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Handbook of Turbomachinery Second Edition Revised - Ventech!

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een employed. This device as employed in a typical centrifugal pump stage<br />

is shown in Fig. 27. The term ‘‘series flow’’ is derived simply from the fact<br />

that the flow required to operate the balance piston is tapped <strong>of</strong>f from the<br />

pump discharge, in effect being in series with the pump discharge. The pump<br />

flow passes first through an annular restriction known as a high-pressure<br />

orifice. As shown in the figure, the flow area <strong>of</strong> this restriction varies with the<br />

axial position <strong>of</strong> the rotor. The flow, which is now at an intermediate<br />

pressure, moves both tangentially and radially inward toward the rotor axis.<br />

The pressure distribution acting over the impeller back face produces an<br />

axial direction force, which, with proper design, acts equally and opposite to<br />

the unbalanced rotor thrust. At a radius close to the rotor axis, the flow<br />

passes through another annular restriction known as a low-pressure orifice<br />

whose area varies inversely with that <strong>of</strong> the high-pressure orifice. The area<br />

downstream <strong>of</strong> the low-pressure orifice is vented to some area <strong>of</strong> lower<br />

pressure, such as the pump inlet or a bearing coolant flow circuit. The end<br />

product <strong>of</strong> a properly designed balance piston is an axial force over the<br />

impeller rear face that varies with rotor position and tends to counter<br />

changes in rotor thrust such that axial equilibrium is maintained. Two<br />

points should be noted here. First, the axial force capability <strong>of</strong> the balance<br />

piston is fixed by such factors as the pump discharge pressure, the<br />

resistances <strong>of</strong> the high- and low-pressure orifices, and the radius at which the<br />

low-pressure orifice is placed. When the balance piston is designed, these<br />

quantities are set (if possible) so as to provide a substantial margin between<br />

the maximum anticipated unbalanced rotor thrust and the force capability<br />

<strong>of</strong> the balance piston. Also, it should be understood that the balance piston<br />

Figure 27 Centrifugal pump stage using a series flow balance piston for control <strong>of</strong><br />

axial thrust.<br />

Copyright © 2003 Marcel Dekker, Inc.

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