Data processing: measuring – calibrating – or testing – Measurement system – Performance or efficiency evaluation
Reexamination Certificate
1997-12-31
2001-07-10
Shah, Kamini (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system
Performance or efficiency evaluation
C702S130000, C702S185000, C702S034000, C702S035000, C073S168000
Reexamination Certificate
active
06260004
ABSTRACT:
CROSS-REFERENCE OF RELATED APPLICATION
Not applicable.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for diagnosing a pump system.
2. Background
Pumps are among the least reliable components in a process plant with the average Mean Time To Repair (MTTR) averaging two years. Recent advances in vibration sensor based condition monitoring are now routinely used to measure the vibration profile of a pump system to determine if large velocity or acceleration vibration levels are present. Such vibration levels are indicative of a failed or failing pump.
Pump maintenance is most often required due to operating a pump under conditions where bearing loads are high and where there is fluid induced damage to the impeller, i.e., cavitation and recirculation. The desirability of operating at the pump's Best Efficiency Point (BEP) is well known in the pump industry. At the to be at a minimum, vibration levels are lowest, and cavitation and recirculation are avoided. Examples of the impact on pump life due to off-BEP operation can be found in
Pump Characteristics and Applications
by M. S. Volk.
No commercial product, method or system has the ability to provide an operator or maintenance engineer with the actual operating pump performance curve and process operating point. An understanding of the operating point or range versus the intended operating range on a pump performance curve is key to operating a pump near its BEP and to diagnosing pump component damage when operating in off-BEP regions.
Vibration monitoring equipment such as that provided by Bently Nevada is well-known for condition monitoring of rotating equipment. The Bently Nevada system consists of sensors (typically accelerometers, displacement sensors, proximity velocity transducers and temperature sensors) that are appropriately mounted to rotary equipment such as turbines, compressors, fans, pumps and drive units such as motors.
Monitoring machine performance through vibration signature analysis is a practice spanning more than two decades. Many standards used for overall vibration measurements are based on specific rotational frequencies and integer multiples of these specific rotational frequencies. Vibration data is routinely collected either manually or with on-line systems from bearings on rotating machines. Bearing vibration measurements should include measurements in both the horizontal and vertical planes of each bearing. At least one axial vibration is made for each shaft.
Vibration readings on the bearing housing, using an accelerometer (acceleration) or a velocity transducer (velocity), provide sufficient data to detect the onset of bearing failure. Displacement or proximity probes measuring the motion the shaft relative to the bearing also provide useful data for diagnosing bearing failure. The motion of the shaft within the bearing as measured by the proximity sensor is commonly called an “orbit”.
Rotating machines and pumps, by their very nature are dynamic machines. Vibration and proximity sensor data is also dynamic and is typically collected as trend data, FFT and waveforms. Most faults are identified by distinct frequency peaks or patterns and therefore frequency bands may be defined which bracket specific faults. These bands may be specifically scanned for amplitude changes which signal the need for further analysis. These scans will include comparing recorded vibration levels against alarm levels as well as a statistical analysis of variation and comparison to baseline values. The defined frequency bands will include the calculated or measured resonant frequencies of the major rotating machine mechanical components such as the shaft, impeller, radial bearings and thrust bearings. Analysis of vibrational data to identify known faults are further described in the CSI Application paper:
“Vibration Monitoring of Common Centrifugal Fans in Fossil Fired Power Generation”.
Further analysis will include a review of the amplitude and phase versus frequency spectra, sometimes a referred to as the Bode plot, for the proximity and vibration sensors. Multiples of the machine component resonant frequencies, commonly termed harmonics, are also examined.
These known vibration monitoring techniques are applied in combination with the rotating machine performance curves to provide for root cause analysis the rotating machine in a method previously not performed.
The sensors are mounted for the purpose of detecting impending motor bearing or pump bearing failures through sensor signal analysis using conventional spectral analysis such as the Fast Fourier Transform (FFT).
The use of a vibration spectra is well known, but such use often requires a human expert to examine the spectra and deduce damage. Expert analysis is required since the frequency components for all of the mechanical components (bearings, impeller, piping, etc.) are all present at the same time. Therefore, discrimination of the vibration by component requires substantial skill. As vibration sensors provide a spectral output (frequency domain), the vibration peaks correspond to a multiplicity of failure modes that may be present at the same time. A single vibration spectra is likely to show the shaft frequency, impeller frequency, radial bearing fundamental frequency, thrust bearing fundamental frequency, motor harmonic frequency, pump/piping resonant frequency, mounting plate frequency, etc. In addition to these fundamental frequencies, the harmonics or multiples, and submultiples of these frequencies will also be present.
Traditional condition monitoring systems are used to detect damage that has already occurred to a rotating machine. The pump diagnostics method is able to detect rotating machine operating conditions that may lead to pump damage. The pump diagnostics method also provides an ability to focus the maintenance engineer or technician to examine a specific area of vibration spectra for evidence of a pending failure detected through knowledge gained from the pump performance signature.
Additional analysis is provided through the measurement of the pump to motor shaft alignment using position or proximity sensors and through the measurement of the shaft “orbit” within the bearing. Temperature measurements are strategically positioned to provide data on bearing “hot spots”. Bently's system provides for off-line or “pseudo real time” acquisition of the above data and field processing of the sensor dynamic data which can be communicated to a centrally located display for viewing either via a proprietary communication or the Modbus protocol. Real time analysis has not been possible in the past due to bandwidth limitations in communications protocols.
Vibration sensors (piezoelectric accelerometers, velocity transducers and proximity sensors) are available from many suppliers such as Bently Nevada, Vibrametrics, Dytram and CSI and are often used with FFT algorithms for determination of vibration spectra for rotary equipment.
Vibration monitoring systems are available in portable “walk around” versions versus in situ systems where the vibration spectra is measured periodically. These portable vibration systems can provide for diagnosis of bearing failure, but are not as effective in determining the “root cause” for the bearing failure.
A diagnostics method is needed that provides for guidance in the repair of a failing pump, but most importantly, provides the operator with a root cause analysis that enables elimination of the cause of failure, which is often operation of the pump outside the BEP range. Available condition monitoring-only solutions provide limited guidance on elimination of the cause of failure as they observe the failure of the mechanical components, but do not identify the operating condition that caused the failure.
a. Electric Motor Diagnostics
Several manufacturers provide partial solutions for the off-line diagnosis of electric motors. Framatome and Liberty Technologies both provide for PC based electric motor diagnostics systems consisting of vibration se
Brunson Thomas A.
Hays Coy L.
Lenz Gary A.
Head Johnson & Kachigian
Innovation Management Group, Inc.
Shah Kamini
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