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hapter IBENEFITS OF PROGRESSINGCA V ITY PU MPS

From the standpoint of flow and pressure range, PC pumps com-pete well with any other type of pum p design. T hey can handlepum p flows from fractions of gallons per minute to several thou -sand gallons per minute. Pressure capability depends on the num -ber of stages (leads of the stator), and typically reaches 800-1,000psi.T h e range of fluids they can handle is eno rm ous , with vis-cosities ranging from waterlike (1 cSt), to fluidlike (clay, cem ent,and sludge with viscosities up to 1 000 000cSt).

Since the rotor and stator have an interference fit (a platedmetallic rotor in an elastomer-lined stator), and low rotatingspeed, the internal shear rates are very low. W hen applied in th efood industry, these pumps are sometimes known to pum p cher-ries or apples, which move through the internal passages with nodamage. The pulsation-free flow and quiet operation of PCpumps are additional advantages for shear-sensitive pumpages.

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PC pumps are excellent self-primers and have good suc-tion characteristics. They are tolerant to entrained air andgases,and produce minimal churning or foaming.

Another major feature of these machines is their high tol-erance to con tam ination and abrasion. Often called a last-resource pu m p, P C pum ps are often utilized for extrem elyabrasive applications, because of a unique p rope rty of the elas-tomer that lines up the stator tube and resists abrasion. Theseelastomers are made from regular rubber (Buna) or moreexotic m aterials, such as Viton, Teflon, and others.

As versatile as they are, PC pum ps do have limitations, pr i-marily size. To prevent flow slip (leakage from hig he r dis-charge pressure back to suction), the n um ber of roto r/sta torleads (stages) must be increased as pressure is increased. T his ,in tu rn , increases the overall leng th of the un it. For such h igh -pressure applications, it is som etimes difficult to retrofit anexisting installation, where sm aller pum ps (such as centrifu-gal) have been operating in the past. However, when space isnot an issue, this limitation is not a factor.

Another reason for PC pumps' larger size is their lowspeed, which requires a gear redu cer (or a belt drive) betweenthe motor driver and the pump. This can result in added cost.However, the recent advent of variable frequency drives(VFDs) has allowed elimination of gear reducers and, at thesame time, has introdu ced a new capability to vary flow whilepum ping against a given pressure.

Another P C pum p limitation is fluid compatibility with theelastomer. Some chemicals may cause problem s with the elas-

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tomer and others may cause swelling. For corrosive applica-tions,Viton or even Teflon stators are selected. Elastom ersalso impose certain temperature limitations, as compared tofully m etallic pumps. Typically, PC pum ps are used where tem -peratures do not exceed 3OO-35OE PC pumps should not rundry, except for a very short tim e, because heat generated at th einterference between the rotor and stator m ay cause elastomerfailures, often called chunking or debonding.