PUMP CHARACTERISTIC CURVES
The performance of a centrifugal pump can be shown graphically on a characteristic curve. A typical characteristic curve shows the total dynamic head, brake horsepower, efficiency, and NPSHr all plotted over the capacity range of the pump.
TOTAL DYNAMIC HEAD VS. CAPACITY
This curve is the plot of the total dynamic head (HT) on y-axis and capacity on x-axis. The head at zero flow is called shut-off head. For a radial flow centrifugal pump, the curve is flat at low flow region and slopes downwards as flow increases. For simplicity we consider on the radial flow centrifugal pump operation. This plot is also called the head – flow curve. This curve gives the “duty condition” of the pump.
POWER VS. CAPACITY
This curve is a plot of the power consumed by the pump (BkW) on y-axis and capacity on x-axis. Please note that this curve shows the electric power consumed by the pump or the electric power delivered by the motor and NOT the hydraulic (PkW) power delivered by the pump.
EFFICIENCY VS. CAPACITY
This curve is a plot of the efficiency by which pump delivers the hydraulic power on y-axis and capacity on x-axis. The efficiency increases at first with increasing flow, levels off at a certain flow rate and falls after the flow rate is further increased. The point where the efficiency is maximum is the called the Best Efficiency Point (BEP) is of critical importance in pump selection and operation as will be discussed later.
NPSHr VS. CAPACITY
This curve is a plot of NPSHr on y-axis and capacity on x-axis. Determining the value NPSHr is a long process and many precautions need to be taken to arrive at the value of NPSHr. This curve is used to check whether the NPSHr of the pump is less than the NPSHa mentioned by the buyer at the desired flow rate. A typical plot of NPSHr vs Q is shown. This plot is dependent on many external factors in addition to pump design.