The different types of head are combined to make up the total system head at any particular flow rate. The descriptions in this section are of these combined or dynamic heads, as they apply to the centrifugal pump.
Total Dynamic Suction Lift (hs)
Total dynamic suction lift is the static suction lift minus the velocity head at the pump suction flange plus the total friction head in the suction line. The total dynamic suction lift, as determined on a pump test, is the reading of a gauge on the suction flange, converted to feet of liquid and corrected to the pump centerline, minus the velocity head at the point of gauge attachment.
Total Dynamic Discharge Head (hd)
Total dynamic discharge head is the static discharge head plus the velocity head at the pump discharge flange plus the total friction head in the discharge line. The total dynamic discharge head, as determined on pump test, is the reading of a gauge at the discharge flange, converted to feet of liquid and corrected to the pump centerline, plus the velocity head at the point of gauge attachment.
Suction lift exists when the source of supply is below the centerline of the pump. Therefore, the static suction lift is the vertical distance in feet from the centerline of the pump to the free level of the liquid to be pumped.
Suction head exists when the source of supply is above the centerline of the pump. Therefore, the static suction head is the vertical distance in feet from the centerline of the pump to the free level of the liquid to be pumped.
Static discharge head is the vertical distance in feet between the pump centerline and the point of free discharge or the surface of the liquid in the discharge tank.
Total static head is the vertical distance in feet between the free level of the source of supply and the point of free discharge or the free surface of the discharge liquid.
Total Head or Total Dynamic Head
Total head (H), or total dynamic head (TDH), is the total dynamic discharge head minus the total dynamic suction head:
TDH = hd + hs (with suction lift)
TDH = hd – hs (with suction head)
The work performed by a centrifugal pump is a function of the total head and the weight of the liquid pumped in a given time period. The pump capacity in gallons per minute and the liquid specific gravity are normally used in the formulas rather than the actual weight of the liquid.
Pump input or brake horsepower (BHP) is the actual horsepower delivered to the pump shaft. Pump output or water horsepower (WHP) is the liquid horsepower delivered by the pump. These two terms are defined by the following formulas:
Reading a Pump Performance Curve
Pump 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.
Figure 3. Sample pump performance curve
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.