Metering Pumps 101
The pumping action of a metering pump is developed by a reciprocating piston, which is either in direct contact with the process fluid or shielded from the fluid by a diaphragm. Diaphragms are actuated by hydraulic fluid between the piston and the diaphragm. Metering pumps generally are used in applications such as hydrocarbon processing, power generation and water and wastewater treatment, where one or more of the following conditions exist:
- Low flow rates in mL/hr or gal/hr are required.
- High system pressure exists.
- High accuracy feed rate is demanded.
- Dosing is controlled by computer, microprocessor, DCS, PLC or flow proportioning.
- Corrosive, hazardous or high temperature fluids are handled.
- Viscous fluids or slurries need to be pumped.
Driver. The pump usually is driven by an AC constant speed motor. Variable speed, pneumatic and hydraulic drivers also are utilized.
Liquid End. The liquid end design and materials of construction are determined by the service conditions and the nature of the fluid to be handled. Temperature, flow rate, fluid viscosity, corrosiveness and other factors are considered.
Drive Mechanism. The drive mechanism translates the rotary motion of the driver into reciprocating movement. Industrial-duty pumps will submerge this portion of the pump in an oil bath to ensure reliability during continuous operation.
Flow Adjustment. Pump flow rate is adjustable by varying stroke length, effective stroke length or stroking speed. Most metering pumps are supplied with a micrometer screw adjustment. The micrometer also can be replaced by an electronic or pneumatic actuator to adjust pump flow rate in response to a process signal.
Metering Pump CharacteristicsThe pumping action is developed by a reciprocating piston. This reciprocating motion develops a flow sine wave. Actual flow rate is determined by a formula (figure 2). Unlike centrifugal pumps, flow rate is not greatly affected by changes in discharge pressure (figure 3).
The metering pump flow vs. stroke characteristic curve is linear. It is not, however, necessarily proportional, in that a 50 percent stroke setting may not equal 50 percent flow. This is due to the fact that the calibration line may not pass through 0 on both axes simultaneously. By measuring flow at two stroke settings, plotting both points and drawing a straight line through them, other flow rates vs. stroke can be predicted accurately. The steady-state accuracy of a correctly installed industrial-grade metering pump generally is +/-1 percent or better (figure 4).
Although a metering pump generally can be adjusted to pump at any flow rate between zero and its maximum capacity, its accuracy is measured over a range determined by the pump's turndown ratio. Most metering pumps have a turndown ratio of 10 to 1, which means that the pump is within its accuracy rating anywhere between 10 percent and 100 percent of capacity. Metering pumps that offer high accuracy and a turndown ratio as high as of 100 to 1 are available from some manufacturers.