Flow Measurements Using Orifice
Objective s
ORMAT To calibrate an orifice meter for flow measurements in small tubing, to become familiar with computer data acquisition and to explore cavitation.
Discussion
In engineering practice, it is rarely possible to measure the rate of flow of a fluid by a direct volumetric or gravimetric determination. Frequently, the metering is accomplished by the observation of some measurable quantity related to the rate of discharge. Orifices, nozzles, Pitot and Venturi tubes produce a differential pressure related to the flow velocity. The differential pressure can be measured with a manometer, pressure gage, or pressure sensor. Pressure sensors are widely used because they provide a voltage output that can be monitored easily by computer.
CEE 331 Software
The Easydata Software can be used to acquire data from a pressure sensor connected to an orifice meter and from a pressure sensor used as a volume detector.
Pressure Sensors
Figure 1. Schematic of the experimental setup.
The pressure sensors used for this experiment measure the differential pressure between two ports. The pressure sensor used to measure the pressure drop across the orifice has a range of 0 to 200 kPa with a corresponding output voltage range of 0 to 100 mV. The pressure sensor used to measure the volume of water in the volumetric detector has a range of 0 to 7 kPa with a corresponding output voltage range of 0 to 16.7 mV. The pressure sensor outputs can be scaled using conversion files. The pressure sensors are connected as shown in Figure 1. The pressure reducing valve is optional.
Orifice Calibration
The change in piezometric head resulting from the flow of a fluid through an orifice is obtained by application of the energy equation between a cross section upstream and a cross section at the vena contracta. Several different locations are commonly used for the pressure taps and the pressures measured are strongly dependent on the pressure tap location. Some kinetic energy is converted back to potential energy downstream from the vena contracta. The pressure tap locations in the laboratory orifice meter are configured as shown in Figure 2.
The orifice equation contains an empirically determined coefficient, Korifice, that is a function of the orifice Reynolds number (based on the orifice diameter).
Figure 2. Location of the pressure taps on the laboratory orifice meter.
1
The orifice coefficient is a weak function of Reynolds number.
Scope of Experiment
In this laboratory you will calibrate one orifice plate and determine the cavitation potential of the orifice.
Procedure
Pressure Sensor Scaling and Monitoring
1) Start the Easy Data software to monitor the pressure sensors.
2) Connect the 200-kPa pressure sensor to the middle row of ports.
3) Connect the 7-kPa pressure sensor to the middle row of ports.
4) Apply scaling to the sensors so that the 7-kPa sensor output is in mL of water using the volumetric detector and the 200 kPa sensor output is in cm. (This may only require opening the correct configuration file.)
5) Set the data rate to 1 Hz.
6) Turn on the “power user” features by pressing the F1 key.
Orifice Calibration
1) Connect the 200-kPa pressure sensor to the orifice meter. The high-pressure port should be connected to the upstream pressure tap (2.5 D upstream from orifice plate). The bore of the orifice plate is 1.59 mm.
2) Connect the high pressure port of the 7-kPa pressure sensor to the port at the base of the volumetric detector.
3) Connect the orifice meter to the supply and drain lines.
4) Insert the drain line into the small tube at the top of the volumetric detector
5) Open the flow control valve very slowly! Water hammer from a surge of water through an air filled orifice tube can damage the pressure sensor.
6) Alternately hold sections of the supply line including the pressure reducing valve vertically so that air is purged from the line.
7) After the tubing has filled with water and there is a small amount of water in the volumetric detector turn off the water and then subtract the offset voltage from both sensors by pressing . Note that there is a zero function that zeros all sensors as well as zero functions that zero individual sensors.
8) Turn on data logging to file .
Data acquisition loop
1) Open the flow control valve approximately 1/4 turn.
2) Zero the volumetric detector sensor . (Don’t zero all of the sensors!!!!)
3) Continue acquiring data for 250 mL or 20 seconds (whichever is shorter).
4) Zero the volumetric detector sensor .(Don’t zero all of the sensors!!!!)
5) Repeat steps 1-4 for the following settings: ?, ?, 1 – 9 turns in increments of 1 turn.
The results of this data acquisition will be a data series that can easily be broken in segments that correspond to different flow rates. The slopes of the data segments from the volumetric detector determine the flow rates. The head losses are obtained from the averages of the 200-kPa sensor data segments.
Cavitation Potential
Your goal is to determine if there is danger of cavitation in the orifice (caused by the pressure approaching the vapor pressure of water). Determine how you could find the velocity and the pressure in the orifice (the lowest pressure point in the system!). Take appropriate measurements so you can answer #6 below. Note that you may have to modify the apparatus.
See if you can hear the sound of cavitation and if you can observe any differences in the water at the maximum flow rate.
Report
Before leaving the lab complete #2 below and show your results to the TA. Make sure you will be able to answer all of the questions below!
Write a discussion and conclusion for the results of this experiment, incorporating answers to the following items. Submit your report electronically as a well formatted Word document and include your Excel spreadsheet as an appendix.
1) Explain how you obtain the actual flow rate for each run from the output of the volumetric detector.
2) The 200-kPa pressure sensor measured the difference in pressure (a drop in the HGL) but the desired term is really a head loss (a drop in the EGL). Why can the pressure sensor measurements be used directly to obtain the head loss?
3) Create a graph showing the relationship between the orifice coefficient and Reynolds number.
4) Identify the major sources of error for this experiment and comment on which errors are potentially most significant.
5) The pressure sensor accuracy is approximately 1% of full scale. What is the flow rate that corresponds to 1% of the full-scale voltage? How does this affect your accuracy at a flow rate of 4 mL/s? (Determine the pressure drop that corresponds to 4 mL/s and then increase the pressure drop by 1% of the full range for the pressure sensor and calculate a new flow rate.)
6) Calculate the velocity in the vena contracta of the orifice at the highest flow rate achieved. Estimate the absolute pressure in the small orifice at the highest flow rate achieved. State your assumptions and show your work. Create a scale drawing of the HGL and EGL for your system from upstream of the orifice to the outlet. Was the orifice in danger of cavitation? If the orifice wasn’t in danger of cavitation, how could you explain your laboratory observations? Don’t forget the distinction between gage and absolute pressure!
Lab Setup
Use 100 mV range for the data acquisition ports.
CEE 332: Hydraulic Engineering Spring 2005
Monroe Weber-Shirk