Modeling Pumps with HydroCAD
can perform complete pump modeling and hydrograph routing,
including the use of separate on/off elevations, pipe losses, tailwater
effects, and even multiple pumps.
Just select "pump" from the list of outlet devices and enter the appropriate
parameters. For further details, including a discussion of tailwater interactions, click
the Help button on the pump screen. Also see the "Pump Examples"
project that is preinstalled with HydroCAD, and the pump
modeling tips near the bottom of this page.
Note: Pump modeling was first available in HydroCAD 9.0, with further enhancements
in HydroCAD 10.0 build 24. Earlier versions of HydroCAD cannot model the
pump switching hysteresis and should not be used to model pumps.
The following reports were generated from the "Pump Examples" project that is
preinstalled with HydroCAD.
1P - Preset WSE with no inflow
hydrograph demonstrates a simple draw-down scenario in which the pond has no
inflow hydrograph, but the starting water surface elevation has been preset
to 105', as shown on the (black) elevation line.
Since this satisfies the
pumps "on" elevation, the pump starts running immediately at time zero.
As the pump operates, the WSE in the pond decreases, increasing the pump
head, which causes the flow rate to steadily decrease. Finally, the
WSE gets to the "off" elevation and the pump turns off, causing the outflow
- Constant inflow
example the pond starts empty, but there is a constant inflow of 0.4 CFS.
The WSE increases steadily until the pump turns on at 105'. With the
pump running the WSE starts to drop until it reaches the "off" elevation at
102.5'. The pond then begins to fill again and the cycle repeats as
long as the inflow in sustained.
Pond 3P -
Typical inflow hydrograph
example we have a typical runoff hydrograph flowing into the pond. The
inflow causes the WSE to increase until it reaches the pump's "on" elevation
of 105' at 15 hours.
Since the inflow initially exceeds the pumps
capacity, the WSE continues to rise until an equilibrium condition is
reached at 18.88 hours. At this time the inflow has fallen below the
pump capacity so the WSE starts to decrease, until it reaches the "off"
elevation at 102.7' and the WSE stabilizes.
4P - Pump cutoff at overflow weir
example demonstrates the use of the high-level cutoff feature added in
HydroCAD 10.0 build 24. Once again, the pond is being supplied with a
typical runoff hydrograph. The WSE increases until it reaches the "on"
elevation of 105', but the WSE continues to rise until it reaches the
maximum elevation of 107.1', at which time the pump shuts off. This
initial pump cycle is shown by the blue (primary) outflow between
approximately 14 and 15 hours.
As the pump is reaching it's maximum cutoff
of 107.1' an overflow weir had begun to handle the higher outflows, as
depicted by the secondary (red) outflow curve. Water continues to flow
over the weir until the WSE drops below the pump maximum of 107.1', and the
pump resumes operation for a final cycle between 32 and 44 hours.
- Two pumps in a "Priority Group"
This pond is configured
with two pumps in a priority group. The lower pump starts to operate
at 15 hours when the WSE reaches it's on elevation of 106', as shown by the
primary (blue) outflow.
The WSE continues to rise until the second pump
turns on at 109' (secondary red outflow) and the lower pump is
automatically turned off.
The higher pump continues to run until the
WSE drops below it's Off elevation of 105'. At this point neither pump
is running and the WSE begins to climb again, until it reaches the on
elevation of 106' for the lower pump which runs again from 22 to 37 hours.
Pump Modeling Tips
In order to model pump switching and respond to tailwater effects, be sure
to use the Dynamic Storage-Indication method (DSI)
for pond routing.
- Make sure the Off Elevation is above the bottom of the pond storage
definition, otherwise the pump may not turn off completely.
- 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.
- For a better view of the pump operation and switching, right-click the
pond hydrograph and select the elevation curve.
- 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.
- To model multiple pumps discharging into a shared manifold, model the
manifold as a separate zero-storage pond with a tube or culvert outlet.
- If you only want to have a single pump operating at any time, select
Priority Group for all the applicable pumps. This will give priority
to the active pump with the highest On Elevation and all lower pumps
will be turned off.
- A pump can also be configured to turn off at a specified Maximum
Elevation. This feature can be used if you prefer to have high flows
handled by another outlet device, such as an overflow weir. Otherwise
this field must be left blank.
- 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.