There are many industrial applications where pumps transfer liquids in a process plant. Liquids are subjected to shear forces passing through the pumps, which may have detrimental effect on certain types of liquids. Typical shear sensitive liquids are liquid mixtures that can form emulsions (multiphase fluids) and non-Newtonian liquids.
A common example of two liquids that can form an emulsion is water and crude oil. Formation of emulsions in the separation process can be a big concern. The higher the shear forces, the smaller the droplets of the dispersed phase, and hence a more stable emulsion. The stability of the emulsion determines how long it takes to separate the phases, i.e., it affects the efficiency of the separation facilities.
Some commonly known non-Newtonian liquids met in the food industry are ketchup and mayonnaise. If non-Newtonian fluids experience high shear forces during transfer, they might alter its properties. Presence of high shear forces during products transfer may diminish its quality.
Due to this, high shear pumps are not desirable in processes where shear sensitive fluids are present.
Low shear pumps for multiphase flow
Choosing the right type of pump is very important. Pumps are selected based on pressure head, capacity, weight and size, process control and price. Transfer of multiphase fluids requires certain consideration regarding the pump selection process. Usually multiphase fluids need to be separated at a certain stage, and therefore emulsification of these fluids during transfer will have negative consequences. Emulsions reduce the efficiency of the downstream separation equipment and increase the pressure drop during pipe transport due to higher apparent viscosity. Emulsion treating operations involve additional capital and operational expenses. This may include the use of auxiliary equipment for emulsion breaking, use of chemicals and energy for heating, and associated operational costs.
The physics associated with droplet development and emulsification inside the pump are complex and require extensive numerical analyses to be predicted, if possible. However, it does seem plausible that two different mechanisms are present at the same time. These are turbulent droplet coalescence and droplet break-up.
The coalescence process in a turbulent flow consists of two sub-processes: collision of droplets and drainage of the fluid film between them. Eq. 1 shows the collision frequency for equally sized droplets in the inertial subrange of turbulence. This equation determines how often two droplets collide.
= energy dissipation rate per unit mass, m2/s3 or W/kg
= droplet diameter, m
= number of droplets of diameter d per unit volume, m-3
The collision frequency increases if the number of droplets, the size of droplets or the energy dissipation rate increases.
The droplet break-up in turbulent flow can be described by formulating the maximum sustainable droplet size that can exist at given flow and fluid conditions:
= maximum droplet diameter, m
= Critical Weber number, -
= interfacial tension between the oil and water phases, N/m
= density of continuous phase, kg/m3
The mean energy dissipation rate per unit mass, , is the parameter that describes the intensity of the turbulence. Identifiable structures in a turbulent flow are called eddies. The turbulent flow consist of eddies of wide size range. As the kinetic energy cascades from large scale eddies down to smaller ones, energy dissipates into heat due to viscous forces. The energy dissipation rate is the parameter used to determine the amount of energy lost by the viscous forces in the turbulent flow. In order to reduce shear forces and the droplet break-up of the dispersed phase, the mean energy dissipation rate must be minimized according to Eq. 2.
There are no good approximations found for the estimation of energy dissipation rate in different types of pumps. Although, some main low shear pump principles can be drawn.
Regarding non-Newtonian fluids, they usually represent one homogeneous phase and do not experience droplet break-up or coalescence. Instead, if a certain threshold of shear rate is reached during the fluid transfer, they start to degrade and loose/alter its properties, which is commonly undesired.
Low shear pump principles
Positive displacement pumps make a fluid move by trapping a fixed amount and forcing (displacing) that trapped volume into the discharge outlet. There are two types of positive-displacement pumps: reciprocating and rotary. Reciprocating pumps use pistons, plungers, or diaphragms to displace the fluid, while rotary pumps operate through the mating action of gears, or screw-type shafts.
