Cover Page for Lab Report Group Portion. Pump Performance

Transcription

1 Cover Page for Lab Report Group Portion Pump Performance Prepared by Professor J. M. Cimbala, Penn State University Latest revision: 02 March 2012 Name 1: Name 2: Name 3: [Name 4: ] Date: Section number: ME 325. Group # Score (For instructor or TA use only): Lab experiment and results, plots, tables, etc. - Procedure portion Discussion Neatness & grammar TOTAL / 50 / 10 / 10 / 70 Comments (For instructor or TA use only):

2 Procedure and Presentations of Results A. The Test Rig and Its Operation A continuous pump flow rig is used in this lab experiment, as sketched in Figure 4. It is basically the same rig as the one that is used in the pipe flow experiment except that the pipe specimens are replaced by a centrifugal test pump, connected by flexible hoses. The test pump has two static pressure taps installed so that the head gain produced by the test pump can be measured directly, using a differential pressure transducer. Quick connect pressure line couplings are used to connect the pressure taps to the pressure transducers, so that the pressure lines can be connected quickly and easily. The volume flow rate through the test rig is measured by the Rosemount magnetic flow meter. Magnetic resonance flow meter Quick connects Pressure tap to high port of Head pressure transducer Flow control valves Pressure tap to low port of Head pressure transducer Test pump Water tank Flow pump Pump Figure 4. Schematic diagram of the pump flow test rig. The test pump is powered by a variable DC supply, so that pump speed can be varied. The shaft rotation speed n as well as the shaft torque, T, are measured directly by the Himmelstein torque and RPM meter. The brake horsepower, bhp, supplied to the pump is calculated from these two measurements, as explained in the Introduction. The back pressure (at the pump outlet) is controlled by a valve. If the valve is closed completely, no water flows through the pump (Q = V = 0), and the net head H is near its maximum value, as shown in Figure 3 of the Introduction. As the valve is opened, Q increases, and H begins to decrease, (the column height difference between the two manometer tubes decreases). The largest volume flow rate, Q max, is achieved when the valve is open enough such that there is no net head gain (or loss) across the pump (free delivery). Note that because the flow pump is capable of supplying a larger head and volume flow rate than the test pump, it is possible to generate conditions in which Q is actually larger than Q max, in which case the test pump supplies a negative net head to the flow in other words, the test pump acts like a minor loss in the system. By carefully adjusting either of the two downstream flow control valves, it should be possible to control the volume flow rate through the test pump so that it spans the desired range of interest, namely from Q = 0 to Q = Q max. B. Calibration of the Head Electronic Pressure Transducer To simplify the task of data collection, the net head across the pump is measured electronically by the computer data acquisition system. The Validyne electronic differential pressure transducer marked Head consists of a thin stainless steel diaphragm within a chamber. Each side of the chamber has a port, which will be connected to one of the pressure taps. Specifically, the low pressure side of the diaphragm will be connected to the static pressure tap at the upstream end of the test pump, while the high pressure side will be connected to the static pressure tap at the downstream end. When the flow loop is running, the test pump is on, and the flow control valves are set properly, the test pump provides a head gain across the pump. The larger pressure downstream of the pump causes the diaphragm inside the pressure transducer to deflect slightly. This deflection is measured electronically and is converted into a DC voltage that is displayed by the Validyne display unit. This voltage is also sent to the computer s Analog-to-Digital (A/D) converter for processing. As presently set up, the A/D converter can read voltages in the range from -5 to 5 volts. However, the Validyne display unit output is an analog voltage that ranges from only -2 to 2 volts. The display unit actually displays the voltage times a factor of 100. For example, a reading of 158 on the display unit corresponds to an analog voltage output of 1.58 volts. A reading of 200 units corresponds to the maximum 2.00 volts of the unit. Thus, to avoid clipping of the signal, never exceed 200 units on the Head display unit while acquiring data. Prior to data collection, the differential pressure transducer must be calibrated to measure the proper head, and to set the span such that nearly the full range of the display unit is utilized (for highest accuracy). The maximum head gain expected in this lab is less than 200 inches of water column, and the unit will be calibrated such that 100 inches of water corresponds to 100 display units, or 1.00 volts. There is a calibration stand in the lab, which is set up to provide 48.0 inches 1

