All rotating equipment lends themselves to vibration measurement and analysis. During normal operation they emit a specific vibration signature. If this signature changes it provides an indication of a change in the health of the machine. When referring to vibration signature or vibration in this document it is considered to mean the defining characteristics (spectrum) of vibration. A vibration spectrum will separate out various components of the overall vibration level to assist in evaluation and diagnosis since many things contribute to the overall vibration level.
- Displacement – An indication of the magnitude of displacement, usually in mils. The severity of displacement can only be determined if the frequency is known.
- Velocity – An indication of the rate of change of displacement, usually measured in terms of peak inches per second. This is the most useful as it characterizes the severity of the destructive energy in the machine
- Acceleration – Is the time rate of change in velocity usually measure in terms of G’s.
The vibration characteristics can be plotted against frequency usually in cycles per minute (cpm) of cycles per second (Hz)
All the above information can be obtained from fixed transducers for real time monitoring or relatively inexpensive handheld analyzers for periodic surveillance.
If vibration is to be used as a predictive maintenance tool the program should consist of three phases.
- Surveillance or monitoring – information gathered at discrete points of time often do not provide adequate information as to assist in the evaluation or resolution of a problem when it is identified. Accordingly a well planned monitoring program records the equipment vibration signature at time of commissioning or re-commissioning and periodically collects the equipment signature which can provide trend data to identify characteristics which may not be of immediate concern but may develop into serious issues.
- Analysis or Diagnosis – with appropriate data it is possible to identify characteristics related to specific components, system influences, electrical effects or hydraulic influences.
- Corrective action or remedy
During operation the vibration spectrum gradually changes as components wear, looseness appears and materials build up.
Vibration in glandless pumps can sometimes be difficult to evaluate because the fluid inside the pump and motor surrounds the rotor and can mask some elements of vibration that would clearly indicate a problem. Further to this, high pressure equipment with its attendant heavy wall vessels further tends to attenuate internal signals. In addition rotating elements supported with hydrodynamic bearings have their vibration signatures best monitored by direct contact.
What we have found from field experience and discussions with end users is that relying on a single numerical value of displacement or velocity does not provide a reliable indicator of machinery health. The reasons for this are as discussed above and:
- Critical values for this equipment tend to be lower than more typical machinery that most people are familiar with.
- The as measured values in situ consist of contributions made internally from mechanical imbalance, electrical imbalance, bearing clearance, wear ring clearance, hydraulic forces and mechanical looseness together, foreign matter with external contributions from structure natural frequency, through the connecting pipe work, hangers and mounting bolts.
- With all these factors to consider and with lower overall limits each unit and each installation position can have significant differences.
As an illustration, one installation establishes their base line signature beginning at the installation of a refurbished unit. Data is obtained at the motor lower cover for two horizontal directions and one axial direction. For each position baseline signatures are different. Having the baseline signatures in hand, levels are set for “Alert High” and “High Danger”.
- “Alert High” for radial vibration levels established at 1.60-1.75 times the mean baseline signature
- “High Danger” for radial vibration levels established at 1.7 times the “Alert High” setting.
- “Alert High” for axial vibration levels established at 2.0 times the mean baseline signature
- “High Danger” for axial vibration levels established at 1.4 times the “Alert High” setting.