This article takes an in-depth look at metering pumps and their use.
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A metering pump is a precision machine that moves or pumps a measured amount of fluid in a predetermined time to create volumetric flow. The term “metering pump” covers a wide range of pumping methods, each designed to meet the needs of an application, operation, or process. The application determines the choice of what metering pump style to use.
The selection criteria for choosing a metering pump include the viscosity of the liquid, temperature, discharge pressure, flow rate, and the types of wetted path materials. The basic structure of a metering pump includes a pump head and motor with a manual or electronic adjustment system.
Metering pumps are often piston-driven positive displacement pumps that use the piston to displace the fluid in the reservoir chamber. Though these piston-driven pumps are common, other styles can be configured for unique circumstances.
A metering pump is a form of reciprocating pump that uses positive displacement to release a precise amount of a liquid with each of its strokes. The rotary motion of the pump is converted into reciprocating motion through the use of a wheel and worm shaft.
These metering pump features are generalizations that can be applied to all metering pumps but appear differently in each of the types.
The principle of a metering pump is to control the amount of liquid that can be moved for a process. Their accuracy guarantees that the precise measured amount will be added at the right time.
Piston metering pumps are positive displacement pumps that use a piston to displace the fluid in the chamber. The pumping chamber determines the flow rate and the amount of liquid to be dispensed. Fluids move through outlet and inlet check valves. When the piston recedes, a vacuum is created in the pumping chamber that opens the inlet check valve to allow the fluid into the pumping chamber.
Once the pumping chamber is full, the piston makes its discharge stroke and pushes the liquid out the outlet check valve.
A diaphragm metering pump has a diaphragm that is compressed and depressed to bring the liquid into the pumping chamber and expel it. Much like a piston pump, the movement of the diaphragm creates suction or a vacuum to pull the fluid in through the inlet valve. Once the chamber is full, the diaphragm sends the fluid out through the outlet valve.
In the design shown below, the plunger is connected to the diaphragm and moves it mechanically. In a hydraulic actuated diaphragm metering pump, the mechanism is not connected to the diaphragm, which is driven by hydraulic fluid. In both processes, moving the diaphragm is key for dispensing fluid.
The elements of a peristaltic metering pump are different from the mechanisms of piston-driven and diaphragm metering pumps. Peristaltic metering pumps have a hose or tubing through which the medium passes. The control of the fluid is completed by rollers that sit on a rotor. Negative pressure is created in the tubing that pulls in the fluid flow. Once the tubing is full, the rotor rotates and shuts off the flow. The rotational movement of the rotor forces the fluid in the tubing in the pumping direction.
As the roller completes its motion, the tubing is relieved, and the hose lines up with the flow. Negative pressure is once again created, allowing the next supply of the medium to fill the tubing.
The motion of a gear metering pump is similar to that of a peristaltic metering pump without the tubing. The intermeshing motion of the gears creates a vacuum into which the fluid flows or is sucked in. As the gears rotate, they move the fluid from the inlet to the outlet valve in a rotating motion.
Gear metering pumps are often used with high-viscosity fluids since their structure can withstand high pressure. The most common use of gear metering pumps is with hydraulic fluid power applications.
There are two forms of syringe metering pumps: infusion and withdrawal. Both are designed to move a specific amount of fluid. With infusion syringe metering pumps, small amounts of fluid are administered at controlled pressure and intervals. With withdrawal syringe metering pumps, fluid samples are removed automatically for medical and pharmaceutical testing.
Infusion syringe metering pumps are mainly used for dosing purposes in hospitals and have specific speed and flow rate regulations.
A syringe metering pump uses a piston to withdraw or infuse the fluid. They work slowly and are not used with automated applications. Syringe metering pumps are very sensitive to pressure changes in conditions with multiple syringe metering pumps. The reservoirs of syringe metering pumps have limited capacity for moving small amounts of fluids.
A bellows metering pump is a positive displacement style pump. The rotation of the motor shaft produces an up-and-down motion on a lever crank system. The motion creates compression and expansion on the bellows that forces fluids in and out.
The operation of a bellows metering pump’s motor extends the bellows to create a vacuum, pulling in the fluid through the inlet valve. As the motor continues its rotation, the bellows retract and displace the fluid, expelling it through the outlet valve.
