Predictive Maintenance (PdM) is the use of nondestructive predictive measurement technologies to detect equipment failures in their early stages of development and reduce the possibility of catastrophic failures during critical operations.

Condition Monitoring (CM) is the well-known term to refer to the process of monitoring certain machinery parameters capable of providing an indication of the machine condition, in order to identify significant changes which are indicative of a developing fault.

In an industrial plant some machines will be more critical than others and their criticality index is often used to determine the degree on condition monitoring or the predictive techniques applied on them. Some of these critical assets will need more than one technique to positively identify their health condition. When taking a critical machine out of production, the maintenance manager wants to be absolutely sure what certainly it needs to be fixed, that the right spares and personnel are available, and how long it will take.

A wide range of condition monitoring techniques is available. The following paragraphs describe the main condition monitoring techniques widely used in the industry, including the machines where they can be applied and the main faults that can be detected.

Vibration analysis

Vibration analysis is in particular the most popular predictive technique and is widely used in rotating and alternating machinery to support maintenance decisions. As a general rule, machines do not break down or fail without some form of warning, which is indicated by an increased vibration level. By measuring and analyzing the characteristic vibrations at different frequencies of a machine, it is possible to determine both the nature and severity of the mechanical defect, and hence predict the machine’s failure.

  • Main faults detected: unbalance, misalignment, eccentricity, looseness, structural issues, coupling issues, blade issues, electrical issues, bearing or journal bearing issues, hydrodynamic issues, gearbox issues, pulley issues and resonance.

Thermography

Thermography (or thermal imaging analysis or infrared (IR) thermography) is a condition monitoring technique that studies radiated energy to determine excessive heat in rotating and non-moving components.

  • Main faults detected: high temperature, hotspot, faulty electrical connection

Oil analysis

Oil analysis (or spectrochemical or spectrographic analysis) is a condition monitoring technique that consists in the examination of the physical, chemical and additive properties to maintain oil stability. It has been in use already for several decades and has proved to be a highly effective management tool for monitoring the health of lubricated machinery and the lubricants themselves. Oil analysis programs usually rely on submitting occasional or periodic oil samples to a laboratory for analysis. It is as well applied to pressure or refrigerant oils in mechanical and electrical equipment, and even to insulation oils in electrical transformers.

  • Main faults detected: Bad oil condition, contaminants and water content, machine wear. Also dielectric strength in insulating oils.

Wear debris analysis

Wear debris analysis (or ferrographic analysis) is a condition monitoring technique that studies oil-wetted parts (shape or morphology, composition, size and concentration) to determine possible wear mechanisms, wear rates and wear sources. Wear debris analysis programs usually consist of submitting occasional or periodic oil samples to a laboratory for analysis.

  • Main faults detected: Bad oil condition, contaminants and water content, machine wear. Also dielectric strength in insulating oils.

Wear debris analysis vs oil analysis: Spectrometry (used for oil analysis) studies particles lower in size than 10 microns, while, on the other hand, ferrography (used for wear debris analysis) studies particles above 10 microns. During normal machine operation small wear debris particles of the order of 1 to 10 microns are generated. O the contrary when abnormal wear begins, large debris particles in the range of 10 to 150 microns are produced.

Airbone ultrasound analysis

Airbone ultrasound analysis (or ultrasonic analysis) is a condition monitoring technique that studies high frequency (ultra-sonic) sound waves (above 20 kHz) to detect frictional forces, electrical issues and leaks.

  • Main faults detected: air leaks,steam leaks in steam traps, bearing issues, electrical issues, lubrication issues, flow related issues

Motor current analysis (MCA)

Motor current analysis is a condition monitoring technique that studies motor currents in the frequency domain. Motor current signatures are measured and converted from the time domain to the frequency domain and analyzed in form of spectra in a similar way as vibration (movement).

  • Main faults detected: stator issues, rotor issues, airgap issues, insulation issues, power circuit issues, power quality issues

Visual inspection

Visual inspection is a condition monitoring technique that refers to any evaluation procedure that uses direct measurements taken by the human eye and/or remote measurements made with non specialized inspection equipment.

  • Main faults detected: rust, missing parts, leaks, breakages, deterioration, dirt

Others

There are other several minor used techniques, as:

  • Performance analysis: is a way of measuring physical performance of both rotating and non-rotating plant assets.

  • Alignment (laser): is a way of measuring misalignment between two or more shafts.

Their objective is provide indicators of their condition that can help us to detect asset failures and improve maintenance. There are also many techniques specifically applied to a type of asset, for example, dissolved gas analysis in transformers.

 

Conclusions

It has long been accepted that condition-based maintenance (CBM) is the most effective and cost-efficient approach to maximizing the life of industrial machinery. Although a strong condition monitoring program does not translate into zero downtime or machinery failures, it will drastically reduce unforeseen problems and allow for improved reliability.

In an effort to establish more accurate maintenance strategies it is often advantageous to utilize multiple CM techniques simultaneously, in an integrated way. It is clear that each technique has its strengths and limitations. For example, vibration based techniques can diagnose the rotating element to suffer from an abnormal behavior and pinpoint the damage location. However, it cannot provide information on lubricant condition.

It is important identify what are the proper techniques necessary to suit the particular characteristics of the critical assets. A reliable and easily accessible equipment history is indispensable when investigating recurring problems and evaluating success of corrective actions.