Water Hammering, and Cavitation in Pumps

Water Hammering, and Cavitation in Pumps

Newtonian and Non-Newtonian Fluids,Water Hammering, and Pump Cavitation

Newtonian and Non-Newtonian Fluids, Water Hammering, and Cavitation in Pumps

While handling pumping systems, it is critical to have a background understanding of some of the important concepts that govern pump performance, efficiency, output, and longevity. The following resource aims to discuss the concepts of Newtonian/Non-Newtonian fluids, water hammering, cavitation in pumps, and how these factors impact modern pumping systems.

Newtonian and Non-Newtonian fluids

In fluid pumping, it is fairly important to understand the properties of the fluids being pumped, transferred, and mixed. This understanding plays a major role in selecting suitable equipment for fluid pumping. Fluid viscosity or thickness determines the way a particular fluid will behave in a pump. When it comes to fluid viscosity, fluids show a non-uniform pattern. Some fluids maintain a constant viscosity while others show a significant change in viscosity as temperature and applied forces change. 

In general, fluids can be divided into two broad categories. Newtonian fluids and Non-Newtonian fluids. Non-Newtonian fluids have further subcategories (more on that later). Pump manufacturers and end-users must have a clear understanding of these fluid types and their behavior. This plays a vital role while selecting and operating pumps.

Understanding ‘Shear’

Before we move on, it is important to understand the concept of ‘shear’. In very simple terms, shear is the relative motion between adjacent layers of a fluid that is in motion. An example of a fluid in motion is when butter is applied to bread. In this case, there is relative motion between two layers of fluid (butter on the knife and butter on the bread). 

In the case of solids, the shear force acts tangentially to the surface of the solid. A solid can resist deformation due to this force but a fluid will flow under its influence. When adjacent layers of a fluid move relative to each other, shear stresses are developed. Shear forces are developed because adjacent layers of a fluid move with different velocities in a pipe (thus there is relative motion between them). Shear rate is defined as a measure of the extent of this relative motion between the layers.

Shear Stress and Shear Rate

Newtonian Fluids

Newton’s law states that the shear stress is directly proportional to shear rate when fluid viscosity is constant. Fluids obeying this law are called Newtonian fluids. In other words, the fluid viscosity does not change no matter how much shear is experienced by it. In the case of Newtonian fluids, kinematic viscosity is considered. This is the viscosity when no external forces are acting on the fluid. In other words, this is the viscosity when only gravity is acting upon a fluid. The kinematic viscosity of Newtonian fluids remains constant even if external forces are applied to the fluid. Examples of such fluids include water, high viscosity fuel, some motor oils, most mineral oils, gasoline, kerosene, most salt solutions in water, light suspensions of dye stuff, kaolin (clay slurry). These fluids have fairly defined and consistent flow characteristics.

Non-Newtonian Fluids

The viscosity of these fluids changes as shear is applied. This makes them much more challenging to pump. In the case of Non-Newtonian fluids, the Dynamic viscosity is considered and it changes when shear is experienced by the fluid. During pump design, the changing viscosity has to be considered for ensuring appropriate flow, pipe size, and optimum pump characteristics (pump speed; inlet pressure requirements; pump pressure drop to initiate flow). Depending upon how the viscosity changes in response to external forces, Non-Newtonian fluids are further divided into sub-categories. These categories are as follows:

Dilatant

The fluid viscosity increases as shear is applied. They are also called shear thickening. If you quickly try to force an object through such a fluid, it will be met with a lot of resistance. On the contrary, slowly introducing the object will give its molecules the time to move away and the object will move while facing much less resistance. Examples of such liquids include:

  • Feldspar 
  • Quicksand 
  • Beach sand 
  • Clay
  • Mica
  • Silly putty 

Pseudoplastic

In the case of pseudoplastics, viscosity decreases when shear is applied. Examples of such cases are:

