Metering Pump Technologies

Metering Pump Technologies

Metering Pump Technologies

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.
    • Metering volume can be controlled electronically through the push of a button. 
    • Since there are no valves, there is no clogging which removes the need for constant maintenance and servicing. 
    • A drift free operation is ensured because of the ceramic body. 
    • A wide range of fluids including acids, bases, and solvents can be used because of the inertness of the ceramic material.
    • Because of a positive displacement design, the pump’s accuracy and precision remain unaffected even as the viscosity of the fluid changes. 
    Covid 19 Response Thank You from Air Logic

    Covid 19 Response Thank You from Air Logic

    Covid 19 Response Thank You from Air Logic

    2020 was a hard year and many of us had to adapt and react quicklyin response to the fight against Covid-19. Air Logic sent our employees this wonderful thank you video. We are proud to have partners like Air Logic and we look forward to many more years and projects together.

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    The Ideal Gas Law & Leak Testing Video

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    In this video we will discuss the ideal gas law and its effects on leak testing.

    During leak testing, changes in pressure can occur due to changes in temperature, volume, and number of moles of the gas. Since parameters like pressure, volume, and amount of air are involved in this process, it is governed by the ideal gas law.

    PV = nRT

    Where:

    P = the Pressure of the air (or gas) enclosed in the container, measured in atmospheres

    V = the Volume of the container occupied by the gas, measured in Liters

    n = the Number of moles of the gas

    R is the Ideal gas constant with a value of 8.314

    & T = the Absolute temperature of the gas, measured in Kelvin

    Changes in pressure, temperature, and volume are related and a change in any single parameter can affect other parameters.

    When the temperature of air rises, the average kinetic energy and velocity of the particles increases.  This raises both the pressure and temperature.

    When the test part is pressurized, the walls of the part can undergo expansion. This causes a change in the volume of the part. With an increase in area, the overall pressure drops. Similarly, a decrease in volume causes an increase in pressure, so long as the temperature and number of moles are constant. If the number of particles increases, this exerted force per unit area also increases.

    Using the ideal gas law, a suitable relation can be derived that allows determination of leak rates through pressure loss in the part. A leak rate can be seen as the volume of gas that escapes through the part per second.

    During a leak test, if the separation of the results, or Delta P, between leaking and non-leaking parts is not great enough, the repeatability of the process will be compromised. This is due to environmental factors influencing the pressure and temperature of the part under test.

    The selection of Fill and Settle times during the test influence the part’s ability to come to equilibrium, which directly affects the test outcome. The greatest contributor to successful test outcomes is the Fill step. At higher test pressures, a longer fill step will allow the thermodynamic process to reach equilibrium. Parts that have more compliance, or capability to experience expansion, also need to reach a point of equilibrium. The ideal gas law illustrates how reducing or nullifying pressure or thermodynamic change will influence test repeatability by causing greater separation in the ΔP, thus improving the test.

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    Ideal Gas Law & Its Effects on Leak Testing

    Ideal Gas Law & Its Effects on Leak Testing

    Pressure, volume, moles, and temperature

    During leak testing, changes in pressure in the test part are of primary interest. When a test part is filled with air (or any other gas), it initially expands inside the part to occupy its volume. When the part finally reaches the test pressure, the air inside contracts. This rapid expansion and contraction of air changes its temperature and volume. The test part also undergoes slight changes in temperature and volume because of changes in temperature and volume of the air. 

    In order to detect the leakage, the changes in pressure must be taken into consideration. The changes in pressure can occur due to changes in temperature, volume, and number of moles of the gas. Since parameters like pressure, volume, and amount of air are involved in this process, it is governed by the ideal gas law. Mathematically, the law is expressed as:

    Ideal Gas Law & Leak Testing Information Video

    P = Pressure of the air (or gas) enclosed in the container

     

    V = Volume of the container occupied by the gas

     

    n = Number of moles of the gas

     

    T = Absolute temperature of the gas

     

    R = Ideal gas constant with a value of 0.0821 dm3 atm K-1 mol-1

     

    According to equation (1), the product of the pressure and volume of any quantity of an ideal gas is equal to the product of the number of moles, ideal gas constant, and the absolute temperature of the gas. It can be seen from equation (1) that changes in pressure, temperature, and volume are related and a change in any single parameter can affect other parameters. 

