These pumps operate by having a rotating member turn inside a housing in such a way as to create trapped liquid through the pump. Figure 10-10 shows several configurations of rotary pumps. Although these pumps may look like centrifugal pumps, their action is that of a positive displacement pump in that the liquid is continually compressed to a high pressure without first being given a high kinetic energy.
Rotary pumps have the same characteristics as reciprocating pumps, except that at low speed leakage between the cavities increases. At very low speeds the reduction in efficiency can be very significant. When compared to reciprocating pumps, rotary pumps require less space, and deliver relatively pulsation-free flow. Their main advantage is that unlike reciprocating and centrifugal pumps, their construction subjects the pumped fluid to a minimum amount of shear or turbulence. Thus, they tend to be used in process applications where one of the other pump types could be expected to shear and disperse one liquid into another making subsequent treating more difficult.
Their disadvantages are that they have close clearances that require that the liquids being pumped have a lubricating value, be non-corrosive, and contain few solids. Therefore, they tend to be limited to relatively solids-free oil or emulsion streams.
In addition to the standard rotary pumps shown in Figure 10-10, another type of positive displacement rotary pump exists in which the pump, motor, and pumped liquid are completely contained within a closed vessel (Figure 10-11). This type of pump is known as a “canned” rotary pump since the motor/pump package is contained within the closed vessel, or “can.” This type of pump is essential for pumping toxic liquids, radioactive waste water, and other liquids that pose serious risks if a shaft seal failure occurs. Although canned centrifugal pumps have existed for many years,they were inefficient and—in some cases—not able to reasonably handle more viscous fluids. Today’s canned rotary pumps are able to accommodate highly viscous fluids. As for pricing, the current canned motor/pump package compares well with conventional shaft-sealed pumps that are flexibly coupled to standard industrial three-phase AC motors.