Centrifugal Pump Basics ‖ What’s Wrong With Your Pump?

How often do users stand in front of their pump and wonder if it is performing to their expectations or design specifications? Do they examine the operating points showing flow [X-axis, gallons per minute (gpm)] and total dynamic head (Y-axis, TDH) and think, "Does this tell the whole story?" Perhaps they question whether the pump is operating according to its factory-tested performance curve and design point, or if a system change was recently made to affect production.

These questions are common, but not easily answered from just the pump's perspective. The performance of a pump is determined by the interaction between its curve and the system curve. System changes, such as valve adjustments, additional lines, or changes to the original design, can shift the system curve and affect the pump's performance. By collecting key data points, users can better understand the performance of their pump and optimize its operation.

Understanding Centrifugal Pump Performance
To assess the true performance of a pump, start by asking the following questions:
Is the pump operating at its best efficiency point?
The best efficiency point (BEP) is the point at which a pump operates most efficiently. Check if the system curve intersects the pump's BEP. Any deviation from this point could indicate changes in the system that affect performance. The further from the BEP the system curve intersects the pump curve, the shorter the mean time between repairs (MTBR) for the pump.
Is the TDH accurately measured?
Using a calibrated pressure gauge or pressure transducer on the suction and discharge sides of the pump to determine the pressure differential is essential to accurately calculate the TDH.
Is a flow meter installed and recording flow?
Make sure there are accurate flow records during pump operation. Determine peak production times and check if increased flow demands are consistent with the pump's published curve. Running away from the BEP can shorten the time between necessary repairs.
Is the pump's current value changing?
A change in current value could indicate that some modification, such as an impeller resizing, has been made. Current values ​​help determine where the pump is on the curve. Modifying the pump or changing the impeller size, material or blade angle will affect performance and the accuracy of the published curves.

Maintenance and setup considerations
Evaluate the maintenance and setup of the pump to understand its condition:
MTBR:
Compare the MTBR of this pump to the other pumps in the system. If this pump requires more frequent service, it may indicate a problem with the way it interacts with the system.
Pumps running in parallel:
Make sure all pumps running in parallel are identical in type, size, impeller diameter, and pump curve. This difference can cause "water grabbing" from one pump to another. If it is a vertical pump, are the impeller clearances set at the same level to maintain the same head? Also, if a variable frequency drive (VFD) is used, check that all pumps are running at the same percentage to avoid similar problems.
Has the pump been recently rebuilt?
If the service was performed at different service intervals or any parameters within the pump were changed during the service (impeller diameter, blade angle, material of construction, etc.), there may be differences between the pumps. When the pump was reinstalled, was the impeller clearance set correctly?

Impact of Changes
System changes can significantly affect pump performance:
Valves:
A change in valve type or opening will change the system curve, which will affect pump performance.
Other equipment in the system:
Small changes to equipment in the system, even small changes such as nozzles, will affect the system curve.
Fluid properties:
Changes in fluid temperature, tank levels, and filter settings can affect TDH calculations and the Net Positive Suction Head Available (NPSHa). Are there areas in the system where foreign matter or air is entering the fluid? These can have a significant impact on the pump and system.
Auditory cues:
Listen for unusual sounds, such as cavitation, which can indicate that the pump needs repair or the system needs evaluation

Make informed decisions
Users can gain a clear understanding of pump performance by systematically evaluating these factors. The next time a user stands in front of their pump, they will be confident that they understand what the pump is doing and how it is performing.