When plant engineers are tasked with selecting and specifying chemical metering pumps, the two common choices that come to mind are either a reciprocating plunger pump or a diaphragm pump. Gear pumps typically are seldom considered as conventional metering pumps have almost become synonymous with reciprocating pumps.
Conventional metering pumps, whether a diaphragm pump of disk or tubular shape or a reciprocating pump, operate with the same principles. The volume of fluid in the pumping chamber is altered by the reciprocating action which also affects the pressure that is exerted on the fluid. A pair of unidirectional check valves control the pumping action.
Limitations of Reciprocating Pumps for Metering
The stroke length, cycle rate, and plunger cross-sectional area determine the fluid displacement. Reciprocating pumps, in theory, displace fluid with each plunger stroke. During actual operation, however, the fluid discharge can vary due to variation in the flexing action of the diaphragm during continual pulses or erratic action of check valves. Reciprocating pumps have other limitations such as poor suction lift, sensitivity to fluid viscosity changes, and delivering output in pulses. The pulsing effect can be minimized by using pulsation dampeners, or multiple pump heads, although these come with added expense. Uniform discharge is often required for optimum results such as in applications that require pH control or where monitoring flow through a flow meter is required.
There are several methods that can be incorporated to vary the stroke length during operation although they increase the complexity of the reciprocating pump’s drive design such as eccentric cam adjustment, direct stroke adjustment, and lost motion devices that limit the plunger movement during part of a return stroke.
Rotary Gear Pumps for Metering – an Alternative Method
A reciprocating pump may be preferable in some applications where high pressure is required, although in many low to moderate pressure applications, considering the rotary gear pump as an alternative should not be overlooked. A rotary gear pump is generally less expensive and provides solutions to many problems encountered with reciprocating metering pumps. When fluids being pumped require corrosion-resistant materials, the cost factor is especially significant. An example is sodium hypochlorite for disinfection in municipal water supply or pulp bleaching in the paper industry.
Rotary gear pumps operate when fluid is picked up at the suction port in the cavities between the housing and the gear teeth and is then carried around the diameter of the gears when released at the discharge port. The gear pump displaces a fairly constant volume per each revolution and provides linear flow without pulsing. The flow rate, or the quantity of fluid delivered, can vary using different methods:
- Variable speed metering
- Bypass metering
- Ratio metering
Rotary gear positive displacement pumps can oftentimes be a good choice for chemical metering applications. The advent of variable frequency drives (VFD) makes metering with a gear pump more reliable and cost effective than it was years ago. Flow meters with closed loop feedback to a controller ensure metering accuracy and repeatability. The instrumentation accuracy contributes to the degree of metering accuracy as well as the level of sophistication in designing the control system.
Some of the most important advantages of using a rotary gear pump in metering applications include:
- Low initial cost
- Minimum maintenance
- Simplicity of the system
- Pulseless steady flow
IPE Offers Metering Pump Solutions
Illinois Process Equipment, IPE, offers a wide selection of quality pumps and process equipment including metering pump solutions. We offer technical assistance in choosing the right type of metering pump for your application. Our pump experts help to evaluate the type of fluid to be pumped, viscosity, desired flow rate, efficiency, and safety requirements to select the right equipment for your process.
There are many applications where a Rotary gear pump such as the Pulsafeeder ISOChem Series, Eclipse, ECO Gearchem, Micropump, Tuthill, and others provide more efficient, quality metering solutions than other options.
IPE offers pumps and process equipment including full system design, installation, testing, and services. We differ from other pump suppliers in our technical expertise and commitment to customers. Contact us to learn more about choosing the right pumps for your process and the advantages of metering with Rotary gear pumps.
