Centre de Recursos per a l’Aprenentatge (CRAI)

Pump and system head curves can be optimized by establishing the operating range of the pumps as developed by way of the system head curve. The pumps must be selected so that:

 The operating points must be near the best efficiency point (BEP). Where possible, the BEP shall be as close as possible to the optimized flow or near the exceedence flow and head point in the system head curve. Exceedence point is defined as the flow in which the pump will operate between 10 to 95 percent at time or operating point of the pump MOST of the time. Refer to Section 2.4.1 for Exceedence Flow Curve.

 The pumps’ operating range must be within the Preferred Operating Range (POR)

From the Preliminary Design Phase (PDP) to the design development phase (DDP), the design engineer is expected to perform several iterations with regards to pump and system curves. With each iteration, the fit of the pump curve may change. If the head losses in the system increase, the pump efficiency may decrease and the pump operating point may change. The design engineer must have the ability to analyze the fit of the pump curve to a system.

Curve shape

When evaluating the fit of a pump curve, the first step is to examine the slope of the curve. An ideal pump curve will always be downward sloping. A flat spot (zero slope) or an upward slope (identified as a dip in the curve) is of concern as this is an unstable operating range. The physics of a centrifugal pump establishes pump head as the independent variable and flow rate as the dependent variable. Meaning the flow rate generated depends on the pump head. In areas where the pump curve is flat or a dip is present, a specific pump head may have a range or multiple possible flow rates. The hydraulics will become unstable and the pump drifts between flow rates. The flat region on the pump curve before the minimum continuous flow region (MCF) is where internal recirculation usually occurs. When the pump is allowed to operate at the flat region of the pump curve, higher vibration is expected due to flow reversal within the water passage of the impeller which would result to cavitation, higher vibration which

ultimately reduce bearing and seal life. Conversely if the operating point along the pump curve exceeds the run-out point on the pump curve at high flow low head, not only that the pump will be operating at the lower efficiency but also the NPSHr is the highest and cavitation could also present. For this reason, a pump should not be selected if there is a flat spot or dip in the performance curve; or the operating range is past the run-out point.

Design Point

The design engineer shall select a pump with the best efficiency at, or close to, the design point. Ideally the design point must be within 25 percent of the best efficiency point. The design engineer shall also verify that multiple pump manufactures are able to meet the required design point.

Operating Range

The design engineer shall check the maximum and minimum operating points along the pump curve.

The system’s maximum and minimum operating points must be within the manufacturer’s recommended pump preferred operating range (POR). The limits of the safe operating range are normally represented on the pump curve by dashed lines near the shut-off head and at the maximum-flow end of the curve.

With regards to multiple manufacturers’ curves, no two manufacturers will have an identical pump performance curve. By specifying certain operating points, such as design points, max flow and min flow, the design criteria may inadvertently exclude certain manufacturers. The design engineer shall review the potential pump curves from various manufacturers to determine what is available. The pump specification should then be modified to include tolerances on the various operating points. The

tolerances shall be adjusted to encompass pump curves from various manufacturers. This process will allow multiple manufactures to bid.

Non-Overloading Horsepower

The design engineer shall review the power requirements for the pump. The required input horsepower to drive the pump is indicated on the pump curve is defined as “brake horsepower” or “shaft

horsepower”. All electric motors are rated based on the output shaft horse power or brake horsepower.

Electric motors are not rated based on the motor input horsepower or kilowatt. Thus the output horsepower of the motor should be the same as the input horsepower of the pump. The design engineer shall verify the required horsepower is non-overloading throughout the entire pump curve (i.e.

power required by the pump does not exceed the nominal rated horsepower of the motor). For mixed flow pumps, the maximum non-overloading horsepower is typically at shut off head. This creates a decision regarding sizing of the electric motors. Covering the entire pump curve may lead to over sizing of the motor. That being said, the shut off head may not be an operating point the pump will ever operate. Unless otherwise stated the pump will never operate near shut off head in which case the motor horsepower can be determine equal to the non-overloading horsepower within the operating range. For variable speed pumping applications, the non-overloading requirement can be limited to the operating range, provided that the pump will never operate beyond this specified range. For pumps driven by VFDs, the motor horsepower shall be determined as follows:

Example:

Calculate the non-overloading or the maximum brake horsepower of the motor (shaft horsepower) throughout the pump curve. Example maximum bhp = 290 hp.

Multiply the brake horsepower by 110% to provide safety factor for any harmonic imparted by the VFD to the motor, which can cause the motor to operate at a higher temperature. Example 290 x 1.10 = 319 bhp.

Use the next standard motor size of 350 bhp. Always remember that the motor rated horsepower is based on how much the motor can deliver in terms of brake horsepower.

System Curve Preparation refer to Figure 4-3

1. The vertical axis of the graph represents head in feet of water. The horizontal axis represents capacity in mgd or gpm.

2. Designate the maximum static head at point “A” at zero flow.

3. Designate the minimum static head at point “B”.

4. Designate the maximum TDH, point “C” at maximum capacity “D”. Designate the intersection of point “C” and “D” as design point “E”.

5. Select the pump which has the best efficiency point (BEP) near the design point. If the optimized flow and head is other than the design point, select pumps so that when they operate at the optimized flow and head, the BEP should be as close as possible to the optimized flow and head point. For this example, the optimized flow and head is at 700 gpm.

6. Select three pumps so that when all three pumps operate at the same time, they can deliver and meet the design point total pump station capacity of 1,700 gpm @ 130 ft TDH. Make sure that the pumps operate within their POR.

7. Check if only one pump is operating, the operating point is not exceeding the maximum POR or within the run-out point in the pump curve “F”)

8. Plot the performance curve of one pump. Curve “G”.

9. Plot the performance curve for two pumps operating. Curve “H”.

10. Plot the system head curve for three pumps operating. Curve “I”.

11. The design capacity of each pump will be approximately the point where a selected pump curve intersects the design head.

12. Make a tentative pump selection from manufacturer’s curves and superimpose the cumulative pump performance curves on the system head curve as indicated in Figure 4-3. Note the maximum range of head that could possibly be imposed on any pump when running alone or if all pumps were operating.

13. Ensure that the selected pumps can operate over the range of head conditions that are possible.

Check NPSH, energy requirements and thrust requirements over the range of head for each type of pump included in the accumulative curves.

14. It is possible to make a detailed investigation of several manufacturers’ curves and specify one or two performance points in addition to the design capacity. For complex application, attach a copy of the pump and system head curve at the end of the specifications. Such an analysis is not necessary during preliminary design and usually it is adequate to specify only the following data.

a. Design capacity and design head (design point) b. Minimum pump efficiency at design point c. Maximum shut off head

d. Range of heads over which pump may operate

e. Maximum required power input at any point within operating range

15. Figure 4-4 shows a typical hydraulic profile of a pump station lifting one from one reservoir and discharging into another. The profile could be applied to sewage applications also. It is important to note that the design head of the pumping units must exceed the mean static lift and should also be designed to exceed the maximum possible total dynamic head

Figure 4-3: Typical System Head-Capacity Schematic

Figure 4-3: Typical System Head-Capacity Schematic

Number of Pumps

Several factors influence the appropriate number of pumping units. There are applications where only one unit is necessary (such as a standby fire pump) but in general, two units (one in service and one standby unit) is the minimum number of units that should be provided for municipal water, wastewater and storm water applications.

On the other hand, there may be circumstances where ten or more units may be warranted.