A plunger metering pump operates like a piston metering pump but has a plunger as the reciprocating member. It is a single-acting pump that discharges fluids during its forward stroke. A suction check valve pulls in the fluid. When the plunger makes its forward motion, the fluid is forced out through a discharge check valve.
The design of plunger metering pumps makes them capable of enduring high-pressure applications that piston metering pumps cannot handle. They can withstand pressures exceeding 50,000 psi and operate at higher speeds than piston metering pumps.
Small metering pumps are miniaturized versions of larger metering pumps and use the same principles of operation. They have a micro pump head, driver, and motor and are suitable for the transport of various forms of liquid media. The motors of small metering pumps can be replaced to fit the needs of an application, which makes them more adaptable and flexible.
Larger versions of metering pumps are capable of being downsized or miniaturized into smaller versions. Multiple small metering pumps can be used in groups to monitor the flow of several fluids simultaneously as a part of a single process.
The mobility of small metering pumps makes them easy to install and configure. Their size makes it possible to adjust their flow rate, stroke length, and the frequency of their cycles. Small metering pumps can weigh as little as 1.6 ounces (46 g).
Chemical metering pumps are designed to add chemicals to a process. The most important feature of chemical metering pumps is consistency, which is necessitated by the types of materials they handle. Although chemical metering pumps are designed to be the most accurate type of pump, no metering pump is 100% accurate. Metering is calculated based on an average over a period of time. Additionally, metering pumps are very sensitive to pressure, which has to be closely monitored during their use.
Too much or too little of a chemical can cause changes that can make a substance too acidic or too basic. Inappropriate and incorrect measurements can ruin the liquid application for which the chemical fluid is being transferred. Piston and diaphragm metering pumps are two types of metering pumps used for chemical applications.
The five types of applications for which chemical metering pumps are used are:
pH Levels: The purpose of a metering pump for water treatment is to control the pH level, the measure of how acidic water is. Chemical metering pumps are meter sulfuric acid or caustic soda, which help balance pH levels.
Coagulating or Flocculating: Water has impurities that have to be removed. Coagulating or flocculating is a chemical process in water to make impurities and particles stick together and settle at the bottom of a stream or container for easy removal.
Enhancement: The final aspect of water treatment is the addition of materials to water to stop processes, improve purity, or kill germs.
Fuel Additives: There is an endless number of fuel additives that are injected into oil and gas using chemical metering pumps.
Mining: Chemical metering pumps are used to remove precious metals from ore using various types of chemicals. Every mining operation has its own proprietary method for extracting raw materials.
Electronic metering pumps operate on direct current (DC) or alternating current (AC). A benefit of electronic metering pumps is their ability to provide greater control over the accuracy of the amount of fluids being dispensed. The pumping action is controlled by an electrical current, which allows for monitoring of the stroke of the pump.
Traditionally, metering pumps have been powered by methods that are used to power other forms of pumps such as pneumatics, hydraulics, manual pumping, or fuel.
Electronic metering pumps have taken the place of these other powering methods and can be integrated into digital and computerized technologies. With the incorporation of touch screens, the flow rate, volume, and cycle of a metering pump can be controlled.
Automatic adjustments can be input to electronic metering pumps to ensure accuracy of the set point regardless of the discharge pressure. The use of electronic metering pumps makes it possible for operators to set metering pumps to a desired discharge rate without the need to calculate capacity due to the changes in stroke length settings.
A solenoid metering pump uses electromagnetic power as its driving force. When the electromagnet is energized, it attracts the plunger on the solenoid. This produces a reciprocating motion to move a diaphragm to pull in the appropriate amount of fluid.
The convenience and ease of operation of a solenoid metering pump is due to its LED display that can be changed using simplified button controls. Stroke lengths, low maintenance, simplification of operation, high efficiency, and pump head structure has increased the use of solenoid metering pumps.
Solenoid metering pumps are used for control of discharge volume or for external control.
A metering pump, or dosing pump, has the ability to dispense fluids, such as acids, bases, corrosives, viscous liquids, or other fluid materials, to meet the requirements of a process or application. They complete repetitive discharges of substances automatically..
The driver provides the power for the pump using AC, DC, pneumatic, or hydraulic power systems.
