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Metering Pump Technologies

Time to Read: 10m 47s

Delivery of a fluid in precise and adjustable flow rates is called metering. A metering pump is a device used to control the flow rate of a fluid. It is used to move a precise volume of liquid in a specified amount of time to provide an accurate volumetric flow rate. Typically, water, chemicals, solutions, and other fluids are moved by a metering pump. The flow rate depends on the outlet pressure and is usually constant over time. A metering pump is designed to deliver a maximum discharge pressure so the selection of a suitable pump depends on the type of application. A vast majority of metering pumps can be categorized as positive displacement pumps. 

Positive displacement pumps

Positive displacement pumps move the fluid by repeatedly enclosing its fixed volume and then moving it mechanically through the system. The fluid moving mechanism (pump action) can be executed through gears, pistons, plungers, rollers, screws, vanes, and diaphragms. The majority of positive displacement pumps can be categorized either as reciprocating type or rotary type.

Reciprocating positive displacement pumps

These types of pumps use a piston, plunger, or a diaphragm to execute a stroke (repeated back and forth movements). These movements are known as reciprocating motion. 

These cyclic movements of reciprocating pumps create a pulsating flow at discharge where the flow accelerates during the compression stroke and decelerates during the suction stroke. This requires different kinds of damping mechanisms to be installed to minimize the vibrations that result from pulsating flows. If left unchecked, these vibrations can have damaging effects on the installations and equipment. Vibrations and pulsating flow can be reduced if multiple pistons, plungers, or diaphragms are used where one group is in compression stroke while the other is in suction stroke.

Reciprocating pumps are ideal for places where accurate metering is required because of their repeatable and predictable motion. Specific quantities of fluids can be pumped by adjusting the stroke length and stroke rate in these pumps.

Piston and plunger pumps

In piston pumps, the initial stroke starts the suction phase. During this stroke, vacuum is created, the inlet valve opens, the fluid is drawn in, and the outlet valve is closed. The next stroke of the piston is in the reverse direction and starts the compression phase. The inlet valve closes while 

the outlet valve opens, allowing the fluid to move out of the chamber. The inlet and outlet valves operate 180 ᵒ out of phase that causes one valve to open while the other is closed. A seal is installed on the piston that prevents leakages. 

The operation of a plunger pump is similar but in this case, the volume of the fluid moved depends on the plunger size. In the case of a piston pump, this volume depends on the cylinder volume. The seal is installed on the cylinder housing instead of the plunger.

Benefits: 

  • Piston and plunger pumps are very suitable for low specific speeds and high-pressure heads.
  • These pumps are self-priming which means that they can evacuate air from the suction side at startup before commencing the normal pumping mode. 
  • They can handle a wide range of pressure and can move thick fluids and abrasives with relative ease. 
  • These pumps are ideal for delivering a known quantity of fluid precisely. They have low energy consumption while having significantly high efficiency.
  • A wide variety of fluids can be pumped including hot, cold, toxic, corrosive, abrasive, viscous, and flammable fluids.  

Limitations: 

  • The piston and plunger pumps can only deliver a pulsating output instead of a smooth continuous flow. No fluid is discharged during the suction stroke which results in a pulsating flow. This type of flow is the cause of vibrations that can cause damage to the equipment. To cater to this issue, additional damping or smoothing equipment needs to be installed. 
  • In the case of large units, often the investment costs for a single pump can be on the higher end. 
  • The moving parts of this pump are in contact with the fluid so there is a greater frequency of mechanical wear. This results in higher maintenance costs.

Diaphragm pumps

In the case of a diaphragm pump, the piston or plunger is replaced by a flexible membrane. This flexible membrane or diaphragm is connected to a rod that is used to expand and compress the diaphragm. The volume of the pumping chamber is increased by expanding the diaphragm. This creates suction and draws the fluid into the chamber. When the diaphragm is compressed, the volume of the chamber is decreased and some fluid is expelled out.

Benefits: 

  • Because of completely airtight seals, diaphragm pumps are often used to pump hazardous fluids.
  • These pumps can handle fluids with high viscosity. Because there are no rotating and close-fitting parts, fluids with high solid and abrasive content can be easily pumped. This also helps these pumps to run dry without the risk of significant damage. 
  • Their efficiency remains constant over time because there is no wear and tear associated with moving parts. 
  • Diaphragms pumps are used for a wide spectrum of applications because of their adjustable flow rate and discharge pressure which can be controlled by adjusting stroke length and rpm. 
  • The maintenance requirements for these pumps are significantly low because of the absence of mechanical seals, couplings, and motors. 
  • These pumps are portable and can be quickly transported where required.
  • Diaphragm pumps come to a halt when the discharge pressure exceeds air pressure. The discharge line is close with no power consumption and no increase in temperatures. 
  • A significantly high discharge pressure can be achieved through these pumps. Mechanical diaphragms pumps can provide a pressure of around 60 to  250 psig while hydraulic diaphragm pumps can pressurize fluids up to 4000 psig.

Limitations:

  • The flow is pulsating. It accelerates and decelerates during compression and suction strokes respectively. A damping mechanism must be installed to reduce pulsation.
  • The check valves located at intake and discharge sections can become clogged if the fluid contains a higher percentage of solid particles. This can result in a loss in the suction and priming ability of the pump which can lead to inaccurate metering. Because of this limitation, these pumps require frequent maintenance.

