Handbook of Civil Engineering Calculations

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Related Calculations. Use this procedure for any centrifugal pump handling
any liquid in any service—power, process, marine, industrial, or commercial. Pump man-
ufacturers can supply a temperature-rise curve for a given model pump if it is requested.
This curve is superimposed on the pump characteristic curve and shows the temperature
rise accompanying a specific flow through the pump.


SELECTING A CENTRIFUGAL PUMP


TO HANDLE A VISCOUS LIQUID


Select a centrifugal pump to deliver 750 gal/min (47.3 L/s) of 1000-SSU oil at a total
head of 100 ft (30.5 m). The oil has a specific gravity of 0.90 at the pumping temperature.
Show how to plot the characteristic curves when the pump is handling the viscous liquid.


Calculation Procedure:



  1. Determine the required correction factors
    A centrifugal pump handling a viscous liquid usually must develop a greater capacity and
    head, and it requires a larger power input than the same pump handling water. With the
    water performance of the pump known—from either the pump characteristic curves or a
    tabulation of pump performance parameters—Fig. 1, prepared by the Hydraulic Institute,
    can be used to find suitable correction factors. Use this chart only within its scale limits;
    do not extrapolate. Do not use the chart for mixed-flow or axial-flow pumps or for pumps
    of special design. Use the chart only for pumps handling uniform liquids; slurries, gels,
    paper stock, etc., may cause incorrect results. In using the chart, the available net positive
    suction head is assumed adequate for the pump.
    To use Fig. 1, enter at the bottom at the required capacity, 750 gal/min (47.3 L/s), and
    project vertically to intersect the 100-ft (30.5-m) head curve, the required head. From here
    project horizontally to the 1000-SSU viscosity curve, and then vertically upward to the
    correction-factor curves. Read CE = 0.635; CQ = 0.95; Cn = 0.92 for 1.Og^. The sub-
    scripts E, Q, and //refer to correction factors for efficiency, capacity, and head, respec-
    tively; and NW refers to the water capacity at a particular efficiency. At maximum effi-
    ciency, the water capacity is given as LOQwyi other efficiencies, expressed by numbers
    equal to or less than unity, give different capacities.

  2. Compute the water characteristics required
    The water capacity required for the pump Qw = QVICQ where Qv = viscous capacity,
    gal/min. For this pump, Qw = 750/0.95 = 790 gal/min (49.8 L/s). Likewise, water head Hw
    = HV/CH, where Hv = viscous head. Or, Hw = 100/0.92 = 108.8 (33.2 m), say 109 ft
    (33.2m) of water.
    Choose a pump to deliver 790 gal/min (49.8 L/s) of water at 109-ft (33.2-m) head of
    water, and the required viscous head and capacity will be obtained. Pick the pump so that
    it is operating at or near its maximum efficiency on water. If the water efficiency Ew = 81
    percent at 790 gal/min (49.8 L/s) for this pump, the efficiency when handling the viscous
    liquid Ev = EWCE. Or, Ev = 0.81(0.635) - 0.515, or 51.5 percent.
    The power input to the pump when handling viscous liquids is given by Pv =
    2^5/3960^, where s = specific gravity of the viscous liquid. For this pump, Pv = (750)
    x (100)(0.90)/[3960(0.515)] = 33.1 hp (24.7 kW).

  3. Plot the characteristic curves for viscous-liquid pumping
    Follow these eight steps to plot the complete characteristic curves of a centrifugal pump
    handling a viscous liquid when the water characteristics are known: (a) Secure a complete

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