  • Grease
  • Soap
  • Sewage 
  • Sludge
  • Molasses
  • Paint
  • Most emulsions
  • Paper pulp
  • Printer’s ink 
  • Starch
Figure 1: Stress response of dilatants and pseudoplastics
Stress over time

Thixotropic

In addition to shear, the viscosity is also time-dependent in the case of thixotropic fluids. The application of shear decreases the viscosity and the viscosity also decreases as time goes by. These fluids include:

  • Silica gel 
  • Greases
  • Bentonite
  • Inks
  • Mayonnaise

Rheopectic

The behavior of these fluids is similar to dilatants (viscosity increases with shear) but the difference is that the viscosity is time-dependent as well as goes on to increase with time. Examples include 

  • Gypsum in water
  • Asphalt 
  • Lard
  • Starch
  • Fruit juice concentrates
  • Molasses
Figure 2: Stress response of Rheopectic and thixotropic fluids
Stress over time

Shear thickening in Non-Newtonian fluids

Under a fast-moving shear force, shear thickening fluids tend to act like solids. The more force is increased, the more their viscosity increases. When pumps are run at high speeds, they produce high pressure as well as fast-moving shear forces. This increases the viscosity of these fluids because the molecules crowd each other under high pressure. On the contrary, molecules have time to move out of each other’s way under low pressure and low velocities which causes a decrease in viscosity. It means that the pump speed and pressure greatly influence the way fluids behave during processing and dispensing. 

A Non-Newtonian fluid tends to resist flow much more than a Newtonian fluid like water. This difference in viscosity has to be taken into account to maximize production efficiency. Viscosity affects how long fluid takes to dispense and its flow rate. Besides viscosity, shear thickening also needs to be considered (it is a fluid’s response to shear forces). For example, ketchup is a shear-sensitive fluid and at rest doesn’t pour well. But in response to squirting (application of forces), it becomes less viscous and pours easily out of the bottle. Honey is not a shear sensitive fluid so squeezing a honey bottle doesn’t affect its viscosity considerably. For pump and piping systems designers, this is important to consider because shear-sensitive fluids like shampoo, ketchup, or egg white need to be handled gently as their composition and integrity can be affected.

Non-Newtonian Fluid

Shear thickening in pumps

As discussed earlier, Non-Newtonian shear-sensitive fluids change in viscosity in response to shear. This means that while passing through the impeller of a pump, the fluid viscosity changes in response to forces exerted by the impeller. Shear thickening fluids (dilatants) increase in viscosity while the viscosity of shear-thinning fluids decreases when forces are applied. Corn starch, for example, is a dilatant, and its viscosity increases when a mixture of cornstarch and water flows out of a pump.

Why are high efficiency pumps important?

Pump efficiency is the ratio of the amount of fluid entering the pump to the amount of fluid leaving it. A pump with higher pump efficiency is more effective for gentle product handling. A low-efficiency pump means that some portion of the product remains in the pump casing and is recirculated. This further impacts the fluid’s viscosity and sometimes the changes in viscosity can be permanent due to overhandling which can result in compromising the product’s integrity. 

To avoid this problem, a positive displacement pump can be used. Such a pump can deliver a constant flow rate at a given pump speed. Even if the viscosity increases, a constant rate of the fluid can be dispensed by ramping up the pump’s horsepower.

Water Hammer and its causes

Water hammer is a surge of pressure that can arise in pumping systems. The pressure is created when the pumping system undergoes an abrupt change in flow. The main causes of water hammering include opening and closing of valves, pump starts and stops, and separation and closure of the water columns. Due to these factors, the water column undergoes a change in momentum and this abrupt change can produce shock waves that travel back and forth within the system. Depending on the magnitude of the shock wave, physical damage in the system can be severe. 

