Centrifugal Pumps – Performance and Selection
Centrifugal pumps are commonly used in many applications due to their versatility and high reliability. Centrifugal pumps are versatile because:
- They can pump a range of different liquids including slurries.
- There are a wide variety of models and sizes available and thus they can be utilised over a large range of different pressures (head) and flow rates.
- They can easily be adapted for upgraded or changed conditions by changing the pump speed or changing the pump impeller size.
- Multiple centrifugal pumps can be used together in different configurations (in parallel or series) to further broaden the range of applications.
Centrifugal pumps are highly reliable because:
- There is a large gap between the impeller and casing and thus no wear caused by contact between the main pump components.
- They are easily adapted to the specific requirements of the pumped media by the addition of different liner materials and / or different materials of construction.
- Only one pump seal is needed where the pump drive shaft enters the pump casing. The seals are commonly mechanical labyrinth type seals or flushed gland packing and require only minimal maintenance.
- The only moving part of the pump is the pump shaft and impeller, and thus it is easy to maintain and replace individual pump components.
When designing a pumping system, the centrifugal pump size must be matched to the pipework system. If the design is incorrect it can lead to issues such as not achieving the required flowrates and / or pressure, pump damage due to cavitation and low energy efficiency.
The requirements of the pumping system are determined by calculations being completed to determine the static pressure (constant with flow e.g. pumping elevation, constant pressure reducing devices etc.) and dynamic pressure (changing with flowrate e.g. pipe friction losses, valves and pipe restrictions) of the pipework system. For a range of flow rates the total pressure is calculated for the system. The results of this can then be plotted on a graph of pressure v’s flow rate to produce the system curve. The pump curve is normally provided by the pump supplier and is also defined over a range of pressure v’s flow plotted on a graph.
To determine if the pump is suitable for the pumping system the pump curve is plotted on the same graph as the pipework system curve. The point at which the two graphs cross each other is pump duty point, this is the pressure and flowrate that the pump will operate at, when connected to that specific pipework system. Several iterations are normally required between selecting the right pump and refining the pipework design until the desired flow rate, pressure and pipe velocities are achieved. An example of a pump curve and system curve plotted together is shown below.
Rapallo has the experience and capabilities to visit your site, measure flow rates with a portable flow meter that straps onto your existing pipes without any modification. With knowledge of the flow rate and pump details, Rapallo can evaluate and troubleshoot any current pump performance issues and recommend improvements to increase plant performance and reliability.
Rapallo is experienced in completing engineering pumping calculations and assessments for both slurry and plain liquid systems. If you have a new pumping application that you want to install or an existing system you want assessed, please contact Rapallo to discuss further. To see more about Rapallo’s design capabilities, visit our website at Engineering Design | rapallo.com.au.