When you are choosing a centrifugal pump, it is important to match it to the application and pumping system both mechanically and hydraulically. If you install a pump that doesn’t match, it can result in ongoing performance issues until the imbalance is corrected.
Matching a centrifugal pump to a system means making sure to consider some important factors. These include the flow rate, the system design, the amount of resistance in the system and the system curve.
The pump has to be the right size for the system. Deviation may force the pump to operate outside its design parameters. This can cause excessive noise, cavitation, vibration or circulation issues.
The head created by the pump is the main determinant of the flow rate in the pumping system.
System resistance head counters both the friction head and the static head in the system.
When the system resistance head is equal to the head created by the pump, it is called the duty point. When selecting a centrifugal pump, it is crucial to identify the accurate duty point.
Net Positive Suction Head (NPSH) Made Available
The NPSH is determined by subtracting the vapour head of the fluid being pumped from the total absolute head at the point of pump suction. It is important to know the NPSH available (NPSHA) when selecting a centrifugal pump for any given application. If the NPSH available isn’t greater than the NPSH required (NPSHR) with an extra margin for safety, the pump will be at heightened risk for cavitation.
In clean water systems that only transport cold water, a measurement called suction lift can be used to simplify calculations. In this case, it is also recommended that a safety margin is added in.
Matching the Requirements to the Right Pump
Once you have determined the total dynamic head, the flow rate and the NPSH or the suction lift, you can choose a pump which is appropriate for your system. The specifications can be presented with a constant speed with different impeller diameters corresponding with different head quantity curves. It can also be presented with a constant impeller diameter and varying speeds.
Either set of parameters will indicate power use, pump efficiency and the NPSH or suction lift required for the flow range of the pump. You can then determine the input power kW and speed (RPM) using the total head, required flow and suction lift from the performance curves.
Ideally, your system would be best served with a pump that has an NPSHA that is greater than the NPSHR by a reasonable margin while running at best efficiency point (BEP). The volute and impeller are operating as designed and the flow of the pump is smooth. Bearing loading, energy dissipation, vibration and shock are at a minimum and the pump should operate for a long time with no downtime.
Sadly, “ideally” isn’t often possible for a number of reasons. There is only a finite number of available pumps. Pump manufacturers would have to custom design individual pumps for individual systems. This is not feasible, practically or economically. Consequently, pumps are selected from units that are commercially available.
Specific speeds can be limited. For example, a duty point with a low flow rate and high head can exceed the limits of impeller design, causing the pump to operate near minimum continuous flow. A limited NPSHA can limit pumping speeds and running speeds.
Because duty requirements can vary to great extents, it can be impossible to find an ideal selection. For some operations, the ideal pump may be too complicated or cost prohibitive. Ultimately, choosing the right centrifugal pump can be a tradeoff between reliability, efficiency and cost.
How Big is the Compromise?
If the duty point rate is from 50-110% of the best efficiency flow rate, the pump is normally considered “acceptable.” If the BEP can’t be matched perfectly, a centrifugal pump whose operation is to the left of the BEP is preferable to one that exceeds the BEP.
We Can Take Care of it For You
Our customer service is the best in the business and we also have access to the manufacturers. If you are having trouble selecting a centrifugal pump, call Pump Solutions Australasia today. We don’t just import pumps, we provide solutions: 1300 793 418.