Metering Pumps

View A Video on Metering Pumps- A Quick Introduction

Several different metering pump mechanisms are used to achieve the movement of exact amounts of fluid. Diaphragm metering pumps utilize a flexible membrane to initiate motion while piston metering pumps use a cylindrical plunger. Bellows pumps employ an accordion style pleated body to achieve displacement while peristaltic pumps use rubber tubing that is compressed by rollers. No matter the specific type, these chemical metering pumps can be used to transport any number of materials including acids, bases, corrosives, viscous fluids and slurries. Large and small metering pumps are therefore popular in a wide variety of industries related to chemical engineering and processing. Far from limited, however, metering equipment is commonly found in pharmaceutical, food and beverage processing, water treatment, medical, commercial vending, irrigation, petroleum, milling, sewage, plastic fabrication, metalworking and laboratory facilities as well. Each industry utilizes the unique ability of metering pumps to cyclically introduce a consistent amount of fluid into a continuous-flow stream ensuring homogeneity. Although micrometer screw adjustment is common, pneumatic and electronic metering pumps are also popular and increase the ability to employ computer and other automated programming in order to adjust flow rates throughout processing. With continuing technological advancements, fluid transfer pumps are utilized in any number of metering systems and lauded for accuracy and precision.

While the means of motion may vary significantly, all metering pumps are composed of relatively few parts. The pump head is a cylindrical cavity which houses the fluids just before they are displaced. A fluid reservoir of variable size is connected via hoses or tubes to an intake check valve attached to the pump head. Opposite the intake point is the exhaust or outlet valve which is connected, again through the use of tubing or hoses, to the main body which holds the process flow to which the fluid is introduced. Each valve has the ability to flow only in one direction. The pump head also houses the diaphragm, bellows or piston that will enact motion, drawing the liquid in and expelling it. A motor or manual pedal is attached to this component depending on the specific design. Bellows and peristaltic pumps differ somewhat in design, but follow the same general operation. Metering pumping is a two part process. The first action is the suction, or intake stroke. At this point, the membrane or piston is withdrawn or the bellows extended. This action creates a vacuum effect which opens the inlet valve and pulls the fluid, be it oil, water, syrup or any other material into the pump head. The inlet valve closes as suction ceases. The motor or manual operator then extends the membrane piston, or compresses the bellows in towards the fluid which is then compressed as much as is possible and pressurized. This pressure forces open the outlet valve and the liquid is expelled due to displacement.

There are two methods for enacting variable fluid metering in pumps such as these. Either the length of the stroke may be changed to pull in differing but ever precise amounts of fluid, or the frequency of the cycle is altered. The two types are distinguished by name as variable displacement constant speed pumps and fixed displacement variable speed pumps. As the names suggest, the former adjusts the displacement, or the stroke in order to control the amount of liquid pumped per minute while the latter allows consumers to adjust the motor speed or cycle in order to enact change in the flow rate. Each model has the capacity to produce an unending continuous flow of metered fluid into an expulsion unit, be it a dialysis machine or beverage dispensing unit. The ability to adjust the stroke involves a more complex mechanism and a larger unit to allow for greater fluid transmission. For this reason, the fixed displacement variable speed type of metering pump is often preferred and commonly seen in industrial and commercial applications.

The wide use of metering pumps is largely reflective of the many benefits they offer. High accuracy dispensing, high system pressure, low flow rates in GPH or mL per hour, the capacity to handle hazardous or corrosive fluids and controlled dispensing that can even be metered by a microprocessor or computer are all common advantages of metering pumps. In order to ensure that manufacturers reap these benefits, however, several facts and features of a specific pump must be considered. To meet the supply of various industries metering pumps are extremely diverse. While most are adjustable, this is only true to a certain extent and care should still be taken when selecting the most appropriate model for a specific task. As the two types of pumps are determined by the way in which they affect flow rate, it is easy to understand why it is such an important consideration. This involves motor capacity as well as stroke length, or the degree to which the membrane, piston or bellows can be extended. Additional considerations include the maximum discharge pressure rating, temperature rating, flow rates, pump apparatus, motor capacity and fluid composition and viscosity. The type of liquid in use is extremely important as it has a profound impact on the functionality of the pump. Most liquids can be only slightly compressed, allowing for easy displacement. The introduction of gaseous bubbles, however, makes displacement more difficult as the air may be able to withstand a great deal of compression, perhaps even beyond the capabilities of the pump therefore leaving it ineffective at expelling the fluid. The material construct of the pump is also important with anti-corrosive and durable metals or plastics most often being used. It is essential to consider every aspect before investing in metering pump equipment.

