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.
