Mumbai University > Electronics and telecommunication > Sem 7 > Applied Hydraulics

Marks: 08

Years: MAY 2016

The characteristics if a centrifugal pump are:

the pump chars. are all related to the flow rate so they are all drawn with the flow rate as the X-axis and each on as Y-axis:

• The head : being maximum at zero flow rate and zero at free delivery and is drawn against the flow rate. note that when drawing the H-Q curve for a pump, the head is the independent variable with Q a dependent one.

• the power : depends upon the power delivered to the fluid and the efficiency of the pump which variant with flow rate, so the power depends on the head and the efficiency at some specified flow rate.

• efficiency : which is dependent on flow rate being zero at zero flow rate or zero head and maximum in between.

• NPSH: divided in two curves one for NSPH available and one for NSPH required which must be controlled to avoid cavitation and you must understand it will from any online website.

Performance characteristics—such as flow, pressure, efficiency and brake horsepower—are shown graphically on a pump curve. The first item to look at is the size of the pump. The size of the pump, 2x3-8 is shown in the upper section of the graph. The numbers 2x3-8 indicates:

The outlet (discharge port) is 2 inches. The inlet (suction port) is 3 inches. The impeller has an 8-inch diameter. Some companies may have the number shown as 3x2-8. The larger of the first two numbers is the inlet. Pump speed (rpm) is also shown in the upper section of the graph and indicates performance at a speed of 3,560 rpm. All the information is representative of this operational speed.

Capacity or flow is shown along the bottom of the curve. The flow levels are shown for the operating speed of 3,560 rpm but indicate the effect of head as the outlet is throttled.

The left side of the performance curve shows the head (feet) generated at the different flow rates. Multiple flow versus head curves are present on the graph (see Figure 3). Each one represents a different (trimmed) impeller size. For this pump, the range of impellers is 5.5 inches to 8.375 inches.

Efficiency curves are overlaid on the graph (vertical lines) and indicate from 64 to 45 percent efficiency for this pump. As head is increased, flow and efficiency decrease.

Brake horsepower is shown with the dashed lines drawn diagonally from upper left to lower right. BHP curves are shown for 7.5 to 30 horsepower. Using the 8-inch impeller with a flow of 250 gpm, the BHP is approximately 25 horsepower.