When evaluating machinery health, the analyst should:
- Visually inspect each machine for the overall condition of the machine, such as leaking seals, other damaged or incomplete components, cracked welds, deterioration in the grouting, missing clamps and hold-downs while visiting the site to collect vibration data.
- Ensure that you can determine the turning speed (TS) at the measurement point location before any critical measurement is taken.
- Once TS is identified, determine TS harmonic relationships
- When harmonics of TS are determined, other vibration peaks of interest and other harmonic relationships may be identified for analysis
Every machine will vibrate when excited by a forcing function. Each machine has one or more natural or resonance frequencies. When any forcing function is near the natural frequency, the resulting vibration will be significantly amplified and could cause premature failure.
A critical speed is when the rotating element is turning at a speed which excites resonance in the machine. Many times a forcing function (such as from a rotating shaft) excites a resonant frequency in another part of the same or nearby machine. This resonant frequency may be identified by an impact test, typically when the machine is off; the machine structure may be “rung” like a bell. Changing a structures stiffness, mass, damping, operating speed and/or reduction of the forcing function will affect the resonance issue and may help sort the problem.
Resonance is an increasing problem in industry because:
- New equipment is often built lighter
- More variable speed machinery is being used in industry
- Machines are often now run at a higher speed
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Detecting imbalance or misalignment is simple with most data collectors but the CSI 2130 can also detect developing faults on bearings and gears. Emerson’s patented PeakVue® processing applies digital technology to detect stress waves – the earliest sign of bearing and gear wear. Demodulation and other analog technologies typically cannot detect such faults until much later – after the machine is already damaged.
PeakVue processing not only offers the earliest warning of developing faults, it also provides an indication of severity. Measurements can be translated into reliable trends to determine the optimal timing for maintenance. With PeakVue, machinery faults are clearly visible in the waveform, opening up new options for fault detection and diagnosis.
Full Spectrum of Measurement
Another unique feature of the CSI 2130 is its exceptional frequency range. Using Emerson’s patented Slow Speed Technology (SST), the CSI 2130 can accurately measure signals on critical low speed equipment that would be out of range for other vibration analysers. The CSI 2130 also boasts the highest frequency range in the market. It can measure signals up to 80,000 Hz, which is important for accurate diagnosis of centrifugal compressors and other high speed machinery.
Variable Speed Analysis
Variable speed analysis is essential to any effective machinery health programme because most critical pieces of equipment must be operated at varying speeds to accommodate the changing production demands. While most vibration systems do not take variable speed into account during data collection, the CSI 2130 automatically adapts all of its diagnostic tools to variable turning speeds during routine data collection. This provides an accurate evaluation of developing problems in the field.
Correlate Vibration and Process Variables to Identify Machine Problems
Use the dual-channel feature of the CSI 2130 to correlate machinery vibration with process variables. This is accomplished by inputting the process information as a volt signal into one channel, while monitoring vibration on the other.
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Heat and fatigue, associated with excessive motor starts or overloading can lead to broken rotor bars in electric motors. Broken rotor bars eventually lead to motor failure, and can cause secondary damage to stator windings. Rotor bar damage can be difficult to detect in a vibration signature. Emerson’s CSI Motor Diagnostic technology, with embedded analysis expertise, helps complete a total Machinery Health Management programme by diagnosing electrical problems that may be missed with a vibration monitoring programme alone.
Motor current analysis provides information on rotor-related electrical faults such as broken rotor bars, high-resistance joints, voids in aluminum cast rotors, and cracked rotor end rings in squirrel-cage induction motors.
In addition to motor current analysis, Emerson’s motor monitoring tools also include motor flux analysis. Monitoring the magnetic flux field of the motor provides the safety and convenience of not having to open motor control cabinets to access electrical wiring directly, and provides detection of additional electrical problems related to the motor stator that are not found in the current signature. Temperature measurements also play an important role in the overall motor condition analysis and are incorporated into the automated analysis.
The tools are non-intrusive and perform motor current analysis, motor flux analysis and temperature measurements while motors remain online. Motor signature data is transferred to AMS Suite: Machinery Health Manager for automated expert analysis, trending, comparison with results from other diagnostic technologies, and implementation of corrective actions.
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Imbalance is the condition of a rotating component where the center of rotation is not the same as the center of mass.
Looseness is characterised by harmonics of TS with raised noise floor. Waveform displays random impacting which may be in a non-repetitive irregular pattern. Highest amplitudes typically occur in the radial direction, specifically the vertical plane for horizontally mounted equipment.
The number of turning speed harmonics and their amplitudes increase with the severity of the problem.
Unstable phase characterises looseness.
- Material buildup
- Broken or missing parts
- Improper assembly or poor design
- Thermal distortion
- Turning speed peak amplitude increases with icrease in speed
- Little axial energy except with overhung machines
- Little or no turning speed harmonics.
- Simple, sinusoidal, periodic waveform
- One event per shaft rotation
- Little or no impacting
Misalignment is when rotational center lines of coupled shafts are not colinear.
High, 2 x Turning Speed (TS), peak characterises offset misalignment; a high 1 x signifies angular misalignment, though both frequently combine to cause 1 x TS and 2 x TS in the spectrum. Highest amplitudes typically occur in the radial direction for horizontally mounted equipment; over hung rotors may exhibit higher amplitudes in the axial plane.
There are different methods which can be used to connect a vibration transducer to a piece of equipment. In order of preference, these are:
The vibration transducer is screwed onto a threaded stud which is attached to the machine, typically by drilling and tapping a hole, or by stud welding. Is commonly used only with inaccessable systems and online systems.
As an alternative to stud mounting, vibration transducers can be glued to the equipment using suitable adhesive.
Commonly used for route based monitoring or other temporary data collection, magentic mounting involves connecting the transducer to a magnetic base which is then magnectically attached to the equipment. This method can only be used if the mounting location is ferromagnetic.
Hand Held Probes
Hand held probes wield the poorest results and are not recommended but are sometimes necessary because of access restrictions or safety concerns.
There are three main types of vibration transducer.
Vibration is movement relative to a reference position, such as the centre line of a shaft on rotating equipment. It is a result of an excitation force or forcing function and may be either random or periodic. This is why vibration analysis for machinery is important to undertake.
Vibration monitoring can often pinpoint a failing element of a rotating machine in time to avoid catastrophic failure and costly replacement of machinery as well as lengthy production interruptions.
- Each machine fault generates a specific vibration pattern.
- The frequency of the vibration is determined by the machinery geometry and operating speed.
- A single vibration measurement provides information about multiple components.
Vibration Monitoring Can Identify The Following Problems
- Belt Problems
- Oil Whirl
- Vane pass
- Rolling element bearing defects
- Electrical Problems
- Sleeve bearing problems
- Flow problems
- Lubrication problems
- Gear problems
- Oil Whip
- Blade pass
- Pipe Strain
Vibration monitoring is used to improve profitability in every major industry in the world.