PIPE-FLO® uses individual data points when modeling the performance of centrifugal pumps instead of developing a mathematical expression to simulate the operation. This approach uses actual manufacturer supplied catalog pump curves for head, efficiency, and NPSHr for a range of flow rates and provides an extremely accurate model of the pump performance[i]. In this article, we will see how the pump performance data is effectively extracted from the catalog curve for insertion into the PIPE-FLO® model. The pump performance data is being read from a paper[ii] pump curve and will provide insight as to how to get an accurate model using a manufacturer’s supplied catalog pump curve.
Figure 1 shows an example pump from the Olympia Pump Company, an End Suction Pump (ESP), with a size of 4×3-7, running at 3550 RPM. From the supplied catalog curve, we can see the pump is available in diameters from 7.1875 inches to 5 inches. In our example, the selected pump has a 7-inch diameter impeller.
Figure 1 – Example of manufacturer’s catalog pump curve showing performance data for a range of flow rates. The highlighted data is used in the pump information to describe the pump curve. The pump’s landmarks are indicated in black.
Catalog Pump Curve Data
The pump catalog data consists of pump information and performance data points. In this instance we are using paper so the process is manual. Manually enter the Pump Data into the dialog box (s). The completed data extracted from this paper curve can be found in Figure 2.
- The pump rotation speed is required data information and is often found in the title block area of the manufacturer’s curve (Figure 1). In this example, the test speed is 3550 rpm.
- The impeller diameter is required data and is taken from the manufacturer’s pump curve. In this example, the manufacturer specified a 7-inch diameter impeller.
The additional data fields are used to provide more pump details.
- The manufacturer in this example is Olympia Pump.
- The manufacturer’s pump size is a 4×3-7.
- The maximum speed is entered if the pump is connected to a Variable Speed Drive (VSD). This provides the maximum speed to be considered when the pump is operated with a VSD, or an operating speed different from the test speed. For this example enter 3600 rpm.
- The minimum speed is entered if the pump is connected to a VSD. This provides the minimum rotation speed to be considered when the pump is operated with a VSD. For this example enter 1200 rpm.
- Maximum and minimum diameter are the limits to consider for using the pump affinity rules for pump performance for a range of impeller diameters. It is recommended that if the pump modified with a different impeller diameter, the pump manufacturer should be contacted to provide a curve a new diameter.
Pump Performance Points
In PIPE-FLO® the maximum number of data points that can be used for a manually entered pump is 15. The more pump performance points entered, the more accurate the pump is represented in the system. When entering the performance points the data should be entered with increasing flow rates.
Within the performance data, there are four landmark points that are used in pump design. These landmark points should be included in all manually entered pump curves when ever possible.
Landmark Point Data
Each pump curve has a set of landmarks used by manufacturers to provide valuable information on pump operation. These pump landmarks consist of the shut-off head, the minimum recommended continuous flow, the Best Efficiency Point (BEP), and the maximum recommended continuous flow. These points are indicated in Figure 1 on the flow axis and as black arrows on the pump curve.
Shut Off Head
The Shut-Off Head is the head produced when the pump is in operation with the discharge valve closed and is a required data point when entering a pump curve. Looking at the attached pump curve, with a 7-inch diameter impeller, we can see the Shut-Off Head equals 210 ft of fluid. The efficiency at this point is automatically entered as 0%. If the NPSHr is to be entered for the remaining performance points the NPSHr value must be entered for the Shut-Off as well.
Minimum Continuous Allowable Flow
The Minimum Continuous Allowable Flow is specified by the manufacturer as the lower limit of flow that will not damage the pump. In this example, the minimum flow is indicated by the red line occurring at 100 gpm, the resulting head is 209 ft, the NPSHr is 9.5 ft, and pump efficiency is 30.7%.
Some manufacturers may not publish a minimum flow value for a pump. If this is the case, you should use the first available data point of efficiency, power, or NPSHr as your Minimum Continuous Allowable Flow.
Additionally, if efficiency or NPSHr data is supplied to the left of the manufacturer’s published Minimum Continuous Allowable Flow it is recommended that this data not be entered.
Best Efficiency Point
When designing a pump for a given service or flow rate, the pump is designed to be most efficient at that flow rate. As the pump operates farther from the designed value, it becomes less efficient. As a result, manufacturers specify a Best Efficiency Point or (BEP) on the pump curve. When entering the pump performance data from a pump curve, care should be given to enter the pump’s performance data for the BEP flow rate. Looking at the manufacturer’s pump curve the BEP flow rate is 443 gpm, the resulting head is 179 ft, NPSHr of 11.4 ft, and an efficiency of 76.1%.
Maximum Allowable Continuous Flow
The manufacturer’s Maximum Allowable Continuous Flow is the last pump performance point on the right side of the pump curve. In this example the maximum allowable flow is 574 gpm, the head is 144 ft, an NPSHr of 16.0 ft, and an efficiency of 70%.
Choosing the Remaining Performance Points
Within PIPE-FLO® you can enter a maximum of 15 performance points since the pump landmarks should be entered that allows up to 11 additional points.
In Figure 1 you can see the performance points selected occur at the intersection of the iso-efficiency line (in green) and the pump head vs. flow curve (in black) which provides nine additional points.
Here are some tips for choosing the additional points:
- Do not choose any data points with flow rates outside of the minimum and maximum allowable values. The manufacturer does not recommend operation of the pump outside this range, so there is no need to enter performance data here.
- The BEP data point must have the highest efficiency value for all the data points, in Figure 1 the BEP point had an efficiency of 76.1% where the adjoining points have efficiency values of 76%.
- Have a majority of data points to the left of the BEP because that is where pumps typically are selected for operate.
- Select points closer to the pump BEP.
- Take the lead from the pump manufacturer catalog curve, they published the data so enter your data using their values.
- The more data entered, the more accurate the program can simulate the operation of the pump.
How Does PIPE-FLO® use the Performance Data
The head and flow values are required for each performance point entered. This data is used to display the pump curve and is used for interpolation for head values between the entered flow data points.
The pump efficiency and NPSHr data are optional data values, but if the efficiency or NPSHr values are entered for any data point it then the rule is to enter the NPSHr values for all data points.
PIPE-FLO® uses pump efficiency data to calculate the power required for pump operation. The efficiency is also required for calculating the coefficients to correct a pump curve for a viscous fluid.
The NPSHr data is compared to the calculated NPSHa at the pump suction. The NPSHr data is also used to ensure there is a sufficient NPSH margin ratio if that is specified for the pump.