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Monday, April 19, 2010

Centrifugal Pump Performance

Volume Flow Rate

Abbreviated to “flow rate” and known traditionally as “pump capacity” Q, this is the volume of liquid per unit time delivered by the pump. To US customary system unit, Q is expressed in US gallon per minute or USgpm, for which the abbreviation of gpm is used (1 US gallon = 231 in3). For very large pumps, the unit ft3/sec are used. The consistent SI unit m3/s are implied when an SI value of Q is, unless the numerically convenient liters per second (l/s) are specifically called out.


Datum for Pump Head

The total head has components of pressure, velocity, and elevation z. because pump head H more precisely ΔH is the difference of total heads evaluated at the discharge flange d and the suction flange s respectively, the elevation of the datum from which Z is measured cancel out. However, for purpose of identification, computing NPSH, and so on, the standard datum as shown is used.

The standard datum for horizontal shaft pumps is a horizontal plane through the centreline of the shaft. For vertical shaft pumps, the datum is a horizontal plane through the entrance eye of the first stage impeller. Because pump head is the difference between the discharge and suction heads, it is not necessary that the standard datum be used, and any convenient datum may be selected for computing the pump head.

Power

In USCS, the pump output is customary given as liquid horsepower (hp) or as water horsepower if water is the liquid pumped.

lhp = QH (sp.gr.)/ 3960

where Q is the gallon per minute, H in feet, and sp.gr. is specific grafity. If Q is in cubic feet per second, the equation becomes,

lhp = QH (sp.gr.)/8.82

In SI, the power P in watts (W) is given by

P = 9797 QH (sp.gr.)

Where Q in cubic meters per second and H is in meters. Where Q is in liters per second and H is in meters.

P = 9.797 QH (sp.gr.)

Efficiency

The pump efficiency, ή is the liquid horsepower divided by the power input to the pump shaft. The latter usually is called the brake horsepower (bhp). The efficiency may be expressed as a decimal or multiplied by 100 and expressed as percent. In this subsection, the efficiency will always be the decimal value unless otherwise noted. Some pump driver units are so constructed that the actual power input to the pump is difficult or impossible to obtain. Typically of these is the “canned” pump for volatile or dangerous liquids. In such case, only an overall efficiency can be obtained. If the driver is an electric motor, this is called the wire to liquid efficiency or, when water is the liquid pumped, the wire to water efficiency.

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