Pump Station and Pump Combination Curves

  Applies To 
  Product(s): WaterGEMS , WaterCAD, SewerCAD, HAMMER
  Version(s): V8i SELECTseries 2 and higher
  Area:  Modeling
  Original Author: Mark Pachlhofer, Bentley Technical Support Group

Overview

This TechNote will explore the new pump station element and pump combination curves feature to provide a better understanding of how they are used and what they can do.

Pump Station Element

The 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 station

  1. Select the pump station icon from the layout toolbar
  2. On the drawing pane, left click once to set the first point for your polygon
  3. Move your cursor to the next position and you should notice a line being drawn
  4. Repeat steps 2 and 3 until you have laid out your polygon
  5. 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 you want to associate to the station and change the field labeled "Pump Station". When you click the field you will have the option to 'select' the actual pump station polygon by clicking on it or choosing the pump station from the drop down menu. See the red box in the screenshot below.  

  

 In order to identify the association of a pump with a pump station in your model, you will look for a dotted line connecting the pump to the pump station. This can be seen in the screenshot below. 

 

Combination Pump Curves

This feature allows for multiple pump curves, efficiency curves, wire-to-water efficiency curves(overall efficiency curve), 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 can select 'Combination Pump Curves'. 
  2. Go to Analysis > Combination Pump Curve. This opens the combination pump curve manager. From here you would click the 'new' icon, which looks like a white piece of paper, and then double click the new pump combination 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 discussion purposes.

 Section1

As you can see in the screenshot above, this section displays the active pump station and shows all the pump configurations that are possible. If you would like to change the pump station you are currently viewing, you would click on the ellipsis button next to the pump station pull down and then select another pump station.

What do the columns mean?

'Active?' - If checked, this combination will display in the graph 
'ID' - This number displays the index on the curve in the graph (e.g. Head[2] would be the curve corresponding to the head of the pump combination with ID = 2) Basically this is a unique identifier corresponding to a specific combination of pumps.

Other - The column(s) succeeding  the 'ID' column are the labels of each unique pump definition found in the current pump station configuration. (e.g. You have a total of 6 pumps in the pump station you are observing. 2 of the pumps share the same pump definition and hence the same pump definition label, call it 'A'.  2 other pumps share the same definition but it is different from the first 2, call it 'B'. The last two pumps each have unique definitions different from A and B and not the same as each other(call them C and D). After the 'ID' column you would therefore have 4 more columns A, B, C, and D.)

The numbers in the cells below these column(s) indicate the number of pumps, using that pump definition, that are running for that particular configuration (row). If there is a zero in a cell this would indicate that all pumps with the definition are off. (e.g. Using the pump station above contains 6 pumps in total but, only four unique pump definitions you might see something like this:

Active? ID A B C D
 X 0 2 1 0 1
X 1 2 2 0 0
X 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 in the graph. You would select the ones you wanted by checking the box next to them.

The four choices are:

  1. Head Curve
  2. Efficiency Curve
  3. 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 efficiency curve. 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 the system head curve box is checked. Here you would select the time you want to see the system head curve displayed 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 system head curve however, involves some more information to be provided. When this box is checked, the user must specify the 'representative pump'. This is the path through the station that the head loss is calculated from and the results from the pump you select usually don't vary that significantly from the other pumps. You can also see the options to specify a 'Maximum Flow' and 'Number of Intervals'.  Maximum Flow will determine the horizontal extent of the system head curve and the number of intervals will specify the number of points along the curve that will be calculated.

Section 3

This section is small, but critical to using the Combination Pump Curves feature. It consists of the compute button and the chart options button. As can be seen above, clicking the chart options button will bring up the chart options settings. Here you can change just about anything you can think of for your graph display. The compute button 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 box
  2. 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 the graph, the X and Y axes, the graphing area, and the legend. Most of the options for these parts of the display can be changed or adjusted using the chart options icon from section 3. The legend is associated with the ID's found in section 1. (i.e. Head |X|, where  X = some number, is referring to the ID given to each pump combination curve as see in section 1)

 

Solving Combination Pump Curves

 Identical Pumps

  • When pumps run in parallel, they all have the same value for head (unless adjacent pipe headloss is significant.)

  • For each head where the flow is > 0, flows from each pump are additive (e.g. A pump station with 3 pumps that 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 pump curve would be (160, 100))

  • Will have 'n' number of combinations where 'n' is the number of pumps in the pump station. (e.g. If you have 3 pumps then you have 3 combinations)

 Non-Identical Pumps

  • The number of combinations is based on the number of pump curves that you have (e.g. 2 pumps with type A 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 head curve?

When computing a model with multiple pumps in parallel, in some situations you may notice that the sum of the pump flows do not match the operating point of that particular combination, in the pump combination tool. This is expected 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 independent from the pumps as you would expect, but the flows plotted on the system head curve need to be routed through a particular path through the pump station. This is why you select a "representative pump", as this is the pump that the system head curve flows are routed through during the system head curve calculation. This means that the system head curve will reflect the head necessary to overcome head losses through the selected pump. In most cases, the head losses through each pump in series will be similar and will be relatively insignificant, so the selection of representative pump would not make a difference. Meaning, if the headloss through each individual pump is the same and close to zero, 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, it could 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 a separate operating point for each one. Therefore in these cases, the intersection between the system head curve and combined pump curve is not the operating point of the station. So, you would not be able to simply add up all the flows and compare to the operating point. In these cases, the representative system head curve can be viewed as an approximation.

Furthermore, if you are comparing the operating point in the Combination Pump Curve tool to the sum of the flows from multiple pumps in the station being turned on, they may not match if there is A) significant headloss between the pumps and the common downstream node or B) no common downstream node (each pump having its own parallel pipe going all the way down the system). The key is that the flows on the system head curve in the combination pump curve tool are forced through the specified "representative pump", as opposed to being evenly dispersed among those pumps. For instance if you have five pumps running in the model, the flow may be 1000 gpm X 5 = 5000 GPM, yet if you look at the combination pump curve for those five pumps together, the system head curve might intersect at something lower such as 4000 gpm. The reason is because that entire 5000 gpm is forced through that single pump, so if the piping just downstream is undersized, you can end up with more headloss compared to the same flow dispersed among the five pumps. So again, in these cases the system head curve can be viewed as an approximation. The reason why the flow is not dispersed among all pumps is because you may not always be looking at the combination pump curve of all the pumps running. You may be looking at only two out of five running for example. You could also look at multiple combination pump curves at the same time, in which case each combination could potentially have a different system head curve.

Note that the system head curve is a property of each individual pump. Calculating the system head curve involves making some assumptions. The head calculations are based on the assumption that all of the flow through the pump station passes through the representative pump which is not accurate if there is a great deal of head loss in the pump station piping. The inaccuracy in this calculation becomes significant when the head loss within the station becomes significant in comparison to the head loss in the distribution systems/pipeline/force main. This is a valid assumption in most cases.

When the head loss in the station piping is large, the user can generate a system head curve for the station, by setting up scenarios for 1, 2, 3…n pumps running; run each scenario; record the flow and pump head value for each combination and connect those points to form a system head curve.

Another idea to consider would be to make the internal piping oversized for the representative pump. Try to make the 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 of 400mm (note that this would be a very rough estimate though, as the capacity of a pipe is not solely based on the area) This would give a more 'correct' result with 4 pumps running and the error for fewer pumps running would typically be fairly small. To more precisely determine the equivalent diameter, you can set up a small example model in WaterCAD or WaterGEMS (for example of minor losses are at play), or use a pressure pipe worksheet in FlowMaster if there are no significant minor losses. Modifying the diameters of the pipes adjacent to the Representative Pump will skew the pump operating point when computing the actual model though, so you'll need to switch the diameters back to the way they were, or set up a separate scenario (with child physical alternative) with the modified diameters, just for the purpose of system head curve calculation.

Viewing Results as a Table

To 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 the series 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 simply export the data to Excel. To do this, go to the Export tab in Chart Option, then click the Data subtab. In the Series pulldown menu, make sure that "all" is selected. In the Format section, choose Excel. Then click Save. The data for the head curves and the system head curve will be transerred to an Excel spreadsheet.

See Also

Understanding System Head Curves in WaterGEMS, WaterCAD, and SewerCAD

System head curve changing depending on status of other pumps

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