The power provided by the driver is in a rotary motion, which has to be changed into a reciprocating motion for use by the pump mechanism. The drive mechanism completes the change of power by taking the rotary motion of the driver and changing it into reciprocating movement. In stressful conditions, this part of the system will be immersed in a bath of lubricant or oil to ensure reliability and continuous performance.
Flow adjustments include adjusting the stroke pump and the number of strokes per minute. A metering pump’s stroke can be adjusted manually, using axial displacement, or automatically.
Manual: Manual adjustment of a metering pump is used to shut down the pump using a screw mechanism.
Axial Displacement: Axial displacement can be completed while the pump is in operation using an N or L shaped crankshaft adjustment or an eccentric cam adjustment.
Automatic: Pneumatic and electronic controls are common types of methods for adjusting the stroke of metering pumps. A pneumatic control changes the stroke by adjusting the source of the air pressure signal, while electronic controls change the electrical signal.
Check valves can be located at the suction end of a metering pump or at its discharge end. The designs of check valves permit the liquid flow to enter the chamber of the metering pump and allow the flow to be discharged. Aside from peristaltic pumps, most metering pumps have ball check valves with one or two balls.
Poppet check valves are also common in metering pumps since they function the same as ball check valves by allowing flow to go in only one direction. The operation of a poppet check valve is very similar to the operation of a plug or stopper for the drain on a sink or bathtub. The name of poppet check valves relates to the poppet that opens and closes to control flow.
Metering pumps are devices that assist manufacturing processes that include blending, purifying, and controlling of fluid operations and applications. Pairing a metering pump with accessories can improve its accuracy and reliability for dosing, preventing downline issues, and reduces maintenance costs.
Each of the different components are designed to take an existing feature and improve on it such that the addition makes the final process of the metering pump better. Most chemical dosing systems benefit from a combination of accessories, whether it is a pressure relief valve, back pressure valve, calibration cylinder, pulsation dampener, pressure gauge, injection quill, or strainer.
If the discharge side of the pump becomes blocked when the pump is operating, the pressure in the pump can continue to build until something breaks. In those situations, a relief valve releases the excess pressure preventing damage to the pump, piping, and other components.
Back pressure valves are placed near the discharge side of the pump to ensure that pressure is held on the discharge side so that the checks within the pump seat properly. This valve releases the fluid when a set pressure is reached, which optimizes both the function of the pump as well as the flow in the pipeline.
The job of a back pressure valve is to perform as a control valve that holds pressure when the suction pressure is greater than the discharge pressure. Without a back pressure valve, a metering pump may have overfeed and be inaccurate. Back pressure valves apply a sufficient amount of pressure to overcome the inertia of the discharged liquid.
Injection quills have a built-in check valve to ensure that the chemical being dosed cannot backflow. They are designed to inject the chemical into the center of the stream to allow for uniform dispersal and homogenous mixing of chemicals. Injection quills include flow calculations to determine the natural frequency. Beveled end injection quills give better dispersion and diminish the possibility of corrosion or fouling from concentrated chemical injectant.
An anti-syphon check valve is a built-in back pressure and check valve that prevents siphoning, a problem where a fluid continues to flow and be sucked out after the pump has stopped. This happens when the discharge side of the pump has lower pressure than the suction side. Anti-syphon check valves prevent overfeeding and backflow in the mainline.
An air chamber is a vibration or pulsation dampening device that reduces the pulsations created by a reciprocating pump and contributes to stable liquid flow. When air is released, it reduces pulsations in the pipeline.
Air chambers work with throttle valves where the liquid passes through after exiting the air chamber. Liquid accumulates in the air chamber in proportion to the pressure loss over the air pressure.
A pulsation damper has the same function as an air chamber and is designed to dampen pulsations and vibrations caused by a metering pump. When the fluid is discharged, it enters the dampener that compresses the gas filled bladder or diaphragm to remove vibrations in the system. Pulsation dampers ensure smooth and continuous flow in the system. They are used with all types of metering pumps.
Calibration cylinders provide readings of the flow rate given the fluid viscosity, pump location in relation to the fluid source, and the amount of wear on pump components. They are installed on the suction side of the pump where the drawdown of the column is timed to calculate the flow rate to ensure proper dosing. Calibration is performed when there is a change in fluid characteristics or any time a pump's operation is changed.