The phenomenon can be understood by an example in which water is pumped in a pipe that has valves on its both ends. The inlet valve is opened and the water column starts traveling towards the discharge valve. At this point, the discharge valve is closed instantly and the leading edge of the water column strikes the closed valve and begins to compress. A pressure wave (shock wave) begins to travel along the backstream (towards the inlet valve). The shock wave travels back and forth between the two valves until it finally diminishes due to friction losses. This water hammer shock wave is so fast that it can make a round trip between the two valves in less than half a second in the case of a 1000 feet pipe. The pressure created by this shock wave depends on the wave velocity (a), the velocity of water in the pipe (V), and the universal gravitational constant (g). Mathematically,

P = a V / 2.31 g

Even at a pipe velocity of just 10 ft/sec, the additional pressure can reach 657 psi. This is a huge amount of pressure that can easily devour pipelines! 

Causes of water hammer

If we examine the causes of this phenomenon, there appear to be three dominant factors:

  • Valve opening and closure
  • Pump start and stops
  • Water column separation and closure

Valve opening and closure

The abrupt closure of valves is one of the primary causes of water hammer. Consider the system in figure 3 below. A branch is feeding the main pipeline and the circuit is in the form of a ‘Tee’. A valve has been installed at the end of the branch. In this case, the primary barrier to water flow is the valve while the secondary barrier is the ‘Tee’.

Figure 3.
Valve opening and closing

If the valve is closed quickly when the water is flowing in the branch line, a shock wave will develop and will travel back and forth between the valve and the Tee. The time of valve closure (i.e. how quickly the valve is closed) also affects the intensity of the water hammer. The following equation gives the relationship between the additional pressure generated by the shock wave (P), flow velocity in ft/sec (V), pipe length between barriers (P), and valve closing time in seconds (t): 

P = 0.07 (VL / t)

The additional pressure is directly related to the flow velocity and length of the pipe between the barriers. It is inversely related to the valve closure time. Since the pipe length is often fixed, the additional pressure caused by the water hammer can be managed by changing the valve closure timing and the flow velocity. 

It is also important to know that water hammering can have more devastating effects in systems designed to operate at low pressures. A 1000 ft pipe with a 5 ft/s flow velocity will experience the same intensity of shockwave whether operating at a pressure of 50 psi or operating at a pressure of 200 psi. The main difference is that the ratio of shock pressure to design pressure will be much higher for a system with a 50 psi design pressure than for a system with a 200 psi design pressure. The damage in the case of the 50 psi system will, therefore, be much greater.

Pump starts and stops

When a pump is started in large pumping systems, it is done so when the discharge valve is closed. The valve is opened slowly only after the pump has reached its maximum speed. Water hammer is significantly reduced when the pumps are started and stopped against a valve that is opened or closed slowly. 

The problem occurs when the pump motor loses power during a power outage. This reduces the water flow at pump discharge and a water hammer is created due to abrupt changes in pressure and kinetic energy of the flowing water. This shockwave can reverse the water direction that can prompt the pump impeller to accelerate in the reverse direction. This creates additional pressure because backflow is reduced when the impeller reaches maximum reverse speed. 

To mitigate this situation, a ‘spring-loaded’ check valve is installed at the pump discharge and a specific value of pressure is maintained at the pump inlet. When the pump starts, it has to create more pressure than that at the pump inlet in order for the flow to initiate. This ensures that the flow is increased gradually and water hammering is not initiated. When the pump stops, the spring-loaded valve is instantly closed. This prevents the water column from changing direction and a relatively uniform pressure is maintained through the pipeline. 

If a normal check valve is used instead of a spring-loaded one, the water column will accelerate backward and slam the check valve close thus initiating a shock wave.

Water column closure and separation

This occurs in a two-phase system when the water column exists as liquid and vapor simultaneously. When the pressure in the pipeline is reduced below the vapor pressure of water, a phase change can occur. This can separate portions of the liquid water column by creating pockets of water vapor in between. When the pressure rises and exceeds vapor pressure, the liquid water column joins again (water column closure) that can create a wave of high pressure. This is how water column closure and separation can cause damage to thin-walled pipes.

