A Comparison Of Positive Displacement And Centrifugal Pump Applications
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Positive displacement and centrifugal pumps sometimes seem to inhabit different worlds. Users are generally much more familiar with one type than the other. This causes some confusion when the user’s expectations are based on experience with one type of the reality of the pump’s operation is based on the other type. Contrasted, herein, will be the differences in operation of the two types of pumps through real world examples where the characteristics of each were used to advantage. Centrifugal and positive displacement pumps operate on completely different principals. With centrifugal pumps, flow results from a pressure differential created by the pump. For a positive displacement pump, pressure differential results from flow created by the pump. This is analogous to moving a truckload of tennis balls from the street to the second story of a warehouse. There are two ways to accomplish this. One could stand in the street and throw the balls through an open second story window. This is the centrifugal pump case. One could also put a few balls into a box and carry the box up the stairs, dump out the balls and bring the box back from another load. This is the positive displacement pump case. Continue the analogy by noting that some balls thrown from the street miss the window and bounce off the wall, like recirculation in the centrifugal pump. Similarly, some balls fall out of the box and roll back down the stairs, like slip in a positive displacement pump. Imagine that as the height of the window increased, more balls will miss and bounce back. Eventually, the window will be too high to throw any balls into, and the “shutoff” point will have been reached. At the same time, the number of ball carried up the stairs will remain relatively constant as the height of the window increases. A few more balls may roll out of the box on the taller stairs, but most of the ball will still reach the top. Five examples of pump applications are examined to illustrate how these different characteristics may be used to good effect. First, a positive displacement pump solved problems for an aluminum can manufacturer because the insensitivity of flow rate to change in pressure (Figure 1, A and B) improved the uniformity of a coating applied to the inside of the cans. Second, a centrifugal pump was favored to boost the suction pressure of a high pressure fuel pump on a gas turbine, because the variable flow characteristics of the centrifugal pump matched the variable demand (Figure1, A and C). Third, positive displacement pumps were preferred for unloading tanker trucks because of their self-priming characteristics and relative insensitivity to large changes in the viscosity or specific gravity of the liquid being pumped. Next, a centrifugal pump was the best choice for a large pump to circulate cooling water, and finally, a centrifugal pump was also preferred for use as a boiler feedpump.
Purcell, John E.; Silvaggio, Joseph A. (1997). A Comparison Of Positive Displacement And Centrifugal Pump Applications. Texas A&M University. Turbomachinery Laboratories. Available electronically from