Measuring and testing – Volume or rate of flow – Using differential pressure
Reexamination Certificate
1998-04-02
2001-02-20
Patel, Harshad (Department: 2855)
Measuring and testing
Volume or rate of flow
Using differential pressure
C073S756000
Reexamination Certificate
active
06189390
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to methods and devices for measuring gas flow, and more particularly to a method and apparatus for measuring gas flow at a plurality of positions in a conduit.
BACKGROUND ART
There are many circumstances where it is necessary to obtain accurate measurements of the velocity and volumetric flow rates of gas through a stack or duct. Frequently, those circumstances relate to measuring and monitoring of pollutant emissions from industrial sources of air pollution. Many of the methods that are used to measure and monitor pollutant emissions under state and federal regulations include provisions and procedures to determine flow rates. As government regulations have evolved and become more stringent, there has been more and more scrutiny placed on the accuracy of those flow rate measurement methods because of their increased impact on the operating and compliance cost of regulated industrial sources. Separately, there are many engineering needs to more accurately measure and observe gas flow characteristics and changes in those characteristics over a cross section of a stack or duct and over a period of time. Those engineering needs include obtaining more refined design data for use in the design of more effective pollution control equipment and to improve the efficiency and effectiveness of industrial processes in general.
The most significant problem in measuring flow rates through a stack or duct is that the actual gas velocity can be different at different cross sectional positions of the stack or duct. In addition, those velocities are not uniform over time and also not predictable even in the steady state operation of the industrial process. The gas flow at individual positions of the cross section may be flowing at some angle to the center line of the duct and that angle may change over short periods of time. These variations across the duct cross section are typically caused by variations in the operation in the industrial process emitting the gas stream and/or the physical structure of the duct through which the gases are flowing.
The United States Environmental Protection Agency (U.S. EPA) has promulgated regulations which include two methods to be used in measuring the flow of gas through a stack or duct for purposes of administering its regulations regarding emission of pollutants to the atmosphere. Those methods are referred to as U.S. EPA Reference Methods 1 and 2. Method 1 is used to determine the locations of positions on the cross section or traverse area of the duct at which readings are to be taken using Reference Method 2. Method 2 typically uses an S-type pitot tube to obtain a differential pressure reading at each of the points on the traverse, all of which are averaged together and used in a calculation to determine the actual volumetric flow rate of the gas through the stack. That flow rate is then used in another calculation which includes the concentration of an individual pollutant being measured (obtained using another reference method) in the calculation of the total emission rate of a source for regulatory compliance purposes. Any error in the measurement of the flow rate, therefore, is directly translated to an error in the measurement of the individual pollutant or pollutants being measured. In addition, these same readings are used to set and check the calibrations of monitors used to continuously measure the flow rate of gases through the stack. The error is similarly translated and can have significant cost and operational impacts on the regulated industrial source.
Method 2 requires that the tester use an S-type pitot tube to obtain differential pressure readings at specified points over a cross section of the duct being tested in order to account for the variability of the flow rate across the cross section. The tester is asked to extend a moveable pitot tube assembly some distance into the duct to a prescribed point, align it parallel to the center line of the duct and hold it there for a number of seconds to allow a second person to take the differential pressure reading from a fluctuating incline manometer. The tester then moves sequentially to each of the other points, makes the alignment and waits a few moments again to allow the second tester to take the readings. This continues until all points on the traverse are read. During this process, the pitot assembly must be removed from the duct completely and moved to another opening or openings to get access to position the pitot tube at all of the traverse points, sometimes taking an extended period of time. Thereafter, repetitions of that process are made to increase the confidence in the accuracy of the measurement by having more data points, to average and to account for variations in the gas flow over time. As a practical matter, there are a number of disadvantages with the use of this manual method in actual field conditions which have a significant impact on the accuracy of the measurements derived. They include:
1. The imprecise method of positioning and aligning the pitot tube assembly sufficiently close to the specified sample point and then repositioning it again at the same point on subsequent repetitions of the traverse;
2. The imprecise method of “eyeballing” to average the fluctuating inclined manometer indication of the differential pressure at each sample point;
3. The reading of the parameters of the gas flow at the individual sample points on a traverse at different times (i.e., sequentially) in a circumstance where instantaneous changes in gas flow parameters over the cross section are common. In other words, using the manual method there is no way to account or adjust for the variations in actual flow parameters at all other points on the traverse when a reading is taken at any one of the sample points at a specific time; and
4. The amount of time required to complete a traverse is significant and therefore costly and as such inhibits the ability as a practical matter to make more repetitions to increase the confidence in the accuracy of the measurements.
Prior art methods of determining gas flow rate, such as those disclosed in Traina, U.S. Pat. No. 5,394,759 and Traina et al., U.S. Pat. No. 5,509,313, involve the use of a single pitot tube to measure differential pressures at a variety of positions. The Traina '759 patent in particular discloses apparatus for automatically moving the pitot tube to a number of different positions in the duct and repositioning it at the same position in subsequent repetitions. The Traina et al. '313 patent discloses a method of measuring “true flow direction rate” at a variety of positions in order to more accurately measure gas flow. The premise of the latter method is that a more accurate measurement of gas flow rate is obtained by finding and adjusting for the angle of the true flow direction at each sample point versus the center line of the duct.
In the method of the Traina '313 patent, a pitot tube is automatically positioned at a sample point and displaced 90 degrees from a null point. Differential pressure readings are then taken and the true flow direction angle is recorded. The differential pressure readings are used to calculate flow rate considering the measured flow direction angle. Even though an automated system is used to control the repositioning of the pitot tube, the methods utilized in the foregoing Traina patents have the same disadvantage of the manual method described above in that they fail to determine flow rate at more than one position at any given time. In addition, it is possible for the true flow direction angle to change in the short time it takes for the pitot tube to be repositioned from the null point reading to the 90 degree position. As noted above, because there are continuous and quickly-occurring variations in gas flow rate and direction in a typical duct, calculations of flow rate based upon measurements at different points and at different times have significant accuracy limitations.
A commercial gas flow monitor system sold
Compliance Instrument, Inc.
Marshall O'Toole Gerstein Murray & Borun
Patel Harshad
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