Cavitation in pumps

When the liquid turns to vapor at low pressure, cavitation occurs in pumps. It happens due to a lack of sufficient pressure at the suction end of the pump or insufficient Net Positive Suction Head (NPSH is the difference between the pressure available at the pump inlet and the vapor pressure of the liquid). Cavitation occurs when the pressure and temperature of the liquid at the suction side of the impeller equals the vapor pressure. This creates air bubbles at low pressure. When the liquid travels from the suction side of the impeller to the delivery side, these air bubbles collapse. The force generated when the bubbles implode is often strong enough to damage pump components, seals, shafts, bearings, and impellers. Cavitation can also cause vibration that causes mechanical damage and thus the service life of pumps is reduced.

cavitation

Why does cavitation occur?

Under normal conditions, liquids have a predictable and steady vapor pressure. When the pressure inside the pump falls below the vapor pressure, cavitation occurs. This drop in pressure is often caused by disruptions in flow. Disruptions in flow can be caused by:

  • Poorly specified pump
  • Higher than expected viscosity
  • Clogged inlet
  • Clogged filters and strainers

In addition, at very high discharge pressures, a portion of fluid cannot discharge from the pump. The undischarged fluid gets trapped between the impeller blades and can cause a pressure drop at high velocity. This pressure drop triggers cavitation.

Recognizing pump cavitation

Typical signs of cavitation include:

  • Vibration
  • Impeller erosion
  • Noise
  • High power consumption
  • Seal or bearing failure

Cavitation usually sounds like gravel or marble moving inside the pipe, pump, or hoses. 

NPSHa and NPSHr

A clear understanding of the NPSHa and NPSHr is critical for understanding the cavitation process. The NPSHa (Net Positive Suction Head Available) is a system-specific value and doesn’t depend on the pump. It is the difference between the pressure at the pump inlet flange and the vapor pressure of the liquid. In short, the head available at the pump suction flange pipework connection is the NPSHa. 

NPSHa = Pressure at pump inlet – vapor pressure

The NPSHr (Net Positive suction Head Required) is not related to the system and is a purely pump specific property. It is the minimum pressure at the pump suction port that will prevent the pump from cavitating. The NPSHr alone is not sufficient to keep the pump from cavitating because it is measured just as cavitation begins to start. Therefore, the NPSHa should always be greater than NPSHr to keep a positive margin and prevent cavitation.

Steps to avoid pump cavitation 

The main focus should be to prevent unnecessary pressure drop. Additional pressure drop can often be avoided by removing as many valves and bends as possible and moving the pump closer to the fluid source. The distance between the fluid source and the pump should be decreased if the suction lift requirement is too high and would cause a pressure drop. 

Sometimes there are blockages in the piping or hoses near the pump so enlarging suction lines can mitigate the problem. The suction lines should also be cleaned regularly. In addition to these basic steps, there are a few other effective ways to curb cavitation. 

Pump selection

Cavitation is increased when the pump head drops so selecting the correct pump ensures a positive margin of the NPSHa above NPSHr. Keeping the pressure at the pump inlet 10% greater than the pump specific NPSHr is a good general rule to prevent cavitation. For an NPSHr of 10 feet, an 11 feet NPSHa is a good value. Contrary to NPSHr (which is a pump specific value and cannot be changed), the NPSHa is a system-specific value and can be increased by the following steps:

  • Elevating the supply tank
  • Raising and maintaining tank liquid level
  • Cleaning debris inside the pipes
  • Reducing piping losses caused by unnecessary valves and fittings

Preventing discharge cavitation

Discharge cavitation is observed when the pressure at the discharge end of the pump is very high. This limits the volume of the fluid exiting from the pump and high-velocity fluid remains trapped between the impeller and housing. This is a major cause of cavitation. Causes of high discharge pressure include pipe blockages, poor piping design, and clogged filters. 

To prevent discharge cavitation, reducers should be kept as close as possible to the pump. In addition, pockets should be avoided that allow air and vapor to accumulate. The control valves (if they are needed) should always be installed on the discharge side instead of the suction side of the pump.

