Often times the distinction between Design Point and Operating Point is unclear during the pump selection process. With Engineered Software, Inc.'s endeavor to continuously improve the functionality of its PUMP-FLO branded pump selection software, an improvement has been made to the calculated Operating Point in the PUMP-FLO Premium product. This point is also often referred to as the Duty Point, and describes the actual flow, head and other performance indicators for the pump under installed conditions. This means that the hydraulic resistance of the piping system in which the pump will be installed is taken into consideration.
The way PUMP-FLO works is the user must first specify some search criteria. At a minimum, this must include the Rated Design Point Flow rate and Total head (see Figure 1 below).
Figure 1: Design Point Search Criteria
PUMP-FLO searches through the selected catalog and creates a list of pumps which meet the search criteria (see Figure 2). In order to calculate the operating point parameters, PUMP-FLO must find the intersection of the selected pump’s performance curve and the installed system’s Resistance Curve (SRC). For this reason, PUMP-FLO allows the user to customize the system resistance curve by specifying a Static head (Figure 1). The SRC is then generated as a second-order curve drawn from the static head through the design point. If the user chooses to leave the Static head field empty, then PUMP-FLO will draw a default SRC from the origin (zero flow, zero head) through the design point. The user also has the option to enter additional SRC curves by clicking the More SRCs button, however the operating point will always be calculated from the default SRC generated from the Search Criteria page.
Figure 2: Pump Selection List
In the pump selection list, note that some of the values for Flow and Head do not match the specified design point flow and head. This is due to the fact that impellers are trimmed according to the manufacturer’s specifications. Often times, the selected trim which meets the search criteria will not go directly through the design point, but rather pass slightly above it. So if the selected pump is installed in the system described by the SRC on the Search Criteria page, then the pump will operate at the intersection of the two curves. This point of intersection reflects the values which will be displayed in the Data Point for the Flow, Head, NPSHr, Efficiency, Power and Motor selection (see Figure 3).
Figure 3: Pump Graph Window
In the graph window for this selected pump, note that the red “knee” indicates the Design Point search criteria of 200 US gpm, at 100 ft of Head. But the minimum diameter impeller which will be able to meet these criteria is 6.75 inches. And the performance curve for this pump and impeller combination intersects the SRC at the Operating Point of 203 US gpm, and 102 ft of Head.