Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Mechanical measurement system
Reexamination Certificate
2001-05-02
2003-04-15
Shah, Kamini (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Mechanical measurement system
C702S138000
Reexamination Certificate
active
06549857
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods for accurate and reliable detection of leaks in pressurized pipe systems containing a liquid such as water, petroleum fuels and products, and other hazardous and non-hazardous substances, and more particularly to a method achieving high performance due to accurate compensation of product temperature changes that occur during a test.
2. Brief Discussion of Prior Art
There are a number of different types of pressure-based methods that are used to detect leaks in underground pressurized pipelines containing petroleum fuel or any type of liquid. A loss of liquid from the line due to a leak will produce a drop in pressure. A leak is declared if the pressure drop exceeds a predetermined threshold value. The magnitude of the pressure drop due to a leak is a function of and proportional to the volume of the liquid in the line, the bulk modulus of the pipeline system, and the initial pressure of the line. Thus, the pressure drop is larger in a smaller line than in a larger line for the same size leak. This is important because the pressure changes due to product temperature changes are independent of line volume.
Conventional Pressure Test
The most common type of pressure test is a pressure-decay or hydrostatic test. The method is to isolate the line to be tested from tanks or other line segments connected to it by valves or valve blinds, fill the line with a liquid or use the existing liquid in the line, pressurize the line, and then monitor the pressure changes over time. This pressure testing method is used for evaluating the structural integrity of a pipe. The main applications of the method are in transportation and transmission pipelines. Because of the inherent errors in this type of test, such a test was not originally intended to be used for leak detection.
Erroneous results occur in a pressure test if (a) any vapor is trapped in the line (or appurtenances attached to the line) or (b) the temperature of the fuel changes during a test. While both effects are acknowledged in the test procedure, no methods are offered to compensate for their effects. Also, the method described in these standards does not indicate what threshold to use to declare that the line is leaking, i.e., how large a pressure drop is required before the line is suspected of leaking. Over the years this method has been frequently applied to a wide range of pipelines, both small and large, but without much success for detecting small leaks.
Ambient Product Temperature Changes
One reason that this approach has not been successful is that a drop or rise in pressure can also occur if the temperature of the fuel (or liquid in the line) is also changing. An increase in temperature will cause the pressure in the line to increase. If these thermally induced pressure changes are large, they can mask the presence of a leak and result in a missed detection. A decrease in temperature will cause the pressure in the line to decrease. If these thermally induced pressure changes are large, they can falsely indicate the presence of a leak and result in a false alarm.
Underground petroleum fuel lines can experience large, nonlinear temperature changes, which produce large, thermally induced pressure changes, because the coefficient of thermal expansion for petroleum fuels is large and the temperature of the product brought into the line can be very different than the temperature of the fuel in the line or the ground surrounding the line. A new temperature condition is generated any time fuel from a storage tank is transferred through a pipe.
FIG.
1
(
a
) is a time series showing the typical thermal behavior of product brought into a line at a warmer temperature than the backfill and soil surrounding the line; FIG.
1
(
b
) shows the time series of the rate of change of temperature. The thermally induced pressure changes are proportional to temperature changes, and scale according to the bulk modulus (compressibility) of the line and the coefficient of thermal of expansion of the liquid in the line. The volume of product in the line affects the rate of change of the temperature in the line. Thus, the two time series in
FIG. 1
also illustrate the thermally induced pressure changes that occur in the line. In the present disclosure, this type of product temperature and line pressure change will be referred to as an ambient thermal change to distinguish it from product temperature and line pressure changes produced by changing the pressure in the line.
The observed curvature in both the temperature (and the pressure) and the rate of change of temperature (or the rate of change of pressure) curves in
FIG. 1
clearly illustrate the nonlinear changes in product temperature that occur during a test. When high performance is desired, testing with conventional pressure-decay methods, which do not compensate directly for the product temperature changes, cannot be initiated until the rate of change of temperature is sufficiently small that the thermally induced pressure changes are negligible. This means that the line must be taken out of service for whatever length of time is necessary to reach this stage of negligible thermal changes. Small pipelines at retail service stations may require a waiting period of 2 to 12 h. The larger lines at bulk fuel storage facilities may require a waiting period of 12 to 36 h, and the larger lines found in airport hydrant systems may require a waiting period of many days or longer.
This approach for minimizing the impact of fuel temperature changes with a waiting period has adverse operational and performance implications. First, transfer operations may need to cease for an unacceptably long period of time. Second, there is no way to guarantee that a presumably adequate waiting period is in fact sufficiently long for thermal changes to dissipate. Third, even if the waiting period is adequate, there is no way to verify quantitatively that the rate of thermal change is negligible or to verify that product temperature has not changed in response to other heat sources and sinks (e.g., heating or cooling of a section of an underground pipe that is exposed to sun or clouds).
The use of a waiting period to minimize the thermally induced errors in a pressure test is only practical for use on pipelines with small diameters or small capacities, such as those found at petroleum fuel service stations. Even for these pipelines, this approach has had only limited success. The use of a waiting period is also not useful if quick tests are to be conducted. The use of a waiting period is not practical for large diameter or large capacity lines found at bulk fueling facilities, in airport hydrant systems, or in transportation or transmission pipelines, because it could take many days or longer for the thermal changes to become negligible.
For these larger pipelines, accurate leak-detection tests can only be performed if the thermally induced pressure changes are compensated for. One compensation approach is to measure the temperature changes of the fuel in the line, estimate by standard hydraulic computations the magnitude of the pressure changes produced by these temperature changes, and then subtract these thermally induced pressure changes from the measured pressure changes. If the temperature compensation is accurate, then only the leak-induced pressure changes remain.
This temperature-compensation approach to performing a pressure test has a number of serious technical and implementation problems. First, it is very difficult to obtain accurate measurements of the fuel temperature changes along the length of the pipe. These measurements are necessary to account for any differences in the temperature conditions along the length of the line. Typically, no more than one temperature sensor is used, even though the pipe may be many miles in length and may be affected by many different thermal environments. Second, the bulk modulus and volume of the product in the line must be accurately known.
Trapped Vapor
The presence of tr
Fierro Michael R.
Maresca, Jr. Joseph W.
Jaffer David
Pillsbury & Winthrop LLP
Shah Kamini
Vista Research, Inc.
Washburn Douglas N
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