The image below is a conceptualization of the positioning of a calibration cylinder in relationship to the valves of a metering pump system and the pump.
As with all pumps, a pressure gauge is designed to measure the pressure of a liquid and to ensure it is within the defined limits. They are a safety device for the protection of pumping systems that may explode if the pressure level is exceeded.
The spring of a pressure gauge reacts to compression or expansion to provide a reading in compliance with Hooke’s law of force. Modern pressure gauges are digital and supply numerical readings. They are completely automated and provide readings that can be sent to laptops or desktop computers.
Strainers for metering pumps have the same function as strainers for every other application, which is to remove particles and debris that may interfere with the operation of the pump. With metering pumps, strainers are placed before the suction side of the piping. They are used when materials are being drawn from the top of a tank, vat, or container where debris may collect.
Strainers can be used to sift out impurities in the fluid or when there is the possibility of contaminants. The most common form of strainer is the Y configuration.
A degassing valve removes dissolved oxygen, carbon dioxide, and other gasses from the liquid flow. The trapped gasses are vented back to the supply tank. Degassing valves are installed at the highest point of the piping system to prevent gas being trapped and creating vapor lock. Degassing connectors ensure proper performance of the valve system and metering pump.
Chemical dosing produces gasses that include sodium hypochlorite, hydrogen peroxide, or sulfuric acid. By movement or compression, the chemicals gas off creating vapors that need to be vented.
The images below depict examples of metering pump installed systems.
Control volume pumps, injection pumps, feed pumps, dosing pumps, or metering pumps are used to inject chemical additives, proportional blendings, and transfer of liquids. They are relied on for their ability to control and adjust the flow rate. The performance of metering pumps depends on their suction and discharge functions, which can be adjusted by changing the length of their stroke and its speed.
Detergent dosing metering pumps are designed for commercial washing machines. They release detergent for every washing cycle to reduce waste. Detergent dosing prevents blockages in machines, removes waste, keeps the laundry process efficient, and protects workers from exposure to detergent and other chemicals.
Automated dispensing of raw products for packaging is a lean production operation. It is a process that does not require human intervention. In the modern era, automated packaging is an all-inclusive method that has all of the steps of packaging in a single set of operations.
Problems with automated dispensing relate to problems when switching from one product to another. This particular issue has been overcome with the use of digitally controlled volumetric adjustments that shorten the time from moving from one product to another.
Metering pumps are used to dispense chemical mixtures into municipal water supplies and wastewater applications. They are chosen for water treatment due to their energy efficiency. The goal of all water treatment plants is to optimize production and to control energy costs. Metering pumps offer functions to help water treatment plants meet and exceed their goals.
Metering pumps inject chemicals, such as methanol, monoethylene glycol, and corrosion inhibitors, into the flow of gas and oil to prevent hydrate formation, wax and scale build up, and corrosion, which hinders flow and lowers production rates. The use of corrosion inhibitors increases safety by preventing leaks and spills.
A necessity for oil and gas metering pumps is durability. It is estimated that metering pumps for oil and gas can last up to 20 years without the need for repair or replacement.
Center pivot irrigation systems require the injection of nitrogen into the system as it pulls water from a pond, river, or local source. Water is pulled from the source and sent into the pivoting points where the water is applied to crops. Metering pumps are set up at the pivot point of the system. The pumps are portable to be changed for every irrigation system or small enough to be a permanent part of the pivot point.
Irrigation metering pumps are attached to 55 gallon drums that contain nitrogen. As the chemical is pulled from the tanks it is distributed by the sprinkler heads. The metering pump provides a mixture of nitrogen to help the crops and prevent the nitrogen from bleeding into the groundwater supply.
A metering pump system is designed to disperse, transfer, and inject a measured amount of a fluid into a process, application, or operation. At the core of a metering pump system is the metering pump that extracts the amount of the fluid to fit the needs of the system.
Metering pump systems are used as part of the dispersion process. Chemical processing, pharmaceutical, food and beverage processing, water treatment, medical, commercial vending, irrigation, petroleum, milling, sewage, plastic fabrication, metalworking, automotive, and analytics depend on metering pumps.
