Figure 1 shows a detailed manufacturer’s pump curve. It includes the flow versus head performance curves as well as isometric curves for efficiency, power, and NPSHr. These curves accurately represent the performance of the indicated pump in accordance with the ANSI/HI 1.6-2000 Centrifugal Pump Tests standard. If you had to size a pump for a system with design conditions of 600 U.S. GPM at 39 feet of head, and you were evaluating this particular pump, you could feel confident in knowing that this pump, with a 10” impeller would successfully fill the bill. It is important that the pump selection and evaluation process accurately reflect the manufacturers published pump curve to aid the customer in evaluating the pump for their application. As we will see, the key to generating accurate curves is in the method with which the pump performance data is saved and displayed.
So how are these curves digitized? Well, there are several ways to accomplish this, all of which involve obtaining data points from the pump manufacturers. These points can either be extracted from the published paper curves, or gathered directly from the manufacturers. One simple method is to take several points along the performance curve, and perform a polynomial regression analysis. This results in a polynomial expression describing the performance curve which can be saved and used in a pump selection program. The original data points are then discarded, and the pump performance data is now represented solely by an equation. Depending on the shape of the original published curve, this polynomial expression may or may not be an accurate representation. A second, more robust method is to collect as many data points as is necessary to accurately fit the shape of the published performance curve, and use these saved data points to represent the original published curve in the pump selection program. A numerical analysis can then be performed on these points such that a curve is drawn which precisely represents the original published curve.
- PUMP-FLO has the ability to define and save as many data points as is necessary to ensure complete accuracy when representing the performance curve’s geometry.
- PUMP-FLO’s proprietary algorithm forces the drawn curve to go through every single data point entered which ensures the curves accurately match the manufacturer’s published data.
- PUMP-FLO’s algorithms have the capability to provide the most accurate efficiency and power curves.
First, look at an example which illustrates the problems associated with the simple polynomial regression method of curve digitization. The following curves depict flow versus head and since they are the primary search parameters, it is crucial that these points match up with the performance curve provided by the manufacturer. In figure 2a, we have a performance curve from a major pump manufacturer displayed in black. On top of that in pink, we have overlaid the curve which was generated by a pump selection tool using a simple polynomial regression.
Now, look at the same performance curve with an overlay of the pump curve generated by PUMP-FLO. The proprietary PUMP-FLO regression forces the curve to pass through every single data point collected. It then smoothes the curve to provide clean, accurate pump performance curves which end users can use with confidence in their various piping system analyses.
Even the best numerical analysis techniques cannot predict anomalies between data points. If the performance curve you are trying to digitize has these types of dips or abnormalities, it is imperative that you are able to collect enough data points in and around that range. As a minimum, you would want to have data points collected at the edges and at the peak (or valley) of the anomaly. Ideally, you might want to add a couple more points in between. The advantage that you get with PUMP-FLO is that you have the flexibility to define as many points as is necessary to accurately describe the entire range of the pump curve, anomalies included.
Power and Efficiency Curves
In addition to the standard flow versus head pump curves, many manufacturers also include curves for pump efficiency, and pump power. It must be stressed that pump efficiency and pump power are functions of each other. The formulas for calculating efficiency and power are:
Another area where PUMP-FLO is going to give superior results is in the digitization of pump efficiency and power data from the manufacturer. PUMP-FLO utilizes various techniques to digitize the curves based on what data is available from the manufacturer. Our most frequently used algorithm provides highly accurate efficiency and power data.
Now look at our previous manufacturer’s performance curve, and see the effect digitizing can have on power and efficiency curve generation. In Figure 4a, we have the manufacturer’s performance and efficiency curves with an overlay of another pump selection software maker’s curves. Notice the efficiencies are well off the mark, particularly in and around the 70%+ operating range. Not only are the iso efficiency curves misaligned, but the percentages do not match up with the published curve at all. The pump’s overall efficiency is overstated by up to 3%. If your customer was to download these performance curves (Figure 4a in black) from your technical literature, and your salesperson were to present the performance curves (Figure 4a in pink) from your internal pump selection software, there’s no way this customer is going to able to tell that these are one and the same pump. This introduces confusion and a lack of confidence in the pump performance data to the customer. In Figure 4b, we have the manufacturer’s performance and power curves with an overlay of the other pump selection software’s curves. The power curves start off tracking well, which we would expect, since those data points are taken directly from the manufacturer. However, notice that the flow range has been severely limited on the 30 hp and 50 hp curves. They do not even extend anywhere close to run out. Once again, if your customer has downloaded the black curves, and your salesperson is presenting the pink curves, the salesperson is not going to be able to properly size a motor because of the lack of information.
The second issue is with respect to the power curves. The truncated power curves shown in the figure above do not extend to run out flow rates. This is important because pump motors are most often sized to the non-overloading power on the design curve, which is frequently at run out. If the customer cannot tell what the power is at run out flow rates, then they will not be able to properly size a motor.
Once again, look at the same performance curves with an overlay of the curves generated by PUMP-FLO. In Figure 5a, we have the manufacturer’s performance and efficiency curves with an overlay of PUMP-FLO’s curves. Note that the iso efficiency curves are dead-on at every single point in the graph, and the efficiency is never overstated. In figure 5b, PUMP-FLO’s power curves also track extremely well with the manufacturer’s, given that they are calculated power curves, and more importantly, the endpoints at the run out flow rates are dead-on. In addition to unmatched pinpoint accuracy of curves, PUMP-FLO also generates an additional horsepower curve above the top impeller diameter based on manufacturers’ data. This allows you to see how close you are to requiring the next higher size motor.