Flow of water

The missing link between the system curve and variable speed pump curve reveals the benefits, and potential pitfalls, of variable speed drives
T Leyden
Publication Date (Web): 11 January 2017
DOI: https://doi.org/10.21139/wej.2017.003

Power consumption is an expense that often drives capital expenditure decisions and maintenance schedules to ensure it is minimised. By thorough understanding of the relationship between the system and the pump performance there is often the opportunity reduce the energy consumed to pump a given volume by modifying the time taken to pump it.

A synergy of equations has been utilised to produce a term that quantifies the Energy density (ED) of any given system flow rate using any given pump; and has been presented in a format intended to have the most relevance, that is, Kilowatt Hours per Mega Litre (kWh/ML).
It can be shown that when a pumping system has been chosen based on peak demand periods that include seasonal demand and firefighting capacity, pumping at a flow rate slower than the design duty often delivers significant energy savings.

The ability to realise these benefits depends on the ability of the pump to operate effectively at the duty identified in the analysis. However, with the increased acceptance of systems that allows pumps to readily be run to 50% of the nominated pump curve speed, often there is only an operational adjustment required to realise these benefits.

Through analysis it can also be determined the flow rate at which the benefit is maximised, and therefore the limit for how slow the pumps should typically be run. There is the potential that some systems are being run too slow on the assumption that as less power is being used instantaneously that it is reducing energy consumption.

There is also the risk that pumps are being inadvertently operated outside their range of application. The equations developed that allow for the ED to be determined also allow for the pump efficiency across a range of flows to be analysed. This can aid in determining the pumping flow rate that coincides with the minimum flow rate of the pump to be identified. 

Thus, the analysis available from the utilisation of the equations presented can potentially assist to optimise pump systems, both by reducing the energy to pump a given volume, and also by minimising maintenance tasks that could have arisen from pumping outside the range of application.

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