Ensuring pump maintenance

Ensuring routine pump maintenance is a proven method to avoid cavitation. The maintenance activities can include:

  • Cleaning blocked filters and strainers to avoid discharge pressure buildup. A maintenance schedule should be set up for regular cleaning. 
  • Checking the pressure sensing equipment. 
  • Ensuring that there are no cracks or collapsed points in the piping and hoses.

 

Installing the system properly

The system should be designed such that the NPSHa is greater than the NPSHr. There are four factors that have to be considered for an appropriate system design:

 

Pump location

The pump should be installed in such an orientation that the pump suction inlet lines have an adequate slope and the pump housing is flooded. It should be ensured that the fluid flows smoothly into the pump suction inlet. 

The pump should be placed at a level that is lower than the fluid level in the supply tank. This pump orientation involves the force of gravity to maintain the flooded section of the pump. This helps to curb cavitation.

Size of the suction pipe

The pump suction diameter should be chosen carefully. This helps to maintain laminar flow. It is best to avoid elbows, valves, reducers, and strainers in the final length of the pipe. 

In addition, the pump casing should not be made to support the weight of the piping. This can cause strain on the pump casing. The piping should be supported on supports and hangers. The suction side of the pump should be always kept larger in size than the pump inlet.

Suction lift

The suction lift is the vertical distance from the water source to the pump inlet. A high suction lift can increase turbulence, ramp up energy demands, and most importantly, decrease NPSHa. The pump should therefore always be installed below the water level in the supply tank.

Friction loss

Friction loss in the piping system results in pressure drop that can cause cavitation. Friction losses are affected by pipe length, diameter, and flow rate. Obstructions in flow velocity can cause pressure changes that can trigger cavitation. 

In conclusion, there are a number of factors that initiate cavitation. If any trouble is encountered, the following list of quick checks can help in troubleshooting:

  • Checking whether the pump is installed high above the fluid source or not
  • Checking whether the pump suction line is sloped properly or not
  • Checking if there are unnecessary fittings on the suction pipe
  • Checking if the pump is running too fast
  • Checking if the suction pipe diameter is too small or the pipe is too long

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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. 
    Soft Gel Capsule Dispensing

    Soft Gel Capsule Dispensing

    Softgel manufacturing requires precise dispensing with excellent accuracy and repeatability. For example, Active Pharmaceutical Ingredients or APIs require the utmost precision so the dosage to consumers is not only safe but effective. In addition to stringent quality standards, pharmaceutical manufacturing must also be economical. Pharmaceutical manufacturing demands speed and convenience to achieve high throughput. In Softgel manufacturing the FDA has strict guidelines on batch uniformity1 that require any chemistry dispensing system to be fast sterile, repeatable and accurate.

    Sterile Dispensing

    An important feature of any dispense system that handles regulated recipes is the use of aseptic materials. The Zaxis eVmP Smart pump heads are made of chemically inert materials such as aluminum oxide, 316SS, or CKC (Ceramic/Kynar®/Ceramic).

    Valveless Design

    The rotating and reciprocating single piston design of the eVmP does not require any valves which limits the number of moving parts exposed to the fluid being dispensed. This allows the pump to be run for millions of maintenance free cycles and facilitates easy disassembly for cleaning. To remove an eVmP pump head, simply remove the two bolts (four on a VS6-Series) holding the head to its drive.

    Other positive displacement pumping technologies that do not both rotate and reciprocate, require valves on the inlet and outlet of the ports. These ports can become clogged and affect performance. For this reason, the valveless design of the eVmP smart pump is a major advantage over traditional pumping technologies.

    Electronic Adjustment

    For a clean in place or CIP protocol, simply maximize the shot volume or displacement by pressing a single button. The eVmP Smart Pump System is controlled by a TSi (Touchscreen Interface) that can communicate up to 32 pumps at one time. Between batch runs, using the TSi, select all pumps and press the dedicated Max Shot button. The preferred cleaner or solvent is then passed through the pump heads cleaning the entire fluid path with zero disassembly required. Traditional pharmaceutical dispensing technology requires manual adjustment to change the shot volume. This is both imprecise and time consuming. The ability to dynamically adjust the dispense volume electronically is a huge advantage for pharmaceutical manufacturing and should not be overlooked.

