In round numbers, the operation of water and wastewater services absorbs 30 to 50 percent of the average municipality’s energy use, and 90 percent of that is put toward running pumps. So it naturally correlates that achieving a highly efficient pumping operation while maintaining a satisfactory flow rate under varying conditions is a major goal for water service managers.
Some of the common, more rudimentary methods of achieving this balance include oversizing the pump to ensure maximum flow at all times or controlling flow with a throttling valve, a bypass valve or by trimming the impeller. More recently, controlling the flow by pairing the pump motor with a variable frequency drive (VFD) has emerged as a more viable solution in many cases. In order to appropriately examine these methods in more detail, it is important to consider the value of system curves and pump curves in understanding the relative merits of each option.
A system curve graphically defines the amount of head pressure required to move a fluid through a hydraulic piping system as a function of flow rate. Head, which is measured in meters or feet, is the maximum height (pressure) a pump can achieve. In wastewater applications this is the vertical lift from the inlet of the pump to the discharge of the pipe. Flow is measured in gallons per minute.
The system curve is derived from the two major system-related aspects that a pump must surmount:
- Static head: the fluid elevation (uphill) differences between pump supply and pump, and the pressure differences between pump supply and pump system
- Frictional head: the frictional impediment to the fluid flowing through the system caused by things such as the type of pipe, diameter of the pipe, flow regulating valves and fittings such as any elbows or tees in the piping route
Each application has its own system curve, depicting the pressure created by the piping system over a range of flow rates based on the unique components of the individual system. The curve is always an upward sloping parabola.
A friction dominated system with little head loss due to fluid level elevation changes results in a steeper curve. A static head dominated system with large head loss due to fluid level elevations results in a flatter curve.
Typically provided by a pump manufacturer, a pump curve describes the relation between the flow rate and the head for the actual pump (essentially data about a given pump’s ability to produce flow against certain head). It is only good for one specific pump, with one impeller diameter, operating at one speed and has one best efficiency point (BEP).
Popular Pump Control Methods & Their Inefficiencies
In examining the common methods water districts employ to match the pump performance to the system requirements, it is important to understand how pump curves relate to operating a pump at its rated speed.
As head increases from the BEP, efficiency decreases as the operating point moves up the pump curve toward shutoff head. As flow increases (which means head has decreased) from the BEP, efficiency decreases as the operating point moves down the pump curve. If a valve is installed to restrict flow or use a bypass to restrict head pressure, the pump curve does not change and the operating point simply moves up and down the pump curve.
Common pump control methods and inefficiencies include: