Nobody wants to be the person responsible for procuring an undersized pump. Your fellow engineers would never let you hear the end of it. Unfortunately, this is the ideology that often leads to pumps being oversized for their respected applications, and the resultant increase in operating and maintenance costs. So if you are careful to select a pump which meets the worst case operating conditions, how then do you know that this pump will also be able to meet the demands of partial flow or varying load conditions?
There are features in PUMP-FLO Premium that you can use to your advantage when evaluating pumps under varying load conditions. You can specify a Secondary Operating Point in the Advanced Criteria during the selection process. Let’s say for example that under a full load Design Flow rate of 450 gpm, your system requires 100 feet of Total head, and that the Static head is 60 ft. This would be your primary rated design point because you want to make sure that your selected pumps can meet the full load scenario. See the rated design point conditions in Figure 1. We will use the Cornell pump catalog, and focus on the Clear Liquids types of pumps.
Figure 1: PUMP-FLO Premium’s Primary Search Criteria
Now, when part of your system is offline or isolated, let’s say your load becomes 250 gpm at 85 feet of required total head. You want to see if the pumps selected can also meet the partial load requirements, so you can specify this as your secondary operating point in the Advanced Criteria. When you click Advanced Criteria at the top of the window, a dialog box pops open in which you can specify those partial load requirements as shown in Figure 2.
Figure 2: PUMP-FLO Premium's Advanced Search Criteria
When you click OK and then Search the catalog, the list generated will include all pumps whose selection window encompasses the primary rated design point, and all of those pumps will also display the secondary operating point. If the secondary operating point is also within the selection window of a particular pump, then the pump will not be flagged. However, if the secondary operating point falls outside of the selection window, then a warning flag will be issued for that pump. If you click on the 3WH30 pump and open up the graph window, you will see both the primary and secondary design points marked off on the graph, with the design impeller curve passing through the primary design point. See Figure 3 below.
Figure 3: PUMP-FLO Premium’s Graph Window with Primary and Secondary Operating points
From the graph, you can see that a System Resistance Curve is automatically drawn from your Static head through your Primary design point. If you isolate one of your end users and run the pump back on its curve to your secondary operating point of 250 gpm, notice that the pump will be generating almost 40 feet of head more than your system requires. If there is no variable speed drive to slow down the speed of this pump, the additional head would have to be dissipated across a control valve or some other restricting device.
Another feature that you can make use of is the additional system resistance curves. If you go back to the Search Criteria page, and click on the More SRCs button, a dialog box will pop up allowing you to enter new SRCs or automatically generate them. Notice that the message “Auto Generated SRC is ON” is displayed indicating that the SRC from your static head and primary rated design point is already on as shown in Figure 4.
Figure 4: System Resistance Curves dialog box
We want to generate an SRC for our secondary operating point, so click the Generate button. This opens the Generate Resistance Curve dialog box where you can specify the SRC for the secondary operating point as shown in Figure 5.
Figure 5: Generate Resistance Curve dialog box
After you have specified the data for the secondary SRC, click the Generate button. The curve data is generated, and you now must give this new curve a name. We have named this curve “Partial Isolation” as shown in Figure 6.
Figure 6: SRC data for secondary operating point
If you click OK and search the catalog again, the pumps in your list will now display the primary SRC and also the secondary SRC. Once again click on the 3WH30 pump and open up the graph window as shown in Figure 7.
Figure 7: PUMP-FLO Premium’s graph window with Primary/Secondary operating points and SRCs
From the graph you can see that this particular pump would be able to accommodate the varying load conditions. But under the partial load condition, there will be considerably more pressure throughout the system unless you can slow down the speed of the pump.