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communities.bentley.comhttp://communities.bentley.com/products/hydraulics___hydrology/w/hydraulics_and_hydrology__wiki/pump-station-and-pump-combination-curves
Applies To Product(s): Bentley WaterGEMS V8i , Bentley WaterCAD,
Bentley SewerCAD Version(s): V8i SELECTseries 2 and higher
Environment: N/A Area: N/A Subarea: N/A Original Author: Mark
Pachlhofer, Bentley Technical Support Group
Pump Station and Pump Combination Curves
OverviewThis technote will explore the new pump station element
and pump combination curves feature to provide a
betterunderstanding of how they are used and what they can do.
Pump Station ElementThe new pump station element provides users
a way to indicate pumps which are located in the same
structure,serving the same pressure zone.
The element can be accessed by clicking on this icon in the
layout toolbar It doesn't contain any data fields that require data
input
It has polygon geometry on the drawing pane
How to layout a pump station1. Select the pump station icon from
the layout toolbar2. On the drawing pane, left click once to set
the first point for your polygon3. Move your cursor to the next
position and you should notice a line being drawn4. Repeat steps 2
and 3 until you have laid out your polygon5. When you get to the
last node and want to finish or close the polygon, right click the
mouse button and
select 'Done'.
How it works
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In order to associate pumps to the pump station element, you
must go into the properties of each pump that youwant to associate
to the station and change the field labeled "Pump Station". When
you click the field you will havethe option to 'select' the actual
pump station polygon by clicking on it or choosing the pump station
from the dropdown menu. See the red box in the screenshot
below.
In order to identify the association ofa pump with a pump
station in yourmodel, you will look for a dotted lineconnecting the
pump to the pump station.This can be seen in the screenshot
below.
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Combination Pump CurvesThis feature allows for multiple pump
curves, efficiency curves, wire-to-water efficiency curves(overall
efficiencycurve), and system head curves to all be displayed on a
single graph.
The combination pump curve feature can be accessed in two
ways.
1. Right clicking on the pump station element. After right
clicking, a context menu will come up and you canselect
'Combination Pump Curves'.
2. Go to Analysis > Combination Pump Curve. This opens the
combination pump curve manager. From hereyou would click the 'new'
icon, which looks like a white piece of paper, and then double
click the new pumpcombination curve.
Both options above result in the following window opening:
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In the screenshot below, I have divided the combination pump
curves window into 4 sections for discussionpurposes.
Section1
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As you can see in the screenshot above, thissection displays the
active pump station andshows all the pump configurations that
arepossible. If you would like to change the pumpstation you are
currently viewing, you would clickon the ellipsis button next to
the pump stationpull down and then select another pump station.
What do the columns mean?
'Active?' - If checked, this combination willdisplay in the
graph 'ID' - This number displays the index on thecurve in the
graph (e.g. Head[2] would be thecurve corresponding to the head of
the pumpcombination with ID = 2) Basically this is aunique
identifier corresponding to a specificcombination of pumps.Other -
The column(s) succeeding the 'ID'column are the labels of each
unique pumpdefinition found in the current pump
stationconfiguration. (e.g. You have a total of 6pumps in the pump
station you are observing.2 of the pumps share the same
pumpdefinition and hence the same pump definitionlabel, call it
'A'. 2 other pumps share thesame definition but it is different
from the first2, call it 'B'. The last two pumps each haveunique
definitions different from A and B andnot the same as each
other(call them C andD). After the 'ID' column you would
thereforehave 4 more columns A, B, C, and D.)The numbers in the
cells below thesecolumn(s) indicate the number ofpumps, using that
pump definition, that arerunning for that particular configuration
(row).If there is a zero in a cell this would indicatethat all
pumps with the definition are off. (e.g.Using the pump station
above contains 6pumps in total but, only four unique
pumpdefinitions you might see something like this:
Active? ID A B C D X 0 2 1 0 1X 1 2 2 0 0X 2 1 1 1 0
Section 2
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In the screenshot above, we can see on the left is where you
would select which curve(s) you wanted to display inthe graph. You
would select the ones you wanted by checking the box next to
them.
The four choices are:
1. Head Curve2. Efficiency Curve3. Wire-to-Water Efficiency
Curve (Overall Efficiency)
4. System Head Curve
NOTE: You do not need to run your model to be able to generate a
head, efficiency, or wire-to-water efficiencycurve. If you would
like to generate a system head curve however, you will need to
run/compute your model.
On the right side of this section you see the "Time (hours)"
list. This list only becomes available to use when thesystem head
curve box is checked. Here you would select the time you want to
see the system head curvedisplayed for and at least one time has to
be checked to plot a system head curve.
The first three choices for curve display are simple because all
you do is check the box next to them. The systemhead curve however,
involves some more information to be provided. When this box is
checked, the user mustspecify the 'representative pump'. This is
the path through the station that the head loss is calculated from
and theresults from the pump you select usually don't vary that
significantly from the other pumps. You can also see theoptions to
specify a 'Maximum Flow' and 'Number of Intervals'. Maximum Flow
will determine the horizontal extentof the system head curve and
the number of intervals will specify the number of points along the
curve that will becalculated.
Section 3
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This section is small, but critical to using the Combination
Pump Curves feature. It consists of the compute buttonand the chart
options button. As can be seen above, clicking the chart options
button will bring up the chartoptions settings. Here you can change
just about anything you can think of for your graph display. The
computebutton is what ties the changes that you make by selecting
to display one or multiple curves to the graph area.You will need
to click the compute button after you make any of the following
changes:
1. If you want to add/remove a pump combination using the 'Is
Active?' check box2. If you want add/remove a type of curve (i.e.
head curve, efficiency curve, system curve, wire-to-water-
efficiency curve)
3. If you want to add/remove a time from the system head
curve
Section 4
The final section, as seen in the screenshot above, is the
graphical display. The 5 main parts are the title of thegraph, the
X and Y axes, the graphing area, and the legend. Most of the
options for these parts of the display canbe changed or adjusted
using the chart options icon from section 3. The legend is
associated with the ID's foundin section 1. (i.e. Head |X|, where X
= some number, is referring to the ID given to each pump
combination curveas see in section 1)
Solving Combination Pump Curves
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Identical PumpsWhen pumps run in parallel, they all have the
same value for head (unless adjacent pipe headloss
issignificant.)
For each head where the flow is > 0, flows from each pump are
additive (e.g. A pump station with 3 pumpsthat have a head of 100
ft. Pump 1, 2, and 3 produce flows of 50 gpm, 50 gpm, and 60 gpm,
respectively.The flow from the 3 pumps will therefore be 160 gpm.
The coordinate for the point on the combined pumpcurve would be
(160, 100))
Will have 'n' number of combinations where 'n' is the number of
pumps in the pump station. (e.g. If youhave 3 pumps then you have 3
combinations)
Non-Identical PumpsThe number of combinations is based on the
number of pump curves that you have (e.g. 2 pumps with typeA
curves, 2 pumps with type B curves, 2 pumps with type C curves)
There are a total of 24 possible combinations as can be seen in
the table below.
1 A 1 B 1 C 2 A 2B
1 A + 1 B 1 B + 1 C 2 A + 1 B 2 B+ 2 C
1 A + 1 C 1 B + 2 C 2 A + 1 C 2B + 1 C
1 A + 2 B 2 A + 1 B + 1 C
1 A + 2 C 2 A + 1 B + 2 C
1 A + 1 B + 1 C 2 A + 2 B + 1 C
1 A + 2 B + 1 C 2 A + 2 B + 2 C
1 A + 1 B + 2 C
1 A + 2 B + 2 C
Why doesn't the sum of the pump flows match the intersection of
the system headcurve?When computing a model with multiple pumps in
parallel, in some situations you may notice that the sum of thepump
flows do not match the operating point of that particular
combination, in the pump combination tool. This isexpected behavior
for a pump station in which the head loss through each pump is
different.
