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 letter: (A, B, ...) _____
Score (For instructor or TA use only):
Lab experiment and results, plots, tables, etc. - Procedure portion / _____ / 50Discussion / _____ / 10
Neatness & grammar / _____ / 10
TOTAL
/ ______/ 70Comments (For instructor or TA use only):
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.
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 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 = = 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, Qmax, 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 Qmax, 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 = Qmax.
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 of water head as a calibration point. In this lab experiment, therefore, the head transducer will be calibrated such that 0.480 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, close all thumb valves, and close both transducer bleed valves. 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 1.
8. 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.
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 = 100.0 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, : ______rotations per minute (rpm)
Volume flow rate, Q or : ______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 : ______m3/s
Calculate the electrical power delivered to the motor, in watts, showing your calculations below:
Calculated : ______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 H2O)
Computer-acquired values:
Net head, H: ______inches of water (in H2O)
Torque, T: ______ounce-inches (oz-in)
Rotation rate, : ______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 H2O). Enter the values recorded above as your first data point.
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 . 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 = Qmax 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.