SUBJECT: The centrifugal pump is drawing
too much amperage. 10-11
There are a couple of things you must keep in mind when
troubleshooting centrifugal pump problems:
- The centrifugal pump always pumps the difference between the
suction and discharge heads. If the suction head increases, the
pump head will decrease to meet the system requirements. If the
suction head decreases the pump head will increase to meet the
- A centrifugal pump always pumps a combination of head and
capacity. These two numbers multiplied together must remain a
constant. In other words, if the head increases the capacity must
decrease. Likewise if the head decreases, the capacity must
- The pump will pump where the pump curve intersects the system
- If the pump is not meeting the system curve requirements the
problem could be in the pump, the suction side including the
piping and source tank, or somewhere in the discharge system.
- Most pumps are oversized because of safety factors that were
added at the time the pump was selected. This means that
throttling is a normal condition in most plants, causing the pump
to run on the left hand side of its curve.
The increased amperage can be caused by a
pump that is too large for the application.
- A large pump was specified in anticipation of future
- The pump was sized for the maximum operating condition, but
does not run anywhere near that point most of the time.
- The capacity requirement has been lowered and the pump is
being throttled rather than cut back the impeller diameter.
- The pump was oversized because of safety factors that were
added at the time the pump was sized.
- Increasing the speed of the pump causes a dramatic change in
the amperage required. The amperage changes by the cube of the
change in speed or impeller diameter. If you double the speed of a
pump you will need eight times the amperage.
The increased amperage can be caused by a
change in the product.
- The motor was sized for a low specific gravity fluid, but the
lines are being flushed or tested with water.
- The specific gravity of the fluid has increased for some
- The viscosity of the liquid is increasing with a change in
temperature. Some viscosities increase with a lower temperature,
some with a higher temperature.
- The viscosity of a liquid can increase with agitation. That is
how cream becomes butter.
The increased amperage is caused by two part
rubbing together as a result of shaft displacement. Here are some
common causes of shaft displacement:
- Pipe strain
- Misalignment between the pump and driver.
- A bent shaft.
- The rotating assembly is not dynamically balanced.
- Water hammer.
- Operating off the BEP.
- Thermal growth.
- Pulley driven pumps.
- Different types of vibration including harmonic, slipstick,
There are many parts that can come into
contact when the shaft displaces.
- The impeller can contact the pump volute or back plate. This
can also happen with an improper impeller adjustment or thermal
- The end of the stuffing box can be hit by the shaft or sleeve.
There is often a close fitting bushing installed in this
- The outside diameter of the rotating mechanical seal and the
inside of the stuffing box.
- A gasket or fitting protruding into the stuffing box that rubs
against the mechanical seal.
- The rotating shaft and the stationary seal face.
- The shaft and the API gland disaster bushing.
- The closed impeller wear rings are a common source of
The increased amperage can be caused by an
increase in bearing loading.
- Check the shaft and housing tolerances along with the
- Cooling a bearing outside diameter causes it to shrink and
- The wrong lubrication level. There is too much lubricant in
The starting procedure could be the
- The radial flow pump is being started with the discharge valve
open. Radial flow pumps use the most horsepower at high
- The axial flow pump is being started with the discharge valve
shut. Axial flow pumps use the most horsepower at high head.
Check to see if there is too much axial
- See if the impeller balance holes are clogged.
- If there is an elbow too close to the suction of a double
ended pump, and the piping is running parallel with the shaft, The
change in velocity of the incoming fluid will cause axial
- Converting packing to a mechanical seal can increase the axial
loading on the bearing
Here are a few more reasons why you might be
using too much amperage.
- Your pump is equipped with an axial flow or high specific speed impeller. These impellers cause the pump to draw more amperage when the pump discharge is throttled.
- The stuffing box packing has been tightened too much.
- An unbalanced mechanical seal is being used in a high pressure
application. There is too much face load
- The impeller has been installed backwards.
- The shaft is running in the wrong direction.
- The open impeller needs adjusting. You have too much clearance
between the impeller and the volute, or back plate, depending upon
the pump design.
Additional pump troubleshooting subjects:
For information about my CD
with over 600 Seal & Pump Subjects
Link to Mc Nally home page