Many real-world applications are designed for system curves that are provided as an envelope. This is due to varying head conditions caused by stormwater expectations, varying reservoir levels, piping conditions over time, or the use of pressurized tanks. The below is an example of a system curve envelope which dictates all the conditions seen. It is up to the system designer to specify the important points the selected pumps need to hit.
For many situations, the use of pumps in parallel or pumps in series can be used to define minimum and maximum conditions considering the use of a Variable Speed Controller in order to cover the points below the pump curves. Below is an example of 3 identical pumps in parallel being sized to cover a system curve with varying head conditions.
When selecting the appropriate pump in a situation like this, the use of a Variable Speed Controller is expected which means it is possible to reach duty conditions that exist within the curve envelope. Using affinity rules will allow the engineer to calibrate the Variable Speed Controller to cause the pumps to hit any desired duty condition within the curve envelope. It is important to remember that at any given time, the only point to dictate pump performance is where the present system curve will intersect the pump curve.
It is important to note that as the flow increases, the pump head produced will decrease as the system flow losses increases. Oversized or undersized pumps will have curves that do not intersect at the required flow rate or will not cross at all. This will show that a different pump should be selected.
A list of frequently asked questions regarding pumping systems can be accessed here at Hydraulic Institute's website.
Additionally, "Pumps & Systems" publishes HI FAQs which can be accessed here.
Last updated on January 15th, 2020