3 of water head as a calibration point. In this lab experiment, therefore, the head transducer will be calibrated such that volts (48.0 display units) corresponds to 48.0 inches of water head. (48 inches per 0.48 volts is the same as 100 inches per 1.0 volts.) To calibrate the transducer, follow these steps: 1. Place both stainless steel calibration containers on the top shelf of the calibration stand, side by side. 2. Check that both manometer tubes coming from the calibration tanks are connected to the pressure transducer inputs (high pressure tank to the high pressure port and low pressure tank to the low pressure port). 3. Add water to the calibration containers (if necessary) until they are about 3/4 full (and approximately equal in level). 4. This step is critical! Any trapped air bubbles in any of the lines will lead to severe calibration errors. Bleed water from the manometer tubes into the return tank by opening the two blue thumb bleed valves (located near the bottom of the manometer) to remove any air bubbles trapped in the manometer tubes. Also open the blue thumb valves that are connected to the Head differential pressure transducer. At the same time, open both bleed valves on top of the Head differential pressure transducer to bleed any air bubbles trapped in the lines or in the transducer chamber itself. Note: These valves are open when the switch handle is vertical, and are closed when the handle is horizontal. Bleed all of the air bubbles out of both lines and both manometer tubes. Lightly tap the control panel to dislodge any bubbles trapped inside. If you are unable to get rid of the air bubbles, get help from your instructor or TA before proceeding. When all air bubbles have been removed, you are now ready to calibrate. 5. Add water to (or drain water from) one of the calibration containers if necessary until the water in both containers is at the same level (approximately 68 inches on the manometer scale, which has an arbitrary datum plane). 6. At this point, there is zero pressure difference across the pressure transducer. If necessary, adjust the ZERO potentiometer on the Head Validyne display unit until the reading is zero. 7. Carefully and slowly move the right hand (low pressure) calibration container from the high shelf to the low shelf. If not done slowly, new bubbles may form in the manometer tube take your time here or you will have to start over at Step Add water to (or drain water from) the low pressure calibration container if necessary until the difference in height between the two manometer tubes is exactly 48.0 inches. 9. At this point, the pressure difference across the pressure transducer is 48.0 inches. Adjust the SPAN potentiometer on the pressure transducer display unit until the reading is 48.0 units. As discussed above, this calibrates the transducer to 48.0 inches of water head per 0.48 volts (100 inches of water per 100 units). 10. Now that the span has been set, the zero may have shifted slightly. Slowly return the low pressure calibration container to the upper shelf. The Head Validyne display unit should return to zero. Repeat Steps 5 through 9 if necessary until both the zero setting and the pressurized setting are correct. 11. The differential pressure transducer is now properly calibrated. (A linear transducer response is assumed, and is quite accurate.) Return the low pressure calibration tank to the upper shelf. 12. Close off the blue thumb valves and then disconnect the tubing from the pressure transducer to the calibration tanks. Instead, connect the high pressure port of the pressure transducer to the quick connect leading from the pressure tap at the downstream (outlet) end of the test pump. Connect the low pressure port of the pressure transducer to the quick connect leading from the pressure tap at the upstream (intlet) end of the test pump. C. Data Collection 1. Add water if necessary to the water tank so that the flow pump is fully submerged. Verify that the test pump is properly installed, and that all the flow control valves are partially open. 2. Turn on the flow pump and make sure there are no significant leaks through any of the connections (some dripping is okay). If leaks persist, call your instructor or TA for assistance. The head reading should be negative, indicating that the test pump is acting as a minor loss. This is expected, of course, when the test pump is not turned on. 3. Bleed the flow system of all air bubbles. Note: This is a very critical step! Air bubbles in any of the pressure lines will lead to gross errors in your data. To bleed the air bubbles properly, close the flow control valve about 95% of the way (not fully closed because this will burn out the flow pump). This lowers the flow rate to a trickle, and increases the head in all the tubes, which aids in purging out any trapped air bubbles in the lines. Open both bleed valves above the pressure transducers. Tap on the tubing quick connects to release trapped air bubbles, and watch them until they move all the way through the pressure line, through the control panel, through the pressure transducer, and out the bleed valve. Close both bleed valves. 2

4 4. Start program PumpLabMeasure from the computer s desktop. In the User Control window, Enter Parameters. In the User Entries window that pops up, Save Results File, and enter a file name for your data set. For uniqueness of your data file, name your file something indicative of your group and of the test specimen, such as Smith_groupC_pump_1000.txt, where Smith is the name of one of the group members, and the 1000 represents the rpm of the first data set. Unless calibrated otherwise, the calibration constant should default to 1.0 V = inches of water head, since the Head transducer is calibrated at 48.0 units (0.480 V) per 48.0 inches of water head. OK. 5. Turn on the test pump, and adjust the speed of the test pump until the rpm is as close as possible to 1000 rpm. Check for any additional air bubbles in the pressure lines, and bleed if necessary. (5) 6. Adjust the valves if necessary so that the Head display is positive (the pump is operating somewhere within its designed operating range). Record the following values from the instruments (manual readings): Manual readings at 1000 rpm: Torque, T: ounce-inches (oz-in) Rotation rate, n : rotations per minute (rpm) Volume flow rate, Q or V : liters per minute (L/min) Motor supply voltage, E: volts (V) Motor supply current, I: amperes (A) (5) 7. Convert the volume flow rate to cubic meters per second, showing your calculations below, including all unit conversions: Calculated Q or V : m 3 /s Calculate the electrical power delivered to the motor, Welectric EI in watts, showing your calculations below: Calculated W electric : W (5) 8. Compare the manual reading of the pump s net head, torque, and rotation rate to those measured and calculated by the computer (take a data point using the data acquisition system be sure to also enter into the computer the volume flow rate displayed on the Rosemount flow meter before starting the data acquisition process. Note that the manual readings fluctuate and are difficult to read the computer averages the voltages for several seconds so that the averaged outputs are much more stable, easier to read, and therefore more accurate. Manual net head calculation: Validyne Head display: units Validyne Head voltage: Volts (V) Calculated net head, H: inches of water (in H 2O) Computer-acquired values: Net head, H: inches of water (in H 2O) Torque, T: ounce-inches (oz-in) Rotation rate, n : rotations per minute (rpm) Make sure that the computer s values agree reasonably well with those recorded manually. If they don t, something is not set correctly ask your instructor or TA for assistance if needed. It is a good idea to take a few data points with the computer to verify that the flow rate and the readings are stable and repeatable. 9. Open an Excel spreadsheet, and list your group members names and date at the top. Type the following column titles near the top of the spreadsheet: Motor voltage (V), Motor current (A), RPM (rot/min), Torque (oz-in), Volume flow rate (L/min), and Net head (in H 2O). Enter the values recorded above as your first data point. 3