    When dispensing chemistry that precipitates or crystallizes, with exposure to atmosphere, it is best to a choose pump head with an integrated washport. The washport acts as an isolation gland between the dispense fluid and atmosphere. This gland introduces an inert fluid to the piston between the fill chamber and the head seals that acts as a piston lubricant and atmospheric barrier. This barrier prevents any crystallization of chemistry that can be caused by evaporation.

    Precision and Accuracy

    The FDA has general drug manufacturing guidelines put in place “to assure batch uniformity and integrity of drug products.” 1

    These guidelines cover things such as:

    • Tablet or capsule weight variation
    • Adequacy of mixing to assure uniformity and homogeneity
    • Dissolution time and rate
    • Clarity, completeness, or pH of solutions

    To achieve the product consistency required by FDA guidelines, a Softgel dispense system must achieve high accuracy with a low relative standard deviation. Positive displacement pumps are preferred for pharmaceutical metered dispensing due to their high accuracy. Positive displacement pumps do not depend on back pressure for flow, they control flow rates by changing the speed at which the pump is driven.2

    Most positive displacement pumps can be classified as rotating, such as a gear or screw pump, or reciprocating, such as a diaphragm or plunger pump. The eVmP uses a unique drive and piston design that both rotates and reciprocates to accomplish both valving and pumping, eliminating valves used in traditional piston pumps. eVmP Smart pumps have 1% or better full-scale accuracy as well as a 0.5 coefficient of variation resulting in accurate and repeatable metered dispensing without the use of valves.

    Another feature created by Zaxis for the eVmP smart pump system is micro stepping. Positive displacement pumps are designed to be viscosity independent but in real world applications the viscosity of a fluid can minutely affect the desired dispense volume. Example: you have set your pump to dispense 50 microliters but during production the volume metered is consistently 49.8 microliters. The micro stepping feature makes slight adjustments to the volume metered allowing you to hit your desired target.

    Achieving High Throughput

    Manufacturing pharmaceuticals demands excessively high throughput and constant changeover. eVmP stands for Electronic Variable Metering Pump and the purpose of the systems patented design is the ability to change dose volume without the need to manually adjust the pump. Along with the stepper or a powerful 1500 RPM servo motor that drives the pump head piston, each eVmP contains a second stepper motor that controls the pump drives angle of articulation. This angle affects the travel distance of the pump heads piston thereby changing the dispense volume. This enables the electronic micro-stepping feature referred to above, the clean in place Max Shot feature and most importantly, the ability to easily and quickly change dispense volumes between batches/recipes.

    Every eVmP Smart pump contains an onboard processor that can store up to 50 programs. These programs can be batch specific with different volumes, speeds, timing, etc…. A common saved program often set is a clean in place program. For example, at the beginning of a shift an operator can select the programs for every batch that will be run that day, with a clean in place program set to run between each batch. The pumps will automatically change over at the completion of every batch leaving the operator nothing to do but switch out the chemistry.

    eVmP Smart Pump System Makes Pharmaceutical Dispensing Easy

    The eVmP Smart Pump System is ideally suited for softgel dispensing. The aseptic materials that are easy to dismantle for an autoclave, or simply clean in place, combined with integrated washports and an auto-agitation feature make eVmPs optimal for handling pharmaceutical chemistry. The eVmP onboard processors are compatible with Digital I/O, RS485 and EtherNet/IP for easy integration into any manufacturing system. The micro stepping feature helps dial in your target for the perfect shot size. The programmable volume creates a highly accurate and highly versatile tool for any dispensing system. The high accuracy and excellent standard deviation attained by the rotating and reciprocating, positive displacement design will expand your pharmaceutical manufacturing core competencies.

