Centrifugal pumps are commonly used for most liquid pumping services. For viscous liquids, positive displacement pumps often perform better. However, many engineers specify positive displacement pumps for services where centrifugal pumps would be more effective. Some engineers believe the technology offers better flexibility or more operational advantages without considering the specific application. The truth is that variable-speed centrifugal pumps can effectively handle many medium-viscosity liquids—even those with suspended solids or other contaminants.
Centrifugal pumps can safely handle liquids with between 7 and 15 percent contamination if special design measures are implemented, such as corrections to the pump curves. In terms of reliability, pump curves deserve more attention no matter the nature of the application. During the selection process, plotting an application's operating points can mean the difference between saving and losing money.
Best Efficiency Point
The best efficiency point (BEP) is the most stable operating condition for a pump. If a pump operates away from the BEP, the resulting hydrodynamic unbalanced load increases. The load usually peaks at shutoff, at which point long-term operation can reduce pump component life and reliability.
Pump design usually determines the best operating range, but pumps should generally operate within 80 percent to 109 percent of the BEP. This range is more ideal than practical, and most operators should decide on an optimized operating range before selecting a pump.
The net positive suction head required (NPSHR) often restricts a pump's operating range with regard to its BEP. At flows significantly higher than the BEP, a significant pressure drop within the suction passages and piping will dip below the NPSHR level. This pressure drop can result in cavitation and damage to the pump.
As pump components wear and degrade, new clearances are opened. The pumped liquid begins to recirculate more often compared with new pumps. Recirculation can have a harmful impact on the pump's efficiency.
Operators should examine pump curves with respect to whole system operation. For example, pumps operating in a closed-loop or recycling service should operate close to BEP or about 5 to 10 percent to the left of the BEP. Based on my experience, closed-loop systems have less attention paid to their pump performance curves. In fact, some operators fail to check alternative operating points or the recycling flow ranges on the pump curve. Recycle service flow can vary widely, which is why operators must locate and evaluate all possible operating points on the pump curve.
Extreme Operating Points
In batch transfer services, pumps move liquid from vessels or tanks with varying liquid level at the suction and discharge. The pumps fill the vessel or tank at the discharge and empty liquid at the suction. Some batch transfer services require control valves, which can significantly change the differential pressure.
The pump head constantly changes, but the rate of change could be high or low.
Batch transfer services have two extreme operating points, one for the highest head and another for the lowest head. Some operators mistakenly match a pump's BEP with the operating point at the highest head, forgetting other head requirements.
The selected pump will operate to the right of the BEP, providing unreliable and inefficient performance. In addition, the selected pump is bigger than what is actually required because the pump train is sized so the operating point with the highest head is near the BEP.
At the lowest head operation point, the wrong pump selection will result in more power consumption, lower efficiency, more vibration, shorter seal and bearing life and less reliability. All these factors contribute to significantly higher initial and operational costs, including more frequent unscheduled shutdowns.
Find the Mid-Point
Optimal pump selection for batch transfer services depends on locating operating points at the highest head to the left of the BEP and at the lowest head to the right of BEP. The resulting pump curve should include operating points that account for several additional factors, including NPSHR. The pump should operate near the BEP most of the time—roughly, the mid-point between the highest and lowest head.
Generally, all operating points should be identified and the pump operation should be evaluated for all possible operating points.
An important consideration would be the pump operation and an estimation of the pump operating point on the pump curve, when the pump is slightly degraded. For some pump applications, such as batch transfer services with great differences between the highest and the lowest head points, variable-speed centrifugal pumps should be used.
Read the second part of this article here.