There are several ways to control two identical, parallel pumps operating under variable frequency control in pumping applications. One method is to size a single drive to handle both pumps and vary the speed of both synchronously. A more efficient method uses two drives to control the pumps. Once one pump reaches its maximum speed, the second pump is brought online and both are operated at synchronous speeds (as in the previous example). Yet another two-drive method keeps one pump at maximum speed and varies the speed of the other. This example can also be achieved with a single drive and a transfer switch. When the drive reaches full speed, the switch causes a contactor to operate that pump across the line, and the drive is transferred to the second pump. The downside to this control scheme is that there is no back up drive.
The first example can work if the normal flow is always greater than the maximum flow of one pump. Otherwise, it can be inefficient. Examples two and three are better alternatives, but which is best? It depends upon the breadth of a pump's hydraulic efficiency and the system conditions in which it is operating. Comparing these two control schemes to see which can provide the best operating conditions is valuable. The beta version of variable frequency parallel pump analyzer (VFPPA) allows for the comparison of the hydraulic efficiencies of identical, parallel pumps operating under synchronous or independent speed control.
Figure 1. VFPPA Data Input Tab
Figure 1 is a screen shot of the data input tab of VFPPA Excel sheet. It shows the required data (yellow cells) and the tabs that are generated. The “One Pump” tab shows the H/Q curves and hydraulic efficiencies of a single pump at speeds of 45 to 60 Hz. The “Two Pumps” tab shows the same information for two pumps running at synchronous speeds. The other tabs show both single and two-pump operations at different speeds and are used if a more detailed view is required. The Average Efficiency Calculator and the Energy Savings Calculator normally seen at the right of the screen are shown in Figure 4 and will be discussed later.
Synchronous Speed Control
The example included with the analyzer is vertical multistage with a BEP flow of 350 gpm and a BEP efficiency of 78 percent. The rather flat H/Q curve is typical of this design. The system curve shows a required static pressure of 206 feet.
Figure 2. Plot produced by two pumps running at synchronous speeds
Figure 2 is the plot produced in the “Two Pumps” tab and shows the H/Q curves produced from 45 to 60 Hz under synchronous speed control. The data labels show the average hydraulic efficiency. As shown, when both pumps run at full speed (60 Hz), they produce a flow of 700 gpm at 206 ft TDH and operate at BEP efficiency (78 percent). The black angled line is the operating point of both pumps at single pump maximum flow (350 gpm). Speed is approximately 54 Hz and efficiency drops to slightly below 60 percent. As flow (speed) increases, so does the average efficiency of the two pumps. The red angled line crosses the system curve at 450 gpm at a speed of approximately 56 HZ, and efficiency is increased to about 67 percent. Let's take a look at individual speed control and compare the two operating efficiencies at 450 gpm.
Individual Speed Control
Figure 3 is the plot produced in the “One Pump” tab. As shown, a single pump operating at 60 Hz will produce a maximum flow of 350 gpm at 78 percent hydraulic efficiency. Under this control scheme, when the pump reaches maximum flow, it is maintained at full speed and the second pump is brought online at some reduced speed. The red angled line crosses the system curve at 100 gpm at a speed of about 52 Hz. This is the flow that must be provided by the second pump to match the 450 gpm flow produced by two pumps running at synchronous speeds. The hydraulic efficiency at this flow point is about 50 percent.
Figure 3. Plot produced by one pump running at 60Hz
Determining the Most Efficient Option
To compare the efficiencies of these two control techniques, the information above is entered into the Average Efficiency Calculator, which is used to determine which control technique is best for a particular pump and application.
The calculator, seen in Figure 4, requires the flow and efficiency of the pump running at full speed and the flow and efficiency of the pump running at a reduced speed. Upon entry, it calculates the percent of total flow contributed by each pump and the average hydraulic efficiency of the two pumps. The calculator shows an average efficiency of 71.8 percent, which is about 5 percent higher than the 67 percent produced at synchronous speed.