs that have become standard throughout the industries in which they are used.
More pumps are incorporating machine languages (MODBUS, PROFIBUS, etc.). With the lack of a global standard, establishing a common protocol has been difficult. A more recent integration of new technology has been the addition of Ethernet connectivity to the pump. This connection may facilitate easy programming, feedback and even control as part of a network.
Connecting to a smartphone, enabling control and programming of pump parameters are the newest trends. It is only a matter of time before more pump manufacturers integrate similar communication connections and expand their capabilities. Technology will continue to advance for metering pumps as they evolve to meet new application challenges.