COVER SERIES | Pump System Optimization
Optimizing Efficiency With Rotodynamic & Positive Displacement Pumps
A look at how viscosity, system pressure, output flow and operating costs influence pump selection.
Randy Bennett | Leistritz Advanced Technologies Corp.
With energy costs continuing to increase, there is a growing need to operate pumping systems as efficiently as possible. Rotodynamic pump system optimization has received a lot of press lately, but analyzing the type of pump used in a certain application has not. A review of how rotodynamic and positive displacement (PD) pumps react to operating conditions and fluid characteristics can assist in the selection of the correct pump to maximize pump efficiency and minimize long-term power consumption.
Pump Operating Principles
A rotodynamic pump operates by accelerating a fluid from the pump suction toward the discharge, where velocity is gradually decreased and converted to pressure. The energy transfer is finite and is split between flow, head (pressure) and friction losses in the pump. A positive displacement pump, on the other hand, moves a fixed volume of liquid per unit time and is not majorly influenced by the system pressure. Pressure is obtained as the liquid is forced through the pump discharge into the system, thereby converting energy to pressure.
For any pump selection, the pump design, fluid viscosity, discharge pressure and speed can all affect the overall system efficiency. However, the actual effect of each element will depend on whether the pump is rotodynamic or positive displacement. In many markets, there are applications that clearly should be rotodynamic and applications that are clearly positive displacement. However, there are also many applications where both types could be considered. While both types may work, the efficiency of each type may vary widely.
Viscosity Considerations
Pumps are typically designed with the potential applications in mind. Rotodynamic pumps are most efficient when pumping water and light hydrocarbons with low-vapor pressures. The majority of positive displacement pumps are most efficient when pumping viscous products. The total efficiency of a PD pump generally increases with increasing viscosity because the slip (the backflow of liquid) inside the pump reduces, leading to a higher volumetric efficiency. Certain PD pumps can handle extremely high-viscosity fluids, potentially reaching several million Saybolt Seconds Universal (SSU) (centistokes [cSt]), depending on the system conditions.
Rotodynamic pumps can see a reduction in efficiency with increasing viscosity due to the friction losses inside of the pump. The efficiency of a rotodynamic pump starts to drop when the fluid viscosity gets above about 500 SSU (108 cSt). Correction factors are applied to the performance data of rotodynamic pumps per industry standards, and the efficiency may continue to decrease as the viscosity increases. This loss can lead to an increase in required power while the flow rate is reduced.
System Pressure Considerations
When selecting any pump, it is necessary to know the operating parameters of the system and the characteristics of the pump over the complete operating range. Rotodynamic pumps are selected to run as close to the best efficiency point (BEP) as possible. If the system pressure varies, the rotodynamic pump will operate further away from the BEP, resulting in a drop in efficiency and output flow.
Positive displacement pumps are also affected by system pressure, but they do not exhibit a rotodynamic-style BEP. As the differential pressure increases, the slip can increase. However, the overall effect on efficiency and output flow is small, so a positive displacement pump will often maintain relatively stable efficiency over a wide operating range.
Adjusting Output Flow
Speed control is commonly used to change the output of a pump. Rotodynamic and PD pumps react differently to a change in speed. Rotodynamic pumps have a BEP at any given speed, and efficiency depends on how closely the operating point matches the pump curve at that speed. If the pump speed changes, the pump curve shifts and may move the operation away from the BEP, resulting in a lower pump efficiency. However, the required power varies approximately with the cube of speed under affinity-law conditions, so the total power draw will most likely be decreased when reducing the pump’s speed. The complete speed envelope and resulting pump curves need to be analyzed when varying the speed of a rotodynamic pump, particularly on the low end, to see the total impact on efficiency and power demand.
Theoretically, the output flow of a PD pump simply varies linearly with speed. Typically, reducing the speed will decrease the volumetric efficiency but increase the mechanical efficiency, and vice versa. The pump’s total efficiency may increase or decrease depending on the effect of the speed change on the volumetric and mechanical efficiencies. Both of these changes are relatively small, so the total efficiency of a PD pump should remain fairly constant. It should be noted that the power draw may also change linearly with speed if the differential pressure remains relatively constant.
The output flow of a rotodynamic pump can also be changed by artificially changing the discharge pressure. This is typically done with pressure control valves or “choking” the discharge line to move the pump on its curve. Increasing the discharge pressure will lower the output of the pump.
However, trying to control a positive displacement pump by throttling the discharge is not recommended, because it only changes the discharge pressure while not significantly changing the flow rate.
This results in wasted energy and the potential to damage the pump or system. Installing speed control for a positive displacement pump will result in lower energy usage by “tuning” the output of the pump to the requirements of the system.
Bypass (recirculation) flow regulation can also be found in positive displacement pump systems. However, this type of flow control can be less efficient, as the pump is running at full speed (and consuming full power) all of the time, even when the total output into the system is lowered.
Long-Term Operating Costs
Selecting the correct style of pump and then operating it properly is critical to minimize the total cost of ownership. Operating costs are typically the largest component of the full life cycle cost, especially for pumps that operate continuously. Energy costs are dependent not only on the best efficiency of the pump, but also on the energy consumed by the pump system (pipe size, etc.) and how much power is wasted across bypass and throttle valves.
When an application includes viscous products, varying pressures and the need to control flow rates, the type of pump and the operating philosophy should be carefully considered.
Randy Bennett is vice president of technology and operations for Leistritz Advanced Technologies Corp. For more information, visit leistritzcorp.com.
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