When choosing a pump for a chemical metering process, the choices often between a diaphragm metering pump or a reciprocating plunger pump with gear pumps are seldom considered. This understanding has grown over the years to the point where the terms metering and reciprocating pump are nearly interchangeable.
Whether diaphragm (disk or tubular shape) or reciprocating plunger, conventional metering pumps function according to the same general principle. The reciprocating movement modifies the volume of the pumping chamber, hence altering the pressure imposed on the fluid being pushed. Controlling the pumping activity are two unidirectional check valves. The cross-sectional area of the plunger, the length of the stroke, and the cycle rate are the three factors that together define the displacement.
Challenges When Metering with Reciprocating Pumps
Theoretically, reciprocating pumps displace a constant amount of fluid with each plunger stroke. In actual operation, the discharge may differ from the ideal due to the erratic action of the check valves or variations in the flexing action of the diaphragm between subsequent pulses.
In addition to problems with check valves, reciprocating plunger pumps have the following limitations: they give output in pulses, have limited suction lift, are sensitive to changes in liquid viscosity, and require a complex drive for variable stroke.
A reciprocating pump discharge is provided in pulses. Utilizing phased duplex or triplex heads or surge chambers can lessen this pulsing effect or slug feeding, albeit at an increased expense. Many applications, including pH control, extremely critical exothermic reactions, and applications in which flow through a rotameter or instrumented flowmeter must be monitored, require uniform discharge for optimal results. For optimal performance, reciprocating plunger pumps often require a flooded suction. In essence, they are pressure intensifiers with low suction lift qualities. With liquids that do not flow freely and liquids with low vapor pressures, a raised reservoir, a pressurized supply tank, or a feed pump may be necessary.
Rotary Gear Pumps for Metering
Rotary gear pumps are positive-displacement pumps that trap equal amounts of fluid between the gear teeth and pump housing for transmission between the intake and discharge ports.
In reciprocating metering pumps, the ability to adjust the stroke length during operation enhances the complexity of the drive system. Available options include direct stroke adjustment, eccentric cam adjustment, and lost-motion devices to restrict plunger movement during a portion of the return stroke.
In many applications, particularly where high pressures are required, the reciprocating pump is preferred, if not the sole option. The rotary gear pump is a viable choice for applications with low to moderate pressure (100 psi or less). The rotary gear pump is preferable to reciprocating metering pumps because it eliminates a number of their disadvantages and is generally less expensive. When corrosion-resistant materials are required for liquids pumped, the cost factor is very critical.
The rotary gear pump operates by sucking fluid into the cavities produced between the gear teeth and housing at the suction port and transporting it around the outer diameter of the gears to the discharge port. By design, it offers a virtually pulsation-free linear flow and displaces a nearly constant volume every rotation.
Accurate and Reliable Metering with Gear Pump Solutions
Rotary gear pumps provide a high level of accuracy in many applications such as low-pressure chemical metering processes. The degree of precision depends on the precision of the instruments and the sophistication of the control system’s architecture.
In metering applications, the benefits of employing a rotary gear pump are low initial cost, minimal maintenance, and system simplicity.
IPE is a leading pump and process equipment supplier, providing pumping solutions to a variety of industrial operations in and around the Chicagoland area. We are set apart from other pump suppliers with our expert technical capabilities and comprehensive services, including full system design, installation, and testing. Contact us to learn more about choosing the best metering pump and process equipment for your application.