The four most common types of metering pump systems are the diaphragm, bellows, peristaltic, and piston.
A diaphragm system uses a vibrating membrane to move fluids. As the membrane retracts, it pulls the fluid into the pumping chamber. The determination as to the amount of the fluid is dependent on the stroke of the mechanism that moves the diaphragm. Once the proper amount is achieved, the membrane releases, sending the fluid out through the discharge or outlet valve.
A bellows metering system consists of connected circular membranes that are joined and sealed. The structure of the bellows is attached at one end to the fluid source. At the other end, there is a mechanical arm that moves the bellows, which is activated by a motor.
When the bellows are pulled, it draws in the fluid until the appropriate amount is reached. At that precise point, the bellows moves in toward the flow to discharge the captured amount out through the outlet valve. Bellows metering pump systems are used for applications where low flow and low pressure is required at a fixed flow rate.
A peristaltic system involves a rotor, tubing, and rollers. The tubing that carries the fluid is placed in contact with the rollers and rotor inside an enclosed housing. As the fluid enters the tubing, its flow is shut off when the amount in the tube is the quantity to be dispensed. With the rotation of the rotor, the proper amount is discharged, and the next amount enters the tubing. The rotation of the rollers and rotor seals and unseals the tubing making it possible to rapidly move fluids into the application.
An advantage of a peristaltic metering pumping system is its ability to move dirty and contaminated materials, which is not possible with other systems. The plastic or rubber tubing is designed to not be influenced by harmful materials, which never come in contact with the working mechanism.
The piston type of metering pump system uses the movement of a piston to create a void or vacuum into which the fluid is pulled. The stroke of the piston is measured to pull in enough fluid to fill the pumping chamber. Once the measured amount is achieved, the piston moves outward to discharge the fluid through the outlet valve.
The inlet and outlet valves on a piston metering pump system are check valves that only allow fluid movement in one direction. When the piston is moving backward to create the void or vacuum, the ball of the inlet check valve rises, letting the fluid flow inward. When the correct amount is reached, and the piston moves forward, the inlet valve ball is forced downward by the flow to seal the inlet while the ball of the outlet valve moves outward to discharge the fluid.
Metering pumps are an important part of dispensing systems. The selection of a metering pump has to be carefully considered to ensure that the one chosen fits the needs of an application.
At the heart of every metering pump is its flow rate, which has to match the application’s needs. The viscosity of a fluid influences the flow rate and determines the stroke force necessary to move a fluid. The flow rate is the speed at which a fluid moves. Its calculation has a significant impact on the performance of a metering pump.
Turndown ratios determine the low end capability of a metering pump and its ability to maintain its accuracy. At any time in the operation of a metering pump, there can be variations in temperature and pressure that can influence the pump’s performance. Properly chosen metering pumps are capable of having adjustments made to their stroke and drive to deliver the set flow rate.
Each application that uses a metering pump has a different required pressure. In the case of water treatment, low pressure is needed at under 100 psi. Other applications, such as oil and gas production, require pressures that can exceed 20,000 psi. The different types of metering pumps are engineered and designed to deal with the varying pressures required.
The capacity of each pump is determined by its gear ratio, piston diameter, and motor RPMS. As the piston diameter changes and the stroke speed increases, the pressure capabilities of a metering pump decrease.
The flexibility of metering pumps is necessary for production operations where raw materials and finished products are being dispensed. When there are product changes, the metering pump system has to be able to adapt to the changes and be configured to meet the needs of the new application.
Metering pumps have to be engineered to meet the shifting and changing demands required of production operations. In the case of small metering pumps, this is easily resolved. Larger more complex pumping operations require more planning. The needs of an operation or production setting can strongly influence metering pump selection.
As with any form of machinery designed for industrial and production operations, metering pumps must be reliable, dependable, and resilient enough to meet the needs of any operation. Where there are chemicals, corrosives, and toxic materials, metering pumps should be chosen in accordance with the materials used to manufacture them. This aspect of the selection process ensures the continuous service of a metering pump and the safety of workers.
Placed in the correct conditions, metering pumps can last as long as twenty years without needing repairs or replacement. This consistency has made them an important part of manufacturing processes.
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