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What is a Metering Pump?

The metering pump is a positive displacement chemical dosing device with the ability to vary capacity manually or automatically as process conditions require.  Approximately 90% of all pumps sold are transfer pumps, which are designed to move liquids from point A to point B.  But metering pumps are specialty pumps: They’re designed for precision-injection of chemicals, acids, bases, corrosives or viscous liquids and slurries.  Metering pumps are required for applications where one or more of the following conditions exist:
 
·         High accuracy feed rate is demanded
·         Low flow rates in ml/hr or GPH are required
·         High system pressure exists
·         Flow rates vary, and must be controlled by computer, microprocessor, DCS or PLC.
·         Corrosive, hazardous, or high temperature fluids are handled
·         Viscous fluids or slurries need to be pumped.
 

Where are Metering Pumps Used?

There are thousands of applications that require corrosion inhibitors, wax inhibitors, biocides, antifreeze, disinfectants, coagulants, oxygen scavengers, polymers, softening agents, acids/bases, process additives and other types of chemicals for their processes.  The main industries that purchase metering pumps include:

  • Industrial water treatment (cooling towers and boilers)
  • Potable water & wastewater treatment
  • Oil and gas production
  • Chemical and petrochemical processing
  • Power generation
  • Agriculture
  • Manufacturing
  • Food & beverage production

How Does a Metering Pump Work? 

The pump’s motor drives a piston to create a vacuum that pulls chemicals into the liquid end of a metering pump from external tanks.  Alternating piston strokes create pressure that closes the inlet valve, opens the outlet valve, and forces the chemical out to the process.  Within the liquid end is a diaphragm, which acts as a barrier between the piston a
nd the process fluid.  Sometimes a diaphragm is mechanically connected to a piston.  Sometimes a diaphragm is hydraulically connected.  The piston's pumping motion is applied to hydraulic fluid, which causes the diaphragm to flex back and forth as the piston reciprocates.  The movement of the piston flexes the diaphragm – the more the diaphragm flexes, the higher the flow rate for the pump.  The rate of flow can be precisely controlled to ensure that the process receives just what it needs, without over/under injecting.

Metering Pump Characteristics

The pumping action is developed by a reciprocating piston.  This reciprocating motion develops a flow that is easily represented by a sine wave.  Actual flow rate is determined by the following formula:

Flow rate = Displacement x Cycles per unit of time.


Unlike a centrifugal pump, the flow rate for a metering pump is not greatly affected by changes in discharge pressure.

Capacity and Pressure Capabilities

Some metering pump applications (such as industrial water treatment) require low pressure (under 100 PSI), while other applications (like flow assurance for offshore oil & gas production) require enormous pressures (exceeding 20,000 PSI). To address these wide ranges, metering pumps are typically designed in product families – with multiple models that can address different flows and pressures.

The maximum capacity of each pump will be determined by gear ratio, piston diameter, and motor RPM.  As the piston diameter and stroking speed increase, pressure capability decreases.


Capacity and Pressure Capabilities
Different metering pumps are designed to address a wide range of pressure requirements.

Metering Pump Basic Components

Driver: The pump is usually driven by an AC constant speed motor. Variable speed, pneumatic, and hydraulic drivers are also utilized.  Smaller pumps use solenoid coils as an economic drive mechanism.

 
Driver 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 assure reliability during continuous operation.  Solenoid pumps use an electro-magnetic coil to directly create linear motion.
 
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, or an electronic or pneumatic actuator to adjust pump flow rate in response to process signal.
 
Accuracy:  The steady state accuracy of a correctly installed industrial grade metering pump is generally + 1.0% or better. Although a metering pump can generally be adjusted to pump at any flow rate between 0 and its maximum capacity, its accuracy is measured over a range determined by the pump's turndown ratio.  Today’s hydraulically actuated metering pumps have a turndown ratio of 1,000-1, which means the pump will accurately dose chemicals anywhere between .1% and 100% of its rated capacity.
 
Liquid End:  The liquid end is referred to as the “wetted” part of the pump.  Its ability to protect plant personnel and the environment are important considerations when dealing with toxic or hazardous chemicals.  Stainless steel, nickel alloys, or plastic materials are used – depending on the application’s specifications, which include: temperature, flow rate, fluid viscosity and the corrosiveness of the materials that will be pumped.
 
Significant enhancements have been made recently with “smart” materials, that enable pump manufacturers to design smaller, more efficient and more powerful pumps that run longer and more reliably than their predecessors.

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