Rotary positive displacement pumps

In these pumps, rotating gears and cogs are used to transfer fluid instead of the backward and forward motion associated with reciprocating pumps. A suction is created at the pump inlet when the rotating gear or cogs develop a liquid seal with the pump casing. After the fluid is drawn inside the pump, it gets enclosed between the rotating teeth of the gear or cogs and is then transferred to the discharge section. During this process, energy is transferred to the fluid, and its pressure increases. The gear pump is one of the most common examples of a rotary positive displacement pump.

Gear pumps

Gear pumps use the rotation of gear teeth to transfer fluids from the inlet section to the discharge section. The gears apply force on the entrapped volume of the fluid and its pressure is increased during the process. A pair of spur or helical gears is normally used in the process. These gears revolve in opposite direction. The fluid is trapped between the teeth of the moving gears and is carried from the suction side to the discharge side. Due to this movement of the gears and low tolerance between the casing and gear teeth, a negative pressure zone is created on the suction side. This draws fluid inside where it is trapped between the gear teeth. The fluid is then carried towards the discharge and its pressure is increased in the process.

Benefits:

  • Gear pumps can operate at high pressures and can be used with high viscosity fluids. 
  • Despite being low cost and having a relatively simple structure, they can deliver results with up to 0.5% accuracy.
  • Their output flow is free from surging pulses that are observed in piston and plunger pumps. The pulses in flow are also less as compared to diaphragm pumps. 
  • The system design is simplified because no vibration-damping equipment is required.
  • Fluid leakage can be eliminated using hermetically sealed magnetic couplings.
  • The operation, handling, and maintenance of these pumps are relatively straight forward. 

Limitations:

  • The low tolerance between the gear teeth and the casing can be affected by abrasive fluids. This can result in leakages and changes in suction pressure. 
  • Abrasive fluids can cause wear on the gear teeth and around the face of the gears. This can lead to frequent maintenance requirements. 
  • Due to wear, the efficiency of these pumps can decrease over time.
  • The phenomenon of flow slip can occur which leads to backflow of the fluid from the discharge side to the suction side.
  • Pump output can be affected as flow slip increases. This can also affect metering accuracy. 
  • Despite being capable of self-priming (because rotating gears can evacuate air of the suction line), this ability can be restricted to 1 ft. lift.

Peristaltic pumps

In peristaltic pumps, the pump casing contains a hose or a tube in which the fluid is contained. The tube is alternatively relaxed and compressed by means of a roller. This creates a vacuum that draws the fluid through the tube.

Benefits:

  • The fluid is always enclosed within the casing and is never in contact with the pump mechanism. This eliminates the chances of the pump contaminating the fluid and vice versa. 
  • The complete closure of the tube when it is squeezed between the roller and track prevents backflow when the pump is not running. This eliminates the need to install separate check valves when the pump is not running. 
  • Because of a lack of seals and valves, these pumps are easy and less costly to maintain. 
  • The pump is capable of self-priming and has the ability to run dry. 
  • Cleaning the tube and hosing is a simple process.
  • Peristaltic pumps can easily handle viscous fluids and slurries.

Limitations:

  • The cyclic action of these pumps creates pulses in the discharge that has to be catered by introducing additional pumping elements and hoses.
  • These pumps are generally limited to 40 to 60 psig discharge pressure. 
  • The flexible tubes and hosing need to be replaced periodically as they are prone to degradation over time.
  • The drive motor is constantly under load because the rotor has to continuously squeeze parts of the tubing. This can increase power costs associated with the motor. 
  • The suction lift is limited by the strength of the tubing wall.

The eVmP Reciprocating and Rotating Pump

The eVmP (electronic variable metering pump) manufactured by Z-axis is an electronically controlled, rotating, and reciprocating pump that combines the benefits of rotating and reciprocating technology. This pump design is a major improvement because it eliminates the need for discharge valves which results in only one moving part (the ceramic piston). This drastically reduces the wear and tear associated with the relative movement of mechanical components and ensures up to 84,000,000 maintenance-free cycles.

The pump technology ensures remarkable accuracy because of extremely tight tolerances between the piston and the liner. This ensures extremely precise metering and volume control of the fluid being dispensed. This valve-less design ensures that the inaccuracies associated with tubing and diaphragm pumps are eliminated. Through its electronically controlled linear stepper actuator, ultrafine adjustments to piston travel distance and angle position are possible. This means that the volume being dispensed can be controlled with the push of a button. This also enables the pump to overcome variations in fluid viscosity and surface tension through dynamic fluid displacement. Overall, the Z-axis eVmP is one of the finest, valveless, positive displacement pumps that is fully automated and provides a safe and accurate way for precise metering and dosing of fluid without the design complexities and additional costs associated with other pumps in the market.

The pump technology ensures remarkable accuracy because of extremely tight tolerances between the piston and the liner. This ensures extremely precise metering and volume control of the fluid being dispensed. This valve-less design ensures that the inaccuracies associated with tubing and diaphragm pumps are eliminated. Through its electronically controlled linear stepper actuator, ultrafine adjustments to piston travel distance and angle position are possible. This means that the volume being dispensed can be controlled with the push of a button. This also enables the pump to overcome variations in fluid viscosity and surface tension through dynamic fluid displacement. Overall, the Z-axis eVmP is one of the finest, valveless, positive displacement pumps that is fully automated and provides a safe and accurate way for precise metering and dosing of fluid without the design complexities and additional costs associated with other pumps in the market. 

Benefits: 

  • Mechanical wear is eliminated because there is just a single moving part. There is no relative motion between parts which is a major cause of wear and tear.
  • Extremely precise and accurate metering and dosing is possible owing to tight tolerances between the piston and the liner wall.
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