CIVE HYDRAULIC ENGINEERING Fall 2015 Pierre Julien Colorado State University

Transcription

1 CIVE HYDRAULIC ENGINEERING Fall 2015 Pierre Julien Colorado State University Problems with are simple, are considered moderate and those with are perhaps the most important and/or challenging. PROBLEM SET #4 PUMPS 1. Determine the power required to pump 2.5 cfs at 68 F from the lower to upper reservoir. Assume that the friction factor is f = 0.015, and sketch the HGL and EGL. 2. What power must be supplied by the pump if water (T = 20 C) is pumped through a 200 mm diameter steel pipe from the lower tank to the upper one at a rate of mcs? Draw the EGL and HGL. [hint: find f from the Moody diagram] D

2 3. At a flow rate of 0.25 m 3 /s, and head losses such that fl/d = 3.5, determine the power required at the pump and plot the EGL and HGL. 4. For the four pumps given in your handout, determine the specific speed of each pump and locate on the pump classification chart from the handout. 5. What type of pump should be used to pump water at a rate of 12 cfs and a head of 25 ft? Assume N = 1,500 rpm. 6. For the pump below, what is the maximum head that can be generated if the speed is increased to 1500 rpm? Also determine the increase in discharge at a given head for the same condition. [hint: look at C H and C Q ]

3 7. English Units - For the system below and the pump curve given, neglect minor losses and assume f = Determine the head losses as a function of discharge and plot on the pump curve diagram below. Determine the discharge and head at the pump when the pipe diameter is 16 inches. What type of pump would you recommend? Repeat with a pipe diameter of 24 in. If the NPSH is 8 for the 16 line, what is the maximum elevation X max for this pump.

4 8. SI Units Find the discharge pumped under the condition shown given the selected pump characteristics. If the motor is located at an elevation of 23 m, which elevation (propeller or motor) should be considered for the NPSH calculation?

5 9. Pump design experience! English units In the following set-up, the pump must provide 0.125m 3 /s and your company (Jim and Joe Eng.) is charged to design the pipe diameter (single value) and the pump size. Jim tells you to assume f = 0.02 and asks you to determine the flow velocity and velocity head in the pipeline for pipe diameters of 8, 10, 12, 15, etc. You can find the head and power at the pump for each diameter. A) Jim asks you to determine the cost for each alternative and he hands you a 2014 table with the cost for galvanized iron pipe and pumps. The cost of pipes is 2 (6$/ft), 3 (10$/ft), 4 (25$/ft), 6 (35$/ft), 8 (60$/ft), 10 (90$/ft), 12 (120 $/ft), 15 (180 $/ft), 20 (360$/ft), 24 (500$/ft), 30 (720 $/ft), 36 (900$/ft). The cost of pumps was: 10Hp ($2,500), 20Hp ($4,000), 30Hp ($5,500), 40Hp ($7,000), 50Hp ($9,000), 75Hp ($12,000), and 100Hp ($20,000). Regardless of pipe diameter, you expect a fixed labor cost of $150,000 for your project. First meeting with your client - Jim has to travel and asks you to make the recommendation to your very important client named Jill. B) The meeting went very well for you and upon return from travel, Jim asks you whether you considered the replacement and energy cost for your project. Well, I guess you didn t, so he tells you that the energy cost adjusted for inflation is constant at about 10 cents per kilowatt-hour. You will need to replace the pumps every 25 years while the pipes should be fine for 50 years. Consider 5% inflation on the above costs for the pump and pipe to start your detailed analysis from For each pipe diameter, plot the total cost of your project. Meanwhile, Jill is asking for a second meeting with you. Second meeting with your client Jill is all excited about what you said last time and she tells you that her very good friend Jack can supply 2,500 ft of 6 diameter pipe at $15/ft with a 50% discount on the purchase of any suitable pump. This looks like a super deal for the District. Jack told her that the best solution would be to design a system with two parallel pipe lines. In cutting the discharge in half your energy costs should drop a lot, he said.

6 C) Conduct your analysis and find out whether the super deal is so great. You make your recommendation in an to Jill. Soon after, you are asked to meet Joe in his office. You worked very hard and feel ready for a promotion, but Joe is very unhappy since Jack called him and there is a big concern that your company may lose this contract to your rival. Jack said that you change your mind all the time and that your analysis is flawed because on governmental contracts like this, you should only consider the cost over the term of elected officials (say 4 years in this case). Can you sort out this situation? D) What did you learn from this problem? 10. With reference to Problem 9. The NPSH of the pump in your final design is 20. Is it true that it would be perfectly fine because your pump is located 15 feet above the intake level? 11. A pump delivers water from a large reservoir to the tank through a 800 ft long 2 ft diameter cast iron pipe. Assume f = 0.02 and an efficiency of 70% to determine: (a) the minimum pressure in the pipe; (b) the time to fill the tank to a 200 ft level, (c) the power to be supplied to the pump when the tank levels are 100 ft and 200 ft respectively.