Water Well Journal
Issue link: https://read.dmtmag.com/i/692787
For our example, four different possible selections were picked from three different manufacturers. We shall refer to Pump #1, Pump #2, Pump #3, and Pump #4. Among these three manufacturers, one of them, the makers of Pump #1 and Pump #2, offer two different bowl options: one with enclosed impellers and one with semi-open impellers, and Pump #4 of- fers a selection at a cost much lower than the three alternatives. The performance curves for each unit are shown: Pump #1 (Figure 1), Pump #2 (Figure 2), Pump #3 (Figure 3), and Pump #4 (Figure 4). The full speed head, efficiency, shutoff head (SOH), and horsepower conditions that correspond to each of our design conditions are shown in Table 1. Immediately, we can eliminate Pumps #3 and #4 from the selection process. The primary reason, both pumps are operating at or below the Minimum Continuous Stable Flow (MCSF) rating for that particular pump when operating at 156 GPM, the minimum design flow. The MCSF flow rating for a vertical turbine pump is generally shown on the pump curve by a red or solid line on the left side of the curve that aligns with various flow rates and is usually designated by the abbre- viation "MCSF." It is established by the manufacturer based on various factors, the primary factor being excessive vibra- tion and thermal heating. Other factors often considered include maximum bearing loads, radial and axial thrust, packing or mechanical seal lubrication, and horsepower dissipation. The bottom line is these red lines are placed on the curve for a reason and de- signers should not allow operation of the pump to the left of that line continuously. This doesn't mean the pump we ulti- mately select should operate at the low flows below the MCSF rating continuously either. In fact, the water system will likely demand much less water during the early morning or late evening hours, which could push the pump flow further below 156 GPM than de- sired. In these cases, the system design will need to incorpo- rate a method to either bypass flow back to the well in order to maintain the MCSF, or shut down the pump and rely on interim storage until the system demand is higher. This is a design and owner decision that cannot arbitrarily be made here but must nonetheless be made. Now that the selection is down to Pumps #1 and #2, the final selection is made. At this juncture, the cost of each unit often enters into the equation. The client's cost for Pump #1 is $15,900 and for Pump #2 is $16,750. I would have leaned to- wards Pump #1 in any event as the efficiency band is wider and the horsepower draw is at or less than 40 BHP (brake horsepower) throughout the operating range, allowing use of a 40 HP motor. The selection for Pump #1 is as follows. WATER WORKS from page 55 Figure 2. Pump #2 with eight stages, closed impellers. K = 7 lb./ft Max. lateral = .75″ waterwelljournal.com 56 July 2016 WWJ