| Applies To | |||
| Product(s): | Bentley WaterGEMS V8i | ||
| Version(s): | V8i SELECTseries 2 | ||
| Environment: | N/A | ||
| Area: | N/A | ||
| Subarea: | N/A | ||
| 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
- Select the pump station icon from the layout toolbar
- On the drawing pane, left click once to set the first point for your polygon
- Move your cursor to the next position and you should notice a line being drawn
- Repeat steps 2 and 3 until you have laid out your polygon
- 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.
- Right clicking on the pump station element. After right clicking, a context menu will come up and you can select 'Combination Pump Curves'.
- 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:
- Head Curve
- Efficiency Curve
- Wire-to-Water Efficiency Curve (Overall Efficiency)
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:
- If you want to add/remove a pump combination using the 'Is Active?' check box
- If you want add/remove a type of curve (i.e. head curve, efficiency curve, system curve, wire-to-water-efficiency curve)
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 based on one single representative pump. 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, so the selection of representative 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, 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 3000 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.
See Also
Haestad Methods Product Tech Notes And FAQs