    1. Food and Drug Administration. Code of Federal Regulations Title 21,   https://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/CFRSearch.cfm?fr=211.110. Revised 1 Apr. 2019.
    2. Science Direct. Positive Displacement Pumps, https://www.sciencedirect.com/topics/engineering/positive-displacement-pumps. Accessed 14 May 2020.
    Manufacturing Single Use Vape Cartridges

    Manufacturing Single Use Vape Cartridges

    Growing Vape Market

    The vape market has been on the rise for several years and is projected to continue to grow at a rapid pace The global e-cigarette and vape market size was valued at USD 10,261.8 million in 2018 and is expected to grow at a CAGR (Compound Annual Growth Rate) of 24.9% from 2019 to 2025 1. Single use vape cartridges are used for both disposable and rechargeable vaporizers. Recent health concerns associated with the growing vape market will drive spur increased regulations for manufacturing vape cartridges. These increased regulations will drive demand for more accurate and reliable manufacturing equipment as seen in the Pharmaceutical manufacturing industry. Zaxis Inc. pumps are already being used to manufacture pharmaceuticals because of the eVmP’s exceptional precision and accuracy. The manufacturing of vape cartridges requires pumps that can handle highly viscous fluids while dispensing very precise doses in each cartridge. Zaxis Inc. metering pumps use positive displacement to pump fluids making them ideal for fluids with low or high viscosity. A natural progression for the vaping industry will be to convert their existing pump equipment to the Zaxis eVmP precision dispensing system.

    Highly Viscous Fluids

    Concentrated plant-based oils such as cannabis and hemp are known to be highly viscous fluids. Manufacturers use a number of different techniques to obtain the correct fluid viscosity for vape cartridges including distillates with thinning agents, cutting/infusing the oil with additives to thin the consistency, or infusing terpenes and strain-specific flavorings.

    On a manufacturing production line all metering and dispensing pumps need to be able to overcome the backpressure the highly viscous fluids create. The Zaxis eVmP VS-Series was design specifically to deal with highly viscus fluids. The servo driven motor has nearly 4 times the torque of a traditional stepper motor and can handle flow rates near 2,000 mL/min and back pressure up to 200 psi. Meaning, the eVmP VS-Series can not only overcome the backpressure created by highly viscous fluids, is can dispense at a faster rate increasing product line throughput.

    Precision Dosing

    When manufacturing single use cartridges precision dosing is essential. In the Vape industry the FDA is particularly strict upon delivering exactly what is on the product label. Inconsistent dosing can upset end users if they receive a less than full product. Inconsistent dosing and overfilling can also have a damaging effect on the financial bottom-line of production.

    The rising market has shined a spotlight on the Vape industry. Consumer safety is a top priority and with that comes higher quality control, including the repeatability of precision mixing ingredients. The FDA states that if you do not report your ingredients or changes to your ingredient quantities “your products may be deemed “misbranded” under federal law and therefore, subject to regulatory action, including seizure and injunction. Submission of false information is also punishable by criminal and civil law.2” If an e-liquid or “juice” mixture diverges from its label by the smallest percentage the FDA often requires destruction of the entire batch including video evidence of its destruction. Precision metering/dosing is vital to the successful and safe manufacturing of e-liquids.

    The Zaxis eVmP system achieves +/- 0.5% full scale accuracy and 0.5 CV with 0.01 µL resolution through the use of a rotating and reciprocating positive displacement design. The use of positive displacement in the eVmP design helps reduce the risk of contamination or bleed over between packages. The difference between the external diameter of the piston and the internal diameter of the liner is near perfect. This means when the piston head is drawn away from the fluid path a vacuum is created ensuring volume remains constant through the entire cycle of operation. Furthermore, the eVmP rotating and reciprocating design renders highly repeatable results. With stainless steel pump head construction and sapphire hard, alumina ceramic, internals Zaxis eVmP pumps will last for millions of maintenance free cycles.

