A pump is hardware that feeds energy to a fluid (e.g. Water) to flow through channels. Pumps are used, for example, to direct water out of the ground, to transport drinking or sewerage water over large distances in combined pipe networks or to discard water from polders. In any practical application, the pump needs to work with its best performance. It is also important to check that the flow rate and head of the pump are within the required specifications, which are normally presented as the Pump Characteristic curves. These plots play an important role in understanding the region in which the pump needs to be operated thus ensuring the life of the pump.
Pump Characteristic Curves
The performance of any type of pump can be shown graphically, which can be based on either the tests conducted by the manufacturer or the simulations done by the designer. These plots are presented as Pump Characteristic Curves. The hydraulic properties of any pump (e.g. Centrifugal Pump) can be described by the following characteristics.
- Q-H Curve
- Efficiency Curve
- Net Positive Suction Head (NPSH) Curve
The Q-H curve gives the relation between the volume flow rate and the pressure head, i.e. the lower the pump head, the higher the flow rate. Q-H curves are provided by the manufacturer of the pump and can normally be considered as simple quadratic curves.
This gives the relation between pump efficiency against the volumetric flow rate. The efficiency of the pump is directly associated with the running cost. Lower efficiency of a pump results in higher power consumption and, in turn, an increase in operating cost to run it. While selecting the pump, it has to be selected based on the Best Efficiency Point known as BEP. However, the pump needs to operate at various off-design conditions and it is preferable to choose the pump with a flatter efficiency curve if the pump is to operate at off-design conditions for a longer duration.
Net Positive Suction Head Curve
The Net Positive Suction Head (NPSH) curve provides information about the suction characteristics of the pump at different flows. At the suction side of a pump, negative pressures can occur due to drop in absolute pressure of the liquid below vapour pressure. This results in formation of bubbles which are carried along with the fluid. As the pressure increases in the pump, these bubbles collapse, resulting in an implosion causing a pitting action on the impeller and casing. This phenomenon is known as cavitation. Even though the cavitation inside a pump cannot be completely avoided, it should be limited as much as possible. A pump should operate in such a way that the available NPSH is always higher than the required NPSH so the excessive cavitation in the pump can be prevented.
To maintain a strategic distance from cavitation, the available NPSH should be larger or equal to the required NPSH. The available NPSH is defined as:
NPSH available = Ha + ho – hv Here, Ha = Suction head at the impeller
ho = Atmospheric pressure
hv = Vapour pressure
Ha = h + V2 / 2g Here, h- pressure at the impeller entrance
V- Velocity of the water at the impeller entrance
How to Generate Pump Curves
Pump curves can be generated by reading measurements of pressure, NPSH, power, efficiency and others, while the pump is operating at different flow conditions. These results are then plotted against the flow rate to generate the pump characteristic curves. This method is useful for pumps in operation, but not entirely helpful for design purposes. Luckily, there is a way to generate these pump curves and more during the design process, to make sure the pump you need is the pump you get. AxSTREAM is the pump design and analysis tool that helps generate the pump characteristic curves. Using AxSTREAM, the pump can be designed from the known specifications as well as analyzed for different off-design characteristics. The performance curves can be generated during such analysis (see Figure 4). The calculations of AxSTREAM have been extensively validated with various experimental data helping the designers to produce these curves with very good accuracy.
One must be wondering how can these curves be utilized in a system design, for example, in designing a pumping station. AxSTREAM NET™ is the one-dimensional thermal fluid analysis tool of SoftInWay, which can be used for designing the pumping station. It consists of various other components apart from the pumps. The entire pumping station can be modeled inside AxSTREAM NET™ including several components such as valves, pipes, accumulators/reservoirs etc. The layout can be analyzed to simulate various situations. The performance characteristics generated in AxSTREAM, and incorporated in AxSTREAM NET™, helps to accurately predict the performance of the pumping station and to optimize it at the system level. A simple example of a pumping station, designed and calculated in AxSTREAM NET™, is shown in Figure 2.