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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

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.

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:

In the screenshot below, I have divided the combination pump curves window into 4 sections for discussionpurposes.

Section1

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

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

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

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

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.

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.

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