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 system requirements.
- 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 increase.
- The pump will pump where the pump curve intersects the system curve.
- 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 needs.
- 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 reason.
- 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, induced, etc…
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 growth.
- The end of the stuffing box can be hit by the shaft or sleeve. There is often a close fitting bushing installed in this location.
- 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 rubbing.
The increased amperage can be caused by an increase in bearing loading.
- Check the shaft and housing tolerances along with the installation method.
- Cooling a bearing outside diameter causes it to shrink and over compress.
- The wrong lubrication level. There is too much lubricant in the bearing
The starting procedure could be the problem.
- The radial flow pump is being started with the discharge valve open. Radial flow pumps use the most horsepower at high capacity.
- 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 thrust.
- 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 thrust.
- 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:
- On February 09, 2018