HydroCAD^® Stormwater Modeling - Since 1986

Modeling Pumps with HydroCAD

HydroCAD can perform complete pump modeling and hydrograph routing, including the use of separate on/off elevations, pipe losses, and even tailwater effects.

Just select "pump" from the list of outlet devices and enter the appropriate parameters, as shown in the screen at the right.

For further details, including a discussion of tailwater interactions, click the help button on the pump screen.

If you're using an older version of HydroCAD without the pump outlet option, see additional notes below.

Pump Modeling Tips

1. In order to model pump switching and respond to tailwater effects, be sure to use the Dynamic Storage-Indication method (DSI) for pond routing.
    
2. Make sure the off elevation is above the bottom of the pond storage definition, otherwise the pump may not turn off completely.
    
3. To avoid short-cycling, make sure you provide an adequate distance (and storage) between the on and off elevations.  If the values are too close (or the same) the pump will cycle on and off rapidly.
    
4. For a better view of the pump operation and switching, right-click the pond hydrograph and select the elevation curve.
    
5. You can have multiple pumps in a single pond.  If the pumps use the same device routing (i.e. primary) the flows will be combined and reported together.  In some cases you may prefer to use separate routing (primary, secondary, etc.) so that you can view the operation of each pump separately.
    
6. To model multiple pumps discharging into a shared manifold, model the manifold as a separate zero-storage pond with a culvert outlet.
    
7. Make sure you don't exceed the flow or head limits of each particular pump.  HydroCAD will issue a warning message if the pump is operating beyond the range of the specified rating curve.

Modeling pumps in older versions

For HydroCAD-8.5 and earlier, a special (user-defined) outlet can be used to simulate some aspects of a pump.  For more complete modeling with less effort, HydroCAD-9.0 or later is strongly recommended when pumps must be included in the model.

A special outlet can be used to model specialized situations where one of the standard outlet devices (weir, orifice, culvert, etc.) may not be appropriate.  Rather than using the built-in discharge equations, a special outlet allows the user to define an arbitrary rating curve of discharge vs. head.

In the case of a pump, we need to define a stage discharge-relationship that produces a sudden increase in flow when the pump turns on.  To configure a special outlet device for this purpose:

1) Edit the desired pond, select the Outlets tab, and create a "Special" outlet.  (This may be the only outlet, or may be combined with an overflow weir, culvert, or any other arrangement of outlets.)

2) On the Special outlet screen, set the "Invert Elevation" to the elevation where you want the pump to turn on.

3) Enter the following values in the head/discharge table:

  Line  Head    Q
   1    0.00  0.000
   2    0.01  x.xxx  <- Enter pump rate here
   3    1.00  x.xxx  <- and here as well

Line #1, in conjunction with the invert elevation, establishes the level at which the pump starts to turn on.  When the water reaches line #2 (1/100 of a foot higher) the pump will be fully "on".  Line #3 is used to prevent extrapolation of the discharge at higher heads, or you can adjust this line if you want to simulate higher pump rates as the pond level increases.

Note: If you're using HydroCAD 7.1 or later, you can omit line #3 and deselect the "Extrapolate" option instead.

After creating the special outlet, OK the pond description, open a report window, and examine the stage-discharge plot.  If you've set up the device correctly, the plot will show no discharge up to the invert, and then a sudden jump to the full pump rate.  The rate should then remain constant as the level increases.

Note: This procedure simulates a fixed on/off point without hysteresis. The full pump rate will be achieved only if the pond level exceeds the turn-on point, which can occur only if the inflow rate exceeds the pump capacity.  If the inflow is less than the pump capacity, the discharge will be reduced accordingly.  For complete pump modeling with separate turn on/off points (hysteresis), use the new pump capability added in HydroCAD-9.

4) If applicable, you can also model an increasing pump rate as the pond fills above the turn-on point.  This is done by adding additional lines (4, 5, 6, etc) to the table shown above.  When entering this data, remember that the pond head is a measure of the pond water level above the invert (turn-on point), while the pumped head is measured from the water surface to the pump discharge:

    Pond Head + Pumped Head = Discharge head (above invert)

When determining discharge from the pump rating table, always use the pumped head given by:

    Pumped Head = Discharge head - Pond Head

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