When generating a combination pump curve, the system head curve
is based on one single representative pump.This means that the
system head curve will reflect the head necessary to overcome head
losses through theselected pump. In most cases, the head losses
through each pump in series will be similar, so the selection
ofrepresentative pump would not make a difference. Meaning, if the
headloss through each individual pump is the
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same, the system head curve will represent the system head curve
of the entire station.
However, if the head loss through one particular pump is
significantly greater than other pumps in the station, itcould have
a relatively large effect on the system head curve. Because of
this, when the pumps are all turned on,their operating points will
be different, since the one(s) with higher head losses will need to
add more head.Because of this, there really isn't a single
operating point for the pump station in this situation, but instead
aseparate operating point for each one. Therefore in these cases,
the intersection between the system head curveand combined pump
curve is not the operating point of the station. So, you would not
be able to simply add up allthe flows and compare to the operating
point. In these cases, the representative system head curve can be
viewedas an approximation.
Furthermore, if you are comparing the operating point in the
Combination Pump Curve tool to the sum of the flowsfrom multiple
pumps in the station being turned on, they may not match if there
is A) significant headloss betweenthe pumps and the common
downstream node or B) no common downstream node (each pump having
its ownparallel pipe going all the way down the system). The key is
that the flows on the system head curve in thecombination pump
curve tool are forced through the specified "representative pump",
as opposed to being evenlydispersed among those pumps. For instance
if you have five pumps running in the model, the flow may be
1000gpm X 5 = 5000 GPM, yet if you look at the combination pump
curve for those five pumps together, the systemhead curve might
intersect at something lower such as 4000 gpm. The reason is
because that entire 5000 gpm isforced through that single pump, so
if the piping just downstream is undersized, you can end up with
moreheadloss compared to the same flow dispersed among the five
pumps. So again, in these cases the system headcurve can be viewed
as an approximation. The reason why the flow is not dispersed among
all pumps is becauseyou may not always be looking at the
combination pump curve of all the pumps running. You may be looking
atonly two out of five running for example. You could also look at
multiple combination pump curves at the sametime, in which case
each combination could potentially have a different system head
curve.
Another idea to consider would be to make the internal piping
oversized for the representative pump. Try to makethe velocity in
those pipes match the single pump velocity. For example, with four
pumps that have 400mm pipes,you might increase the pipes to 800mm
for the representative pump, since 800mm has 4 times the area
of400mm. This would give the 'correct' result with 4 pumps running
and the error for fewer pumps running wouldtypically be fairly
small.
Viewing Results as a TableTo view the results as a table, you
would use the Chart Options. First click the Chart Options
button.
Next, go to the Series tab and select the data you want to view,
such as Head |0| or System Head.
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Now go to the Data tab to view the data.The X-Y data in the
table is only from theseries that is selected in the Series
tab.Each series must be viewed separately.This data can be copied
out to Excel.
A simpler approach would be to simplyexport the data to Excel.
To do this, go tothe Export tab in Chart Option, then clickthe Data
subtab. In the Series pulldownmenu, make sure that "all" is
selected. Inthe Format section, choose Excel. Thenclick Save. The
data for the head curvesand the system head curve will betranserred
to an Excel spreadsheet.
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See AlsoProduct TechNotes and FAQs
Haestad Methods Product Tech Notes And FAQs
External LinksWater and Wastewater Forum
Bentley Technical Support KnowledgeBase
Bentley LEARN Server
Pump Station and Pump Combination CurvesOverviewPump Station
ElementHow to layout a pump stationHow it works
Combination Pump CurvesSection1What do the columns mean?
Section 2Section 3Section 4
Solving Combination Pump CurvesIdentical PumpsNon-Identical
PumpsWhy doesn't the sum of the pump flows match the intersection
of the system head curve?
Viewing Results as a TableSee AlsoExternal Links