5 10. Sample many more data points at various flow rates, but at a constant test pump rotational speed of 1000 rpm. Start with the flow control valve open enough to get nearly zero net head, and then slowly close the valve. At each valve setting, use the computer to measure net head H, torque T, and rotation rate n. Other parameters, such as the volume flow rate of water, motor voltage, and motor current must be read from the instruments and entered manually into the spreadsheet. Note: As H changes, the load on the test pump is changed, and its rpm may drift. Adjust the pump speed as required (with the potentiometer) in order to maintain a constant rpm throughout the test, as best as possible (it is somewhat sensitive and difficult to control precisely). The manometer tubes may need to be bled occasionally to free trapped air bubbles. Be sure to take enough data points for meaningful results; take data from Q = Q max to Q = 0. Never close the control valve completely for more than a few seconds, as this can cause pump burnout! It is OK to cycle the flow rate up and down, taking data along the way. Note: Monitor the Head Validyne display unit to ensure that the reading never drops below zero units or rises above 200 units. Repeat some of your data points more than once to check for repeatability of the measurements. 11. Increase the rotational speed to about 2000 rpm. Repeat the data collection for the full range of valve settings. Make sure to begin at Q = Q max, which corresponds to zero head loss over the pump, and continue until Q = 0. You will need to open the valves around the pump to achieve Q max. 12. When finished, turn both pumps off. Do not turn off the computer or the Himmelstein unit or the magnetic flow meter, and do not drain the water from the lower tank unless told to do so by your instructor or TA. You may exit the data acquisition program (Quit). E. Presentation of the Data (5) 1. Being careful with unit conversions, add columns to your spreadsheet and calculate angular velocity (rad/s), net head H (m), volume flow rate V (m 3 /s), supplied motor power W electric (W), water horsepower delivered by the pump W water horsepower (W), pump efficiency pump (dimensionless), and pump-motor efficiency pump-motor (dimensionless). Show sample calculations for all parameters here: Sample Calculations: (5) 2. Generate a plot of net head as a function of volume flow rate. Include both sets of data (at both rpm settings) on the same plot (use a different symbol for each rpm, and be sure to include a figure caption, figure number, and legend on your plot). 4

6 (5) 3. Generate a plot of pump and pump-motor efficiency as a function of Q for both sets of data on the same plot. (5) 4. Add additional columns, as needed, to your Excel spreadsheets in order to calculate the nondimensional pump parameters C Q and C H for each data point, and for each rpm case. Note: For the pump used in this experiment, the impeller diameter D = 3.5 in. Show sample calculations of C Q and C H in the space below, showing all units, unit conversions, etc. Make sure all your units cancel, as they must since these are dimensionless parameters. Sample Calculations: (5) 5. Make printouts of both of your Excel data files. Attach these tables, along with a title and table number for each, to this report. Record the table numbers here as well. See Tables. (5) 6. Generate a plot of C H, with all the data from both rpm settings on the same plot. Use different symbols for each case so that the data are distinguishable. Number, label, and attach your plot to your report and record the number here. (5) 7. In a similar manner, generate a plot of pump and pump-motor for all cases. Number, label, and attach your plot to your report and record the number here. 5

7 Discussion (5) 1. Explain how this lab has helped you to understand the performance characteristics of centrifugal pumps. Specifically, discuss the trade-offs between high volume flow rate and large pump head. (5) 2. Do the predictions of dimensional analysis really work? Specifically, do the data for C H from different pump speeds collapse onto the same curve when plotted nondimensionally? Why or why not? What about the data for pump and pump-motor? Compare and discuss. 6