    Not only is the FDA scrupulous when it comes to ingredients and dosages, the Vape industry also has a discerning customer base that is well informed about their products and often demand high standards that preserve a plant’s unique profile, flavor, and complexity. Precisely mixing each element of a juice as a distinct individual dose can be far more exact than big batch mixing producing a very high-quality product and reducing the margin of error and potential waste.

    No One Wants a Leaking Pen

    Many of the first Vape pens came with a warning not to lay them down or tip them over as the oil from the tank would often leak out. Cartridge design has evolved over the last few years and with the vape market on the rise the development and refinement of cartridge design is a continual process. There are several different leak tests that can be performed to validate a cartridge design. Closing off the vents and running a pressure decay test through the mouthpiece will validate all of the seals and ensure the tank is leak tight. Then opening the vents and running a mass flow test through the cartridge is especially helpful in validating that an atomizer filter design will not leak fluid.

    The Isaac HD and Zaxis 7i are multi-function leak testers that have the capability of running multiple kinds of tests in the same unit. Both leak testers come with electronic regulators and up to 100 stored programs. This mean the air pressure setting will automatically adjust from test to test saving time and removing user error. Finding a desired flow rate for a cartridge is a delicate and precise task as the flow of air should be unimpeded while the flow of liquid should not pass through the saturated filter in full drops. The intuitive user interface makes test parameters easy to adjust and customize. The ability of the Isaac HD and Zaxis 7i to set and sense the difference between such tight parameters allows designers to develop cartridges that function perfectly without leaking.

    Conclusion

    The Zaxis eVmP VS-Series is ideal for handling the highly viscous fluid and extreme precision required for manufacturing single-use vape cartridges. The VS-Series servo motor technology is not only powerful enough to overcome the backpressure created by highly viscous fluids, but it also has the speed and accuracy to increase quality throughput. The Zaxis line of leak testers can validate the design of any vape cartridge as well as determine if there are any cartridge leaks or defects.

    1. Grand View Research. “E-cigarette And Vape Market Size, Share & Trends Analysis Report By Distribution Channel (Online, Retail), By Product, By Component, By Region, And Segment Forecasts, 2019 – 2025” Grand View Research, July. 2019 https://www.grandviewresearch.com/industry-analysis/e-cigarette-vaping-market
    2. FDA “ How Do I Comply with FDA’s Tobacco Regulations?” FDA  27, Aug. 2019 https://www.fda.gov/tobacco-products/compliance-enforcement-training/manufacturing 
    Soft Gel Manufacturing

    Soft Gel Manufacturing

    Accuracy and Repeatability with the Zaxis Variable Metering Pump

    Modern Soft Gel Manufacturing is an incredibly long, sophisticated process, which relies on many of the electro-mechanical and chemical technologies. For many years, the encapsulation and filling of the active pharmaceutical ingredients (API), relied on old mechanical technologies, that no longer can keep up with the industry demands. The Zaxis eVmP pump has been the choice to replace mechanical filling methods, with a dynamic, electronically controlled, positive displacement, ceramic piston pump.

    Pump Head and Gel CapsulesIn addition to being very accurate, the patented eVmP pump relies on the repeatability of sapphire hard, ceramic internals, and no valves.  These wetted path parts are designed for millions of maintenance free cycles, because they resist wear.  The repeatability and accuracy are very important in the control of API fluids, but the eVmP offered much, much more.

    The Sanitary Pump Head design is made of 316SS, and is easily disassembled for cleaning, which can take place after each batch or run of product.  Once cleaned, an eVmP pump head must return to the encapsulation area quickly, and without compromise to precious downtime.

    With the modular design and electronic controls, including Ethernet IP, the pump is quickly getting back to another formulation and production run. Zaxis eVmP pumps can be configured in single OEM or Dual modules, including IP45, 316SS enclosures.

    For the latest in API dispensing technology, used in soft gel encapsulation, please come see us at Booth 1365 at Interphex, in NYC, April 2-4,  where you can see the eVmP in action and talk to one of our application engineers.