Reciprocating pumps are regarded as low shear pumps, as in principle they transfer fluids in and out of a chamber with the help of check valves. These valves usually resemble some sort of orifice that opens and closes during the chamber filling and discharge. The fluid flow experiences local velocity- and pressure variations when passing through these restrictions. This type of pump is known for the pulsating performance. These process fluctuations cause shearing of the fluids, unless fluid flow rates are very low.
Rotary pumps displace a fixed quantity of fluid for every revolution of the driver shaft. They have different pumping elements such as vanes, lobes, gears, and screws. Transferred fluids experience low shear as they move along the pump in closed cavities, created by pumping elements. Some slip of the fluids occurs during pumping. Slip is the quantity of fluid that leaks from the higher-pressure discharge area to the lower-pressure suction area. It occurs because all rotary pumps require some clearance between the rotating elements and the pump housing. This clearance provides a leak path between the discharge and suction sides. The slip is the biggest source of shear in rotary pumps. A pump with large clearances, because of machining tolerances or wear, exhibits a proportionally larger slip. Therefore, originally operated as low shear, rotary pump starts to wear after some time and may increase shearing of the transferred fluid with pump age.
Kinetic-energy pumps are velocity type pumps in which kinetic energy is added to the fluid by increasing the flow velocity. This increase in energy is converted to a gain in potential energy (pressure) when the velocity is reduced prior to or as the flow exits the pump into the discharge pipe.
This pump type is generally not viewed as low-shear, since the fluid experiences high velocity fluctuations when transferred. Velocity of the fluid is proportional to the turbulence and energy dissipation rate. However, some level of shear is required to promote the coalescence where multiphase flow is present. The configuration of the typical centrifugal pump has recurrent structures that can accommodate small droplets and promote coalescence while pumping at certain process conditions.
Experimental results of low shear pump operation
There are several studies performed on shearing characteristics of pumps while transferring multiphase mixtures. The pump’s shearing effect can be evaluated by comparing the outlet droplet size distribution of the dispersed phase to the fixed inlet size of dispersed droplets.
Progressive cavity pumps
A comparative study of five different pump types showed that all pump types experienced consistent droplet break-up. The progressive cavity pump showed least droplet shear. The testing of a twin lobe pump revealed that, at constant differential head, the degree of droplet break up decreases with increasing flow rate. This trend was seen for all the positive displacement pumps. Reported field tests off progressive cavity pumps concluded that no significant droplet shearing was observed in the field conditions. Average droplet size of the dispersed phase on the outlet side of the pump was around 95% of the inlet droplet size.
An experimental study of gear pumps showed that higher shear intensity (higher RPM of the pump) led to reduced outlet droplet size. It was observed that at pump speeds over a certain threshold, the droplet size distribution was not affected further. Possible explanation for this effect was that the residence time of the droplets in the pump was no longer higher than the required break-up time for the given droplet size. Another practical observation was that higher dispersed phase concentration resulted in higher average outlet droplet size. This trend points to the possible coalescence effect that takes place in the gear pump.
Single stage centrifugal pumps
As mentioned, centrifugal pumps are not usually considered low shear, as they develop high velocities of the fluid flow. Simultaneously, some level of shear forces inside the pump may promote the coalescence. Different studies were performed in order to investigate the possibility of low shear operation for this type of pump. Following requirements were drawn for optimal centrifugal pump operation concerning shearing:
Coalescing multistage centrifugal pumps
A coalescing multistage centrifugal pump has several pump stages with specially designed impellers. The new impeller configuration (reduced diameter) improves droplet coalescence significantly in multiphase flow compared to the standard configuration. Pump test results for oil-water mixtures show that the coalescing centrifugal pump consistently increases the average droplet size for various process conditions. Additional conclusions are that the coalescing effect of this pump is increased when 1) the dispersed phase concentration is increased and 2) average inlet droplet size is reduced. CFD modelling of this coalescing multistage centrifugal pump revealed recurrent turbulent structures in the passage between the diffuser and the return vanes. These structures have a retaining effect on the dispersed droplets. The retaining effect proved having a higher impact for the smaller droplets, increasing their residence time in the pump and therefore increasing the possibility of coalescence.
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