Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
2002-03-29
2003-10-28
Williams, Hezron (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
C073S659000, C073S592000
Reexamination Certificate
active
06637267
ABSTRACT:
FIELD OF AND BACKGROUND OF THE INVENTION
The invention relates generally to a diagnostic system and a diagnostic method for a valve, which can be actuated by a positioner via a drive.
In many areas of process and power technology, trouble-free operation of a plant depends on the performance of the control and check valves used. To avoid costly unscheduled interruptions of plant operations, valve damage should be detected as early as possible, e.g., before the failure of a valve causes the plant to be shut down. For instance, defective valve seats can cause leakage flows that produce broadband sound emission. Recording and evaluating the sound emitted by a valve can consequently be used for early detection of valve damage. Thus, since valve faults can lead to system damage and increased follow-up costs, diagnostics, possibly with automatic detection and programmable evaluation of the faults, are highly useful. Statistical evaluation of the diagnostic data can be used to optimize the maintenance processes and effectuate timely replacement of damaged valves as well as to evaluate and classify the valve manufacturers with respect to quality or to evaluate the suitability of certain valves for different process types.
European Patent Number EP 0 637 713 A1 discloses a diagnostic system with a structure-borne noise sensor, which is mounted to the housing of a valve and the signal of which is supplied to a device for detecting and storing structure-borne noise spectra. Characteristic curves of the sound level versus sound frequency are different in an intact, defect-free, valve than they are in a defective valve. Forming a surface integral and defining an acceptable deviation makes it possible to detect a defective valve. This measurement is suitable, in particular, to determine valve wear caused by corrosion, cavitation, or erosion. For evaluation, the structure-borne noise spectrum in a frequency range of between 2 kHz and 10 kHz is evaluated.
The German Utility Model Application with the official file number 299 12 847.4 proposes an acoustic sensor, particularly an ultrasonic sensor, for acoustic valve diagnostics having a substantially cup-shaped housing, in the interior of which a piezo-electric measuring element is arranged. To improve immunity of the device against electromagnetic fields, a shield with a sleeve is provided, one base of which is sealed with an insulating disk that is provided with an electrically conductive coating. The measurement electronics are galvanically decoupled relative to the mounting means and thereby relative to a mounting location. To fix the device to the mounting location, the bottom of the cup-shaped housing is provided with a coaxially arranged threaded stem on its exterior. The acoustic sensor permits detection of the structure-borne noise that is generated in a valve body by flow sounds in a frequency range greater than about 100 kHz without cross sensitivity for electromagnetic fields. For further information on the construction of the acoustic sensor, see the aforementioned utility model application.
The German patent application with the official file number 199 24 377.8 proposes a diagnostic system for a valve that can be actuated by a positioner via a drive and comprises a device for detecting, storing and evaluating the structure-borne noise spectra measured on the valve. To permit particularly reliable valve diagnostics, a structure-borne noise spectrum that is recorded for a slightly open intact valve can be stored in the detection, storage and evaluation unit. For the diagnosis, a structure-borne noise spectrum recorded for a closed valve is compared with the stored spectrum and the result of the comparison is used as a criterion for valve leakage.
OBJECTS OF THE INVENTION
An object of the present invention is to provide a diagnostic system and a diagnostic method, which are distinguished by improved reliability of the diagnostic result.
SUMMARY OF THE INVENTION
To address the above and other deficiencies in the prior art, a diagnostic system, in particular for a valve, is proposed which has a sensor operable to sense structure-borne noise in the valve and an evaluation unit operable to evaluate a recorded measurement signal. The evaluation unit is configured such that a spectral region of the measurement signal above a first limit frequency, where the first limit frequency is greater than 50 kHz, is evaluated for fault detection and a fault indication signal is generated if an intensity of the measurement signal in the spectral region exceeds a threshold value.
Also, a method for determining a fault in a component such as a valve through which a gaseous or a liquid material flows is proposed in which the method includes measuring an acoustic signal generated in the vicinity of the valve as the gaseous or liquid material flows through the valve, separating the acoustic signal into a low frequency portion and a high frequency portion, and determining a likelihood value representing a likelihood that the valve has a fault, wherein the likelihood value is based on the high frequency portion of the acoustic signal.
According to one embodiment of the invention, a distinction is advantageously drawn between a lower spectral noise region, which essentially comprises the operating noise of the valve, and an upper spectral region, which comprises primarily fault-related noise in certain operating states. A frequency range separating these two spectral regions can be selected between 50 kHz and, for instance, 200 kHz, since the operating noise occurs primarily in the range of less than 120 kHz. In any case, a spectral region of the measurement signal above about 50 kHz, is evaluated for fault detection.
The present embodiment is based on the discovery that fault-related noise with respect to gases is primarily produced by ultrasonic flow and, with respect to liquids, primarily by cavitation. Ultrasonic flow is produced as a result of even the smallest valve leaks. Along edges and narrow points leaks cause compression waves and refraction waves in gaseous media. Extremely rapid, spontaneous compression waves, alternating between local ultrasonic flow and subsonic flow in the gas, result in high-energy, broadband ultrasonic emissions, the spectral frequencies of which are comparable to those of cavitation in liquids. Cavitation is defined as the formation and subsequent condensation of vapor bubbles in flowing liquids caused by changes in velocity. Cavitation occurs when the pressure locally falls below the vapor pressure of the liquid as the flow accelerates, so that vapor bubbles form. Subsequent deceleration causes the static pressure to increase above the vapor pressure so that the vapor bubbles condense again. Due to the sudden reduction in volume, this results in an abrupt collision of the liquid particles that previously surrounded the vapor bubble and strong pressure surges. These pressure impulses produce an acoustic signal with a spectral distribution that is similar to that of white noise, i.e., it is possible to detect signal components up into the high frequency ranges.
An increase in cavitation can have a number of causes: e.g., abrasive wear, deposits, or valve seat damage. Particularly in a closed valve, the occurrence of cavitation noise is a clear indication of leakage flow of a valve that no longer seals properly. The intensity of the cavitation noise is a function of the pressure on the valve and the process medium flowing through the valve.
Flow through leaks in a valve produces operating noise as well as fault-related noises. The fault-related noises are ultrasonic noises, which are largely independent of the state of aggregation of the medium, liquid or gaseous, and of the type of the medium, and which resemble one another with respect to their frequency distribution. In regard to the spectral intensity distribution, this applies especially to frequencies above 100 kHz. On the other hand, the difference in the amplitudes of the sound spectra of different low-viscosity liquids and gases, especially in the frequency range b
Fiebelkorn Klaus-Dieter
Klebert Gerhard
Puettmer Alf
Fayyaz Nashmiya
Siemens Aktiengesellschaft
Sughrue & Mion, PLLC
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