hatm = 33.94 ft
NPSHA = 33.94 - 1.23 -24.39 = 8.32 ft
Then we calculate the discharge head:
hgd = (26 x 2.31)/1.221 = 49.19 ft
hd = 49.19 + 8.18 + 1 = 58.37 ft
H = 58.37 - (-24.39) = 82.76 ft
From the motor performance curve (Fig. 4) and the amperage data, you can see that the pump is operating at less power than predicted by the pump performance curve (Fig. 2).
Figure 4. Motor Performance Curve
The motor curve shows that the pump actually is operating at less power than the pump performance curve (Fig.2) predicts.
A performance curve reading at 17.5 amps corresponds to approximately 11.5 hp vs. a performance curve expectation of approximately 14 brake horsepower (BHP). Plotting the flow rate and total head on the performance curve shows the pump is operating well off the stated performance curve. The NPSHA of 8.32 ft is well below the 10.7 ft required by the pump at 240 gpm. This performance reduction likely is the result of cavitation.
Many users do not believe their pumps are cavitating because they cannot hear the characteristic "gravel" sound in the pump. After the performance breaks down, the pump might go into full-sheet cavitation. In this extreme form of cavitation, the entire vane becomes enveloped with vapor bubbles displacing a large volume of liquid. When this severe form of cavitation happens, the noise often drops off because the bubbles do not collapse until they are far downstream. Many users have no idea the pump is cavitating.
The reduction in horsepower also is a clue that the pump is pumping two-phase flow (vapor bubbles and liquid). The fluid density reduction causes a reduction in horsepower demand.
If the pump in this example had been operating satisfactorily and then stopped working, the user should have asked: "What changed?" Are suction strainers present in the system? Could they be clogged?
In one case, an investigation revealed a debris-filled suction strainer on a cooling tower pump's inlet. Neither operations nor maintenance knew the strainer existed until a system performance evaluation showed the pump was cavitating severely. The cooling tower had been cleaned three times to improve performance before we found the strainer.
If you have a similar pump problem, ask a few key questions. Did your facility recently clean a tank or another vessel, depositing slag or scale into the pump or strainer? Has the process changed? Is the liquid hotter? Did the filter size or liquid level change?
As a rule of thumb, pumps operate best when the flow rate is between 80 percent to 120 percent of the BEP flow rate. Chronic pump or seal failures often are a sign of operation problems.
By evaluating the actual operating point of your pumps and correcting operating point problems, you can drastically improve availability and slash maintenance and repair costs. Relatively inexpensive methods often can be used to correct operation problems and improve pump performance. I will introduce some of these methods in a follow-up article slated for Chemical Processing's September issue.
Case is president of MechTronix Engineering, Elk Grove, Calif. The company specializes in rotating equipment design, testing, condition monitoring, analysis and education. Contact him at email@example.com.