PUMPFAQs®
Q. What are the most probable causes of insufficient
discharge flow or pressure in centrifugal pumps?
A. Before looking at the pump, v 2 2 check the measurement of total h = h + d vs g 2g + Z
head and rate of flow. The proper
equation for total head h is in the
formula to the right.
Many users ignore the
change in velocity head and elevation head which often makes a
significant difference in the total
head. See ANSI/HI 1. 6 Centrifugal Pump Test for extensive detail
on this subject.
Next, we should look at the pump system and check for
the following:
• Air leaks in the suction piping.
• System head higher than anticipated (low flow and high discharge pressure).
• Insufficient NPSH available.
• Foot valve too small.
where:
hg = pressure gage head in feet in both suction and discharge
v = liquid velocity in feet per second in both suction and discharge
g = gravity constant 32.2-ft/s
Z = elevation of pressure gauge in feet in both suction and discharge above a reference point
= symbol for change
• Vortex at suction supply.
• Suction or discharge valve partially closed.
Finally, looking at the pump:
• Air leaks through the shaft seal or stuffing-box.
• Pump speed too low.
• Pump wearing rings worn (see Figure 1.78B).
• Damage to the impeller or internal blockage.
• Impellers loose on shaft (primarily multistage).
• Impeller installed backward (primarily double suction).
• Wrong direction of rotation.
Q. We have a 100-hp end suction pump that has a broken
shaft. What are the causes of shaft breakage?
Figure 1.78B
A. Pump shafts usually break in the fatigue failure mode.
High stress starts a small crack, which slowly progresses as
each rotation of the shaft causes a fluctuation or reversal of the
stress. The crack becomes larger until the final failure occurs.
The cause of the original crack can be from unexpected
bending of the shaft. Coupling misalignment could be a contributor. Large radial forces from the impeller when operated
near shutoff are another possible cause.
The original crack may start from a point of stress concentration such as a shaft keyway. The sharp corner in the keyway
causes a stress riser in the shaft which progresses to a small
crack and ultimately shaft failure.
