2

FIGURE 11 Viscous fluid effects on centrifugal pumps—adapted from Hydraulic Institute ANSI/HI 2000 Edition Pump Standards, Reference 7.

activity within the pump. (At the critical point, the liquid and gas phases are identical, and therefore both have the same specific volume.) An example is the pumping of liquid hydrogen, for which an inducer is unnecessary until much higher values of Q,ss are reached. Moreover, inducers—typically limited to Hss-values of about 10 (Nss,(USJ = 27,000) in water—can, at sufficiently low tip speed, operate at zero NPSH, which corresponds to an infinite value of ß 8.

Pumping Entrained Gas In addition to the liquid's own vapor (which is the gas involved in the NPSH-effects discussion), many pumping applications deal with a different gas; that is, a different substance from the liquid being pumped. The effects of this gas on performance arise from a) the volume flow rate of the gas at the inlet, b) the pressure ratio of the pump, which determines how far into the impeller this gas volume persists; that is, how much it gets compressed, and c) how much of the gas dissolves in the liquid as the pressure increases within the pump, which depends on both the solubility and the degree of agitation of the fluid produced by the pump. The set of fluid properties associated with these gas-handling phenomena are represented by {gp} in Eq. 34, the dimensionless form (Eqs. 35 and 43) of this set being {rp}. Generally, for typical commercial centrifugal pumps, the performance under such conditions usually manifests itself as a loss of pressure rise, which is reasonably stable up to an inlet volume flow rate fraction of gas to liquid of 0.04 to 0.079. Inducers can handle larger inlet volume fractions of gas, and, under Dalton's law for partial pressures, the liquid's own vapor also occupies the volume of the gas bubbles. Single and multistage centrifugal pumps have been built that handle far greater gas volume than these single-stage values10,11; moreover, multiphase rotary positive displacement screw pumps can handle gas volume fractions up to 1 (100 percent gas)11.

Effects of Slurries and Emulsions Finally there is the influence of the dimensionless quantities { } in Eq. 43. Impeller and casing design are altered so as to reduce wear-producing velocities if the pumpage is a slurry of solids contained in a carrier liquid. (Slurry pumps are usually single-stage machines with a collector or volute casing sur rounding the impeller.) This usually means a smaller impeller eye diameter (which, as can be seen in Figure 6, reduces the inlet relative velocity W^) and a larger radial distance from the impeller to the surrounding volute because the circumferential velocity component Vs of the fluid emerging from the impeller (also seen in Figure 6) slows down with increasing radial position and is then lower in the volute passageway12. Performance also is altered, depending on the composition and concentration of the slurry. These are complicated non-Newtonian flows and are covered in detail elsewhere in this book in conjunction with a thorough treatment on solids-handling pumps. Emulsions are another example of such flows, many of which are destroyed by excessive local shear in the fluid. For this reason, screw pumps are sometimes utilized for emulsions rather than oversized, slow-running centrifugal pumps. Except for thin layers of the fluid at the clearances, most of the flow in a screw pump experiences very little shear in comparison to the flow through a centrifugal pump.

Electromagnetic Effects Not appearing in Eq. 43 are quantities associated with electromagnetic phenomena. For example, electric current flowing radially outward through fluid contained in an axially directed magnetic field is capable of producing a rotating flow. Called a hydromagnetic pump, this device is therefore "centrifugal," yet it has no moving parts. Such pumps have been used for liquid metals and could be made reasonably efficient for any pumpage with high conductivity.

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Survival Treasure

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