     

    According to the Kinetic Molecular Theory (KMT) of gases, temperature is proportional to the average kinetic energy of a given sample of gas. This can be expressed as: 

    Equation (1) relates the actual experimental behavior of gases while equation (2) relates the results of experimental analysis using KMT. Studying equations (1) and (2), we can see how pressure, temperature, and volume are inter-related during leak testing. This means that changes in temperature, test part volume, and number of moles of a gas can affect the part’s pressure during leak testing. 

    Effect of temperature changes on the test part’s pressure

    When the temperature of air increases, the average kinetic energy and velocity of the particles increases. This is expressed in the equation (2) above. These particles now have a greater velocity, and they hit the walls of the part with greater force. The pressure exerted by these particles is in essence the force exerted per unit area of the walls of the part. So the pressure increases with an increase in temperature. Pressure is directly related to the temperature when the volume of the container and number of moles of gas are constant. 

     

    Effect of volume changes on the test part’s pressure

    When the test part is pressurized, the walls of the part undergo expansion. This causes a change in volume of the part. When the part expands and its volume increases, the pressure inside drops. This happens because the increase in volume increases the area of the walls. So the force per unit areas (pressure) drops because the force is constant. At constant temperature, the particles have the same average kinetic energy (or velocity) and they exert the same force on the walls. With an increase in area, the overall pressure (force per unit area) drops. Similarly, a decrease in volume causes an increase in pressure if the temperature and number of moles are constant. 

      

    Effect of number of moles on the test part’s pressure

    An increase in the number of moles of the gas increases the pressure inside the part when the temperature and volume are kept constant. Pressure is generated inside the part when the particles exert force on the walls of the part. With an increase in the number of particles (number of moles) increases, this exerted force per unit area (pressure) also increases. 

     

    Accounting for the leak rates caused by pressure changes

    In order for detecting the leak rates correctly, the fluctuations in pressure caused by the factors explained above should be allowed to settle. Oftentimes, this is not the case and in practice, leak testing is carried out without a proper fill and settle time. In order to account for these pressure changes, an adequate relation is needed that accounts for the pressure losses. 

     

    Deriving the relation of pressure loss to leak rate 

    Using the ideal gas law, a suitable relation can be derived that allows determination of leak rates through pressure loss in the part. A leak rate can be seen as the volume of gas that escapes through the part per second. This relation can then later be modified to account for the effects of changes in pressure, temperature, volume, and number of moles.

     

    From the ideal gas law, the number of moles in the test part can be expressed by: 

    If,

    NL = Number of moles of gas lost

    P = PATM = Atmospheric pressure

    V = L.R = Leak rate or volume of gas escaping per second

    Then equation (3) becomes: 

    Therefore, the remaining number of moles NR will be:

    Using ideal gas law, at a constant temperature, the remaining pressure after time (t) can be given by:

    Putting the value of NR from equation (5) in equation (6) and solving for PR, we get:

    Solving for leak rate (L.R) yields:

    Modified equation for accommodating the pressure losses. Equation (7) gives the relation for calculating leak rates when the test volume, temperature and PATM are considered constants. As described above, the actual leak rates (or pressure changes) are affected by changes in volume, and/or temperature. 
    To accommodate these pressure changes, a master part or ‘non-leak part’ is put under test. Despite no real leakage, some pressure change occurs in this non-leak part because of the reasons described above. This pressure loss is called the ‘zero offset’ factor. Equation (7) can then be modified after considering the ‘zero offset’ factor. The modified equation can be used to determine leak rates in test parts. The zero offset factor or the pressure loss of the non-leak part is subtracted from the pressure change of the test part in consideration. By doing this, the pressure changes due to temperature and volume are accommodated for and correct leak rates can be determined. The modified equation is:

    Where,

    L.R (t)       Leak rate (scc/s)

    t                 Time (sec)

    ΔP test          Pressure loss in the test part during the test (psi)

    ΔP non-leak    Pressure loss in the non-leak part (psi)

                  Volume (cubic cm)

     

    Process repeatability

    If the difference between ΔP test and ΔP non-leak is not great enough, the repeatability of the process will be compromised by environmental factors influencing the part under test. The selection of Fill and Settle times during the test will directly affect the test outcome. The greatest contributor to successful outcomes will be from the Fill step. Parts that have more compliance (capability to experience expansion) will need to reach a point of equilibrium. At higher test pressures, a longer fill step will allow the thermodynamic process also to reach equilibrium. Reducing or nullifying these two factors will cause a greater difference in the ΔP, thus improving the test.