Water is pumped between the two tanks described in Example 12.4 once a day, 365 days a year, with each pumping period lasting two hours. The water levels in the two tanks remain essentially constant. Estimate the annual cost of the electrical power needed to operate the pump if it were located in your city. You will have to make a reasonable estimate for the efficiency of the motor used to drive the pump. Due to aging, it can be expected that the overall resistance of the system will increase with time. If the operating point shown in Fig. E12.4c changes to a point where the flowrate has been reduced to \(1000 \mathrm{gpm}\), what will be the new annual cost of operating the pump? Assume that the cost of electrical power remains the same.

Fig. E12.4c

A centrifugal pump having the characteristics shown in Example 12.4 is used to pump water between two large open tanks through \(100 \mathrm{ft}\) of 8-in.-diameter pipe. The pipeline contains four regular flanged \(90^{\circ}\) elbows, a check valve, and a fully open globe valve. Other minor losses are negligible. Assume the friction factor \(f=0.02\) for the \(100-\mathrm{ft}\) section of pipe. If the static head (difference in height of fluid surfaces in the two tanks) is \(30 \mathrm{ft}\), what is the expected flowrate? Do you think this pump is a good choice? Explain.

Example 12.4

Water is to be pumped from one large, open tank to a second large, open tank as shown in Fig. E12.4a. The pipe diameter throughout is 6 in., and the total length of the pipe between the pipe entrance and exit is \(200 \mathrm{ft}\). Minor loss coefficients for the entrance, exit, and the elbow are shown, and the friction factor for the pipe can be assumed constant and equal to 0.02 . A certain centrifugal pump having the performance characteristics shown in Fig. E12.4b is suggested as a good pump for this flow system.

With this pump, what would be the flowrate between the tanks? Do you think this pump would be a good choice?

Fig. E12.4a

Fig. E12.4b

Transcribed Image Text:

Head, ft Efficiency, % 100 66.5 Head Efficiency- 84% System curve (Eq. 4)- Q. gal/min 1 1 Operating point 1600 2400