PUMP CURVES
Just like their counterpart, the fan curves, the "pump curves" or "pump performance curves" signify how much pressure or 'head' the pump will produce at a given flow rate, rpm and impeller diameter. 'Pump curves provide a treasure trove of knowledge about how the flow rate changes when the system pressure gets affected in particular arrangement'. The volute surrounding the impeller transfers this velocity energy of the fluid to Pressure head and hence gives us the increase in pressure obtained in the pump discharge.
The pump curve is generally plotted on a static pressure VS flow graph.
Now that we know what the pump curve looks like, let us look at how the affinity laws discussed earlier affect the pump curves.
​
Let us try to recall one of the pump laws we studied earlier.
The resulting changes in the pump curves with change in impeller speed and pump diameter are given as different curves on the same pump chart.
IMPORTANT TERMS AND PHENOMENONS
CAVITATION:
The phenomenon can be very easily explained. Water at a lower pressure area changes to gas at a lower temperature. This causes bubbles to be formed at this stage. When this water flows into a high pressure area suddenly, the boiling temperature increases and the water bubbles want to revert to the liquid state. Thus the entire gas bubble reverts to a tiny liquid molecule, and consequently pulls in material from the surrounding areas. If the bubble collapses next to a pipe wall, it damages the pipe and creates a cavity. This phenomenon is called CAVITATION.
NET POSITIVE SUCTION HEAD:
Cavitation can occur at the inlet of the pump. The suction of the inlet produces bubbles which explode when they enter the pump and damage the impeller.
In order to avoid cavitation, the pressure at the pump inlet must be greater than the vapor pressure of the liquid at this point by an amount termed as the Net Positive Pressure Required (NPSR) or Net Positive Suction Head Required (NPSHR).
The difference in this pressure and the vapor pressure is termed as the Net Positive Suction Head Available(NPSHA)
NPSR – VP = NPSA
Thus, the NPSA should always be greater than the NPSR. If not so, there may be problems like inefficient pumping and cavitation which will increase electricity and maintenance expenses.
​
The NPSHR is usually calculated and listed on the pump curved for reference by pump manufacturers. The line on the bottom right in the diagram below signifies this given property of the pump.
PUMP POWER CURVE
The power curve plotted on the graph below signifies the amount of power actually transferred to the water and is termed as Water Horse Power (WHP). It is the total energy imparted to the water by the pump.
It is the output power of the pump
Pump input power:
Pump input power is the power consumed by the motor which drives the impeller of the pump. The pump motor draws energy in the form of electricity and this is termed as the input power of the the pump.
This energy can be termed as the input power of the pump.
The above mentioned diagram shows the constant power lines plotted as dotted lines on the chart. Where these lines intersect the pump curve, the power of the pump at that operating point is specified. These are hence called the 'power lines'.
PUMP EFFICIENCY CURVES:
Pump efficiency is the ratio of input power provided to the motor and the output power given to the water. Thus,
​
​
​
​
This the efficiency of the pump at any given working condition.
The value of the efficiency for a given pump and varying parameters like flow rate, pressure head, impeller size, speed etc. are plotted to get the Pump Efficiency curves. the pump efficiency curves appear as bands and help to determine optimum working conditions for the pump.
