The system curve tool in MEboost can generate a fluid system curve approximation as well as up to four pump curves on the same chart. You can also generate a combined pump curve where pumps are either in parallel or series. In this article, we'll show how calculations are made and how to generate the system and pump curves in MEboost.
Before we get too far, let's define the system and pump curves along with other terms. Consider the chart below where we have a system curve approximation and a pump curve drawn.
The system curve is a plot of frictional losses (in head or pressure) for a system versus flow rate. You could generate a system curve by calculating friction losses at many flow rates and plotting the results. MEboost approximates the system curve by using one known condition of head loss and flow rate which we'll describe in more detail later.
This approximation is useful to generate quick results for initial design. As the design progresses, you should calculate a more accurate system curve.
The system curve is independent of the pump(s) used.
The static head is the difference in elevation between the source and destination reservoirs. The static head offsets the system curve. If the source reservoir is below the destination, the system curve is shifted up as shown in the example above. If the source reservoir is above the destination, the system curve is shifted down.
The pump curve, aka pump performance curve, is a plot of pump head vs. flow rate.
The operating point is where the system and pump curves intersect. This is the head and flow rate for the given pump/system combination. Changing the pump or system can change the operating point.
System Curve Approximation
Let's say we have a fluid system that's comprised of pipe, valves, and other items that impose friction head loss on the pumped fluid. If we know friction head and flow rate at one operating point, we can approximate head at other flow rates using this equation.
The system curve approximation is based on the following assumptions:
- Pressure at the source and destination reservoirs are equal.
- The fluid level of the source and destination reservoirs remain constant, i.e. constant static head.
- The velocity head is neglected.
- Friction factor remains constant. While the system curve is drawn from of flow rate of zero to a specified maximum, it may only be valid for a reasonable range around operating point 1 flow rate. A large change in flow rate will likely cause a change in friction factor.
Combined Pump Curves
When there is more than one pump, MEboost allows the creation of a combined pump curve. All pumps can be either in series or parallel.
In a series configuration, one pump feeds the next pump, and so on. The combined curve is determined by adding the head of each pump at a given flow rate.
In a parallel configuration, all pumps share a common pump inlet and outlet. The combined curve is determined by adding the flow rate of each pump at a given head. The maximum head of the combined curve is limited to the maximum head of the pump with the least maximum head.
In both configurations the pump curve data is linearly interpolated to add heads or flow rates.
Creating Pump and System Curves in MEboost
To illustrate how to create the curves in MEboost, we'll use an example. Suppose we have a system with a known operating head at the pump discharge of 100 at a flow rate of 50. We want to plot the approximated system curve along with two pump curves and their combined pump curve in parallel.
Using the pump manufacturer's performance curves, we read off head for various flow rates and enter the data in a worksheet.
For each pump, the first column must contain flow rate data. The second column must contain the head data. The two columns must be adjacent.
Now we can use MEboost to generate the chart. Click the System Curve button on the MEboost ribbon.
The System Curve form will appear. The known operating point data is entered, and the pump data ranges are selected as shown below.
Known Operating Point
As we stated earlier, we know an operating point where head is 100 and flow rate is 50. We enter these values in the appropriate boxes. For our example, there is no elevation difference between the source reservoir and the destination reservoir. Therefore the static head is zero.
Pump Curve Data
We've already put the pump curve data in a worksheet. For each pump, either type the cell range in the box, or use the minimize button next to the box and select the range. You can plot up to four pumps.
We want to show each pump curve and the combined curve on the chart, so we'll leave them checked. The pumps are in parallel, so we'll leave parallel selected.
The chart is generated in a new worksheet. The report can be in the current workbook, or in a new workbook.
We also need to specify the maximum flow rate to plot. The system curve will be plotted to this value. If a pump's flow rate exceeds this value, it will be truncated. For our example, we'll plot up to a flow rate of 70.
When everything is entered, click the Create Report button to generate the report.
For clarity, only the chart is shown below. The report also includes all data used to generate the chart.
If pump 1 was operated by itself, the operating point would be where the pump 1 curve intersects the system curve. If pump 2 was operated by itself, the operating point would be where the pump 2 curve intersects the system curve.
If both pumps are operated in parallel, the operating point would be where the combined curve intersects the system curve. Note that the maximum head of the combined curve is limited to the maximum head of pump 1. Since pump 1 has the lowest maximum head, the combined curve is limited to this value.
To understand why this is the case, remember that in a parallel configuration, each pump shares the same outlet. If the system curve was shifted to where pump 2 was generating head greater than 150, pump 1 could not overcome this head and pump fluid. This also means that all pumps in parallel must have curves that overlap when viewed from the y-axis in order to generate a combined curve.