Acoustics – Sound-modifying means – Muffler – fluid conducting type
Reexamination Certificate
2000-12-19
2003-03-18
Dang, Khanh (Department: 2837)
Acoustics
Sound-modifying means
Muffler, fluid conducting type
C181S267000, C181S268000, C073S861180
Reexamination Certificate
active
06533065
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a silencer and method for use with an ultrasonic meter that reduces noise in the ultrasonic range of frequencies generated by other equipment in the flow stream. More particularly, the invention relates to a silencer and method for use with an ultrasonic meter that is capable of reducing ultrasonic noise under high-pressure operating conditions. Still more particularly, the invention relates to a silencer and method for use with an ultrasonic meter that also acts as a reasonable flow conditioner.
2. Background of the Invention
In pipeline operations and other industrial applications, meters must be capable of accurately measuring the flow rate of gases or liquids moving through piping or tubing systems. In natural gas pipelines, for example, these flow rate measurements may be relied upon for custody transfer, leak detection, control, or for other indications.
For custody transfer operations, the meter is the point where custody transfer occurs, such as when gas is delivered into or out of a pipeline system through the meter as it measures the passing flow rate. By accurately measuring the flow rate for a given time period, the volume of gas that passes through the meter can be determined, and a custody transfer volume ticket can then be prepared. The pipeline transportation fee is based on the volume of product moved through the system, i.e. the custody transfer volume. Thus, a custody transfer metering system is commonly referred to in the pipeline industry as the “cash register,” and pipeline operators take great care to maintain its measurement accuracy.
Measurement systems comprising two or more meters may perform a pipeline leak detection function. A pipeline typically operates in a “packed” or full-line condition. Therefore, as gas is pumped into the system through the inlet meter, gas is simultaneously delivered out of the system through the outlet meter, and the measurements taken at each meter are compared. This “meter-in, meter-out” approach provides two modes of leak detection. First, the flow rate measured by the inlet meter should match the flow rate measured by the outlet meter within a certain accuracy tolerance, taking into account characteristics that may cause flow rate deviations, such as elevation differences or product temperature variations. Second, by measuring flow rate over a given time period, the volume moved through each meter can be determined, and the inlet and outlet meter volume measurements should correlate over that time period. A measurement discrepancy could indicate a pipeline leak, although the storage of gas in the pipeline (line packing) makes short-term leak measurements difficult. Nonetheless, early leak detection enables a pipeline operator to locate and repair the problem more quickly, thereby minimizing the environmental and public safety impacts of a leak. Thus, accurate metering systems are necessary for profitable, safe, and reliable pipeline system operations and other industrial applications.
Flow meters are available in many different forms. Most conventional meters, such as turbine meters, are inserted directly into the flow stream where the gas drives a rotor mounted within a meter housing. The meter measures the number of rotations per unit time, which is proportional to the gas flow rate. These meters are fairly expensive and require regular calibrations to maintain accuracy over a long time period. They are also intrusive to the flow stream and include moving parts with close internal tolerances that are susceptible to damage from gas flow stream contaminants.
The ultrasonic meter is often a preferable metering device in gas flow streams because it overcomes the problems of conventional in-line meters by measuring flow rate in a non-invasive fashion, with considerable accuracy, and with no moving parts. An ultrasonic meter includes two or more transducers that emit ultrasonic waves into the flow stream and measure the propagation time of each wave to determine the flow rate of the passing gas stream. An ultrasonic wave is a sound wave having a frequency above the audible sound range, and more particularly, having a frequency >20 kHz. A typical ultrasonic meter emits ultrasonic waves at frequencies between 50 kHz and 300 kHz, and preferably between 80 kHz and 180 kHz. U.S. Pat. No. 4,646,575 (hereby incorporated herein by reference for all purposes) discloses an ultrasonic meter and many of its features.
An ultrasonic measurement system may include a silencer placed between the meter and other equipment in the measurement flow stream. The silencer reduces stray ultrasonic noise that interferes with the accuracy of the ultrasonic meter. Such stray ultrasonic noise is commonly produced during gas distribution where the gas pressure is dropped precipitously and generates substantial noise (i.e. enough to interfere with measurements). A pressure-regulating valve that reduces the pressure of multiple incoming flow streams as the gas is combined into a common supply pipeline, or reduces the pressure from a main supply grid to local distribution, is another source of ultrasonic noise. Environmental regulations set upper limits on the acoustic noise level that industrial equipment can emit. To avoid excess acoustic noise, a pressure-regulating valve may be designed, for example, to reduce gas pressure by variably restricting small holes drilled into a rigid steel plate to reduce, as far as possible, the emission of sound waves in the acoustic range of frequencies. However, because the gas flow approaches supersonic velocity as it moves through these drilled holes, the pressure regulating valve instead generates high levels of broad band ultrasonic noise. This ultrasonic noise propagates through the gas to interfere with the ultrasonic flow meter signals, resulting in a poor signal to noise ratio and a loss of measurement accuracy.
Silencers are designed to attenuate the wave energy of stray ultrasonic noise by reflection, absorption or both. PCT Application WO 97/31365 (the contents of which are hereby incorporated herein by reference for all purposes) discloses one type of ultrasonic silencer that uses a diffuser arrangement, such as a perforated tubular body, with a multiplicity of small-area surfaces that frequently reflect the ultrasonic waves. These reflections result in destructive interference between the acoustic paths, thereby effectively damping the ultrasonic noise. The noise is attenuated by scattering the ultrasonic energy from the wave and reflecting it in many different directions. The diffuser surfaces of the silencer are preferably at least partially curved, leading to the formation of vortices inside the gas flow that likewise cause acoustic path interference to reduce the ultrasonic noise. These gas vortices can introduce undesirable flow disturbances into the measurement path, thus requiring the silencer to be located a minimum distance away from the meter. This distance requirement may be undesirable when space is limited.
A second type of silencer relies on absorption to attenuate stray ultrasonic noise. This silencer is a foam plug, formed of an open-cell material that is inserted into the flow stream for the gas to pass through before entering the measurement flow path. The foam plug attenuates noise by converting the ultrasonic energy into thermal energy through friction loss in the interstices of the material. Although this is an effective ultrasonic silencer, high-pressure loss is observed as the gas flows through the foam plug. Furthermore, the foam plug acts to filter dirt and absorb liquids in the flow stream. Thus, the open-cell foam plug silencer is only suitable for use in clean gas service and in systems that can accommodate high pressure loss through the silencer.
French Publication No. 2,737,564 (the contents of which are hereby incorporated herein by reference
Conley & Rose, P.C.
Dang Khanh
Daniel Industries Inc.
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