Images Provided by Primary Fluid Systems Inc.

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Types of Metering Pumps

• Bellows pumps are a specific type of metering pump which use accordion like pleated materials to enact the positive displacement of fluids. In many applications this type of pump is preferred even to the more traditional piston metering pump as it offers high reliability and clean operation.
  
• Centrifugal pumps use a revolving disk attached to a drive shaft that transports fluid without pulsation as it spins.  
  
• Chemical metering pumps are precise instruments used to move chemicals.
• Diaphragm metering pumps employ flexible membranes to alter the volume of a controlled chamber in order to effectively displace fluids with accuracy and precision.
  
• Dispensing pumps are designed to dispense a precise amount of liquid or gas.
• Electronic metering pumps are fluid transfer pumps that operate off of electrical currents. These currents power the actuating devices responsible for the pumping action of a metering pump.
• Fluid transfer pumps encompass a broad range of devices designed with the intent to move any number of fluids from one place to another without specific concern for the modulation or transfer of power associated with this motion.
• Gear pumps use gears to trap fluid between the teeth of two or three rotating gears. Gear pumps are ideal for high system pressure applications and are typically magnetically powered.
• Liquid pumps use displacement to move fluids from one place to another.
• Metering equipment includes any number of devices used in the metered pumping of precisely measured fluids from an area of high pressure to an area of low pressure.
• Metering systems include all apparatus implicated in the precision transfer of carefully measured fluids through positive displacement. Systems such as these are responsible for the maintenance of process streams of variable chemical composition and monitor liquid flows.
• Peristaltic pumps are a unique type of metering pump that uses rollers to mimic the contracting and relaxing used in the gastrointestinal tract in order to move fluids from one place to another.
  
• Positive displacement pumps transfer liquids and gases from an area of high pressure to an area of low pressure.
  
• Piston metering pumps use a reciprocating plunger to displace liquid through the machine. A rigid piston assembly gives them the maximum pressure and accuracy of metering pumps.
• Small metering pumps are miniaturized positive displacement units designed for the movement or transfer of exact amounts of fluid with extremely accurate flow rates. While ‘small' is a relative term, it can describe the physical size of metering equipment, which may weigh as little as 39 grams or a low flow rate.

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Metering Pump Terms

Air Bound - The situation in which a centrifugal pump is filled with air, which prevents a vacuum from forming. In this case, water will not flow through the pipe.
 
Back Pressure - The amount of pressure a pump is required to overcome to dose at the point of injection, measured in bar.
 
Capacity - The water handling ability of a water pump.
 
Cavitation - A situation in which vacuum pockets form in a pipe, eventually causing the pipe to implode.
 
Dead Head - A situation in which a pump attempts to run against a closed valve or against any other condition preventing discharge.  
 
Discharge Hose - A hose that moves the discharged water from the pipe.
 
Drain Plugs - Plugs that are removable from the pipe, used to drain water during inactivity.
 
Electronic Controller - An entire control system that supplies feedback to the pump to sustain the accurate chemical dosage, which can increase efficiency.
 
Flooded Suction - A condition wherein a pump is located below the liquid being pumped.
 
Flow Switch - A device that supplies a positive indication that the fluid is flowing and the pump is running.
 
Pressure - Force applied per unit area.
 
Prime - Creation of a vacuum in a pump.
 
Pump Housing - The casing of the pump, made of plastic, stainless steel, aluminum or iron.
 
Pumping Level - The lowest level of water during the pumping process.
 
Shock Mounts - Rubber mounts that are used to help control a pump's vibration.
 
Strainer - A filter at the end of the suction hose that prevents large objects from entering.
 
Thermal Overload Sensor - A sensor in a pump that shuts down the system when the operating temperature exceeds the limit of safety.
 
Viscosity - The degree to which a fluid resists flow when pressure is applied.

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